GB1597395A

GB1597395A - Process for the digestion of sludge

Info

Publication number: GB1597395A
Application number: GB4813/78A
Authority: GB
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1977-02-08
Filing date: 1978-02-07
Publication date: 1981-09-09
Also published as: IT1105624B; NL190236C; AR214769A1; FR2379485B1; AU517067B2; SE7801404L; MX150615A; JPS5756400B2; BR7800726A; SE428125B; BE863735A; NO151358C; IT7847961A0; AU4028578A; NL190236B; CA1096064A; JPS5399653A; DE2805054B2; NO780424L; NL7801399A

Abstract

Process for digesting sludge by oxygenation in a first closed zone, at a temperature of 45 to 75 DEG C, and then subjecting the partially stabilised sludge originating from these zones to an anaerobic digestion in a second closed zone, at a temperature of 30 to 60 DEG C. This process can be applied to the removal of waste water. <IMAGE>

Description

(54) PROCESS FOR THE DIGESTION OF SLUDGE (71) We, UNION CARBIDE CORPORATION, a corporation organized and existing under the laws of the State of New York, United States of America, whose registered office is, 270 Park Avenue, New York, State of New York 10017, United States of America.

(Assignees:- Michael Stephen Gould and Ladislas Charles Matsch), do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates generally to a process for warm digestion of sludge, carried out under aerobic and anaerobic conditions.

With continued growth of industry and population, the problems associated with wastewater disposal are correspondingly increased. Although physical, chemical and biological treatment systems have been developed which can efficiently treat polluted waters to produce an effluent suitable for release to natural receiving waters, almost all of the basic wastewater treatment systems currently in use, including clarification, chemical precipitation, biological filtration and activated sludge, convert the water pollutants into a concentrated form called sludge. Particularly in the activated sludge process, which is among the most popular of conventionally employed wastewater treatment systems, there is usually a significant net positive production of volatile suspended solids (MLVSS), i.e., the rate of cell synthesis exceeds the rate of cell destruction. Therefore, an increasing inventory of sludge builds up and the excess activated sludge must be discarded from the process continuously or periodically.

As the overall volumes of wastewater requiring treatment increases, particularly under the impetus of increasingly stringent pollution control legislation, the quantity of waste sludge produced by the above-mentioned wastewater treatment processes is correspondingly increased. Accordingly, it is highly desirable to process this waste sludge in such manner that it can be readily and economically disposed of without creating further pollution of the ecosphere. While much effort has been spent in development of improvements in sludge treatment technology as well as in refinement of existing sludge treatment processes, there still exists a great need for better and more efficient sludge treatment systems.

The basic aim of all sludge treatment processes is to economically and efficiently reduce and stabilize sludge solids. In addition, the sludge treatment system should desirably also produce an end product which is fully suitable for final disposal without further physical or chemical treatment. In conventional practice sludge disposal is commonly carried out by either ocean dumping, combustion or land spreading. In many instances, land disposal is employed and is particularly attractive due to minimal long-term environmental effects. In fact, land spreading of sludge may be highly advantageous in promoting reconditioning of the soil. However the use of land spreading as a final sludge disposal method requires a well-pasteurized end product, so that the concentration of pathogenic organisms in the sludge is sufficiently low to avoid a potential health hazard in disposition of the sludge.

Traditionally, three distinct processes have been widely utilized for treating waste sludge: oxidation ponds, anaerobic digestion and aerobic digestion.

Oxidation ponds are generally employed in the form of comparatively shallow excavated basins in the earth which extend over an area of land and retain wastewater prior to its final disposal. Such ponds permit the biological oxidation of organic material by natural or artifically accelerated transfer of oxygen to the water from the ambient air. During the bio-oxidation process, the solids in the wastewater are biologically degraded to some extent and ultimately settle to the bottom of the pond, where they may become anaerobic and be further stabilized. Periodically the pond may be drained and the settled sludge dredged out to renew the volumetric capacity of the pond for further wastewater treatment, and the withdrawn sludge is utilized for example for landfill. Oxidation ponds thus represent a functionally simple system for wastewater and sludge treatment. The use of oxidation ponds, however, has limited utility, since their operation requires sizable land areas.

Moreover, no significant reduction of the level of pathogens in the sludge is accomplished by this treatment and disposal method.

Anaerobic digestion has generally been the most extensively used digestion process for stabilizing concentrated organic solids, such as are removed from settling tanks, biological filters and activated sludge plants. In common practice, the excess sludge is accumulated in large domed digesters where the sludge is fermented anaerobically for 20-30 days. The major reason for commercial acceptance of anaerobic sludge digestion are that this method is capable of stabilizing large volumes of dilute organic slurries, results in low biological solids (biomass) production, produces a relatively easy dewaterable sludge and is a producer of methane gas. Additionally, it has been variously alleged that anaerobic digestion produces a pasteurized sludge. Even though this pasteurizing capability of anaerobic digestion is questionable, anaerobic digestion is widely used in practice because it reduces the solid residue to a reasonably stable form which can be discarded as land fill without creating a substantial nuisance. The anaerobic digestion is characteristically carried out in large scale tanks which are more or less thoroughly mixed, either by mechanical means or by the recycling of compressed digester gas. Such mixing rapidly increases the sludge stabilization reactions, by creating a large zone of active decomposition.

As indicated above, anaerobic digestion has commonly been practiced with long retention times on the order of 20 - 30 days, without any heat imput to the system. It has been found by the prior art that elevated temperatures in the mesophilic range of 30 to 40"C facilitates reduction of the retention time requirement, to about 12 - 20 days. Such reduction in treatment time is a consequence of the fact that the rate of activity of the organisms responsible for digestion is greatly influenced by temperature, and that in the 30 to 40"C temperature range highly active mesophilic microorganisms are the dominant microbial strain in the sludge undergoing digestion. The best temperatures for mesophilic digestion are in the range of about 35 to 380C, with minimum retention times on the order of 12 - 15 days. Temperatures up to 350C increase the rate of digestion and may allow shorter retention times, but at the expense of system operating stability while temperatures below 35"C require longer retention times.

Methane gas is produced during anaerobic digestion and is characteristically used in combustion heaters to offset heat losses of the anaerobic digestion system operating at elevated temperature. However, seasonal temperature variations and fluctuations in the suspended solids level of the influent sludge have a significant effect on both the methane gas production and the amount of heating which is necessary to maintain the digestion zone at the desired elevated temperature operating level. As a result, if elevated temperature conditions are to be maintained year round in the anaerobic digestion zone, an auxiliary heat source is generally an essential apparatus element of the sludge digestion system.

Since the rates of anaerobic digestion and resultant methane gas formation are strongly influenced by the suspended solids content of the sludge undergoing treatment and by the temperature level in the digestion zone, it is in general desirable to feed as concentrated a sludge as possible to the digester, thereby minimizing heat losses in the effluent stabilized sludge stream discharged from the anaerobic digester while maximizing methane production in the digester. However, even with such provisions elevated temperatures are difficult to maintain economically in the anaerobic digestion zone, expecially during winter months. Furthermore, even comparatively small temperature fluctuations in the anaerobic digestion zone may result in disproportionately severe process upset and souring of the digester contents, as is well known.

In the anaerobic digestion process, the sludge solids being treated undergo essentially three distinct sequential treatment phases: first, a period of solubilization, secondly, a period of intensive acid production (acidification), and finally, a period of intensive digestion and stabilization (gasification). Each of these steps is characterized by the production of various intermediate and end products in the digestion zone. Under normal operating conditions, all three phases occur simultaneously. The primary gases produced during the final gasification phase are methane and carbon dioxide, which normally form more than 95% of the gas evolved, with 65-70% comprising methane. Production of methane gas in anaerobic digestion results from the breakdown of many compounds by numerous interdependent biochemical reactions which take place in an orderly and integrated fashion. The complex organic species in the sludge are converted by a variety of common bacteria called acid-formers to volatile acids and alcohols, without production of methane. These products from the acid-forming phase are then converted to methane gas by another variety of bacteria known as methane-formers.

The facultative acid-forming bacteria utilized in anaerobic digestion are hardy and highly resistant to process changes in their environment. Methane-forming bacteria, on the other hand, require anaerobic conditions and are extremely sensitive to process changes in their environment. For such reasons, oxygen should not be present in the anaerobic digestion zone. The inadvertent introduction of air to the digester will adversely affect methane fermentation, as well as creating a potentially hazardous situation due to combination of the combustible methane gas with oxygen. In addition, methane-forming bacteria are sensitive to such process conditions as pH variations and presence of heavy metals, detergents, ammonia and sulfides. In this respect, temperature stability of the anaerobic digestion zone is particularly important. The methane-formers necessary in the digestion process are highly susceptible to temperature fluctuations, which decrease their activity and viability, resulting in excessive relative growth of acid-formers. This in turn results in inadequately stabilized sludge and a sludge product which is unsuitable, without further treatment, for landfill or similar disposal. Further, these methane-formers have a relatively low rate of growth and this factor necessitates the long retention times employed for anaerobic digestion even at mesophilic temperatures. Due to this low growth rate, there is danger of washing the methane-forming organisms out of the digester if the sludge solids retention time therein is reduced beyond the previously described retention time lower limits.

Inasmuch as the anaerobic digester thus requires long retention times to insure the presence of adequate methane-formers and the influent sludge flow rate to the digestion zone is in general quite low, the tankage requirements for the digester are very large. Operation at elevated temperature is thus difficult, requiring large imputs of heat to the digester together with close control of the digester temperature level. As previously discussed, the prior art, faced with these considerations, has utilized the methane produced by the anaerobic digestion process as heating fuel for the digester, to maintain constant elevated temperature even under extreme ambient temperature fluctuations. Such use of methane has proven effective in minimizing the large heating energy requirements of the process.

As an alternative to the foregoing methods, biodegradable sludge can be digested aerobically. Air has commonly been employed in practice as the oxidant for this purpose. It is known that aerobic digestion proceeds more rapidly at elevated temperatures. As temperature rises from 35"C, the population of mesophilic microorganisms decline and thermophilic forms increase. The temperature range of 45"C to 75"C is often referred to as the thermophilic range where thermophils predominate and where most mesophils are extinct. Above this range, the thermophils decline, and at 90 C, the system becomes essentially sterile. Because of the more rapid oxidation of sludge, thermophilic digestion achieves more complete removal of biodegradable volatile suspended solids (VSS) than the same period of digestion at ambient temperature. A more stable residue is obtained which can be disposed of without nuisance. It is also established that thermophilic digestion effectively reduces or eliminates pathogenic bacteria in the sludge, thereby avoiding the potential health hazard associated with its disposal.

When diffused air systems are used to supply oxygen for digestion, the heat losses tend to be very large. As a result, aerobic digestion using air is typically restricted to digestion with mesophilic microorganisms. Air systems thus cannot operate efficiently in the thermophilic temperature range. Air contains only 21% oxygen and only about 5-10So of the oxygen component is dissolved. As a result, a very large quantity of air must be used to supply the oxygen requirements, and the sensible heat of the "spent" air and the latent heat required to saturate the spent air with water vapor are substantial. At ambient temperature (20"C or lower) sludge residence time in aerobic digestion with air is typically 12-20 days and huge tanks are required to retain the sludge. Even though steps are taken to suppress the rate of heat losses by conduction, convection and radiation, the large exposed area for heat transfer results in heavy heat losses.

As a result of the above listed heat losses in air digestion, autothermal heat effects are minor, and an unecomonical quantity of external heat is needed to sustain temperatures at beneficial levels.

It is known that the heat losses in aerobic digestion can be greatly reduced by using oxygen-enriched gas rather than air. If the oxygen is utilized effectively, the amount of gas which must be fed to and vented from the digester is considerably smaller compared to air, because much or all of the nitrogen has been preliminarily removed. Heat losses due to sensible warmup of the gas and to water evaporation into the gas are decreased. These reductions in heat losses are sufficient for autothermal heat alone to sustain temperature at levels appreciably higher than ambient, so that the digestion zone is able to operate efficiently in the thermophilic temperature regime with minimal input of external heat to the process. Since thermophilic stabilization is much more rapid than mesophilic stabilization the necessary residence time in the aerobic digestion zone is greatly reduced in the thermophilic mode. This in turn permits the use of smaller basins which further reduces heat losses to the surroundings. Because of the faster rate of oxidation of sludge, thermophilic aerobic digestion can achieve suitable high biodegradable volatile solids reduction, as for example, 80-90% reduction levels, in comparatively short sludge retention periods on the order of 3 to 10 days.

Despite its substantial attractiveness, thermophilic aerobic digestion has several associated disadvantages relative to anaerobic digestion. First, since the thermophilic aerobic digestion process is oxidative in character, the process produces a biooxidation reaction product gas containing carbon dioxide and water vapor which have no end use utility but rather are desirably vented to the atmosphere. By contrast, anaerobic digestion produces methane gas as a reaction by-product which may be exported from the treatment facility and is also useful as combustion gas for satisfying the heating energy requirements associated with digestion at elevated temperatures. In addition, the aerobic digestion zone requires a much greater energy expenditure, for mixing and gas-sludge contacting, than is required in the anaerobic digestion system for mixing of the digester contents.

According to one aspect of the present invention there is provided a process for digestion of sludge, comprising the steps of: (a) introducing as fluids said sludge and aeration gas comprising at least 50 percent oxygen (by volume) to a first covered digestion zone and mixing same to maintain the dissolved oxygen content (DO) of the mixed liquor at least at 2 mg/l and the total suspended solids content (MLSS) of the sludge at least at 20,000 mg/l; (b) maintaining sludge in said first digestion zone during step (a) at a temperature in the thermophilic range of from 45" to 75"C; (c) continuing step (b) for sludge retention time (duration) of from 4 to 48 hours to partially reduce the biodegradable volatile suspended solids content of the sludge introduced to said first digestion zone: (d) separately discharging partially stabilized sludge and oxygen-depleted digestion gas of at least 21% oxygen purity from said first digestion zone; (e) introducing said partially stabilized sludge from step (d) to a second covered digestion zone; (f) maintaining sludge in the second digestion zone under anaerobic conditions at a temperature of from 30 to 600C for sufficient solids retention time (duration) to further reduce the biodegradable volatile suspended solids content of the sludge, to less than about 40% of the biodegradable volatile suspended solids content of the sludge introduced to said first digestion zone in step (a), and form methane gas, and discharging further stabilized sludge and said methane gas from the second digestion zone.

In one version of such process in step a) said sludge and aeration gas are mixed and simultaneously one of said fluids is recirculated against the other fluid in the digestion zone to maintain the dissolved oxygen content (DO) of the mixed liquor at least at 2 mg/l.

According to a second aspect of the present invention there is provided a process for BOD-removal from wastewater in a covered aeration zone and digestion of activated sludge with oxygen gas, including the steps of: (a) introducing first gas comprising at least 40% oxygen (by volume) and mixing same as the aeration gas with the wastewater and recycled sludge in said covered aeration zone to form mixed liquor and simultaneously continuously recirculating one of such fluids against the other fluid in the aeration zone in sufficient quantity and rate to maintain the dissolved oxygen content (DO) of the mixed liquor at least 0.5 mg/l, separating the mixed liquor into purified liquid and activated sludge, and discharging unconsumed oxygen-containing gas from the aeration zone at rate such that its oxygen content is not more than 40% of the total oxygen introduced to the digestion zone; (b) returning at least about 85% by weight of the activated sludge to the aeration zone as said recycled sludge; (c) providing second gas comprising at least 80% oxygen (by volume) and including part of first gas; (d) introducing said second gas and the unreturned activated sludge from step (b) to a covered digestion zone and mixing same to maintain the dissolved oxygen content of sludge at least at 2 mg/l and the total suspended solids content (MLSS) of the sludge at least at 20.000 mg/l; (e) maintaining the sludge in the digestion zone during step (d) at a temperature in the thermophilic range of from 45" to 75"C; (f) separately discharging partially stabilized sludge and oxygen-depleted digestion gas of at least 40% oxygen purity from said digestion zone at rate such that the oxygen content of the oxygen-depleted digestion gas is at least 35% of the oxygen content of said second gas entering said digestion zone; (g) providing said oxygen-partially depleted digestion gas from step (f) as at least the major part of said first gas introduced to said covered aeration zone in step (a); characterized by the steps of: (h) continuing step (e) for sludge retention time (duration) of from 4 to 48 hours to partially reduce the biodegradable volatile suspended solids content of the sludge introduced to said first digestion zone; (i) introducing said partially stabilized sludge from step (f) to a second covered digestion zone; (j) maintaining the sludge in the second digestion zone under anaerobic conditions at a temperature of from 30 to 60"C for sufficient sludge retention time (duration) to further reduce the biodegradable volatile suspended solids content of the sludge to less than about 40% of the biodegradable volatile suspended solids content of the activated sludge introduced to said digestion zone in step (d), and form methane gas, and discharging further stabilized sludge and said methane gas from the second digestion zone.

In one version of such a process the mixed liquor temperature in step a) is at least 150C and in step d) said second gas and the unreturned activated sludge from step b) are mixed and one of each fluids is simultaneously recirculated against the other fluids in the digestion zone in sufficient quantity and rate to maintain the dissolved oxygen content of sludge at least at 2 mg/l.

According to a third aspect of the present invention a process for digestion of sludge, characterised by the steps of: (a) introducing as fluids said sludge and aeration gas comprising at least 50 percent oxygen (by volume) to a first covered digestion zone and mixing same to maintain the total suspended solids content (MLSS) of the sludge at least at 20,000 mg/l: (b) maintaining sludge in said first digestion zone during step (a) at a temperature in the thermophilic range of from 45" to 750C; (c) continuing step (b) for sludge retention time (duration) of from 4 to 48 hours to partially reduce the biodegradable volatile suspended solids content of the sludge introduced to said first digestion zone; (d) separately discharging partially stabilized sludge and oxygen-depleted digestion gas of at least 21% oxygen purity from said first digestion zone; (e) introducing said partially stabilized sludge from step (d) to a second covered digestion zone; (f) maintaining sludge in the second digestion zone under anaerobic conditions at a temperature of from 30 to 60"C for sufficient solids retention time (duration) to further reduce the biodegradable volatile suspended solids content of the sludge, to less than about 40% of the biodegradable volatile suspended solids content of the sludge introduced to said first digestion zone in step (a) and form methane gas, and discharging further stabilized sludge and said methane gas from the second digestion zone.

According to a fourth aspect of the present invention a process for digestion of sludge, characterized by the steps of: (a) introducing said sludge and aeration gas comprising at least 20 percent oxygen (by volume) to a first digestion zone and mixing same therein in sufficient quantity and rate for aerobic digestion of the sludge while maintaining the total suspended solids content (MLSS) of the sludge at least of 20,000 mg/l: (b) maintaining sludge in said first digestion zone during step (a) at a temperature in the range of from 35 to 750; (c) continuing step (b) for sludge retention time (duration) of from 4 to 48 hours to partially reduce the biogradable volatile suspended solids content of the sludge introduced to said first digestion zone; (d) discharging partially stabilized sludge from said first digestion zone; (e) introducing said partially stabilized sludge from step (d) to a covered second digestion zone; (f) maintaining sludge in the second digestion zone under anaerobic conditions at a temperature of from 25 to 60 for sufficient solids retention time (duration) to further reduce the biodegradable volatile suspended solids content of the sludge to less than about 40% of the biodegradable volatile suspended solids content of the sludge introduced to said first digestion zone to step (a) and form methane gas, and discharging further stabilized sludge and said methane gas from the second digestion zone.

According to a fifth aspect of the present invention a process for digestion of sludge comprising the steps of: (a) introducing said sludge and aeration feed gas comprising at least 20 percent oxygen (by volume) to a first digestion zone and mixing same therein in sufficient quantity and rate for aerobic digestion of the sludge while maintaining total suspended solids content (MLSS) of the sludge at least at 20,000 mg/l and temperature of the sludge in the range of from 35 to 750C in said first digestion zone; (b) conducting the aerobic digestion of step (a) so as to reduce the volatile suspended solids content of the sludge introduced to said first digestion zone by from 5 to 20 percent and thereby form partially stabilized sludge, and discharging said partially stabilized sludge from said first digestion zone; (c) anaerobically digesting the partially stabilized sludge discharged from said first digestion zone in a covered second digestion zone while maintaining temperature of the sludge therein in the range of from 25C to 60"C for sufficient solids retention time (duration) to further reduce the biodegradable volatile suspended solids content of the sludge introduced to the first digestion zone in step (a), and form methane gas; and (d) separately discharging further stabilized sludge and said methane gas from said second digestion zone.

As used herein, the terms "sludge" and "activated sludge" means a solids-liquid mixture characterized by a sludge solids phase and an associated liquid phase, in which the sludge solids are at least partially biodegradable, i.e., capable of being broken down by the action of living microorganisms. Biodegradable sludges are generally characterized according to their biodegradable volatile suspended solids content (VSS). As used herein "biodegradable volatile suspended solids content" is essentially the maximum reduction in solids achievable by aerating the sludge with O2-containing gas at ambient temperature, e.g., 20"C and DO of at least 2 mg/l. Maximum reduction of solids is assumed to be reached after 30 days aeration. Specifications for such determination are contained in "Water Pollution Control", Eckenfelder, W. W. and Ford, D.L., The Pemberton Press, 1970, Page 152. By determining VSS levels on the fresh sludge and again after 30 days aeration, the biodegradable fraction of the total VSS may be calculated as: VSS(Fresh)-VSS (30 days) VSS (Fresh) The term "stabilized sludge" refers to sludge having a reduced biodegradable volatile suspended solids content subsequent to and as a result of digestion treatment. "Sludge retention time"as used herein means the average duration of time in which the sludge is contained in a given digestion zone, as calculated by the following formula: Vd T = Qs where T = sludge retention time (days, or hours); Vd = volume of sludge in the digestion zone undergoing treatment, (ft3); and Qs = volumetric flow rate of sludge fed to the digestion zone, (ft3/day, or ft3/hr).

The present invention is based on the surprising discovery that an aerobic digestion zone operating in the thermophilic temperature regime may be integrated with a downstream anaerobic digestion zone to provide partial digestion of sludge in each of the sequential zones, and that such integration provides process improvement beyond that which would be expected based on consideration of the respective digestion steps in the treatment process taken alone, as will be shown more fully hereinafter.

The prior art has not sought to combine elevated temperature aerobic and anaerobic digestion of sludge in the manner contemplated by the present invention for numerous reasons. First, the tankage associated with the anaerobic digestion process, as discussed earlier herein, is extremely large and it has been found necessary to produce large amounts of methane for heating fuel to insure economic operation of the huge digester tanks. The combination of an anaerobic digester with an aerobic digestion step would thus appear undesirable due to considerations of overall tankage requirements for the combined process, which one would expect to be larger than the tankage associated with either digestion process alone. Such combination thus appears on its face to merely duplicate the functions normally associated with each of the aerobic and anaerobic digestion processes. at an increased cost of equipment without expected benefit in tr would appear undesirable due to the expected inability of the anaerobic digestion step to provide only partial digestive treatment of sludge in the combined system, at sludge retention time levels less than the long retention times characteristic of conventional anaerobic digesters operating alone. As discussed earlier herein, long retention times are necessary in the anaerobic digestion step to obtain efficient methane production and sludge stabilization. If anaerobic retention time were reduced below its normal full-treatment level in a combined aerobic/anaerobic digestion process, so as to secure only partial digestion in the anaerobic step, one would expect an excessive depletion of the methane-formers in the short retention time anaerobic step, by loss of these slow-growing species in the effluent from the digester, with resulting inadequacy of sludge stabilization in the combined process.

In addition to the foregoing reasons, the combined aerobic/anaerobic digestion system would appear disadvantageous from the point of operating stability, since each of the aerobic and anaerobic digestion steps alone requires close operating temperature control when operating at elevated temperature levels, so that coupling of the two respective processes would appear to require still tighter temperature control with a potential for increased adverse effect from temperature instability and fluctuation.

Finally, a combined aerobic/anaerobic digestion system would appear to be disadvantageous based on a consideration of potential carryover of residual dissolved oxygen from the upstream aerobic step to the downstream anaerobic segment of the process. As indicated hereinabove, methane-forming bacteria present in the anaerobic digestion zone are strictly anaerobic in character and are extremely sensitive to changes in their environment. It is well established that any significant introduction of oxygen into the anaerobic digestion zone will adversely affect sludge stabilization by methane formation and create the danger of evolution of oxygen from the liquor to the methane-containing gas phase and formation of a combustible gas mixture in the digester.

In contrast to the foregoing anticipated behavior, it has unexpectedly been found that the deployment of a thermophilic aerobic digestion zone upstream of an elevated temperature anaerobic digestion zone and operation of these respective zones in accordance with the process of the present invention not only provides an operable and economic sludge treatment system but results in a digestion system with unique overall process improvements relative to prior art processes, due to the synergism which is achieved between the aerobic and anaerobic digestion segments in the instant process. For example, the process of the instant invention is able to provide a thermal operating stability in the overall sludge digestion system which is not possible to achieve in either constituent digestion step operating alone. Furthermore, the integrated digestion process according to the present invention produces a highly stabilized sludge which is also completely pasteurized, despite a reduction in the retention time in the overall process beyond that which would be expected based on consideration of retention time requirements of each of the separate partial digestion steps taken alone. Particularly surprising is the finding that the anaerobic digestion zone in this process is able to operate at sludge retention time levels substantially less than are required for full stabilization treatment of sludge in conventional anaerobic digesters operating alone, and that such operation is achieved without loss of utility or treatment efficiency such as would be expected. As an example of retention times suitably employed under the invention, a pilot plant system embodying the instant process has been satisfactorily operated with a sludge retention time in the aerobic first step of 24-48 hours and a retention time in the anaerobic second step as low as 4 - 5 days. The foregoing advantages are realized in the present invention along with a substantial reduction in system tankage requirements relative to a conventional anaerobic digester system, but with retention of an unexpectedly large portion of the methane production capacity of the conventional anaerobic digester taken alone, as will be shown in greater detail hereinbelow.

However, by way of example, the system of the present invention may employ about 60% of the tankage required by a prior art anaerobic digestion zone, yet retain approximately 75 percent of the methane production capacity of the latter. Such improvement yields substantially greater production of methane than is required for process heating fuel requirements, with the result that a significantly larger amount of high methane-content off gas is available for export from the sludge digestion facility relative to the prior art anaerobic digestion system. Finally, no significant carryover of oxygen from the first digestion zone sludge to the gas phase in the second digestion zone has been found to occur in the instant process.

The reasons for the unexpected advantages of this invention, as described above, are not fully understood. It may be that absence of significant carryover of oxygen from the first digestion zone to the second is due to the fact that the elevated temperature conditions in the second digestion zone create sufficiently intense anaerobic bacteriological conditions to thoroughly deplete the dissolved oxygen content of the sludge passed from the first to the second digestion zone, before any appreciable evolution of dissolved oxygen to the gas phase in the second digestion zone can occur. The striking low sludge retention times in the instant process, particularly in the anaerobic digestion step, together with the thermal stability characteristic of the process and the unexpectedly high methane production capacity of the anaerobic step, may be a consequence of a chemical or biological acclimatization of the sludge and microorganisms in the aerobic digestion zone which provides enhancement of efficiency of the subsequent anaerobic treatment step. Nonetheless, we do not wish to be bound by any particular theory by way of explanation of such performance behavior and, accordingly, the foregoing should not be construed in any limiting manner as regards the present invention, subject only to the essential steps and features disclosed and claimed herein.

Referring now to the drawings: Figure 1 is a schematic flowsheet of a digestion process according to one embodiment of the instant invention, wherein heat is recovered from the effluent streams from each of the first and second covered digestion zones.

Figure 2 is a schematic flowsheet according to another embodiment of the invention, wherein oxygen-depleted digestion gas discharged from the first covered digestion zone is utilized in secondary treatment oxygenation of BOD-containing water.

Figure 3 is a schematic flowsheet according to yet another embodiment of the invention, wherein sludges from primary and secondary wastewater treatment steps are passed to the sludge digestion zones.

Figure 4 is a graph of the temperature of the influent sludge to the first digestion zone which is necessary to maintain a 50"C operating temperature in the first digestion zone, plotted as a function of the total suspended solids content (MLSS) of the influent sludge to the first digestion zone.

Figure 5 is a schematic flowsheet of still another embodiment of the invention, wherein a minor portion of the influent sludge to the process system is diverted to the second digestion zone.

Referring now to Figure 1 a schematic flowsheet of a process according to one embodiment of the instant invention is shown such as is suitable for sludge treatment with a first thermophilic aerobic digestion step followed by mesophilic anaerobic digestion.

Sludge, which may derive from a source such as a primary sedimentation tank, the clarifier in an activated sludge wastewater treatment plant, or a trickling filter, or from some other sludge-producing system, enters the process in line 8 and is sequentially heated in heat exchangers 22 and 15, as for example to a temperature of 30 - 35"C, prior to introduction to the first covered digestion zone 10, to maintain the temperature in the zone at a thermophilic level in the range of from 45" to 750C. The ambient temperature sludge in line 8 is first heated in heat exchanger 22 by passage of the sludge in indirect heat exchange countercurrent flow relationship with the further stabilized sludge discharged from second covered digestion zone 20 in line 24. In this manner heat is recovered from the further stabilized sludge and the resulting cooled stabilized sludge is discharged from the heat exchanger 22 and passed out of the system in line 25 to final disposal or other end use. The further stabilized sludge entering the heat exchanger 22 in line 24 may suitably be at a temperature of 35 - 49"C so that the influent sludge exiting the heat exchanger in line 9 is warmed to temperature of 28 - 30"C. From line 9 the partially warmed influent sludge is further heated in heat exchanger 15 to a temperature of 30 to 350C by indirect countercurrent flow heat exchange with the partially stabilized sludge discharged from the first digestion zone 10 in line 14 and passed from the heat exchanger in line 16 to the second digestion zone 20.

As an alternative to the above-described heat exchange with stabilized sludge product streams from the respective digestion zones, the influent sludge may be heated prior to introduction to the first digestion zone by indirect heat exchange with a suitable externally supplied heating medium such as steam or hot water, although heat recovery from the warm digestion zone product streams is preferred since it efficiently serves to conserve heat within the process and minimizes heating energy requirements for the process. Although heating of the influent sludge prior to its introduction to the first digestion zone is not essential in the broad practice of the present invention, it may be desirable in practice to maximize the thermal efficiency of the elevated temperature process. The desirability of such heating of the sludge, as will be discussed more fully hereinafter, depends on the influent sludge solids content, sludge retention time in the aerobic digestion zone, and other process parameters.

The further heated sludge discharged from the heat exchanger 15 in line 11 is introduced to first digestion zone 10 along with aeration gas from line 17 as the process fluids for the first digestion step. The aeration gas in line 17 comprises at least 50 percent oxygen (by volume), and preferably at least 80 percent oxygen in order to provide suitably high mass transfer driving force and rate of oxygen dissolution in the sludge at the high sludge temperatures in the first digestion zone contemplated under the present invention. Line 17 is connected to a source of oxygen-containing aeration gas (not shown) which may for example comprise a cryogenic oxygen plant or supply vessel or an adiabatic pressure swing adsorption air separation unit, as commonly employed in the art to supply oxygencontaining gas. As shown, the oxygen-containing aeration gas in line 17 may also be heated by heater 19 to assist in maintaining the temperature in the digestion zone 10 at the desired process level.

In the aerobic digestion zone 10, the sludge and aeration gas fluids are mixed and one of such fluids is simultaneously recirculated against the other fluid therein in sufficient quantity and rate to maintain the dissolved oxygen content (DO) of the sludge at least at 2 mg/l and the total suspended solids content (MLSS) of the sludge at least at 20,000 mg/l.

Such mixing and fluid recirculation is effected by the contacting means 12 which may in practice comprise a submerged turbine and gas compressor, coupled to the gas head space in the digestion zone and to the gas sparger, for recirculating the oxygen-containing aeration gas against the sludge, or, alternatively, a surface aeration device for recirculating the sludge against the aeration gas in the gas head space in digestion zone 10. Recirculation of one of the sludge and aeration gas fluids against the other fluid in the aerobic digestion zone is preferred in practice to obtain high dissolved oxygen content in the sludge and high utilization of oxygen in the aeration gas. Nonetheless, such recirculation is not essential in the broad practice of the present invention and in some instances it may be possible to obtain adequate dissolved oxygen in the sludge and high utilization of oxygen in the aeration gas with a once-through flow of aeration gas through the aerobic digestion zone.

The dissolved oxygen content of the sludge in the first digestion zone should be at least at 2 mg/l in order to make available to the thermophilic microorganisms in the sludge sufficient oxygen for the high rate decomposition of the biodegradable sludge constituents which is carried out by these microorganisms. The totak suspended solids content (MLSS) of the sludge is at least 20,000 mg/l so as to obtain a high rate of sludge biodegradation in the aerobic digestion zone, as based on kinetic considerations, and concomitantly, to maintain the aggregate heat of reaction from the exothermic aerobic digestion at a sufficient level to keep the temperature of the sludge in the first digestion zone within the thermophilic regime, and to obtain a satisfactory degree of partial sludge stabilization in the digestion zone at short retention times.

Under the foregoing process conditions sludge is maintained in the first digestion zone for digestion at a temperature in the thermophilic range of from 45" to 750C, for rapid oxidation of the sludge VSS content. The thermophilic digestion step is continued for a sludge retention time in the first digestion zone of from 4 to 48 hours, to partially reduce the biodegradable volatile suspended solids content of the sludge introduced to the first digestion zone. The sludge retention time in the aerobic digestion step should be at least 4 hours in order to obtain a sufficient extent of partial stabilization in the first digestion zone.

at retention times below 4 hours, the extent of sludge stabilization required in the subsequent anaerobic treatment step becomes disproportionately large relative to the stabilization level in the aerobic first step and the overall system retention time and tankage requirements begin to approach those of the conventional anaerobic digestion system, with increasing loss of the unexpected improvement in these process variables characteristic of operation at retention times at or above 4 hours. For correspondingly similar reasons, the sludge retention time in the aerobic digestion zone should not exceed 48 hours. Above such value, the extent of sludge stabilization in the aerobic digestion zone becomes unduly large with regard to the residual stabilization in the downstream anaerobic step, and the retention times necessary in the latter step to maintain a suitable large population of methaneformers in the digestion zone tends to become significantly longer than is desirable for efficient operation, and again there is increasing loss of the unexpected improvement of the overall system retention time and tankage requirements achievable in the retention time range of 4 to 48 hours. Preferably, the retention time is in the range of from 12 to 24 hours, based on the foregoing considerations.

Following the above described aerobic digestion treatment, partially stabilized sludge is discharged from the aerobic zone 10 in line 14 and oxygen-depleted digestion gas of at least 21% oxygen purity, and preferably at least 40% oxygen purity, is separately discharged from the aerobic zone in line 18. The oxygen purity level of the digestion zone vent gas in line 18 may be readily maintained at the appropriate level by suitable regulation of the relative rates of aeration gas introduction via line 17 and venting in line 18, as for example by gas flow control valves in either of the inlet or vent gas lines, coupled in controlled relationship with an oxygen purity analyzer disposed in the vent line 18, in a manner well known to those in the art.

It has been found that by maintaining the sludge in the aerobic digestion zone at the thermophilic process conditions according to the present invention, substantially complete pasteurization of the sludge is achieved. Partially stabilized sludge discharged from the aerobic zone 10 in line 14 has a temperature in the thermophilic range of between 45"C and 75"C. Inasmuch as this embodiment of the invention employs mesophilic anaerobic digestion in the second covered digestion zone 20, heat is desirably removed from the partially stabilized sludge in line 14 to ensure efficient operation of the anaerobic sludge treatment step. Accordingly, the sludge in line 14 is flowed through the heat exchanger 15 in indirect heat exchange relationship with the partially warmed influent sludge entering heat exchanger 15 in line 9. The cooled partially stabilized aerobically treated sludge then flows through line 16 for introduction to the second covered digestion zone 20.

Alternatively, the partially stabilized sludge in line 14 could be cooled by an externally supplied cooling medium such as the clarified effluent of a wastewater treatment plant.

Additionally, in winter operation, there may be no need to utilize a heat exchange step for cooling of the partially stabilized sludge stream, since heat losses to the environment from the second digestion zone and the sludge stream flowing from the first to the second digestion zone may satisfactorily compensate for such cooling.

The partially stabilized sludge introduced to the second digestion zone from line 16 is maintained therein under anaerobic conditions at temperatures of from 30 to 45"C for sufficient sludge retention time (duration) to further reduce the biodegradable volatile suspended solids content of the sludge, to less than about 40%, and preferably less than 20%, of the biodegradable volatile suspended solids content of the sludge introduced to the first digestion zone, and form methane gas.

In the broad practice of the present invention the temperature of the sludge in the second covered digestion zone is maintained in the range of 30 to 600C, which includes both operation in the mesophilic range of 30 to 450C and operation in the thermophilic range of 45" to 600C. For highly efficient operation, the anaerobic zone in mesophilic operation is maintained at a sludge treatment temperature of between 35"C and 40"C, and preferably between 37"C and 38"C. A preferred operating temperature range for anaerobic thermophilic digestion is from 45" to 50"C. Operation in the foregoing preferred temperature ranges provides particularly rapid degradative action of biodegradable volatile solids by the microbial strains involved.

In the operation of the anaerobic digestion zone 20, the digestion zone contents are continuously mixed by agitation means 21, thereby creating a large zone of active decomposition and significantly increasing the rate of the stabilization reactions. Retention time of the sludge in the second digestion zone may suitably lie in the range of from 4 to 12 days and preferably in the range of from 5 to 9 days. Sludge retention times in the second digestion zone of less than 4 days may be undesirable because below such value, the retention time tends to become increasingly inadequate to support a large viable population of methane formers in the anaerobic step, with consequent adverse effect on the overall sludge stabilization performance of the digestion system. On the other hand, at sludge retention times in the anaerobic digestion step of greater than 12 days, the retention time for the second digestion zone becomes superfluously long, and the synergistic retention time and tankage requirement benefits realized by the integrated process of the invention in the broad retention time range of 4 - 12 days becomes increasingly difficult to achieve.

After anaerobic treatment of the sludge in the second digestion zone 20 is complete, the further stabilized sludge produced thereby is discharged from the second digestion zone in line 24 and heat exchanged for recovery of heat content against the influent sludge feed in heat exchanger 22 prior to final discharge from the process in line 25. The methane gas formed in the second digestion zone 20 as a product of the biochemical reactions conducted therein is discharged from the anaerobic treatment step in line 23 having flow control valve 26 disposed therein.

As discussed earlier herein, continuous operation of a high rate anaerobic digester at optimum above-ambient temperatures has been inherently difficult to maintain in conventional practice. Ambient temperature fluctuations typically cause variation in both the temperature of the influent sludge and the heat leak of the digester tank, which in turn results in undesirable temperature fluctuations within the digestion tank. Such variations in temperature, as discussed, influence the relative growth rates of the acid-forming and methane-forming bacteria. The acid-forming bacteria are typically very hardy and moderate temperature fluctuations do not alter their metabolic activity to any significant degree. Methane-forming bacteria, on the other hand, are extremely sensitive to environmental conditions. If the maintenance of constant temperature in the anaerobic digestion zone is upset by temperature flucutations of only a couple of degrees, instability in the activity and growth of the methane-formers will result. In consequence, the activity of the acid-formers dominates with an attendant accumulation of the acidic intermediate products of decomposition and lowering of the pH level in the digestion zone. As the pH drops, the activity of the methane-formers is further reduced and a severe upset to the process is brought about.

Another problem associated with conventional anaerobic digestion systems is the susceptibility of the methane-forming bacteria to the presence of toxic metals such as copper. Even very small quantities of these metals in the sludge will inhibit the activity of the methane-formers. As in the previous instance, after continued feeding of toxic metal concentrations to the anaerobic digestion zone, the acid-formers begin to dominate thereby producing an excess in the acidic intermediate products of decomposition, which in turn lowers the pH in the digestion zone and further inhibits the activity of the methane-formers.

This cumulative effect inevitably leads to severe process upset.

The solution for the above-described process upset conditions in the conventional anaerobic digestion zone is generally the addition of large quantities of lime to the digester in order to increase the buffering, and thereby raise the pH level in the digester. This corrective measure may work if the digester has received a shock load of some inhibitory material that can be flushed out of the system or assimilated therein. In addition, by increasing the pH and decreasing the influent feed rate, it is sometimes possible to bring the digester experiencing such upset back into operation. The foregoing corrective measures, however, are in general only suitable in the case of short-term fluctuations or process upsets and are not usually advantageous in the case of long-term fluctuations or upset conditions.

In the process of the present invention, control and maintenance of elevated digestion zone operating temperatures with minimal temperature fluctuations regardless of climatic conditions is achieved through the integration of a thermophilic aerobic digestion step with a subsequent anaerobic sludge treatment step. In the process of this invention, the thermophilic aerobic digestion step is generally capable of furnishing more than enough heat to thermally stabilize the anaerobic step, by virtue of the heat content of the partially stabilized sludge stream which is flowed from the aerobic digestion zone to the anaerobic step. As a result, temperature upsets in the anaerobic zone of the instant process can be virtually eliminated by varying such process parameters as sludge retention time in the aerobic zone, the solids content of the sludge fed to the aerobic zone, and the amount of heat exchange warming of the feed sludge prior to its introduction to the aerobic zone. The anaerobic digestion zone in the present invention can typically operate within one centigrade degree of the optimum temperature condition regardless of seasonal ambient temperature changes.

In addition to the foregoing, the process of this invention may in many instances exhibit an increased tolerance for toxic metal components in the sludge fed to the digestion system.

Although a conventional anaerobic digestion system cannot tolerate toxic metal species the presence of such contaminants in the sludge fed to the process of this invention has a comparatively reduced effect on the stability and methane producing capacity of the anaerobic digestion step. Present evidence indicates that these metal species are toxic only when in a soluble ionic form. It is believed that during the aerobic digestion step in the instant process these metal ions complex with solid material in the sludge undergoing treatment, with the result that these metal species also remain complexed through the anaerobic step in the process. In consequence, the anaerobic portion of the integrated process of this invention is able to operate more stably with respect to the inhibition normally associated with the presence of heavy metals in the influent sludge to the sludge treatment process.

Another substantial benefit provided by the integrated process of this invention, beyond that attributable to operating temperature stability and resistance to toxic metal inhibitory effects, is its ability to accommodate a sporadic upset, such as a shock load, without loss of process efficiency. In a conventional anaerobic digestion zone not only does the initial solubilization phase of the digestion process occur rapidly, but microbial utilization of this material by mesophilic and facultative acid-forming bacteria also occurs at a high rate.

Upon the incidence of a sudden high solids loading on the conventional anaerobic digestion system, solubilization and acidification occur at a faster rate than the methane-forming bacteria can use the acidic intermediate products. As a result, accumulation of acidic constituents occurs in the digestion zone, the pH in the digestion zone falls and a souring of the digester contents is prone to occur. In the instant process, however, the upstream thermophilic aerobic step not only promotes rapid solubilization of biodegradable species in the sludge but may also tend to reduce the population of mesophilic acid-forming bacteria.

Subsequently, the anaerobic digestion step allows the regrowth of these organisms, however, their population is typically smaller and more in balance with the population of methane-forming organisms. Accordingly, upon the incidence of a shock loading to the aerobic zone a resulting rapid solubilization occurs in the aerobic digestion as well as stabilization of the most volatile portion of the sludge, thereby smoothing out the shock and greatly diminishing its effect on the downstream anaerobic zone. In the further treatment step the anaerobic zone receives a partially stabilized sludge on which the acid-forming bacteria and methane-forming bacteria can grow in balance.

Inasmuch as the first step of the sludge treatment process of the present invention involves an aerobic thermophilic digestion zone, the process disclosed and claimed in U.S.

Patent No. 3,926,794 issued December 16, 1975 to N. P. Vahldieck, incorporated herein to the extent pertinent, may advantageously be employed in conjunction with the process of the instant invention for treatment of wastewater containing biodegradable suspended solids for BOD removal therefrom by the activated sludge process and treatment of the resultant waste activated sludge by the process of this invention.

By way of background, activated sludge secondary treatment of wastewater is conventionally carried out in the following manner. BOD-containing wastewater, as for example municipal sewage, may first be subjected to treatment steps such as degritting and primary sedimentation to separate a primary sludge comprising biodegradeable suspended solids from the wastewater and to thereby form solids - depleted primary effluent which then enters the secondary treatment system. In the aeration zone by suitable conduit means for recirculating aeration gas to the lower portion of the zone for release as small gas bubbles through a conventional type sparger device. Alternatively, the aforementioned mixing means may be also be employed for fluid recirculation, as in the case of surface aeration impellers. Aerating devices are commonly rated by the so-called "air standard transfer efficiency" which identifies the capability of the device to dissolve oxygen from air into zero - DO tap water at one atmosphere pressure and 20"C. Suitable devices are those which have an air standard transfer efficiency of at least 1.5 lb. O2 per HP-hr and preferably at least 3.0. For these purposes the power used in rating the device is the total power consumed both for agitating the liquor and for gas-liquor contacting.

The aforementioned oxygen is introduced and one of the fluids is simultaneously continuously recirculated against the other fluids in sufficient quantity and rate to maintain the dissolved oxygen content (DO) of the mixed liquor at least at 0.5 mg/l. Also, the liquor temperature is preferably maintained at least at 150C., so that means may be needed in cold weather to prevent lower temperature in aeration zone 102, as for example means for heating the incoming wastewater in line 101. Design and operation of wastewater aeration zone 102 may be as described in any of U.S. Pat. Nos. 3,547,811; 3,547,812; or 3,547,815.

The oxygenated mixed liquor is discharged from covered aeration zone 102 and passed through conduit 104 for separation into purified supernatant liquid and activated sludge in clarifier 105. Unconsumed oxygen-containing gas is discharged from aeration zone 102 through conduit 119 and may for example be vented to the atmosphere. This gas is discharged from the aeration zone at rate controlled so that its oxygen content is no more than 40% of the total oxygen introduced to the covered aerobic digestion zone (discussed hereinafter). Returning now to clarifier 105, supernatant purified liquid is discharged through conduit 106 and activated sludge drawn off through conduit 107 containing concentrated microorganisms in the concentration of about 10,000 to 40,000 mg/l total suspended solids content (MLSS). The major part of the activated sludge, eg. at least 85% is returned through conduit 108 and pump 109 to the aeration zone, preferably at a flow rate relative to the BOD-containing wastewater such that the recirculating sludge/BOD containing wastewater volume ratio is 0.1 to 0.5. The flow rates into the covered aeration zone 102 are preferably such that the total suspended solid concentration (MLSS) therein is 4,000 - 12,000 mg/l and the volatile suspended solid content (MLVSS) is 3,000 - 10,000 mg/l.

The liquid-solid contact time in aeration cone 102 for organic food absorption-assimilation is between 30 minutes and 24 hours. This time varies depending upon the strength (BOD-content) of the wastewater, the type of pollutant, solids level in aeration and temperature, all of which is understood by those skilled in the art.

Not all the sludge separated in clarifier 105 is returned to the aeration zone 102 for two reasons. First, the activated sludge process produces a net yield of mocroorganisms because the mass of new cells synthesized from impurities in the wastewater is greater than the mass of cells autooxidized during treatment. Second, the wastewater normally contains non-biodegradable solids which settle and accumulate with the biomass. Therefore, a small fraction of the activated sludge must be discarded in order to balance the microorganism population and the food (BOD) supply and in order to suppress the accumulation of inert solids in the system. Sludge wasting will usually comprise less than 3% of the total separated sludge and rarely more than 15%.

While the waste sludge is a small fraction of the total solids separated in the clarifier, it nevertheless is often a large absolute quantity of material. Regardless of quantity, its disposition represents a significant part of the cost of wastewater treatment, and in addition, poses a serious ecological problem. The sludge is putrescible and is highly active biologically, and often contains pathogenic bacteria. Potentially, the sludge is useful as fertilizer and/or land fill, but before such use, it must be well stabilized to avoid nuisance and health hazards, and its high water content (e.g. 96-98%) must be reduced.

The waste sludge, (which contains primary sludge desired from the wastewater introduced to the process in line 101) from the clarifier 105 is withdrawn from the sludge recirculation loop in conduit 111, containing 10,000 to 40,000 mg/l MLSS, and initially at about the same temperature as the wastewater in aeration zone 102, e.g., 15 to 25"C, and is passed to a thickening tank 151. The thickening tank 151 concentrates the sludge to between 20,000 and 60,000 mg/l MLSS and passes the thickened sludge underflow via conduit 152 to the sludge digestion system.

In some instances, such as high ambient temperature wastewater treatment operation, thickening of the waste sludge from the clarifier may not be necessary and the sludge in conduit 111 may suitably by-pass tank 151 through conduit 153 and subsequently enter conduit 152 for passage to the sludge digestion system. The thickener overflow (supernatant liquid) is passed via conduit 150 to aeration zone 102, as previously described.

The thickened sludge in line 152 may be heated if necessary before introduction into aerobic digestion zone 110, by methane boiler 130. Alternatively, the sludge could be heat exchanged with the stabilized sludge effluent from the anaerobic digestion zone 120b, in a manner similar to that illustratively described earlier herein in connection with the embodiment shown in Figure 1. Waste sludge is introduced to the first covered digestion zone 110 either continuously or intermittently from line 152. The aerobic digestion zone 110 is maintained at a temperature level in the thermophilic regime of from 45" to 75"C. If autothermal operation in the aerobic digestion zone is achieved, the temperature of the sludge in the digestion zone may suitably be maintained at temperatures in the range of from 55" to 65"C. It is difficult to maintain digestion temperatures above 65"C.

autothermally. The elevated temperature in the first covered digestion zone 110 can also be obtained by supplying external heat, as for example by a suitable heated fluid circulating in a heat exchange means (not shown) disposed internally in the digestion zone. Because of the coating and plugging tendency of the solids, heat transfer surfaces disposed within the digester should not be intricate or closely spaced and may advantageously be embedded in or bonded to the wall of the tank.

Second oxygen gas comprising at least 80% oxygen (by volume) is introduced to covered heated first digestion zone 110 through conduit 117. As discussed hereinafter, this gas is sufficient in quantity to provide part of the first oxygen gas introduced to aeration zone 102 through conduit 118.

Preferably, the elevated temperature in first covered digestion zone 110 is obtained autothermally without need for heat exchangers such as 130. The concentrated sludge characteristically obtained in the oxygen aeration process of U.S. Pat. 3,547,813 is very favorable to autothermal operation because of the reduced water content relative to biodegradable "fuel" content. Moreover, high solids concentrations reduce digester size and hence reduce conductive heat losses through the walls of the digester tank. As previously indicated, the total suspended solids content (MLSS) of the sludge in the digestion zone should be at least 20,000 mg/l, based on such considerations.

Upper limits on aerobic digester solids concentration are generally imposed by two factors. Broadly, the maximum concentration depends upon capability of conventional sedimentation and thickening devices to reduce water content. Flotation devices, centrifugal separators, and gravity thickeners often produce 60,000 mg/l total suspended solids concentrations. Solids levels can be further increased by admixture of primary sludge or concentrated waste from a source other than the wastewater. The second factor which limits solids concentrations is the increasing difficulty in dissolving oxygen and mixing solids in the digester. A preferred upper limit is 60,000 mg/l to insure adequate DO uniformly distributed through the sludge. Moreover, in most uses of the invention, temperatures corresponding to near-maximum rates of aerobic thermophilic digestion can be reached autothermally at solids levels no higher than 60,000 mg/l. Further increase in solids concentration would shift more CO2 into the gas space of the digester and unnecessarily reduce the oxygen partial pressure of the aeration gas.

Digester tank construction also affects autothermal temperature levels and concrete walls are preferred over metal because of the lower conductive heat loss through concrete. Heat loss can be further reduced by embedding the tank below grade and mounding earth against any exposed vertical wall of the tank. Thermal insulation such as low-density concrete or foamed plastic can be applied over a metal coster if required.

It is also preferable to practice the invention in aerobic and anaerobic digesters having a surface-to-volume ratio less than 0.8 ft2/ft3 (2.62 m2/m3). For these purposes, 'surface" refers to the entire wall surface area of the covered digester including top, bottom and side walls. Surface-to-volume ratios larger than 0.8 result in large heat conduction losses through the walls in relation to the quantity of heat necessary to be maintained in the digester. Such heat loss is likely to necessitate thermal insulation on walls exposed to ambient atmosphere. In addition, larger surface-to-volume ratios usually imply digester tanks with narrow dimensions which are difficult to aerate and/or mix uniformly.

Retention time of the sludge in the aerobic digester also affects the autothermal temperature levels which can be maintained It will be appreciated that numerous factors enter the relationship of retention time and temperature, such as degradibility of the sludge and strength (solids level) of the sludge. In the broad practice of the present invention. the sludge retention time in the first digestion zone is from 4 to 48 hours, and preferably 12 to 24 hours.

First digestion zone 110 is provided with mechanical agitation means 112 which may be the same type employed as means 103 in aeration zone 102, together with means for continuously recirculating one of the second gas and activated sludge fluids against the other fluids in the digestion zone.

The second gas comprising at least 80ago oxygen is introduced to the covered aerobic digestion zone 110 in sufficient quantity and rate to maintain the dissolved oxygen content of the sludge at least at 2 mg/l.

Oxygen-depleted digestion gas of at least 40% oxygen purity is discharged from the covered digestion zone 110 through conduit 118 at rate such that its oxygen content is at least 35% of the oxygen content of the oxygen feed gas entering through conduit 117. The gas in conduit 118 is introduced to covered aeration zone 102, as at least a major part of the aforementioned first gas supplying the oxygen requirement for biochemical oxygenation of wastewater. If needed, a supplementary external source of oxygen-containing gas may be supplied to augment the oxygen-containing gas stream in line 118.

After the desired level of aerobic digestive treatment is completed in zone 110, partially stabilized sludge, which however is substantially completely pasteurized, is discharged from the first covered digestion zone 110 in line 114 and passed to the anaerobic treatment portion of the integrated system. In this embodiment the second anaerobic digestion zone comprises an acidification sub-zone 120a and a methane fermentation sub-zone 120b. The partially stabilized sludge in line 114 from the first digestion zone 110 is introduced to the acidification sub-zone 120a and maintained therein for sludge retention time of from 24 to 60 hours as required for sludge acidification. The contents of sub-zone 120a are continuously mixed by agitation means 121a to maintain a uniform rate of degradation of carbohydrates, fats and proteins to lower fatty acids therein. After completion of the necessary retention time in sub-zone 120a, the acidified sludge is discharged therefrom in line 126. Since the temperature of the sludge exiting zone 120a is still at elevated temperature in or near the thermophilic temperature range of 45C to 750C and thus is above optimum methane-forming levels, the temperature of the acidified sludge is typically lowered to ensure satisfactory operation of methane-forming sub-zone 120b. Accordingly, the sludge in conduit 126 is passed through heat exchanger 115 against a coolant stream flowed through the heat exchanger in conduit 160. The resulting partially cooled, partially stabilized sludge discharged from heat exchanger 115 then flows through conduit 127 to methane fermentation sub-zone 120b. The coolant heat exchange medium in line 160 may suitably comprise a cooling water stream, as for example a portion of the effluent from the secondary clarifier from line 106 or, as in the previously described embodiment, the influent sludge feed stream to the digestion system.

The anaerobic digestion sub-zone 120b comprises the methane-forming digestion step of the process. For optimal operation, the sludge in the methane fermentation sub-zone is maintained at a temperature of between 35"C and 40"C, and preferably between 37"C and 38"C. The contents of zone 120b are continuously mixed by agitation means 121b, thereby creating a large zone of active decomposition and significantly increasing the rate of the stabilization reactions therein. Sludge retention time in the methane fermentation sub-zone is preferably between 4 days and 8 days under the previously discussed considerations governing the anaerobic second digestion zone sludge retention duration. Methane gas produced by the biochemical reactions occurring in sub-zone 120b is discharged therefrom in conduit 128 having flow control valve 129 disposed therein. A portion of this discharged methane gas may be passed to the boiler 130 in conduit 132, while the remaining portion is withdrawn from the process in conduit 131 to further treatment and/or other end use steps.

The further stabilized sludge, containing no more than 40% of the original biodegradable volatile solids content of the influent sludge and preferably no more than 20%, is discharged from the process in conduit 133.

Figure 3 is a schematic flowsheet of another embodiment of the invention wherein sludge from primary and secondary wastewater treatment steps are passed to the sludge digestion system. This embodiment illustrates a process sequence under the instant invention in which a thermophilic aerobic first digestion step is integrated with a thermophilic anaerobic second digestion step. Heretofore, thermophilic anaerobic digestion has been little more than a laboratory curiosity. The problems attendant conventional mesophilic anaerobic operation discussed earlier herein of inherent thermal instability and extreme sensitivity to change in process conditions, are present in thermophilic anaerobic digestion to an even more critical extent. In fact, it is because of the erratic operating stability of thermophilic anaerobic digestion that this sludge treatment process has received little attention to date in commercial sludge digestion applications. These problems of operating instability and undue sensitivity to process fluctuations are overcome in the thermophilic aerobic/ anaerobic embodiment of the invention in the same manner as described earlier herein in connection with the embodiments of the instant invention employing an anaerobic mesophilic second digestion step.

In the Figure 3 system, raw wastewater composed, for example, of municipal sewage, industrial wastewater, and storm water flows through conduit 240 into the primary sedimentation zone 241. Sedimentation zone 241 may suitably consist of a gravity clarifier of a conventional type well-known in the art. In the sedimentation zone the influent wastewater is separated into a reduced BOD-containing primary effluent, which flows by conduit 201 into aeration zone 202, and a settled sludge underflow, removed from zone 241 via conduit 242. The aeration zone 202 also receives oxygen-containing aeration gas in conduit 218, sludge thickener supernatant liquid in conduit 250 and return activated sludge in conduit 208. A fluid mixing and recirculation means 203 is disposed in aeration zone 202 for mixing of the various fluids introduced to the aeration zone to form mixed liquor and simultaneously continuously recirculating one of the mixed liquor and oxygen-containing aeration gas fluids against the other fluids therein. As discussed earlier herein, the fluid mixing and recirculation means may suitably comprise a submerged gas sparger in combination with a sub-surface mixing impeller, or a surface aeration impeller device.

After the requisite aeration period, e.g. 2-6 hours, a BOD-depleted mixed liquor and an oxygen-depleted aeration gas of at least 21% oxygen (by volume) are discharged from the aeration zone 202 in conduits 204 and 219, respectively.

The BOD-depleted oxygenated mixed liquor in conduit 204 is passed to the secondary sedimentation zone 205 wherein activated sludge is separated from the purified liquid, with the latter being discharged from the process in line 206. The settled activated sludge is withdrawn from the secondary sedimentation zone in line 207. A major portion of this withdrawn sludge is recirculated as the recycle sludge to the aeration zone 202 in line 208 having recycle pump 209 disposed therein. The remaining unreturned portion, which may comprise between 3% and 10% of the sludge in conduit 207, is flowed in conduit 252 to the sludge thickener 251.

Sludge thickener 251 comprises a further sludge settling thickening zone which concentrates the sludge to between 2% and 6% solids, i.e., an MLSS level of between 20,000 and 60,000 mg/l. The thickened sludge underflow is flowed in line 245 and joined with primary sludge in line 242 from the primary sedimentation zone 241 to form the combined sludge stream in line 211. The supernatent liquid from the sludge thickener 251 is passed in line 250 to the aeration zone 202, as previously described.

The combined sludge stream in line 211 may be partially warmed, if desired, by indirect heat exchange with the warm stabilized sludge discharged from the second digestion zone 220, as described more fully hereinafter, and is then flowed to the first digestion zone 210 in conduit 248. Prior to introduction to the first digestion zone 210, the sludge in conduit 248 may be further heated by the methane-fired heater 231, which receives methane gas for combusion from conduit 227.

If ambient temperature conditions are sufficient to eliminate the necessity of heating the influent sludge to the digestion system, it may be by-passed around heat exchanger 244 and heater 231 by the by-pass conduits 261 and 263, respectively.

In the first covered digestion zone 210, thermophilic aerobic digestion of the influent waste sludge is carried out. Aeration gas containing at least 50% oxygen (by volume) and preferably at least 80% is delivered to the digestion zone 210 in conduit 217, and mechanical agitation means 212 mix and simultaneously continuously recirculate the influent sludge mixture against the oxygen-containing gas. The aeration gas feed rate and the energy input to mechanical agitation means 212 are such that a dissolved oxygen concentration of at least 2mg/l is maintained in the sludge in first digestion zone 210.

Sludge is retained in the first digestion zone 210 at temperature of 45" to 75"C for a duration of from 4 to 48 hours to partially reduce the biodegradable volatile suspended solids content (VSS) of the sludge. Partially stabilized sludge is discharged from the aerobic digestion zone 210 in line 216 and the oxygen-depleted digestion gas is separately discharged from the digestion zone in line 218.

From line 216 the partially stabilized sludge from the first digestion zone is introduced to the second covered digestion zone 220.

The second digestion zone 220 comprises a thermophilic anaerobic digester. For optimal operation the sludge in this digestion zone is maintained at temperature in the anaerobic thermophilic temperature range of between 40"C and 60"C, and preferably between 45"C and 50"C. As a result of thermophilic operation in both the first and the second digestion zones in this embodiment of the invention, the partially stabilized sludge from the first digestion zone may be passed directly to the second digestion zone as shown without heating or cooling heat exchange between the zones if the thermophilic temperatures in the respective zones are sufficiently closely aligned. Alternatively, it may be desirable in some cases to operate the second digestion zone at sufficiently higher or lower temperatures relative to the first aerobic digestion zone so that interzone heating or cooling of the partially stabilized sludge from the anaerobic digestive step is advantageous. Heating may be carried out by a methane-fired heater similar to heater 231; cooling may be carried out by heat exchange of the partially stabilized sludge from the first digestion zone with the influent sludge flowed to the digestion system, as described hereinabove in connection with the embodiments of the invention shown in Figures 1 and 2. Additionally, since it is even more critical in thermophilic anaerobic digestion than in mesophilic anaerobic digestion to ensure that temperature fluctuations do not occur in the digestion zone, it may be preferable to provide a well-insulated tank as the sludge treatment volume for the thermophilic anaerobic digestion step, thereby providing a safeguard against severe climatic variations.

In the second digestion zone 220, the sludge is continuously mixed by mechanical agitation means 221 to maintain a high rate of stabilization. Methane gas produced as a result of the biochemical reactions occurring in the anaerobic digestion is discharged from the second digestion zone in conduit 223. This methane gas may be mixed with oxygen-containing gas such as air or the oxygen-depleted digestion gas from the aerobic digestion zone and combusted as fuel to provide heat for maintaining sludge in one or both of the digestion zones at elevated temperature. In the process as shown a portion of the methane gas from conduit 223 is passed by conduit 227 to the methane boiler 231 and combusted to provide heat for maintaining sludge in the first digestion zone at temperature of from 45C to 75"C. The retaining portion is discharged from the process system in conduit 228. The further stabilized sludge from the anaerobic digestion zone, containing no more than 40% of the biodegradable volatile solids content of the influent sludge to the digestion system in line 248 and preferably no more than 20%, is discharged from the second digestion zone through conduit 225, passed through heat exchanger 244 for recovery of heat from the discharged sludge and finally discharged from the process system in conduit 243.

The nature of the biological activity in the aerobic digestion zone in the Figure 3 embodiment just described is significantly different than the biological activity in the aerobic zone of the earlier described Figure 2 embodiment of the invention, by virtue of the difference in sources of the sludge. In the Figure 2 embodiment, the sludge passed to the digestion system as the influent feed therefor is solely activated sludge from the secondary wastewater treatment system, whereas in the Figure 3 embodiment the influent sludge comprises both the secondary sludge from the activated sludge treatment step and also the primary sludge from the raw wastewater sedimentation step. Since the organic material of secondary sludge is primarily viable microorganisms, aerobic digestion of this sludge comprises the various biochemical reaction steps of cell lysis, assimilation of the lysis products for synthesis of new viable material, and respiration. Primary sludge, on the other hand, is primarily composed of non-viable organic material, which the micro-organisms present in the sludge are able to use as food. Accordingly, during the aerobic digestion of a primary sludge the microbial population of the sludge experiences a substantial cell synthesis phase in addition to cell lysis, assimilation of lysis products and respiration. As a result, aerobic digestion of primary sludge takes place with a greater level of both cell synthesis and cell respiration than is present in aerobic digestion of secondary sludge.

Furthermore, aerobic digestion of primary sludge results in a smaller net reduction of biodegradable volatile solids than does aerobic digestion of secondary sludge based on a comparable sludge retention time for digestion. The net reduction of biodegradable volatile solids in the sludge during digestion represents a difference in the competing digestive processes of cell synthesis and cell respiration.

Cellular respiration in the sludge digestion process is exothermic in character and, for the reasons discussed above, primary sludge exhibits a higher heat generating capacity per unit weight of biodegradable volatile suspended solids removed in digestion than does secondary sludge. Accordingly, a lower net reduction in volatile suspended solids is required to achieve and maintain a given temperature level in the aerobic digestion step with primary sludge than with secondary sludge. Thus, the Figure 3 embodiment of the invention, wherein the sludge to the digestion system comprises both primary and secondary sludge. may be operated at a given temperature with a lower level of volatile suspended solids reduction in the aerobic digestion zone than the aerobic zone in the Figure 2 embodiment processing only secondary sludge. A lower biodegradable volatile solids reduction in the aerobic zone of the digestion system in turn requires that the sludge retention time in the anaerobic digestion zone be correspondingly increased to obtain a given overall level of volatile suspended solids removal. Since an increased portion of the overall volatile suspended solids removal is effected in the anaerobic digestion zone in such a case, the system processing primary sludge can therefore obtain an increased level of methane generation in the anaerobic digestion zone relative to the digestion system processing only secondary sludge. Thus, the Figure 3 embodiment is inherently capable of providing greater quantities of methane than the Figure 2 system, but at a cost of increased sludge retention time in the aerobic digestion zone in the former case.

With respect to the foregoing discussion, the capacity for heating the influent sludge to the digestion system prior to introduction of the sludge to the aerobic digestion zone is provided in each of the previously described illustrative embodiments of the invention.

Such heating may or may not be necessary in a given application depending upon various factors such as sludge solids content, ambient temperature, aerobic digestion zone sludge retention time and the type of sludge involved. Figure 4 is a graph of the temperature of the influent sludge to the first digestion zone which is necessary to maintain a 50"C operating temperature in the first digestion zone for a 24 hour sludge retention time, plotted as a function of the total suspended solids content (MLSS) of the influent sludge to the first digestion zone. Curve A represents a combined primary and secondary sludge stream having a volatile suspended solids/total suspended solids (VSS/TSS) ratio of 0.75 and a biological heat content of 20,000 BTU/lb. volatile suspended solids (VSS) removed. Curve B represents an uncombined secondary sludge having a VSS/TSS ratio of 0.79 and a heat content of 14,000 BTU/lb. VSS removed.

As shown by the respective curves of this graph, the combined sludge represented by curve A requires a higher aerobic digestion zone inlet temperature at a given total suspended solids level than does the secondary sludge of curve B. Accordingly, heating of the influent sludge to the digestion system prior to its introduction to the first digestion zone may be particularly desirable in the practice of the invention where the feed to the digestion system contains a significant portion of primary sludge from a wastewater treatment facility.

The graph of Figure 4 further indicates that thermophilic operation may be achieved without the need for heating of the influent sludge to the digestion system prior to introduction to the aerobic digestion zone, when the influent sludge is sufficiently thickened. For example, if a combined sludge (curve A) with a 4% total solids concentration is to be digested, the temperature of the sludge introduced to the thermophilic aerobic zone need only be about 15"C.

All of the previously described embodiments of the invention produce a completely pasteurized sludge product, since in each of these cases all of the influent sludge to the digestion system is passed through the thermophilic aerobic zone in which the high temperatures employed provide complete sludge pasteurization. However, there may be applications in which the final disposition of the sludge does not require a completely pasteurized product, or where the sludge itself does not require pasteurization because of the absence of any appreciable concentration of pathogens therein. Figure 5 is a schematic flowsheet of another embodiment which is within the broad scope of the present invention, wherein a minor portion of the influent sludge to the process system is diverted to the second digestion zone, which is suitable for such applications. In the Figure 5 embodiment, a major portion of the influent sludge entering the process system in line 311 is fed to the first covered digestion zone 310 by line 331. Prior to introduction to first digestion zone 310, the sludge in line 331 can be heated, if desirable, by methane-fired boiler 330.

Oxygen-containing aeration gas, comprising at least 50% oxygen (by volume), and preferably at least 80%, is introduced to the aerobic digestion zone 310 through conduit 317. The sludge flowing into this zone is suitably mixed and continuously recirculated against the oxygen-containing aeration gas therein by the agitation means 312, to maintain a dissolved oxygen content in the sludge of at least 2mg/l. Sludge is maintained in the aerobic digestion zone at temperature of between 45" and 75"C for a retention time of between 4 and 48 hours. Oxygen-depleted digestion gas is discharged from the first digestion zone in line 318 and sludge, partially depleted in biodegradable suspended solids content and fully pasteurized, is separately discharged from the digestion zone in line 316.

The partially stabilized sludge in line 316 is then introduced to the second covered digestion zone 320 operating in the mesophilic temperature range. Since the tempera:-u-e of the sludge discharged from the first digestion zone is between 45"C and 75"C, its temperature must be lowered prior to introduction to the second digestion zone so that efficient operation of the mesophilic anaerobic digestion process in the second digestion zone can be maintained. In the illustrative embodiment, the minor portion of the influent sludge to the process by-passes the methane boiler 330 and aerobic digestion zone 3'0 in conduit 329 and is mixed directly with the warm sludge in conduit 316. The flow rate of the influent sludge bypass stream is adjusted so that the temperature of the combined sludge stream introduced to the anaerobic digestion zone 320 is sufficient to maintain an operating temperature in zone 320 of between 35"C and 40"C.

In the second digestion zone the sludge is mixed by recirculation of methane gas against the sludge therein in order to actively maintain the stabilization rate in the second zone at high levels. Methane gas produced as a result of the biochemical reactions occurring in the second digestion zone 320 is discharged therefrom in conduit 323. A side stream of this gas is diverted into flow loop 340 having compressor 326 disposed therein and the resultant compressed methane gas is introduced into the sludge in the second digestion zone, as for example by sparging means (not shown), to effect the aforementioned sludge mixing and recirculation. From line 323, a portion of the methane gas may be passed in line 327 to the methane-fired boiler 330 and the remainder is discharged from the process system in line 328. The further stabilized sludge, containing less than 40% of the original biodegradable volatile suspended solids content of the influent sludge to the process system in line 331, is discharged from the second digestion zone in conduit 325, to further treatment (e.g., dewatering) and/or final disposal.

The advantages of this invention are illustrated by the following examples: Example I This example compares the performance of the instant invention operated according to the Figure 2 embodiment with a conventional high rate anaerobic system. The further description will be based on treatment of waste sludge from a 10 million gallon per day (MGD) wastewater treatment plant, and referenced to the Figure 2 schematic flowsheet.

A combined 50-50 primary and secondary sludge initially at 180C is fed to the digestion system of the Figure 2 process in conduit 111. The sludge, having a total suspended solids content of 39,400 mg/l and a volatile suspended solids/total suspended solids fraction of 72%, is fed to the system at a flow rate of 0.09 MGD. To maintain the sludge in aerobic digestion zone 110 at a 50"C operating temperature with 24 hour sludge retention time, the influent sludge is heated to about 23"C by the methane boiler 130. Based on a 50% conversion efficiency of the methane gas fuel value to heat, approximately 25,000 cubic feet per day of the methane gas produced in the anaerobic digestion zone is needed to supply the boiler 130.

Approximately 8% volatile suspended solids (16% biodegradable volatile suspended solids; the biodegradable volatile suspended solids are approximately 50% of total volatile suspended solids) reduction is obtained in the aerobic digestion so that a partially digested sludge with a volatile suspended solids content of 26,100 mg/l is fed by conduit 114 to acidification sub-zone 120a. This sub-zone is operated at thermophilic temperature with a 24 hour sludge retention time. A 10% reduction of the influent volatile suspended solids fraction is effected in this stage. A sludge with a volatile suspended solids content of 23,500 mg/l is then discharged to the methane fermentation sub-zone 120b in conduit 126.

Sufficient heat is removed from the discharged sludge in heat exchanger 115 to ensure an operating temperature in the methane fermentation sub-zone 120b of 38"C.

The methane fermentation sub-zone is operated with a 5 day sludge retention time, resulting in an overall volatile solids reduction of 40% for the integrated system (biodegradable volatile suspended solids reduction of 80%). The methane fermentation sub-zone produces approximately 73,000 cubic feet of methane gas per day, amounting to a total fuel value of 43 million BTU per day. Since 25,000 cubic feet of methane gas per day is needed to operate methane boiler 130, 48,000 cubic feet of methane gas per day, amounting to a total fuel value of 29 million BTU per day, is available for export from the sludge digestion system.

If the 0.09 MGD of combined sludge on which the above description is based is instead passed to a conventional high rate anaerobic digestion tank approximately a 13 day sludge retention time would be necessary to achieve the same volatile solids reduction. Although 128,000 cubic feet of methane gas per day, amounting to about 77 million BTU per day, is produced by the conventional high rate digester tank, approximately 60 million BTU per day of heating is needed, at a 50% conversion of fuel value to heat, to maintain optimum operating temperature conditions in the high rate tank. Thus, the conventional system, as compared to the above-described embodiment of the present invention, requires 86% more tankage, based on retention time requirements, and has available for export approximately 40% less methane under normal operating conditions.

Example II This example describes a specific operation of the present invention according to the Figure 5 embodiment. The influent sludge feed comprises 0.06 MGD of a combined 50-50 primary and secondary sludge from a wastewater treatment facility. The influent sludge stream in line 311 at 20"C and 4% total suspended solids (VSS/TSS= 0.75) is divided, with 0.046 MGD flowing in line 331 directly to the thermophilic aerobic digestion zone and 0.014 MGD forming the bypass stream in conduit 329. As indicated by Figure 4, there is no need in this instance to heat the sludge prior to introduction thereof to the thermophilic aerobic digestion zone. The retention time in the first digestion zone 310 is approximately 24 hours and thermophilic temperatures are reached autothermally. A pasteurized sludge at temperature of 50"C is discharged from the aerobic digestion zone in conduit 316 and is mixed with the cool bypass stream from conduit 329. This combined sludge stream then flows to the anaerobic digestion zone 320 in which the sludge is maintained in the absence of oxygen for approximately 8 days resulting in approximately 40% overall volatile solids reduction (80% biodegradable volatile suspended solids reduction). The anaerobic digestion zone produces methane gas at a rate of approximately 72,000 cubic feet per day, amounting to about 40 million BTU per day. All of this methane is available for export from the process system.

If the 0.06 MGD of combined influent sludge feed is instead passed to a conventional high rate anaerobic digestion tank, approximately a 15-day retention time would be necessary to achieve the same volatile solids reduction. Although 90,000 cubic feet per day of methane gas, amounting to about 50 million BTU per day, is produced by the conventional anaerobic digestion tank, approximately 45 million BTU per day is needed, at a 50% conversion of fuel value to heat, to maintain optimum anaerobic operating temperature conditions in the high rate tank. Therefore, in this case, a conventional anaerobic digestion system requires approximately a 65% longer sludge retention time but generates only a net gas energy equivalent of 5 million BTU per day compared to 40 million BTU per day for the combined system. Therefore, after using the internally generated methane gas as a source of heat, the conventional system has substantially less methane gas available for export than does the process of the present invention.

Example III This example compares the performance of the instant invention when operated according to the Figure 1 embodiment with a conventional high rate anaerobic system.

A secondary sludge from an oxygenation wastewater treatment system, initially at 150C, is first heated in heat exchanger 22 with anaerobic digester effluent and then further heated with thermophilic aerobic digester effluent in heat exchanger 15. The first heat exchange step in heat exchanger 22 raises the temperature of the influent sludge from 15 C to about 25"C while lowering the temperature of the stabilized sludge effluent from the anaerobic digestion zone 20 from about 35"C to 250C. The second heat exchange step in heat exchanger 15 increases the influent sludge temperature to about 30"C while the sludge discharged from the aerobic digestion zone 10 is reduced in temperature from about 50"C to 45"C. The influent sludge having a total solids content (MLSS) of 34,400 mg/l and a volatile suspended solids/total suspended solids fraction of 78% is introduced to the first digestion zone 10 at a rate of 0.06 MGD. A 50"C operating temperature is maintained with a 24 hour sludge retention time in the aerobic first zone.

Approximately 16% volatile suspended solids reduction (32% biodegradable volatile suspended solids reduction) is achieved in the aerobic stage so that a partially stabilized sludge with a volatile suspended solids content of 22,500 mg/l is introduced, after heat exchange with the influent sludge in heat exchanger 15 to the anaerobic digestion zone in conduit 16.

The anaerobic digestion zone operates with an 8 day retention time, resulting in an overall volatile suspended solids reduction of 42% (biodegradable volatile suspended solids reduction of 84%) for the integrated system. The anaerobic digestion zone 20 produces approximately 51,800 cubic feet of methane gas per day, amounting to a total fuel value of about 28 million BTU per day. All of this methane gas is available for export from the digestion system.

If the 0.06 MGD of influent sludge to the digestion process described above was instead passed to a conventional high rate anaerobic digestion tank, at least a 14 day sludge retention time would be necessary to achieve the same volatile suspended solids reduction.

Although 84,600 cubic feet of methane gas per day is produced in such conventional high rate system, amounting to about 47 million BTU per day, approximately 45 million BTU per day is needed, at a 50% conversion of fuel value to heat, to maintain optimum operating temperature conditions in the high rate tank. The conventional anaerobic system therefore requires about 55% more tankage, and has about 26 million BTU of methane gas per day less to export, than does the corresponding above-described system of the present invention.

WHAT WE CLAIM IS: 1. A process for digestion of sludge, comprising the steps of: (a) introducing as fluids said sludge and aeration gas comprising at least 50 percent oxygen (by volume) to a first covered digestion zone and mixing same to maintain the dissolved oxygen content (DO) of the mixed liquor at least at 2 mg/l and the total suspended solids content (MLSS) of the sludge at least at 20,000 mull; (b) maintaining sludge in said first digestion zone during step (a) at a temperature in the thermophilic range of from 45" to 75"C; (c) continuing step (b) for sludge retention time (duration) of from 4 to 48 hours to partially reduce the biodegradable volatile suspended solids content of the sludge introduced to said first digestion zone; (d) separately discharging partially stabilized sludge and oxygen-depleted digestion gas of at least 21% oxygen purity from said first digestion zone; (e) introducing said partially stabilized sludge from step (d) to a second covered digestion zone; (f) maintaining sludge in the second digestion zone under anaerobic conditions at a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. If the 0.06 MGD of combined influent sludge feed is instead passed to a conventional high rate anaerobic digestion tank, approximately a 15-day retention time would be necessary to achieve the same volatile solids reduction. Although 90,000 cubic feet per day of methane gas, amounting to about 50 million BTU per day, is produced by the conventional anaerobic digestion tank, approximately 45 million BTU per day is needed, at a 50% conversion of fuel value to heat, to maintain optimum anaerobic operating temperature conditions in the high rate tank. Therefore, in this case, a conventional anaerobic digestion system requires approximately a 65% longer sludge retention time but generates only a net gas energy equivalent of 5 million BTU per day compared to 40 million BTU per day for the combined system. Therefore, after using the internally generated methane gas as a source of heat, the conventional system has substantially less methane gas available for export than does the process of the present invention. Example III This example compares the performance of the instant invention when operated according to the Figure 1 embodiment with a conventional high rate anaerobic system. A secondary sludge from an oxygenation wastewater treatment system, initially at 150C, is first heated in heat exchanger 22 with anaerobic digester effluent and then further heated with thermophilic aerobic digester effluent in heat exchanger 15. The first heat exchange step in heat exchanger 22 raises the temperature of the influent sludge from 15 C to about 25"C while lowering the temperature of the stabilized sludge effluent from the anaerobic digestion zone 20 from about 35"C to 250C. The second heat exchange step in heat exchanger 15 increases the influent sludge temperature to about 30"C while the sludge discharged from the aerobic digestion zone 10 is reduced in temperature from about 50"C to 45"C. The influent sludge having a total solids content (MLSS) of 34,400 mg/l and a volatile suspended solids/total suspended solids fraction of 78% is introduced to the first digestion zone 10 at a rate of 0.06 MGD. A 50"C operating temperature is maintained with a 24 hour sludge retention time in the aerobic first zone. Approximately 16% volatile suspended solids reduction (32% biodegradable volatile suspended solids reduction) is achieved in the aerobic stage so that a partially stabilized sludge with a volatile suspended solids content of 22,500 mg/l is introduced, after heat exchange with the influent sludge in heat exchanger 15 to the anaerobic digestion zone in conduit 16. The anaerobic digestion zone operates with an 8 day retention time, resulting in an overall volatile suspended solids reduction of 42% (biodegradable volatile suspended solids reduction of 84%) for the integrated system. The anaerobic digestion zone 20 produces approximately 51,800 cubic feet of methane gas per day, amounting to a total fuel value of about 28 million BTU per day. All of this methane gas is available for export from the digestion system. If the 0.06 MGD of influent sludge to the digestion process described above was instead passed to a conventional high rate anaerobic digestion tank, at least a 14 day sludge retention time would be necessary to achieve the same volatile suspended solids reduction. Although 84,600 cubic feet of methane gas per day is produced in such conventional high rate system, amounting to about 47 million BTU per day, approximately 45 million BTU per day is needed, at a 50% conversion of fuel value to heat, to maintain optimum operating temperature conditions in the high rate tank. The conventional anaerobic system therefore requires about 55% more tankage, and has about 26 million BTU of methane gas per day less to export, than does the corresponding above-described system of the present invention. WHAT WE CLAIM IS:

1. A process for digestion of sludge, comprising the steps of: (a) introducing as fluids said sludge and aeration gas comprising at least 50 percent oxygen (by volume) to a first covered digestion zone and mixing same to maintain the dissolved oxygen content (DO) of the mixed liquor at least at 2 mg/l and the total suspended solids content (MLSS) of the sludge at least at 20,000 mull; (b) maintaining sludge in said first digestion zone during step (a) at a temperature in the thermophilic range of from 45" to 75"C; (c) continuing step (b) for sludge retention time (duration) of from 4 to 48 hours to partially reduce the biodegradable volatile suspended solids content of the sludge introduced to said first digestion zone; (d) separately discharging partially stabilized sludge and oxygen-depleted digestion gas of at least 21% oxygen purity from said first digestion zone; (e) introducing said partially stabilized sludge from step (d) to a second covered digestion zone; (f) maintaining sludge in the second digestion zone under anaerobic conditions at a

temperature of from 30 to 600C for sufficient solids retention time (duration) to further reduce the biodegradable volatile suspended solids content of the sludge, to less than about 40% of the biodegradable volatile suspended solids content of the sludge introduced to said first digestion zone in step (a), and form methane gas, and discharging further stabilized sludge and said methane gas from the second digestion zone.

2. A process according to claim 1 wherein said sludge introduced to said first digestion zone has a total suspended solids content of between 20,000 and 60,000 mg/l.

3. A process according to claim 1 or 2 wherein the sludge retention time (duration) of said sludge in said first digestion zone is from 12 to 24 hours.

4. A process according to any one of claims 1 to 3 wherein said sludge is heated prior to said introduction to said first digestion zone, to maintain said temperature in step (b).

5. A process according to any one of claims 1 to 4 wherein said temperature in step (f) is maintained in the range of from 35 to 400C, for mesophilic digestion in said second digestion zone.

6. A process according to any one of claims 1 to 4 wherein said temperature in step (f) is maintained in the range of from 45" to 500C, for thermophilic digestion in said second digestion zone.

7. A process according to any one of claims 1 to 6 wherein the sludge retention time of sludge in step (f) is sufficient to further reduce the biodegradable volatile suspended solids content of the sludge to less than about 20% of the biodegradable volatile suspended solids content of the sludge introduced to said first digestion zone in step (a).

8. A process according to any one of claims 1 to 7, wherein the sludge retention time of sludge in step (f) is from 4 to 12 days.

9. A process according to any one of claims 1 to 8, wherein each of said first and second digestion zones has a surface-to-volume ratio less than 0.8 ft.2/ft3.

10. A process according to any one of claims 1 to 9, wherein the sludge in said second digestion zone is mixed by recirculation of methane gas against the sludge therein.

11. A process according to any one of claims 1 to 10 wherein said aeration gas is heated prior to said introduction to said first digestion zone, to maintain said temperature in step (b).

12. A process according to any one of claims 1 to 11 wherein said sludge is heated prior to said introduction to said first digestion zone by indirect heat exchange with said further stabilized sludge discharged from said second digestion zone.

13. A process according to claim 12 wherein said temperature in step (f) is maintained in the range of from 35 to 40"C and said heated sludge is further heated prior to said introduction to said first digestion zone by indirect heat exchange with said partially stabilized sludge discharged from said first digestion zone.

14. A process according to any one of claims 1 to 13, wherein said second digestion zone comprises an acidification sub-zone and methane fermentation sub-zone, partially stabilized sludge from said first digestion is introduced to said acidification sub-zone and maintained therein for sludge retention time of 24 to 60 hours for sludge acidification, acidified sludge is discharged from said acidification sub-zone and introduced to said methane firmentation sub-zone and maintained therein at temperature of from 35 to 400C for sludge retention time of from 4 to 8 days.

15. A process according to claim 14 wherein sludge in said methane fermentation sub-zone is maintained at a temperature of from 37 to 380C.

16. A process according to claim 14 or 15 wherein sludge in said acidification zone is maintained at temperature of between 45" and 75"C and acidified sludge discharged from said acidification sub-zone is cooled to temperature of from 35 to 40"C prior to introduction to said methane fermentation sub-zone.

17. A process according to any one of claims 1 to 16 comprising treatment of wastewater containing biodegradable suspended solids for BOD removal therefrom, including the steps of: separating a primary sludge comprising said biodegradable suspended solids from said wastewater to form solids-depleted primary effluent; mixing said primary effluent and recycle sludge and aerating same at sufficient rate and for sufficient time to form mixed liquor of reduced BOD content; separating the mixed liquor into purified liquid and activated sludge; and returning at least a major portion of the activated sludge for mixing with said primary effluent as said recycle sludge, wherein said primary sludge and unreturned activated sludge are introduced to said first digestion zone in step (a), as the sludge therefor.

18. A process according to any one of claims 1 to 17 wherein said methane gas discharged from said second digestion zone is mixed with oxygen-containing gas and combusted as fuel to provide heat for maintaining sludge in at least one of said first and second digestion zones at elevated temperature.

19. A process according to claim 18 wherein said methane gas and oxygen-containing gas are mixed and combusted as fuel to provide heat for maintaining sludge in said first digestion zone at temperature of from 45" to 750C.

20. A process as claimed in any one of claims 1 to 17, wherein in step a) said sludge and aeration gas are mixed and simultaneously one of said fluids is recirculated against the other fluid in the digestion zone to maintain the dissolved oxygen content (DO) of the mixed liquor at least at 2 mg/l.

21. A process for BOD-removal from waste-water in a covered aeration zone and digestion of activated sludge with oxygen gas, including the steps of: (a) introducing first gas comprising at least 40% oxygen (by volume) and mixing same as the aeration gas with the wastewater and recycled sludge in said covered aeration zone to form mixed liquor and simultaneously continuously recirculating one of such fluids against the other fluid in the aeration zone in sufficient quantity and rate to maintain the dissolved oxygen content (DO) of the mixed liquor at least 0.5 mg/l, separating the mixed liquor into purified liquid and activated sludge, and discharging unconsumed oxygen-containing gas from the aeration zone at rate such that its oxygen content is not more than 40% of the total oxygen introduced to the digestion zone; (b) returning at least about 85% by weight of the activated sludge to the aeration zone as said recycled sludge; (c) providing second gas comprising at least 80% oxygen (by volume) and including part of first gas; (d) introducing said second gas and the unreturned activated sludge from step (b) to a covered digestion zone and mixing same to maintain the dissolved oxygen content of sludge at least at 2 mg/l and the total suspended solids content (MLSS) of the sludge at least at 20,000 mg/l; (e) maintaining the sludge in the digestion zone during step (d) at a temperature in the thermophilic range of from 45" to 75"C; (f) separately discharging partially stabilized sludge and oxygen-depleted digestion gas of at least 40% oxygen purity from said digestion zone at rate such that the oxygen content of the oxygen-depleted digestion gas is at least 35% of the oxygen content of said second gas entering said digestion zone; (g) providing said oxygen-partially depleted digestion gas from step (f) as at least the major part of said first gas introduced to said covered aeration zone in step (a); characterized by the steps of: (h) continuing step (e) for sludge retention time (duration) of from 4 to 48 hours to partially reduce the biodegradable volatile suspended solids content of the sludge introduced to said first digestion zone; (i) introducing said partially stabilized sludge from step (f) to a second covered digestion zone (j) maintaining the sludge in the second digestion zone under anaerobic conditions at a temperature of from 30 to 60"C for sufficient sludge retention time (duration) to further reduce the biodegradable volatile suspended solids content of the sludge to less than about 40% of the biodegradable volatile suspended solids content of the activated sludge introduced to said digestion zone in step (d), and form methane gas, and discharging further stabilized sludge and said methane gas from the second digestion zone.

22. A process as claimed in claim 21, wherein the mixed liquor temperature in step a) is at least 15"C, and wherein in step d) said second gas and the unreturned activated sludge from step b) are mixed and one of such fluids is simultaneously recirculated against the other fluids in the digestion zone in sufficient quantity and rate to maintain the dissolved oxygen content of sludges at least at 2 mg/l.

23. A process for digestion of sludge, characterized by the steps of: (a) introducing as fluids said sludge and aeration gas comprising at least 50 percent oxygen (by volume) to a first covered digestion zone and mixing same to maintain the total suspended solids content (MLSS) of the sludge at least at 20,000 mg/l: (b) maintaining sludge in said first digestion zone during step (a) at a temperature in the thermophilic range of from 45" to 75"C; (c) continuing step (b) for sludge retention time (duration) of from 4 to 48 hours to partially reduce the biodegradable volatile suspended solids content of the sludge introduced to said first digestion zone (d) separately discharging partially stabilized sludge and oxygen-depleted digestion gas of at least 21% oxygen purity from said first digestion zone; (e) introducing said partially stabilized sludge from step (d) to a second covered digestion zone; (f) maintaining sludge in the second digestion zone under anaerobic conditions at a temperature of from 30 to 600C for sufficient solids retention time (duration) to further reduce the biodegradable volatile suspended solids content of the sludge, to less than about 40% of the biodegradable volatile suspended solids content of the sludge introduced to said first digestion zone in step (a) and form methane gas, and discharging further stabilized sludge and said methane gas from the second digestion zone.

24. a process for digestion of sludge, characterized by the steps of: (a) introducing said sludge and aeration gas comprising at least 20 percent oxygen (by volume) to a first digestion zone and mixing same therein in sufficient quantity and rate for aerobic digestion of the sludge while maintaining the total suspended solids content (MLSS) of the sludge at least of 20,000 mg/l; (b) maintaining sludge in said first digestion zone during step (a) at a temperature in the range of from 35 to 75 ; (c) continuing step (b) for sludge retention time (duration) of from 4 to 48 hours to partially reduce the biogradable volatile suspended solids content of the sludge introduced to said first digestion zone; (d) discharging partially stabilized sludge from said first digestion zone; (e) introducing said partially stabilized sludge from step (d) to a covered second digestion zone; (f) maintaining sludge in the second digestion zone under anaerobic conditions at a temperature of from 25 to 60 for sufficient solids retention time (duration) to further reduce the biodegradable volatile suspended solids content of the sludge to less than about 40% of the biodegradable volatile suspended solids content of the sludge introduced to said first digestion zone to step (a) and form methane gas, and discharging further stabilized sludge and said methane gas from the second digestion zone.

25. A process for digestion of sludge comprising the steps of: (a) introducing said sludge and aeration feed gas comprising at least 20 percent oxygen (by volume) to a first digestion zone and mixing same therein in sufficient quantity and rate for aerobic digestion of the sludge while maintaining total suspended solids content (MLSS) of the sludge at least at 20,000 mg/l and temperature of the sludge in the range of from 35 to 75"C in said first digestion zone; (b) conducting the aerobic digestion of step (a) so as to reduce the volatile suspended solids content of the sludge introduced to said first digestion zone by from 5 to 20 percent and thereby form partially stabilized sludge, and discharging said partially stabilized sludge from said first digestion zone; (c) anaerobically digesting the partially stabilized sludge discharged from said first digestion zone in a covered second digestion zone while maintaining temperature of the sludge therein in the range of from 25 to 600C for sufficient solids retention time (duration) to further reduce the biodegradable volatile suspended solids content of the sludge introduced to the first digestion zone in step (a), and form methane gas; and (d) separately discharging further stabilized sludge and said methane gas from said second digestion zone.

26. A process for digestion of sludge substantially as hereinbefore described in any one of the foregoing Examples.

27. A process for BOD removal substantially as hereinbefore described in any one of the foregoing Examples.

28. A process for digestion of sludge substantially as hereinbefore described with reference to and as illustrated in any one of the accompanying drawings.

29. A process for BOD removal substantially as hereinbefore described with reference to and as illustrated in any one of the accompanying drawings.