CH627719A5 - Sludge digestion process - Google Patents

Description

The invention relates to a hot sludge digestion process, implemented in aerobic and anaerobic environments.

The problems associated with the disposal of wastewater are increasing in proportion to the constant development of the industry and the growth of the population. Although physical, chemical and biological treatment means have been developed to effectively treat polluted water in order to produce an effluent that can be discharged into natural water, most of the main waste water treatment means currently used, including clarification, chemical precipitation, biological filtration and the use of activated sludge, transform water polluting substances into a concentrate called mud. As regards in particular the activated sludge process, which is one of the most commonly used processes for the treatment of waste water, there is generally a positive, clean and sensitive production of volatile solids in suspension, that is to say that is, the speed of cell synthesis is greater than that of cell destruction. This therefore results in a constant increase in the volume of sludge, and excess activated sludge must be removed continuously or periodically.

As the overall volume of wastewater to be treated increases, in particular as a result of increasingly stringent pollution control regulations, the amount of sludge to be released produced by the wastewater treatment processes listed above above, grows correspondingly. Therefore, it is very desirable to treat this effluent sludge so that it can be disposed of easily and inexpensively without further polluting the environment. Although much has been done to improve sludge treatment technology as well as to improve existing sludge treatment processes, there is still a need to improve the efficiency of sludge treatment facilities.

The main purpose of all sludge treatment processes is to reduce and stabilize the sludge solids in an inexpensive and efficient manner. In addition, it is desirable that a sludge treatment plant also provides a final product which is entirely suitable for final disposal without any other physical or chemical treatment. In conventional practice, sludge removal is generally carried out by dumping into the sea, combustion or spreading. In many cases, land disposal is used and is of particular interest because of its weak long-term effects on the environment. In fact, the spreading of sludge can be advantageous to favor the reconstitution of the grounds. However, the use of spreading as a final sludge disposal process requires a well pasteurized end product, so that the concentration of pathogenic organisms in the sludge is low enough to avoid any danger to the health of the people performing the sludge. sludge removal.

Three distinct methods have been conventionally used in a very intensive manner for the treatment of effluent sludge, namely the use of oxidation ponds, anaerobic digestion and aerobic digestion.

Oxidation ponds are generally used in the form of relatively shallow basins, dug in the ground and extending over a certain surface of the ground, so as to retain the wastewater before its final evacuation. These ponds allow the biological oxidation of organic matter by natural or artificially accelerated transfer of oxygen from the ambient air to water. During the bio-oxidation process, the solids present in the wastewater undergo some biological degradation and end up depositing on the bottom of the pond where they can become anaerobic and can be stabilized. The pond can be drained periodically and the deposited sludge removed by dredging so that the volumetric capacity of the pond is renewed for the treatment of other wastewater. The removed sludge is used for backfilling, for example. The oxidation ponds therefore constitute a facility operating simply for the treatment of wastewater and sludge. However, the use of these oxidation ponds is limited because they require large areas on the ground. In addition, this treatment and disposal process does not allow a significant reduction in the content of pathogenic organisms in the sludge.

Anaerobic digestion is the most widely used digestion process to stabilize concentrated organic solids such as those removed from sedimentation tanks, biological filters and activated sludge plants. In current practice, excess sludge is accumulated in large dome digestion equipment where it is subjected to anaerobic fermentation for 20 to 30 days. The main reasons for the industrial success of anaerobic digestion of sludge are that this process makes it possible to stabilize large quantities of diluted organic sludge, only results in a low production of materials of biological nature (biomass), makes it possible to obtain sludge the water of which can be relatively easily removed and also allows the production of methane. In addition, it has been more or less asserted that anaerobic digestion makes it possible to obtain pasteurized sludge. Although this ability to pasteurize is questionable, anaerobic digestion is widely used in practice because it reduces solid residues to a relatively stable form, allowing them to be discharged in the form of embankments without creating a significant nuisance. Anaerobic digestion is generally carried out in large tanks, in which more or less significant mixing is carried out either by mechanical means or by recycling compressed digestion gases. This mixing rapidly accelerates the stabilization reactions of the sludge by creating a large zone of active decomposition.

As indicated previously, the current practice of anaerobic digestion includes long residence times, of the order of 20 to 30 days, without supplying heat to the system. It has appeared in the prior art that high temperatures, comprised in the mesophilic range of 30 to 40 ° C., favor a reduction in the residence time which can be reduced to approximately 12 to 20 days. This reduction in treatment time results from the fact that the speed of activity of the organisms responsible for digestion depends substantially on the

5

10

15

20

25

30

35

40

45

50

55

60

65

627719

4

temperature and that, in a temperature range of 30 to 40 ° C, very active mesophilic microorganisms constitute the dominant microbial strain used in the digestion of sludge. The best temperatures for mesophilic digestion are between approximately 35 and 38 ° C, with a minimum residence time of the order of 12 to 15 days. Temperatures up to 35 ° C lead to an increase in the rate of digestion and can allow a reduction in residence time, at the expense, however, of the operating stability of the system, while temperatures below 35 ° C require longer residence times.

Methane gas is released during anaerobic digestion and is generally used in combustion heaters to compensate for heat losses from the anaerobic digestion plant operating at high temperature. However, seasonal temperature variations and fluctuations in the suspended solids content of the sludge have a relatively large effect on the production of methane and on the amount of heat required to maintain the digestion zone at the desired high temperature. Operating. Therefore, when high temperatures are to be maintained throughout the year in the anaerobic digestion zone, it is generally essential that the sludge digestion facility has an auxiliary heating source.

Since the anaerobic digestion and resulting methane release rates are highly dependent on the suspended solids content of the treated sludge and the temperature prevailing in the digestion zone, it is generally desirable to feed the digestion equipment with a sludge as concentrated as possible in order to reduce heat losses in the effluent stream of stabilized sludge, leaving the anaerobic digestion equipment while bringing to a maximum value the production of methane in this equipment. However, even within these parameters, it is difficult to maintain low temperatures in the anaerobic digestion area inexpensively, especially during the winter months. In addition, even relatively small variations in temperature in the anaerobic digestion zone can, in a well known manner, cause disproportionate disruption of the process and acidification of the contents of the digestion equipment.

In the anaerobic digestion process, the solids of the sludge being treated are essentially subjected to three consecutive and distinct phases of treatment, namely a period of solubilization, a period of intensive acid production (acidification) and finally a period intensive stabilization and digestion (gasification). Each of these phases is characterized by the production of various intermediate and final products in the digestion zone. Under normal operating conditions, the three phases occur simultaneously. The primary gases produced during the final gasification phase include methane and carbon dioxide which normally constitute more than 95% of the gases released, of which 65 to 70% consist of methane. The production of methane in the gaseous state during anaerobic digestion results from the decomposition of many compounds under the effect of a large number of biochemical reactions dependent on each other and taking place in an orderly and integrated manner. The complex organic bodies contained in the sludge are transformed by various common bacteria, called acidogenic bacteria, into alcohols and volatile acids, without producing methane. These products resulting from the acidogenic phase are then transformed into methane gas by another variety of bacteria called methanogenic bacteria.

Acidogenic bacteria that can be used in anaerobic digestion are very resistant to changes in conditions occurring during the course of the process in their environment. In addition, methanogenic bacteria require anaerobic conditions and are extremely sensitive to variations in process conditions in their environment. For these reasons, it is desirable that the anaerobic digestion zone does not contain oxygen. The accidental introduction of air into the digestion system interferes with the fermentation causing the release of methane and also creates a dangerous situation resulting from the combination of combustible methane gas with oxygen. In addition, methanogenic bacteria are sensitive to processing parameters such as pH variations and to the presence of heavy metals, detergents, ammonium oxide and sulfides. In this regard, the temperature stability of the anaerobic digestion zone is particularly important. The methanogenic bacteria necessary for the digestion process are very sensitive to temperature variations which decrease their activity and their viability, which leads to a relatively excessive growth of the acidogenic bacteria. This results in the production of poorly stabilized sludge and final sludge which is unsuitable, without further treatment, for backfilling or other forms of disposal. In addition, methanogenic bacteria have a relatively slow growth rate and this factor requires the long residence times practiced during anaerobic digestion, even at mesophilic temperatures. Due to this low growth rate, there is a risk of elimination of methanogenic organisms from the digestion plant by washing if the residence time of the sludge solids in this plant is reduced to a value below the limits indicated above. . Whereas the anaerobic digester therefore requires long residence times to ensure the presence of methanogenic organisms and that the flow rate of the sludge arriving at the digestion zone is generally very low, it is necessary that this digester has a capacity very large storage. Operation at high temperature is therefore difficult and it requires a significant supply of heat to the digester, as well as precise regulation of the temperature of this digester. As indicated above, in the prior art, taking into account these considerations, the methane produced by the anaerobic digestion process has been used as heating fuel for the digester in order to maintain a constant high temperature, even during extremely large variations in Room temperature. This use of methane has been shown to be effective in reducing the significant demand for heating energy in the process.

As an alternative to the methods described above, it is possible to carry out aerobic digestion of the biodegradable sludge. Air has been commonly used for this purpose as an oxidant. It is known that aerobic digestion takes place faster at high temperatures. When the temperature rises above 35 ° C, the population of mesophilic microorganisms decreases and the thermophilic forms increase. The temperature range between 45 and 75 ° C is often considered the thermophilic range, in which thermophilic organisms are predominant while most of the mesophilic organisms have disappeared. Above this range, thermophilic organisms decrease and, at 90 ° C, the system becomes essentially sterile. Due to the speed of oxidation of the sludge that it allows, thermophilic digestion achieves a more complete elimination of volatile solid biodegradable in suspension than that obtained during the same digestion period at room temperature. A more stable residue is also obtained, which can be removed without nuisance. It is also established that thermophilic digestion effectively reduces or eliminates pathogenic bacteria present in the sludge, which eliminates the health dangers associated with the evacuation of this sludge.

When air diffusion devices are used to supply the oxygen used during digestion, heat losses tend to be very large. Therefore, aerobic digestion using air is generally limited to the use of mesophilic microorganisms. Installations using air cannot therefore operate efficiently in the thermophilic temperature range. The air contains only 21% oxygen and only about 5-10% of the oxygen component is dissolved. Therefore, a very large amount of air must be used to meet oxygen requirements, and the sensible heat of the used air and the latent heat necessary to saturate the used air with water vapor are important.

10

15

20

25

30

35

.40

45

50

55

60

65

5

627719

your. At room temperature (20 ° C or below), the residence time of the sludge in the aerobic digester using air is generally 12 to 20 days, and very large tanks are necessary to contain the sludge. Although measures are taken to mitigate heat loss by conduction, convection and radiation, the large area uncovered for heat transmission causes very significant heat loss.

Due to the heat losses indicated above, occurring during air digestion, the effects of adiabatic heating are small and an expensive amount of outside heat is required to maintain temperatures at attractive values.

It is known that it is possible to significantly reduce the heat losses occurring during aerobic digestion by using an oxygen-enriched gas instead of air. If oxygen is used efficiently, the amount of gas to be introduced into and removed from the digester is significantly less than that of air, since most or all of the nitrogen has been removed beforehand. . The heat losses due to appreciable heating of the gas and to the evaporation of water in the gas are reduced. These reductions in heat loss are sufficient to allow the adiabatic heat alone to maintain the temperature at values substantially above room temperature, so that the digestion zone can operate effectively in the thermophilic temperature range, with minimal intake of heat outside the process. As thermophilic stabilization is much faster than mesophilic stabilization, the time required to stay in the aerobic digestion zone is significantly reduced in thermophilic mode. This makes it possible to use smaller basins and, therefore, to reduce heat loss to the environment. Due to the higher speed of sludge oxidation, aerobic digestion at thermophilic temperatures achieves a high and suitable decrease in biodegradable volatile solids, for example 80-90% reduction rates, for periods residence of relatively short sludge, of the order of 3 to 10 days.

Despite its definite interest, thermophilic aerobic digestion has several disadvantages compared to anaerobic digestion. Firstly, because the aerobic thermophilic digestion process has an oxidizing character, it gives off a gas resulting from a bio-oxidation reaction and containing carbon dioxide and water vapor which cannot be used , but which it is rather desirable to release into the atmosphere. On the other hand, the anaerobic digestion process makes it possible to obtain, as reaction by-product, a gaseous mixture which can be used outside the treatment installation and also useful as combustion gas to meet the energy needs of heating associated with digestion at high temperatures. In addition, the aerobic digestion zone requires a greater expenditure of energy for mixing and bringing the gas and sludge into contact than that required for an anaerobic digestion installation for mixing the contents of the digester.

The invention therefore relates to an improved process for digestion of sludge, and in particular a process carrying out aerobic digestion and hot anaerobic digestion, in order to use the advantages of these two modes while attenuating their disadvantages.

The sludge digestion process according to the invention is defined in claim 1. By way of example, one embodiment of this process consists in introducing sludges and aeration gases comprising at least the fluids 50% by volume of oxygen in a first closed digestion zone, kneading them to maintain the content of dissolved oxygen sludge at least 2 mg / 1 and the total content of suspended solids at least 20,000 mg / 1. The sludge is kept in the first digestion zone during this operation at a temperature in the thermophilic range, between 45 and 75 ° C.

The mixing in the digestion zone, at the thermophilic temperature, is continued during the residence time of the mud, which is from 4 to 48 h, in order to partially lower the content of the mud,

introduced into the first digestion zone, in volatile biodegradable solid matter in suspension. Then, the partially stabilized sludge and the oxygen-depleted digestion gas, so that its pure oxygen level is at least 21%, are discharged separately from the first digestion zone. This stabilized, partially discharged sludge is then introduced into a second closed digestion zone. The sludge is kept in this second digestion zone under anaerobic conditions and at a temperature of between 30 and 60 ° C., for a time sufficient to again lower the content of the sludge in biodegradable and volatile solid matter in suspension unless about 40% of the content of the same solids presented by the sludge introduced into the first digestion zone. In addition, methane gas is released in this second digestion zone. Then, the more stabilized sludge and the methane gas are discharged from the second digestion zone.

The expressions sludge and activated sludge used in the present specification designate a mixture of solids and liquid characterized by a solid phase consisting of sludge and by an associated liquid phase and in which the solids of the sludge are at least partially biodegradable, c ' that is, they can be broken down under the action of living microorganisms. Biodegradable sludge is generally characterized by its content of biodegradable volatile solids in suspension (VSS). The expression “content of biodegradable and volatile solid matter in suspension” essentially designates the maximum reduction in solid matter that can be obtained by aeration of the sludge with gas containing oxygen at ambient temperature, for example 20 ° C., and having a dissolved oxygen content of at least 2 mg / 1. A maximum reduction in solids is assumed to be achieved after 30 days of aeration. Details concerning this determination are given in the article "Water Pollution Control", by Eckenfelder, W. W. and Ford, D. L., "The Pemberton Press", 1970, p. 152. By determining the contents of volatile suspended solids in fresh sludge and sludge which has been subjected to aeration for 30 days, the biodegradable fraction of the total solid content can be calculated as follows:

VSS (fresh) - VSS (30 days)

VSS (fresh)

The term stabilized sludge designates sludge in which the content of biodegradable and volatile solid matter in suspension has been reduced by a digestion treatment. The expression sludge residence time used in this specification designates the average duration during which the sludge is maintained in a given digestion zone, this duration being calculated by the following formula:

Qs

0U- t is the sludge residence time

(days or hours);

Vd is the volume of sludge contained in the digestion zone and during treatment (m3); and Qs is the volumetric flow rate for supplying sludge to the digestion zone (m3 / d or m3 / h).

The invention is based on the astonishing observation that an aerobic digestion zone, operating in thermophilic temperature regime, can be associated with an anaerobic digestion zone placed downstream, so that a partial digestion of the sludge is carried out in each of the successive zones. This association or integration of two digestion zones provides an improvement over the results that were expected by considering separately the respective stages of digestion entering the treatment process as described in more detail below.

5

10

15

20

25

30

35

40

45

50

55

60

65

627,719

6

No attempt has been made, in the prior art and for many reasons, to combine aerobic and anaerobic digestion at high temperature of the sludge in the manner carried out in the process of the invention. The first of these reasons is that the capacity of the tanks used in the anaerobic digestion process is extremely large, as indicated previously, and that it was necessary to produce large quantities of methane used as heating fuel to ensure an inexpensive operation. large digestion ponds. The combination of an anaerobic digester with an aerobic digester therefore appeared undesirable when considering the overall capacity requirements of the combined process, this capacity being considered to be greater than that used in one or the other, alone, of the digestion. This association therefore seems, apparently, to result in a simple doubling of the functions normally performed by each of the aerobic and anaerobic digestion processes, which leads to an increase in the cost of equipment without foreseeable increase in the effectiveness of the treatment.

In addition, the association of an anaerobic digester with an aerobic digester appeared undesirable because of the planned impossibility for the anaerobic digester to carry out only a partial treatment of sludge digestion throughout the installation, during residence times shorter than the long periods characterizing conventional anaerobic digesters operating alone. As described above, long residence times are necessary during anaerobic digestion to obtain stabilization of the sludge and sufficient methane production. If the residence time in the anaerobic digester is reduced below the normal duration corresponding to the complete treatment in a combined aerobic / anaerobic digestion process, in order to achieve only partial digestion in the anaerobic mode, provision may be made for excessive depletion of methanogenic organisms during the short duration of the anaerobic mode by loss of the corresponding organisms with slow growth in the digester effluent with, as a result, poor stabilization of the sludge throughout the process.

In addition to the reasons given above, the disadvantages that a combined aerobic / anaerobic digestion installation seemed to present from the point of view of operating stability, since each of the aerobic and anaerobic digestion operations alone requires precise control of the temperature. working when the temperature values are high, so that the combination of the two methods seemed to require an even more precise adjustment of the temperature and, therefore, seemed to present a risk of increased harmful effects due to instability and temperature variations.

Finally, a combined aerobic / anaerobic digestion process seemed to be disadvantageous when considering the possible transfer of residual dissolved oxygen from aerobic digestion performed upstream to anaerobic digestion performed downstream. As previously indicated, the methanogenic bacteria present in the anaerobic digestion zone are strictly anaerobic and extremely sensitive to variations in their environment. It is recognized that any significant introduction of oxygen into the anaerobic digestion zone harms the stabilization of the sludge by the formation of methane and creates a danger of passage of oxygen from the liquor into the gas phase containing methane, and of formation of a combustible gas mixture in the digester.

Contrary to the forecasts indicated above, it appeared that the establishment of an aerobic thermophilic digestion zone upstream of an anaerobic digestion zone at high temperature and that the use of these respective zones according to the process of invention not only make it possible to obtain a functional and inexpensive installation for treating sludge, but also result, in a digestion installation having significant general improvements compared to the installations of the prior art, from the synergism obtained between the digestion modes. aerobic and anaerobic. For example, the method according to the invention makes it possible to obtain thermal working stability, in the entire sludge digestion installation, which it is impossible to achieve separately in one or the other forms of digestion. In addition, the combined digestion process according to the invention makes it possible to obtain a highly stabilized sludge which is also completely pasteurized, despite a reduction in the sludge residence time below the minimum time generally considered to be respected for each of the forms of digestion taken separately. In particular, it has surprisingly appeared that the anaerobic digestion zone used in the process according to the invention can work with residence times of the sludges substantially shorter than those required to obtain total stabilization of the sludges in anaerobic digesters classics working alone, and that this is achieved without loss of usefulness or effectiveness of treatment, as might be expected. As an example of the residence times used in a suitable manner in the process according to the invention, a pilot installation, implementing this process, worked satisfactorily with a residence time of the sludges from 24 to 48 h in the first stage, or stage working in aerobic mode, and 4 to 5 days in the second stage, or stage working in anaerobic mode. In addition to the advantages indicated above, the method according to the invention allows a significant reduction in the capacity requested compared to a conventional anaerobic digestion installation, but with retention of a large part of the methane production capacity of the digester classical anaerobic considered alone, as described in more detail below. However, by way of example, the installation according to the invention can use approximately 60% of the capacity required for an anaerobic digestion zone of the prior art, while retaining approximately 75% of the methane production capacity of this zone. Such an improvement allows a production of methane appreciably greater than that required for heating, so that the quantity of methane gas which can be used outside the sludge digestion installation is much greater than that which can be obtained with a anaerobic digestion installation of the prior art. Finally, no significant transfer of oxygen from the first digestion zone to the gas phase present in the second digestion zone appeared in the method according to the invention.

The reasons why the method according to the invention has the unexpected advantages indicated above are not fully understood. It is possible that the absence of significant oxygen transfer from the first digestion zone to the second is due to the fact that the high temperatures prevailing in the second digestion zone create anaerobic bacteriological conditions sufficiently intense to completely exhaust the oxygen. dissolved in the sludge passing from the first to the second digestion zone before there can be a relatively large passage of the dissolved oxygen towards the gas phase present in the second digestion zone. The surprisingly short sludge residence time in the process according to the invention, in particular in the anaerobic digestion zone, as well as the characteristic of thermal stability of the process and the surprisingly high methane production capacity of anaerobic digestion can be a consequence of chemical or biological acclimatization of sludge and microorganisms in the aerobic digestion zone, resulting in an increase in the effectiveness of subsequent anaerobic treatment. Nevertheless, it is obvious that the invention is not limited to a particular theory and that the preceding explanations are given for information only and are in no way limitative.

The invention will be described in more detail with reference to the accompanying drawings by way of non-limiting examples and in which:

the flg. 1 is a diagram of a first embodiment of an installation implementing the digestion method according to the invention in which heat is recovered from the effluent streams from each of the first and second closed digestion zones;

fig. 2 is a diagram of an alternative installation implementing the method according to the invention, in which the digestion gas depleted in oxygen and discharged from the first closed zone of

5

10

15

20

25

30

35

40

45

50

55

60

65

7

627719

digestion is used for secondary treatment oxygenation of water with biological oxygen demand;

fig. 3 is a diagram of another embodiment of the installation according to the invention, in which the sludge from the primary and secondary stages of wastewater treatment is introduced into the digestion zones;

fig. 4 is a graph showing the temperature of the sludge arriving at the first digestion zone, which it is necessary to maintain at an operating temperature of 50 ° C., as a function of the total content of suspended solids in said sludge,

and fig. 5 is a diagram of another embodiment of the installation according to the invention, in which a small part of the sludge arriving at the installation is diverted towards the second digestion zone.

Fig. 1 is a diagram of a first embodiment of an installation implementing the method according to the invention and suitable for the treatment of sludge by a first aerobic thermophilic digestion, followed by an anaerobic mesophilic digestion. The sludge, which can come from a source such as a primary sedimentation tank, the clarification tank of a wastewater treatment plant with activated sludge, a percolator filter or any other sludge production facility, arrives at the installation according to the invention via a pipe 8 and are heated successively in heat exchangers 22 and 15 to be brought, for example, to a temperature of 30 to 35 ° C, before being introduced into the first closed area 10 digestion in order to maintain the temperature of this zone at a thermophilic level between 45 and 75 ° C. The sludge at room temperature present in line 8 is firstly heated in the heat exchanger 22 by indirect exchange of heat against the current with the stabilized sludge coming from the second closed digestion zone 20 and circulating in a pipe 24. Thus, the heat of the stabilized sludge is recovered and said stabilized sludge thus cooled is discharged from the heat exchanger 22 passing through a pipe 25 to be finally discharged or otherwise used. The stabilized sludge arriving in the heat exchanger 22 via line 24 can advantageously have a temperature of 35 to 40 ° C., so that the tributary sludge, leaving the heat exchanger via line 9, is brought to a temperature from 28 to 30 ° C. The partially heated tributary sludge passing through line 9 is further heated in the exchanger 15 which brings them to a temperature of 30 to 35 ° C by indirect heat exchange against the current with the sludge partially stabilized, discharged from the first digestion zone 10 and passing through a line 14, a line 16 passing this sludge from the heat exchanger 15 into the second digestion zone 20.

As an alternative to the heat exchanges described above, carried out with the stabilized sludge streams coming from the corresponding digestion zones, the tributary sludges can be heated before being introduced into the first digestion zone by indirect heat exchange with a medium. suitable heating, coming from the outside, for example steam or hot water,

although heat recovery from the hot sludge streams from the digestion zones is preferred, since it allows efficient heat conservation inside the installation and reduces the heating energy requirements of the installation. Although the heating of the tributary sludges before their introduction into the first digestion zone is not essential in the general practice of the process according to the invention, it may in fact be desirable to maximize the thermal efficiency of the process to high temperature. The desirability of this sludge heating depends, as described in more detail below, on the solids content of the tributary sludge, the sludge residence time in the aerobic digestion zone and other treatment parameters.

The more heated sludge, discharged from the heat exchanger 15 passing through line 11, enters the first digestion zone 10 with an aeration gas arriving through a line 17, this gas constituting the treatment fluid used in the first stage of digestion. The aeration gas from line 17 comprises at least 50% oxygen, by volume, and preferably at least 80% oxygen, in order to exert a suitably high mass transfer driving force and have a high rate of dissolution in the sludge at the high temperatures of the latter in the first digestion zone concerned. Line 17 is connected to a source of aeration gas containing oxygen (not shown) which may include, for example, a cryogenic oxygen plant or a supply bottle or a separation device. air by absorption and adiabatic pressure oscillations, as is generally the case for supplying oxygen-containing gas. As shown, the aeration gas from line 17 can also be heated by a device 19, so that the temperature of the digestion zone 10 can be easily maintained at the desired treatment value.

In the aerobic distillation zone 10, the sludge and the aeration gas are mixed and one of these fluids is recycled at the same time against the other fluid in an amount and at a rate sufficient to maintain the dissolved oxygen content ( DO) sludge at least 2 mg / 1 and the total suspended solids content (MLSS) of sludge at least 20,000 mg / 1. This mixing and recycling of the fluids is carried out using a contact device 12 which, in practice, may include a submerged turbine and a gas compressor, communicating with the upper free space, containing the gases, of the digestion zone and with the gas injection device to recirculate the oxygen-containing aeration gas, against the flow of sludge or, alternatively, with a surface aeration device which recycles the sludge against the aeration gas present in the free upper space of the digestion zone 10. The recirculation of one of the fluids constituted by sludge and the aeration gases against the other fluid in the aerobic digestion zone is advantageous in practice for obtaining a high content of dissolved oxygen in the sludge and efficient use. oxygen present in the aeration gas. However, this recirculation is not essential to the general practice of the invention and, in certain cases, it is possible to obtain a suitable dissolution of the oxygen in the sludge and a very efficient use of the oxygen. aeration gas by single crossing of the aerobic digestion zone by the aeration gas. The dissolved oxygen content in the sludge present in the first digestion zone must be at least 2 mg / 1 so that the thermophilic microorganisms present in the sludge have sufficient oxygen to rapidly decompose the biodegradable constituents of the sludge. The total content of suspended solids (MLSS) in the sludge is at least 20,000 mg / l, so that the biodegradation of the sludge in the aerobic digestion zone is rapid, this biodegradation being based on kinetic phenomena and, in at the same time, so as to also maintain the total heat of reaction of aerobic exothermic digestion at a level sufficient to maintain the temperature of the sludge present in the first digestion zone in the thermophilic range and to obtain a satisfactory degree of partial stabilization of the sludge in the digestion zone, for a short residence time.

Under the processing conditions indicated above, the sludge is kept in the first digestion zone at a temperature in the thermophilic range of 45 to 75 ° C, so that the volatile solids oxidize quickly. The thermophilic digestion is extended for a period of stay in the first digestion zone from 4 to 48 h, in order to partially lower the content of volatile and biodegradable solid matter in suspension in the sludge introduced into the first digestion zone. The sludge residence time in the aerobic digestion zone must be at least 4 h to obtain sufficient partial stabilization in the first digestion zone. When the residence time is less than 4 h, the importance of the stabilization of the sludge must be

5

10

15

20

25

30

35

40

45

50

55

60

65

627719

8

performed during the subsequent anaerobic treatment operation becomes disproportionate to the level of stabilization obtained in the first aerobic digestion and the overall residence time in the installation and the storage capacity requested approach those of the conventional anaerobic digestion installation, which attenuates the advantages presented by the method according to the invention during residence times of 4 h or more. For similar reasons, the sludge residence time in the aerobic digestion zone should not exceed 48 h. Above this value, the stabilization of the sludge carried out in the aerobic digestion zone becomes too great compared to the stabilization that remains to be carried out in the anaerobic digestion zone located downstream, and the residence times required in this last operation to maintain a sufficiently large population of methanogenic organisms in the digestion zone tend to become significantly longer than those desirable for efficient operation. As previously, it results in a reduction in the advantages presented by the method according to the invention concerning the residence time in the entire installation and the storage capacities when the residence time is between 4 and 48 h. According to the preceding considerations, the residence time is preferably between 12 and 24 h.

After the aerobic digestion described above, the partially stabilized sludge leaves the aerobic digestion zone via line 10 and the digestion gas depleted in oxygen, containing at least 21% of pure oxygen and, preferably, at least 40 % of pure oxygen, is discharged separately from the aerobic digestion zone by a pipe 18. The level of oxygen purity of the gases leaving the digestion zone by the pipe 18 can be easily maintained at the desired value by a suitable setting relative flow rates for the introduction of the aeration gas via the line 17 and the outlet via the line 18, for example using flow control valves mounted in one or the other of the inlet lines or outlet and controlled by an oxygen purity analysis device, placed in the outlet line 18, in a manner known to those skilled in the art.

It appeared that by maintaining the sludge present in the aerobic digestion zone under the thermophilic conditions determined according to the invention, an almost complete pasteurization of this sludge is obtained. The partially stabilized sludge, leaving the aerobic digestion zone 10 via line 14, has a temperature in the thermophilic range between 45 and 75 ° C. Whereas this form of installation according to the invention performs an anaerobic mesophilic digestion in the second closed digestion zone 20, it is desirable that a certain part of the heat is eliminated from the partially stabilized sludges passing through the pipe 14 in order to ensure the effectiveness of anaerobic sludge treatment. Consequently, the sludge passing through line 14 passes through the heat exchanger 15 where it performs an indirect heat exchange with the partially heated tributary sludge which arrives in this exchanger 15 through line 9. The sludge partially stabilized and cooled , having been subjected to aerobic treatment, then flow through line 16 in order to enter the second closed digestion zone 20. As a variant, the partially stabilized sludge passing through line 14 can be cooled by a cooling medium coming from the outside, for example the clarified effluent from a wastewater treatment installation. In addition, in winter, it may be unnecessary to carry out a heat exchange to cool the partially stabilized sludge, since the heat lost to the environment by the second digestion zone and by the sludge flow flowing from the first to the second digestion zone can satisfactorily compensate for such cooling.

The partially stabilized sludge introduced into the second digestion zone via line 16 is maintained in this zone under anaerobic conditions and at temperatures of 30 to 40 ° C., for a sufficient time so that their content of volatile and biodegradable solids in the suspension is further reduced and brought to a value less than about 40%, and preferably less than 20%, of the content of the same solid materials presented by the sludge arriving in the first digestion zone. In addition, these conditions are met to allow the release of methane gas.

In the general practice of the process according to the invention, the temperature of the sludge present in the second closed digestion zone is maintained in the range from 30 to 60 ° C which includes both the mesophilic range from 30 to 45 ° C and the thermophilic range from 45 to 60 ° C. For a very efficient operation, the anaerobic zone used in mesophilic mode is maintained at a treatment temperature between 35 and 40 ° C, and preferably between 37 and 38 ° C. A preferable temperature range used for anaerobic thermophilic digestion is 45 to 50 ° C. Work carried out in the preferred temperature ranges indicated above produces a particularly rapid decomposition of volatile solids biodegradable by the microbial strains concerned.

In the operation of the anaerobic digestion zone 20, the elements contained in this zone are continuously mixed by a stirring device 21, so as to form a large zone of active decomposition and to significantly increase the speed of the stabilization reactions. The residence time of the sludge in the second digestion zone can advantageously be between 4 and 12 days, and preferably between 5 and 9 days. It is undesirable for the sludge residence times in the second digestion zone to be less than 4 days, since below this value, the residence time is less and less suitable for maintaining a large viable population d methanogenic organisms in the anaerobic phase with, as a consequence, a harmful effect on the overall stabilization of the sludge carried out in the digestion installation. Furthermore, when the residence times of the sludge in the anaerobic digestion zone are greater than 12 d, they are unnecessarily long and the advantages resulting from both residence times and reduced storage capacities, presented by the combined process according to the invention in the case where the residence times are between 4 and 12 d, become difficult to obtain.

After the sludge has been subjected to anaerobic treatment in the second digestion zone 20, the stabilized sludge thus obtained is discharged from this second zone 20 by a pipe 24 and their heat is recovered by passing through the tributary sludge, inside from the heat exchanger 22, before the stabilized sludge is finally evacuated from the installation by line 25. The methane gas produced in the second digestion zone 20 by the biochemical reactions which take place there is evacuated from the zone anaerobic treatment by a line 23 carrying a valve 26 for adjusting the flow.

As described above, continuous operation of an anaerobic digester with a high flow rate, working at optimum temperatures above ambient temperature, is difficult to maintain in conventional practice. Variations in ambient temperature generally cause variations in the temperature of the tributary sludge and heat loss from the digestion tank, which results in undesirable changes in temperature inside the digestion tank. These temperature variations, as described above, have an effect on the relative growth rates of acidogenic and methanogenic bacteria. Acidogenic bacteria are generally very resistant and moderate variations in temperature do not significantly alter their metabolic activity. On the other hand, methanogenic bacteria are extremely sensitive to the surrounding conditions. In the case where the constant temperature of the anaerobic digestion zone varies only by 2 ° C, instability results in the activity and growth of methanogenic organisms. As a result, the activity of acidogenic organisms becomes predominant and this results in an accumulation of acidic decomposition intermediates and a lowering of the pH in the digestion zone. When the pH decreases, the activity of the methanogenic organisms decreases more and there is a major disruption of the process.

5

10

15

20

25

30

35

40

45

50

55

60

65

9

627719

Another problem with conventional anaerobic digestion devices is the sensitivity of methanogenic bacteria to the presence of toxic metals such as copper. Even very small amounts of these metals in the sludge inhibit the activity of methanogenic organisms. As in the previous case, after the anaerobic digestion zone has received toxic metal concentrations continuously for a certain time, the acidogenic organisms become predominant and cause the production of an excess of acidic decomposition intermediates, which causes a decrease in pH in the digestion zone and further inhibits the activity of methanogenic organisms. This cumulative effect inevitably leads to a major disruption of the process.

The solution to the disruption conditions of the process described above in the conventional anaerobic digestion zone generally consists in introducing large amounts of lime into the digester in order to increase the buffering effect and, consequently, to raise the pH inside the digester. This intervention can be satisfactory if the digester receives a large load of inhibition material and if this material can then be removed by washing or assimilated by the digester. In addition, by raising the pH and decreasing the flow rate of supply of tributary sludge, it is sometimes possible to restart the digester having undergone such an upheaval. However, the corrective measures indicated above are generally only suitable for short-term process variations or disruptions and are generally not advantageous in the case of long-term variations or disruptions.

The method according to the invention makes it possible to adjust and maintain high temperatures in the digestion zone with minimal variations, whatever the climatic conditions, by the association of the aerobic thermophilic digestion phase with a consecutive phase of anaerobic treatment. sludge. In the process according to the invention, the aerobic thermophilic digestion phase generally makes it possible to supply a quantity of heat greater than that sufficient to thermally stabilize the anaerobic phase, due to the heat of the stream of partially stabilized sludge flowing from the aerobic digestion zone towards the anaerobic digestion zone. Consequently, the temperature variations can be theoretically eliminated from the anaerobic digestion zone used in the process according to the invention, by the variation of treatment parameters such as the residence time of the sludge in the aerobic zone, the content of matter solids from the sludge arriving at the aerobic zone and the extent of the heat exchange causing heating of the tributary sludge before their introduction into the aerobic zone. The anaerobic digestion zone used in the process of the invention can generally operate, to the nearest degree, at the optimum temperature, whatever the seasonal variations in ambient temperature.

In addition, the method according to the invention can, in many cases, have an increased tolerance to toxic metallic components present in the sludge arriving at the digestion plant. Although a conventional anaerobic digestion installation cannot tolerate the presence of toxic metals, when such contaminating substances are present in the sludge arriving at the installation implementing the method according to the invention, their effect on the stability and methane production capacity during anaerobic digestion is relatively reduced. This indicates that these metals are toxic only in a soluble ionic form. It is believed that during the anaerobic digestion phase of the process according to the invention, these metal ions form complexes with the solid materials present in the sludge being treated with, as a result, the maintenance of these metal bodies in a complex form for the anaerobic phase of the process. Consequently, the anaerobic phase of the combined process according to the invention may exhibit greater stability with regard to the inhibition normally associated with the presence of heavy metals in the sludge flowing to the treatment installation according to the invention.

Another important advantage presented by the combined process according to the invention, in addition to that attributable to the stability of the working temperature and to the resistance to the inhibitory effects of toxic metals, is its attitude to tolerate sporadic upheavals, for example periodic charging, without loss of efficiency. In a classic anaerobic digestion zone, not only does the initial solubilization phase of the digestion process occur quickly, but the microbial use of this material by mesophilic and, where appropriate, acidogenic bacteria also occurs at high speed. Upon the appearance of a sudden high load of solids on the conventional anaerobic digestion device, the solubilization and acidification take place at a rate higher than that at which methanogenic bacteria can use acid intermediates. Consequently, the accumulation of acid constituents occurs in the digestion zone, the pH of the latter decreases and an acidification of the contents of the digester tends to appear. However, in the process according to the invention, the aerobic thermophilic phase, carried out upstream, not only promotes rapid solubilization of the biodegradable materials present in the sludge, but also tends to decrease the population of acidophilic mesophilic bacteria. Then, the anaerobic digestion phase allows a new development of these organisms. However, their population is generally lower than that of methanogenic organisms and more balanced in relation to it. Consequently, upon the appearance of a periodic load on the aerobic digestion zone, rapid solubilization occurs in aerobic digestion, as well as stabilization of the greater volatile part of the sludge, which attenuates the shock and Significantly reduces its effects on the anaerobic zone located downstream. During the subsequent operation, the anaerobic zone receives partially stabilized sludge on which the acidogenic and methanogenic bacteria can grow in a balanced manner.

Whereas the first phase of the sludge treatment of the process according to the invention is carried out in an aerobic thermophilic digestion zone, the process described in American patent N ° 3926794 can be advantageously used with the process according to the invention for the treatment wastewater containing biodegradable solid matter in suspension, so that their biochemical oxygen demand is eliminated by the implementation of the activated sludge process, and that the activated sludge thus obtained is treated by the process according to the invention.

By way of explanation, the secondary treatment of activated sludge resulting from wastewater is generally carried out as follows. Wastewater with a biological oxygen demand, for example sewage, can first be subjected to treatment operations such as sand removal and primary sedimentation, in which a primary sludge, comprising biodegradable solids in suspension, is separated from the waste water, these operations also making it possible to obtain a primary effluent depleted in solid matter, then introduced into a secondary treatment device. During the secondary treatment, the primary efBuent and the recycled sludge are mixed and aerated at a sufficient speed and for a sufficient time to present a reduced biochemical oxygen demand. Then, the mixed liquor is separated into a purified liquid and into an activated sludge, at least a significant portion of which is returned to be mixed with the primary effluent in order to constitute the recycled sludge indicated above. This wastewater treatment can be suitably used with the process according to the invention, the primary sludge and the activated sludge not returned being introduced into the first digestion zone used in the process according to the invention, in order to constitute the tributary.

As described in the aforementioned US Patent No. 3,926,794, the gaseous oxygen is used for thermophilic digestion of activated sludge in a closed and hot digestion zone and the gases leaving this digestion zone are used so as to constitute at least the main part of the aeration gases from a closed cooling zone used in the secondary treatment of

5

10

15

20

25

30

35

40

45

50

55

60

65

627719

10

activated sludge from wastewater. The aforementioned US patent indicates that in order to obtain a high mass transfer driving force, necessary for efficient dissolution of oxygen during aerobic digestion at high temperature, oxygen gas having a relatively high purity must be used as aeration gas during digestion. At elevated temperatures, the increased rate of biochemical degradation of sludge in the aerobic digestion zone has the effect of producing relatively large amounts of carbon dioxide as a gaseous reaction product. As the solubility of carbon dioxide is relatively low at the high temperatures used during aerobic thermophilic digestion, a significant amount of this carbon dioxide is released in the gas phase present in the aerobic digester and therefore reduces the effective concentration of oxygen in the gas aeration contained in the digester. In addition, at high thermophilic temperatures, the concentration of oxygen in the gas phase in the digestion zone is further reduced by the water vapor present in the aeration gas and resulting from the relatively high water vapor pressure. encountered at such temperature levels.

The effects indicated above significantly reduce the driving force for mass transfer of oxygen from the gas phase to the sludge in the thermophilic digestion zone. The driving force for mass transfer of oxygen in the liquid phase of the sludge is also reduced, due to the low solubility of oxygen at high thermophilic temperatures. For these reasons, the aforementioned US patent indicates that it is desirable for the oxygen-containing gas which has been introduced into the thermophilic digestion zone to have an oxygen concentration of at least 80% by volume. Within such working limits, the spent aeration gas, coming from the thermophilic digestion zone, can advantageously consist of oxidizing gas used in a device for secondary treatment of activated sludge. Fig. 2 is a diagram of a variant of the installation implementing the method according to the invention. This figure shows how the process described in the aforementioned American patent N ° 3926794 can be advantageously used with the process according to the invention.

As shown in fig. 2, water having a certain biochemical oxygen demand, for example waste water, enters an aeration zone 102 through a conduit 101. A first gas, comprising at least 40% oxygen by volume, enters zone 102 by a conduit 118 (shown in dotted lines) and activated recycled sludge also enter zone 102 by a conduit 108 on which a pump 109 is mounted. The supernatant liquid coming from a sludge thickener 151 is also introduced into the closed aeration zone 102 by a conduit 150. In this figure, the liquid and sludge flow conduits are shown in solid lines, while the gas flow conduits are shown in dotted lines. For clarity, the valves are not shown. Their use for implementing the method according to the invention is however obvious to those skilled in the art. The currents indicated above are intimately mixed in the aeration zone 102 by a mechanical stirring device 103. The latter may include motor-driven rotors placed near or below the surface of the liquid. The gaseous oxygen can be introduced through the conduits 118, above or below the liquid. This device is well known to those skilled in the art and it must be chosen so as to establish a large contact surface between the fluids, with minimal work. In the case where the gaseous oxygen is injected in the form of bubbles or diffused in the liquid, the bubbles must be small so that their total surface is large and their buoyancy is low. The dissolution of oxygen is also favored by immersion of the device for dispersing the gas at a depth, in the liquid, to which the hydrostatic effect is sensitive.

A device provides continuous recirculation of a fluid against the other fluids in the aeration zone 102. For example, a compressor (not shown) can be connected to the space of the aeration zone containing the gases, using a suitable duct, in order to recycle the aeration gases to the lower part of the zone so that they are released in the form of small bubbles by a conventional device. As a variant, the kneading device indicated above can also be used for recirculating a fluid, as is the case with surface aeration rotors. Aeration devices are generally evaluated by their standardized air transfer efficiency which identifies the possibility of the device to dissolve the oxygen in the air in running water with zero dissolved oxygen content, under a pressure of 1 bar and at 20 ° C. Devices suitable for the method according to the invention have a standardized air transfer efficiency of at least 0.875 kg of 02 / kWh and preferably of at least 1.75 kg of 02. For this purpose, the power used when estimating the device is the total power consumed to stir the liquor and to establish contact between the gas and the liquor.

The oxygen indicated above is introduced and one of the fluids is recycled at the same time and continuously against the other fluids, in sufficient quantity and at a rate to maintain the dissolved oxygen content (DO) of the liquor mixed with at least 0.5 mg / 1. In addition, the temperature of the liquor is preferably kept at least 15 ° C, so that it may be necessary, in cold weather, to implement a device preventing the temperature from falling in the aeration zone 102 , for example a device for heating waste water arriving via line 101. The design and operation of the waste water aeration zone 102 can be as described in American patents Nos. 3547811, 3547812 and 3547815.

The mixed and oxygenated liquor is discharged from the aeration zone 102 and passes through a conduit 104 in order to be separated in a clarification tank 105 into a purified supernatant liquid and into activated sludge. The unconsumed gas, containing oxygen, is evacuated from the aeration zone 102 by a conduit 119 and can be discharged into the atmosphere, for example. This gas leaves the aeration zone at a determined flow rate, so that its oxygen content does not exceed 40% of the total oxygen introduced into the closed aerobic digestion zone (described below). With regard to the clarification tank 105, the purified supernatant liquid flows out through a line 106 and the activated sludge is evacuated therefrom through a line 107 which contains microorganisms in a concentration corresponding to a total solids content. suspension (MLSS) of approximately 10,000 to 40,000 mg / 1. Most of the activated sludge, for example at least 85%, is returned by the conduit 108 and the pump 109 to the aeration zone, preferably at such a rate, compared to the wastewater presenting a biochemical oxygen demand. , that the ratio between the volume of sludge recirculated and that of wastewater is between 0.1 and 0.5. The flow rates of arrival into the closed aeration zone 102 are preferably such that the total concentration of suspended solids (MLSS) is from 4000 to 12000 mg / 1 and that the content of volatile suspended solids (MLVSS) or from 3,000 to 10,000 mg / 1. The contact time between liquids and solids in the aeration zone 102 to allow organic assimilation and absorption is between 30 min and 24 h. This time varies according to the strength (biochemical oxygen demand) of the wastewater, the type of polluting substances, the rate of solid matter in aeration and the temperature, these criteria being known to those skilled in the art.

The separated sludge in the clarification tank 105 is not entirely returned to the aeration zone 102 for two reasons. The first of these reasons is that the activated sludge process allows an excess production of microorganisms, because the mass of new cells developed from impurities present in wastewater is greater than the mass of cells oxidizing them. - even during treatment. The second reason is that wastewater normally contains non-biodegradable solids which deposit and accumulate with biomass. Therefore, a small fraction of the activated sludge must be discharged

5

10

15

20

25

30

35

40

45

50

55

60

65

11

627719

in order to balance the population of microorganisms and the supply of nutrients for these microorganisms (biochemical oxygen demand), and also to suppress the accumulation of inert solid matter in the installation. The discharged sludge generally constitutes less than 3% and rarely more than 15% of the total separated sludge.

Although the sludge discharged constitutes a small fraction of the totality of the solid substances separated in the clarification tank, they nevertheless constitute, in absolute terms, a significant quantity of matter. Whatever this quantity, the disposal of this material is an important part of the cost of treating wastewater and, moreover, it poses a serious ecological problem. The sludge is putrescible and has a great biological activity, and it often contains pathogenic bacteria. The sludge can be used as fertilizer and / or backfill. However, before being thus used, they must be well stabilized in order to avoid any nuisance and not to present a danger to health, and their high water content (for example from 96 to 98%) must be reduced.

The residual sludge from the clarification tank 105 is removed from the recirculation loop by passing through a conduit 111. The total concentration of this sludge in suspended solids is from 10,000 to 40,000 mg / l. The sludge is initially at approximately the same temperature as the waste water present in the aeration zone 102, for example from 15 to 25 ° C., and it passes into a thickening tank 151. In this basin 151, the sludge is concentrated to an MLSS value of between 20,000 and 60,000 mg / 1 and the thickened sludge flows from below through a conduit 152 to the digestion installation.

In certain cases, for example when the treatment of wastewater is carried out at a high ambient temperature, the thickening of the residual sludge coming from the clarification tank may not be necessary and the sludge passing through the conduit 111 can avoid without problem the basin 151 passing through a conduit 153, then through conduit 152 which leads them to the digestion installation. The overflow of the thickener (supernatant liquid) flows through a conduit 150 towards the aeration zone 102, as indicated previously.

The thickened sludge passing through line 152 can be heated, if necessary, by a methane boiler 130, before being introduced into the aerobic digestion zone 110. As a variant, the sludge can be heated by heat exchange with the stabilized effluent sludge coming from the anaerobic digestion zone 120b, as described above with regard to the embodiment shown in FIG. 1. The residual sludge passes continuously or intermittently from the pipe 152 into the first closed digestion zone 110. The aerobic digestion zone 110 is maintained at a temperature in the thermophilic range, between 45 and 75 ° C. When adiabatic functioning of the aerobic digestion zone is obtained, the temperature of the sludge in this zone can advantageously be maintained at a value between 55 and 65 ° C. It is difficult to maintain temperatures in a self-sustaining manner digestion above 65 ° C. The high temperature prevailing in the first closed digestion zone 110 can also be obtained by external heating, for example by a suitably heated fluid which circulates in a heat exchanger (not shown) arranged inside the digestion zone. Due to the tendency of solids to form coatings and plugs, the heat transfer surfaces placed in the digester should not be complex or close together, and it may be advantageous to drown them in the wall of the tank or fix them to this wall.

A second gas containing at least 80% oxygen, by volume, is introduced into the first covered and heated zone 110 for digestion by a conduit 117. As described below, this gas is introduced in an amount sufficient to constitute part of the first gas containing oxygen introduced into the aeration zone 102 through the conduit 118.

The high temperature of the first closed digestion zone 110 is preferably obtained in an adiabatic or self-sustaining manner, without the need to use heat exchangers such as that shown in 130. The concentrated sludge obtained for example in the oxygen aeration process described in US Pat. No. 3,547,813, are very favorable to operation at self-maintained temperature, because of their water content which is reduced relative to their fuel content biodegradable. In addition, high concentrations of solid matter make it possible to reduce the size of the digester and, consequently, the heat losses by conduction produced through the walls of the digestion basin. As indicated above, the total content of suspended solids (MLSS) in the sludge present in the digestion zone must be at least 20,000 mg / 1, taking these considerations into account.

The upper limits of the concentration of solids inside the aerobic digester are generally determined by two factors. In general, the maximum concentration depends on the ability of conventional sedimentation and thickening devices to decrease the water content. Flotation devices, centrifugal separators and gravity thickeners often provide total suspended solids concentrations of 60,000 mg / l. These concentrations can be further increased by adding and mixing primary sludge or concentrated residue from sources other than wastewater. The second factor limiting the concentration of solids is the increasing difficulty in dissolving oxygen and mixing the solids in the digester. It is preferable that the upper limits be 60,000 mg / l to ensure a uniform and suitable distribution of the dissolved oxygen in the sludge. In addition, in most uses of the process according to the invention, temperatures corresponding to the almost maximum speeds of aerobic thermophilic digestion can be obtained by self-maintenance at concentrations of solid materials not exceeding 60,000 mg / l. A further increase in the concentration of solids causes an increase in the release of C02 in the free space of the digester and unnecessarily reduces the partial pressure of oxygen in the aeration gas.

The construction of the digester basin also affects self-sustaining temperature levels and concrete walls are preferred over metal, due to the lower conduction heat losses occurring through concrete. Heat losses can be further reduced by burying the pool and forming an earth mound against any vertical wall of the pool that is exposed. Thermal insulation, for example made of low density concrete or cellular plastic, can be applied to a metal cover, if necessary.

It is also preferable to implement the method according to the invention in aerobic and anaerobic digesters having a surface / volume ratio of less than 2.62 m2 / m3. In this respect, the term surface designates the entire surface of the walls of the covered digester, including the top, the bottom and the side walls. Area / volume ratios greater than 2.62 m2 / m3 result in high heat losses by conduction through the walls compared to the amount of heat to be maintained in the digester. Such heat losses may require the application of thermal insulation on the walls subjected to ambient air. In addition, larger surface / volume ratios generally result in small digestion ponds, in which it is difficult to achieve uniform aeration and / or mixing.

The residence time of the sludge in the aerobic digester also affects the temperature levels that can be maintained in a self-sustaining manner. It should be noted that many factors enter into the relationship between the residence time and the temperature, for example the ability of the sludge to decompose and the strength (concentrations of solid matter) of said sludge. In general practice of the method according to the invention, the residence time of the

5

10

15

20

25

30

35

40

45

50

55

60

65

627719

12

sludge in the first digestion zone is 4 to 48 hours, and preferably 12 to 24 hours.

The first digestion zone 110 is equipped with a mechanical stirring device 112 which may be of the same type as the device 103 used in the aeration zone 102, as well as with a device continuously recycling one of the fluids formed. by the second gas and the activated sludge against the other of these fluids, inside the digestion zone.

The second gas, which comprises at least 80% oxygen, is introduced into the covered area 110 of aerobic digestion in an amount and at a rate sufficient to maintain the content of dissolved oxygen in the sludge at least 2 mg / 1.

The oxygen-depleted digestion gas, so as to have an oxygen purity of at least 40%, leaves the covered digestion zone 110 by a conduit 118 at a rate such that its oxygen content is at least 35 % of the oxygen content of the oxygenated gaseous charge arriving via line 117. The gas present in line 118 enters the aeration covered area 102 so as to constitute at least a main part of the first gas cited satisfying the demand for oxygen required for biochemical oxygenation of wastewater. If necessary, an additional external source of oxygen-containing gas can be used to increase the flow of oxygenated gas passing through conduit 118.

When the desired level of aerobic digestion is obtained in the zone 110, the partially stabilized sludges which are however almost completely pasteurized flow from the first zone covered with digestion 110 via the conduit 114 and pass into the anaerobic treatment part of the combined installation. In this embodiment, the second anaerobic digestion zone comprises an acidification sub-zone 120a and a fermentation sub-zone 120b, with release of methane. The partially stabilized sludge present in the conduit 114 and coming from the first digestion zone 110 penetrates into the acidification sub-zone 120a and remains there for a period of 24 to 60 h, necessary for the acidification of the sludge. The content of sub-area 120a is continuously kneaded by a stirring device 121a, so that a uniform rate of breakdown of carbohydrates, fats and proteins into lower fatty acids is maintained. At the end of the required residence time in the sub-zone 120a, the acidified sludge flows out of it through a conduit 126. The temperature of the sludge leaving the sub-zone 120a being still high and included in the thermophilic temperature range of 45 to 75 ° C close to this range and, therefore, this temperature being higher than the values which are optimally suitable for the formation of methane, the temperature of the acidified sludge is generally lowered so that the functioning of the sub-zone 120b methane formation is satisfactory. Consequently, the sludge present in the conduit 126 passes through a heat exchanger 115 where it circulates against the current with respect to a cooling fluid passing through a conduit 160. The partially stabilized and partially cooled sludges thus obtained leave the heat exchanger 115 by a conduit 127 which directs them to the sub-zone 120b of methane formation by fermentation. The cooling fluid, passing through the heat exchanger through line 160, can advantageously comprise cold water, for example in the form of part of the effluent arriving through line 106 of the secondary clarification tank or, as in the embodiment described above, this cooling fluid can be the flow of sludge arriving at the digestion installation.

The anaerobic digestion sub-area 120b constitutes the seat of the digestion phase with release of methane. For optimal treatment, the sludge present in this sub-zone is maintained at a temperature between 35 and 40 ° C, and preferably between 37 and 38 C. The content of sub-zone 120b is mixed continuously, by a device 121b of agitation, so as to form a large zone of active decomposition and to substantially increase the speed of the stabilization reactions occurring in this zone. The residence time of the sludge in the fermentation sub-zone is preferably between 4 and 8 days under the conditions indicated above determining the residence time of the sludge in the second anaerobic digestion zone. The methane gas produced by the biochemical reactions occurring in the sub-area 120b is discharged through a conduit 128 on which a valve 129 for adjusting the flow rate is mounted. Part of this methane can be directed to the boiler 130 through line 132, while the remaining part is removed by line 131 in order to undergo other treatments and / or be used for other purposes. The more stabilized sludge thus obtained, which contains not more than 40%, and preferably not more than 20%, of the initial content of volatile and biodegradable solids, presented by the tributary sludge, is discharged from the installation in passing through a conduit 133.

Fig. 3 is a diagram of another embodiment according to the invention, in which the sludge resulting from primary and secondary treatment of wastewater passes through a digestion installation. This embodiment implements a form of the method according to the invention, in which a first thermophilic aerobic digestion is associated with a second anaerobic thermophilic digestion. Until now, anaerobic thermophilic digestion has been little more than a laboratory curiosity. The problems arising from the conventional anaerobic mesophilic treatment described above, due to thermal instability and the extreme sensitivity to variations in conditions, arise in an even more accentuated manner in anaerobic thermophilic digestion. In fact, this sludge treatment process has not been implemented in industrial sludge digestion applications due to the lack of stability of thermophilic anaerobic digestion. These problems of instability and undesirable sensitivity to variations in the parameters of the process are resolved with the aerobic / anaerobic digestion process according to the invention, in the same manner as described above for the embodiments using a second phase of anaerobic digestion. mesophile.

In the installation shown in fig. 3, the raw wastewater comprising, for example, urban sewage, industrial wastewater and rainwater, arrives in the conduit 240 in a zone 241 of primary sedimentation. This zone 241 can advantageously include a gravity clarification tank of the conventional and known type. In the sedimentation zone, the tributary wastewater is separated into a primary effluent with low biochemical oxygen demand, flowing through a conduit 201 to an aeration zone 202, and an underlying stream of deposited sludge leaving the zone 241 by a conduit 242. The aeration zone 202 also receives an oxygenated aeration gas arriving by a conduit 218, a supernatant sludge thickening liquid which arrives by a conduit 250 and activated sludges returning by a conduit 208. A mixing and recycling device 203 is placed in the aeration zone 202 in order to mix the various fluids which are introduced therein to form a liquor and to circulate at the same time, continuously, one of the fluids, constituted by the mixed liquor and the oxygenated aeration gases, against the other of these fluids. As described above, the mixing and recirculation device can advantageously comprise a submerged gas diffuser associated with a mixing rotor placed below the surface, or else with a rotor ventilation device placed on the surface. At the end of the requested aeration period, for example from 2 to 6 h, the mixed liquor, with low biochemical oxygen demand, and the aeration gas depleted in oxygen so that its oxygen content, by volume, is at least 21%, are evacuated from the aeration zone 202 by conduits 204 and 219, respectively.

The mixed oxygenated liquor passing through the conduit 204 arrives at a zone 205 of secondary sedimentation in which activated sludge is separated from the purified liquid, the latter being evacuated by a conduit 206. The activated sludge deposited is withdrawn from the secondary sedimentation zone in passing through a duct 207. A significant portion of this sludge is recirculated to constitute recycled sludge towards the aeration zone 202 by the

5

10

15

20

25

30

35

40

45

50

55

60

65

13

627,719

conduit 208 which carries a recycling pump 209. The remaining part of the sludge, that is to say the non-recycled part, which can constitute from 3 to 10% of the sludge passing through the conduit 207, flows through a conduit 252 to the thickener 251 of sludge.

The thickener 251 comprises another thickening zone by sedimentation of the sludge, which concentrates the said sludge to a value between 2 and 6% of solid matter, that is to say to a total concentration of solid matter in suspension ( MLSS) between 20,000 and 60,000 mg / 1. The thickened lower sludge flows through a conduit 245 and joins the primary sludge in a conduit 242 coming from the zone 241 of primary sedimentation to form the stream of mixed sludges passing through the conduit 211. The supernatant liquid from the thickener 251 passes by a conduit 250 in the aeration zone 202, as indicated above.

The stream of mixed sludge passing through conduit 211 can be partially heated, if desired, by indirect heat exchange with the stabilized and hot sludge from the second digestion zone 220, as described in more detail below, then it flows to the first digestion zone 210 passing through a conduit 248. Before entering the first digestion zone 210, the sludge from the conduit 248 can be heated again using a heating device 231 to combustion of methane, receiving methane through a conduit 227.

If the ambient temperature is sufficient so that it is not necessary to heat the tributary sludge from the digestion installation, this sludge can bypass the heat exchanger 244 and the heating device 231 by passing through conduits 261 and 263 of bypass, respectively.

In the first covered digestion zone 210, the tributary sludge is subjected to aerobic thermophilic digestion. An aeration gas containing at least 50% oxygen, by volume, and preferably at least 80% oxygen, arrives at the digestion zone 210 by a conduit 217 and a mechanical stirring device 212 which mixes and circulates the mixture of sludge against oxygenated gas at the same time and continuously. The flow rate of the aeration gas and the energy supplied to the mechanical stirring device 212 are such that a concentration of at least 2 mg / 1 of dissolved oxygen is maintained in the sludge present in the first zone 210 digestion.

The sludge is kept in the first digestion zone 210 at a temperature of 45 to 75 ° C for a period of 4 to 48 h, so that the volatile and biodegradable solids in suspension (VSS) of the sludge are partially reduced. The partially stabilized sludge is discharged from the aerobic digestion zone 210 by a conduit 216 and the digestion gas, depleted in oxygen, is removed separately from the digestion zone by a conduit 218.

The conduit 216 passes the partially stabilized sludge from the first digestion zone into the second covered digestion zone 220.

The second digestion zone 220 constitutes a thermophilic anaerobic digester. To be subjected to an optimal treatment, the sludge present in this digestion zone is maintained at a temperature included in the thermophilic range of 40 to 60 ° C, and preferably between 45 and 50 ° C. Due to the thermophilic temperatures prevailing in the first and second digestion zones of this embodiment according to the invention, the partially stabilized sludge, coming from the first digestion zone, can pass directly into the second digestion zone, as shown, without being heated or cooled by heat exchange between said zones when the thermophilic temperatures prevailing in the corresponding zones are sufficiently close. Alternatively, it may be desirable in some cases to work in the second digestion zone at temperatures sufficiently higher or lower than those used in the first aerobic digestion zone, so that it is advantageous to heat or to cool the partially stabilized sludge, having just been subjected to an anaerobic digestion and passing from one zone to another. The heating can be carried out by a methane device similar to the heating device 231, and the cooling can be carried out by heat exchange between the partially stabilized sludge from the first digestion zone and the tributary sludge which flows towards the installation. digestion, as described above for the embodiments according to the invention shown in FIGS. 1 and 2. In addition, since it is even more critical to prevent temperature variations in the thermophilic anaerobic digestion zone than in the mesophilic anaerobic digestion zone, it may be preferable to use a well-insulated basin for the treatment of sludge by anaerobic thermophilic digestion, so as to provide protection against significant climatic variations.

In the second digestion zone 220, the sludge is continuously kneaded by a mechanical stirring device 221 so that a high stabilization speed is maintained. The methane gas resulting from the biochemical reactions which occur during anaerobic digestion is evacuated from the second digestion zone by a conduit 223. This methane can be mixed with oxygenated gas, for example air or the digested gas, depleted oxygen from the aerobic digestion zone, and it can be burned as fuel to heat the sludge at a high temperature in one or both digestion zones. In the process as shown, part of the methane gas from line 223 passes through line 227 into the methane boiler 231 where it is burned in order to produce heat keeping the sludge present in the first digestion zone at a temperature between 45 and 75 ° C. The part of the methane retained is evacuated from the installation by a conduit 228. The more stabilized sludge, coming from the anaerobic digestion zone and containing not more than 40% and, preferably, not more than 20% of the biodegradable volatile solids present in the sludge arriving via line 248 at the digestion plant, are discharged from the second digestion zone through a line 225, pass through a heat exchanger 244 where their heat is recovered , and are finally evacuated from the installation by passing through a conduit 243.

The nature of the biological activity occurring in the aerobic digestion zone of the installation shown in fig. 3 and described above is significantly different from that of the biological activity occurring in the aerobic zone of the embodiment described above with reference to FIG. 2, due to the difference in source of sludge. In the embodiment shown in FIG. 2, the sludge arriving at the digestion installation only includes activated sludge coming from a device for secondary treatment of wastewater, whereas in the embodiment shown in FIG. 3, the tributary sludge includes both the secondary sludge from the treatment of activated sludge and the primary sludge from the sedimentation of raw wastewater. The organic matter present in secondary sludge being mainly made up of viable microorganisms, aerobic digestion of this sludge includes the various biochemical reactions including cell lysis, the assimilation of lysis products for the synthesis of a new viable material , and breathing. In addition, primary sludge mainly consists of non-viable organic matter which the micro-organisms present in the sludge can feed on. Consequently, during aerobic digestion of a primary sludge, the microbial population of this sludge is subjected to a certain phase of cell synthesis in addition to cell lysis, the assimilation of the products of lysis and respiration. Consequently, aerobic digestion of primary sludge is accompanied by greater cell synthesis and cellular respiration than that present in aerobic digestion of secondary sludge. In addition, aerobic digestion of primary sludge causes a smaller net reduction in biodegradable volatile solids than that obtained by aerobic digestion of secondary sludge, for the same duration of digestion. The net reduction in volatile and biodegradable solids from sludge during digestion represents a

5

10

15

20

25

30

35

40

45

50

55

60

65

627719

difference between cell synthesis and cellular respiration in the comparative processes of digestion.

The cellular respiration occurring in the sludge digestion process is of an exothermic nature and, for the reasons indicated above, the primary sludges give off a greater quantity of heat per unit weight of volatile and biodegradable solid matter in suspension, eliminated during digestion, only secondary sludge. Consequently, to obtain and maintain a given temperature in aerobic digestion, the net decrease in volatile solids in suspension must be less for a primary mud than that required for a secondary mud. Consequently, the embodiment according to the invention shown in FIG. 3, in which the sludge arriving at the digestion plant comprises both primary sludge and secondary sludge, can work at a given temperature with a lower level of reduction of the volatile solids suspended in the aerobic digestion zone that in the aerobic digestion zone of the installation shown in fig. 2 which only treats secondary sludge. A lower reduction of volatile and biodegradable solids present in the aerobic digestion zone also requires that the residence time of the sludge in the anaerobic digestion zone be increased correspondingly to obtain a given overall level of solids elimination and volatiles in suspension. Since, in this case, a greater part of the overall elimination of suspended solids and volatiles is carried out in the anaerobic digestion zone, the installation treating primary sludge can therefore produce more methane in the zone anaerobic digestion compared to the digestion plant treating only secondary sludge. Therefore, the embodiment shown in FIG. 3 can produce larger quantities of methane than those shown in FIG. 2, but at the cost of increasing the residence time of the sludge in the aerobic digestion zone of the installation shown in FIG. 3.

As is apparent from the preceding description, each of the embodiments described above performs heating of the sludge arriving at the digestion installation before their introduction into the aerobic digestion zone. This heating may not be necessary in a given installation, depending on various factors such as the solids content of the sludge, the ambient temperature, the residence time of the sludge in the aerobic digestion zone and the type of sludge treated. Fig. 4 is a graph showing the temperature of the sludge arriving at the first digestion zone, this sludge having to be maintained at 50 ° C. in this first zone for 24 h, as a function of the total content of said sludge in suspended solids (MLSS) . Curve A corresponds to a stream of mixed primary and secondary sludge, having a volatile suspended solids / total suspended solids (VSS / TSS) ratio equal to 0.75 and a biological enthalpy of 46,000 kJ / kg of solid matter volatile in suspension (VSS) eliminated. Curve B corresponds to unmixed secondary sludge having a VSS / TSS ratio of 0.79 and an enthalpy of 32200 kJ / kg of VSS eliminated.

As shown by the curves of this graph, the mixed sludge, corresponding to curve A, requires a higher temperature upon arrival at the aerobic digestion zone, for a given total content of suspended solids, than the corresponding secondary sludge to curve B. Consequently, the heating of the sludges arriving at the digestion installation, before their. introduction into the first digestion zone, may be particularly desirable during the implementation of the method according to the invention with a digestion installation containing a large portion of primary sludge from a wastewater treatment installation.

The graph in fig. 4 also shows that it is possible to carry out a thermophilic treatment without the need to heat the sludge arriving at the digestion plant before their introduction into the aerobic digestion zone when said sludge is sufficiently thickened. For example, in the case of mixed sludge (curve A) having a total solids concentration of 4%, it is sufficient that their temperature, upon arrival in the aerobic thermophilic digestion zone, is approximately 15 ° C.

All the embodiments described above make it possible to obtain completely pasteurized sludge, because, in all cases, all of the sludge arriving at the digestion installation passes into the aerobic thermophilic digestion zone where the high temperatures used complete the pasteurization. sludge. However, there may be applications in which the final evacuation of the sludge does not require a complete pasteurization of the latter or in which it is not necessary for the sludge to be pasteurized because of the absence of any appreciable concentration of pathogenic organisms in this sludge. Fig. 5 is a diagram of another embodiment according to the invention, in which a small part of the sludge arriving at the treatment installation is diverted towards the second digestion zone. This embodiment is suitable for such applications. As shown in fig. 5, a significant portion of the sludge arriving at the treatment installation via a conduit 311 passes into the first covered digestion zone 310 using a conduit 331. Before entering the first digestion zone 310, the sludge contained in the conduit 331 can be heated, if desired, by a methane boiler 330.

An aeration gas containing oxygen, namely at least 50% oxygen, by volume, and preferably at least 80% oxygen, is introduced into the aerobic digestion zone 310 by a conduit 317. The sludge flowing in this zone is suitably kneaded and circulated continuously against the current with respect to the oxygenated aeration gas by means of a stirring device 312, in order to maintain a content in sludges dissolved oxygen of at least 2 mg / 1. The sludge is kept in the aerobic digestion zone at a temperature between 45 and 75 ° C for a period of 4 to 48 h. The digestion gas, depleted in oxygen, leaves the first digestion zone by a conduit 318 and the sludge, completely pasteurized and partially depleted in biodegradable solid matter in suspension, is discharged separately by a conduit 316 from the digestion zone.

The partially stabilized sludge, passing through the duct 316, is then introduced into a second covered digestion zone 320 working in the mesophilic temperature range. The temperature of the sludge leaving the first digestion zone being between 45 and 75 ° C, it must be lowered before the introduction of said sludge into the second digestion zone, so that it is possible to maintain therein l efficiency of the mesophilic anaerobic digestion process. In the embodiment shown, the small part of the sludge arriving at the treatment installation passes through a conduit 329 mounted as a bypass of the methane boiler 330 and of the aerobic digestion zone 310, and it is mixed directly with the sludge hot passing through the duct 316. The flow rate of the affluent sludge passing in bypass is adjusted so that the temperature of the mixture of sludges arriving in the anaerobic digestion zone 320 is sufficient to maintain in this zone 320 a temperature between 35 and 40 ° C.

In the second digestion zone, the sludge is mixed by recycling methane gas against the current with said sludge,

in order to actively maintain at a high value the speed of stabilization in the second zone. The methane gas resulting from the biochemical reactions which take place in the second digestion zone 320 is evacuated from the latter by a conduit 323. A part of this gas is discharged into a loop 340 comprising a compressor 326 and the compressed methane gas thus obtained is introduced into the sludge contained in the second digestion zone, for example using a diffuser (not shown), in order to carry out the mixing and recycling of the sludges indicated above. Part of the methane gas passing through line 323 can be directed through a line 327 to the boiler 330, and the remaining part is evacuated from the treatment installation via a line 328. The more stabilized sludges, including the solids content

14

5

10

15

20

25

30

35

40

45

50

55

60

65

15

627719

volatile and biodegradable in suspension is less than 40% of the initial content of the sludge arriving at the treatment installation via line 331, is discharged from the second digestion zone via line 325 in order to undergo other treatments (for example drying out) and / or to be evacuated.

The advantages of the process according to the invention will be described in the following examples, which are in no way limitative.

Example 1:

This example compares the results obtained with the installation according to the invention, shown in FIG. 2, and a conventional high-speed installation working in anaerobic mode. The following description is based on the treatment of sludge from an installation treating 37,850 m3 / d of waste water, and relates to the installation shown in FIG. 2.

A mixture of equal parts of primary and secondary sludge, having an initial temperature of 18 ° C, arrives via line 111 at the digestion installation shown in FIG. 2. The sludge, which has a total suspended solids content of 39,400 mg / l and whose volatile suspended solids constitute 72% of the total suspended solids, arrives at the facility at a rate of 340 m3 / d. To maintain the sludge in the aerobic digestion zone 110 at a temperature of 50 ° C for 24 h, the tributary sludge is brought to a temperature of about 23 ° C by the methane boiler 130. Considering that 50% of the combustible value of methane gas is transformed into heat, the boiler 130 must be supplied with approximately 710 m3 / d of gaseous methane produced in the anaerobic digestion zone.

An approximately 8% decrease in volatile suspended solids (16% biodegradable volatile suspended solids; volatile biodegradable suspended solids constituting approximately 50% of all volatile suspended solids) is obtained during digestion aerobic, so that the partially digested sludge, having a content of volatile solid matter in suspension of 26100 mg / 1, is brought by the conduit 114 to the acidification sub-zone 120a. This sub-zone works at a thermophilic temperature with a sludge residence time of 24 h. It achieves a 10% reduction in the influx fraction of volatile suspended solids. Sludge, having a volatile solid content in suspension of 23,500 mg / l, is then directed via line 126 to the fermentation sub-zone 120b with release of methane. A sufficient part of the heat is removed from the sludge discharged into the heat exchanger 115 to ensure a working temperature in the fermentation sub-zone 120b of 38 ° C.

The residence time of the sludge in the fermentation sub-zone is 5 d, which results in an overall reduction of volatile solids by 40% for the entire installation (reduction of 80% for biodegradable volatile solids in suspension). The fermentation sub-zone produces around 270 m3 of methane gas per day, which corresponds to a total calorific value of 45 million kJ / d. Since boiler 130 requires 710 m3 / d of gaseous methane, it is possible to use outside the sludge digestion plant, 1360 m3 / d of gaseous methane, which corresponds to a total calorific value of 30.6 million kJ / d.

In the case where the 340 m3 / d of mixed sludge, on which the preceding description is based, are treated in a conventional anaerobic digestion tank with high flow rate, a residence time of approximately 13 d is required to obtain the same reduction. volatile solids. Although the conventional high-flow digestion basin produces 3620 m3 / d of methane gas, which corresponds to approximately 81 million kJ / d, it must be heated with approximately 63 million kJ / d, for a conversion rate of 50% of the calorific value in heat, to maintain an optimal temperature in this basin. Consequently, compared to the installation according to the invention described above, the conventional installation requires an increase in capacity of 86%, based on the required residence time, and it produces approximately 40% less methane that can be used. outdoors, under normal working conditions.

Example 2:

It applies to the operation of the embodiment according to the invention shown in FIG. 5. The sludge load includes 227 m3 / d of an equal mixture of primary and secondary sludge from a wastewater treatment plant. The stream of tributary sludge passing through conduit 311, having a temperature of 20 ° C and having a total content of suspended solids of 4% (VSS / TSS = 0.75), is divided so that 174m3 / d s 'flow directly through line 331 into the aerobic thermophilic digestion zone and 53 m3 / d constitute the bypass current passing through line 329. As shown in fig. 4, it is not necessary, in this case, to heat the sludge before its introduction into the aerobic thermophilic digestion zone. The residence time in the first digestion zone 310 is approximately 24 h and the thermophilic temperatures are reached autonomously. Pasteurized sludge, having a temperature of 50 ° C, flows from the aerobic digestion zone through line 316 and is mixed with the cold stream of bypass sludge from line 329. This mixture of sludge flows towards the anaerobic digestion zone 320 in which the sludge is kept in the absence of oxygen for approximately 8 days, which causes a reduction of approximately 40% in the totality of volatile solids (reduction of 80% of volatile and biodegradable solids in suspension). The anaerobic digestion zone produces methane gas at a flow rate of approximately 2040 m3 / d, which corresponds to approximately 42 million kJ / d. All of this methane can be used outside the treatment facility.

If the 227 m3 / d of mixed sludge is passed through a conventional high-speed anaerobic digestion tank, a residence time of approximately 15 days is necessary to obtain the same reduction in volatile solids. Although such a conventional basin produces 2550 m3 / d of methane gas, which corresponds to approximately 53 million kJ / d, for a factor of conversion of calorific value into heat of 50%, it takes approximately 47.5 million kJ / d to maintain optimal temperature conditions for the anaerobic treatment carried out in the high-flow tank. Consequently, in this case, a conventional anaerobic digestion installation requires a sludge residence time approximately 65% greater than that required in the installation according to the invention, while producing only a net quantity of equivalent gas. at an energy of 5.25 million kJ / d, while the combined installation according to the invention produces a quantity of gas equivalent to 42 million kJ / d. Consequently, after having used as the heat source the methane gas which it produces itself, the conventional installation supplies substantially less methane which can be used outside than the installation according to the invention.

Example 3:

It compares the efficiency of the installation according to the invention in the embodiment shown in FIG. 1 to the performance of a conventional anaerobic high flow treatment installation.

Secondary sludge, originating from a wastewater treatment plant by oxygenation and initially having a temperature of 15 ° C., is first heated in the heat exchanger 22 with the effluent from the anaerobic digester, then again heated in the exchanger 15 with the effluent from the aerobic thermophilic digester. The first heating carried out in the exchanger 22 raises the temperature of the tributary sludge from 15 ° C to about 25 ° C while lowering the temperature of the stabilized effluent sludge from the anaerobic digestion zone from about 35 to 25 ° C. The second heating carried out in the exchanger 15 raises the temperature of the tributary sludge to about 30 ° C, while the temperature of the sludge from the aerobic digestion zone 10 is reduced from about 50 to 45 ° C. Tributary sludge with a total material content

5

10

15

20

25

30

35

40

45

50

55

60

65

627719

16

solids (MLSS) of 34400 mg / 1 and a fraction of volatile suspended solids / total suspended solids equal to 78%, are introduced into the first digestion zone at a flow rate of 227 m3 / d. The sludge residence time in this first aerobic zone is 24 h and the treatment temperature is maintained at 50 ° C. In the aerobic phase, there is obtained a reduction of approximately 16% of the volatile solids in suspension (reduction of 32 % of volatile biodegradable solids in suspension), so that partially stabilized sludge, having a content of volatile solids in suspension of 22,500 mg / 1, is introduced via line 16 into the anaerobic digestion zone after heat exchange with the tributary sludge in the exchanger 15.

The residence time in the anaerobic digestion zone is 8 d, which results in an overall reduction of volatile suspended solids by 42% (84% reduction of volatile biodegradable solids in suspension) in the combined installation according to the invention. The anaerobic digestion zone 20 produces approximately 1466 m3 / d of methane gas, which corresponds to a total calorific value of approximately 29.5 million kJ / d. The

20

all of this methane can be used outside the digestion plant.

If the 227 m3 / d of tributary sludge is treated in a conventional anaerobic digestion tank with high flow rate instead of being treated in the digestion installation according to the invention described above, a residence time of at least 14 d is necessary to obtain the same reduction in volatile suspended solids. Although such a conventional high-flow installation produces 2395 m3 / d of methane gas, which corresponds to approximately 49.6 million kJ / d, for a conversion rate of calorific value into heat of 50%, it takes approximately 47.5 million kJ / d for maintaining optimal temperature conditions in the high-flow basin. The conventional anaerobic installation must therefore have a capacity of approximately 55% greater than that of the installation according to the invention, and it provides daily approximately 27.5 million kj less per day, from gaseous methane, than the corresponding installation according to the invention described above.

It goes without saying that numerous modifications can be made to the process described and shown, without going beyond the ambit of the invention.

2 sheets of drawings

Claims (21)

627719 1. Sludge digestion process, characterized in that the sludge is oxygenated in a first covered digestion zone, at a temperature of 45 to 75 ° C, to partially reduce the content of the sludge in volatile and biodegradable solids by slurry, and in that the sludge is then subjected to anaerobic digestion in a second covered digestion zone, at a temperature of 30 to 60 ° C., to further reduce the content of the sludges in volatile and biodegradable solids to a value less than 40% of the content in the sludge introduced into the first digestion zone. 2. Method according to claim 1, characterized in that it consists: a) introducing, as fluids, said sludge and an aeration gas comprising at least 50% oxygen, by volume, into a first closed digestion zone and mixing them to maintain the dissolved oxygen content of the mixed liquor at least 2 mg / 1 and the total content of suspended solids in the sludge at least 20,000 mg / 1; b) maintaining the sludge in said first digestion zone, during operation a), at a temperature in the thermophilic range between 45 and 75 ° C; c) to continue operation b for a period of 4 to 48 hours in order to partially reduce the content of sludge, introduced into the first digestion zone, in volatile and biodegradable solids in suspension; d) discharging the partially stabilized sludge and the oxygen-depleted digestion gas separately from said first digestion zone so as to have an oxygen purity of at least 21%; e) introducing said partially stabilized sludge from operation d into a second closed digestion zone; f) maintaining the sludge, placed in the second digestion zone, under anaerobic conditions and at a temperature of 30 to 60 ° C., for a time sufficient to further reduce the content of said sludge in volatile and biodegradable solids by suspension in order to bring it to less than 40% of the content of volatile and biodegradable solid matter in suspension of the sludge introduced into the first digestion zone during operation a, and also to produce methane gas, the sludge more stabilized and the methane gas thus obtained being discharged from the second digestion zone. 2 3 627719 and in activated sludge, the gas containing unused oxygen being discharged from the aeration zone at a rate such that its oxygen content does not exceed 40% of all the oxygen introduced into the digestion zone; b) returning at least 85% by weight of the activated sludge to the aeration zone to form said recycled sludge; c) using a second gas comprising at least 80% oxygen by volume and comprising a part of the first gas and introducing the gas mixture into the digestion zone; d) discharging the partially stabilized sludge and the oxygen-depleted digestion gas separately from the first digestion zone so as to have an oxygen purity of at least 40%, at a rate such as the oxygen content of said gas depleted digestion is at least 35% of the oxygen content of the second gas introduced into said digestion zone, and e) using the digestion gas, partially depleted of oxygen and coming from operation d, as 'it is at least the main part of the first gas introduced into the closed aeration zone during operation a. 3. Method according to claim 2, characterized in that the total content of suspended solids in the sludge introduced into the first digestion zone is between 20,000 and 60,000 mg / 1. 4. Method according to claim 2, characterized in that the residence time of the sludge in the first digestion zone is from 12 to 24h. 5 5. Method according to claim 2, characterized in that the sludge is heated before its introduction into the first digestion zone, in order to maintain said temperature during operation b. 6. Method according to claim 2, characterized in that the temperature is maintained, during operation f, between 35 and 40 C, so that a mesophilic digestion is carried out in said second digestion zone. 7. Method according to claim 2, characterized in that a temperature of 45 to 50 ° C is maintained during operation f, so that a thermophilic digestion is carried out in said second digestion zone. 8. Method according to claim 2, characterized in that the residence time of the sludge during operation f is sufficient to further reduce the content of said sludge in volatile and biodegradable solid matter in suspension to a value less than 20% of the content of volatile and biodegradable solids in suspension, presented by said sludge introduced into the first digestion zone during operation a. 9. Method according to claim 2, characterized in that the residence time of the sludge during the operation f is from 4 to 12 d. 10 10. Method according to claim 2, characterized in that each of the first and second digestion zones has a surface area to volume ratio of less than 2.6 m2 / m3. 11. Method according to claim 2, characterized in that the sludge present in the second digestion zone is kneaded by recirculation of methane gas against the current with respect to said sludge. 12. Method according to claim 2, characterized in that the aeration gas is heated before being introduced into the first digestion zone, in order to maintain said temperature during operation b. 13. The method of claim 2, characterized in that the sludge is heated before being introduced into the first digestion zone by indirect heat exchange with said more stabilized sludge, coming from the second digestion zone. 14. The method of claim 13, characterized in that the temperature during operation f is maintained between 35 and 40 ° C, said heated sludge being further heated before being introduced into the first digestion zone by indirect exchange heat with said partially stabilized sludge from the first digestion zone. 15 15. The method as claimed in claim 2, characterized in that the second digestion zone comprises an acidification sub-zone and a methane production sub-zone by fermentation, the partially stabilized sludge, coming from the first digestion zone, being introduced into said acidification sub-zone and maintained in this sub-zone for a period of 24 to 60 hours in order to be acidified, said acidified sludge leaving said acidification sub-zone and entering the sub-zone of fermentation where they are maintained at a temperature of 35 to 40 ° C for a period of 4 to 8 days. 16. Method according to claim 15, characterized in that the sludge present in the fermentation sub-zone is maintained at a temperature of 37 to 38 ° C. 17. The method of claim 15, characterized in that the sludge present in the acidification sub-zone is maintained at a temperature of 45 to 75 ° C, the acidified sludge leaving this sub-zone being cooled to a temperature of 35 to 40 ° C before being introduced into the fermentation sub-zone. 18. The method as claimed in claim 2, comprising the treatment of waste water which contains biodegradable solid matter in suspension, in order to eliminate the biochemical oxygen demand, the method being characterized in that it consists in separating primary sludge, containing biodegradable solid matter in suspension, waste water to form a primary effluent depleted in solid matter, mixing this primary effluent and recycling and aerating the sludge at a sufficient rate and for a sufficient time to form a mixed liquor having a low biochemical oxygen demand, to separate the mixture into purified liquid and activated sludge, and to return at least a significant part of the activated sludge to form said recycled sludge which is mixed with • the primary effluent, said primary sludge and activated sludge not returned being introduced into said first digestion zone during the operation. 19. Method according to claim 1, characterized in that the elimination of the biochemical oxygen demand of the wastewater is carried out in a closed aeration zone and comprises the operations consisting: a) introducing an aeration gas comprising at least 40% oxygen by volume and mixing it, as the first gas, with the waste water and recycled sludge, inside said closed aeration zone, to form a mixture, one of these fluids being at the same time recirculated continuously, against the current with respect to the other fluid, in the aeration zone, in sufficient quantity and at a rate to maintain the content of the mixture of dissolved oxygen at least 0.5 mg / 1, the mixture being separated into purified liquid 20. The method of claim 2, characterized in that the methane gas discharged from the second digestion zone is mixed with oxygenated gas and burned as fuel to produce heat keeping the sludge at a high temperature in at least one said first and second digestion zones. 20 25 30 35 40 45 50 55 60 65 21. The method of claim 20, characterized in that the methane gas and the oxygenated gases are mixed and burned as fuel to produce heat keeping said sludge in the first digestion zone at a temperature of 45 to 75 ° C.