WO1990009964A1

WO1990009964A1 - Biological treatment of sewage sludge or similar waste matter

Info

Publication number: WO1990009964A1
Application number: PCT/AU1990/000060
Authority: WO
Inventors: Frederick Carl Miller
Original assignee: La Trobe University
Priority date: 1989-03-02
Filing date: 1990-02-15
Publication date: 1990-09-07

Abstract

An aerobic batch composting process for the biological treatment of sewage sludge or similar waste material, comprising subjecting a batch of the sludge or similar material in intimate admixture with a batch of recycled composting bulking agent to aeration in an enclosed environmentally controlled reactor, by means of an air recirculation system, said admixture having sufficient porosity and aggregate structure for aeration dispersal of the excess waste heat generated by the composting and oxygenation of the composting material, the aeration controlling the temperature of the admixture to below about 60°C and maintaining vertical temperature gradients of the admixture within a range of about 45°-60°C for a period of time after peak composting activity is reached so as to obtain a relatively dry product of about 25-50 wt. % moisture content that is biologically stable and contains an acceptably low level of pathogens.

Description

BIOLOGICAL TREATMENT OF SEWAGE SLUDGE OR SIMILAR WASTE MATTER

BACKGROUND OF INVENTION

This invention relates to the biological treatment of sewage sludge or similar waste matter, and provides an aeration process for the control and management of a biological composting process for sewage sludge or ^'"'similar wastes, in which the sewage sludge or similar waste in admixture with a compost bulking agent as hereinafter described is aerated in an enclosed reactor for temperature control of the biological treatment.

Sewage sludge is a water-based sludge material derived from the settleable fraction of sewage. Sewage commonly comes from the water-carriage disposal of domestic waste but can also come from industries producing a primarily organic waste. Sewage sludges at a treatment facility can be characterised as: (a) primary - sludges derived directly from the settling of fresh sewage; (b) secondary - sludges derived from a biological sewage treatment system; (c) digested - sludges derived from the anaerobic digestion of primary and/or secondary sludges; (d) septage - sludges derived from septic tank systems and normally brought in by private haulers; and (e) chemical - sludges derived from chemical coagulation of raw sewage or other treated waters.

Sewage sludges contain ash, organic fibres, proteins, fats, and carbohydrates. Raw (untreated) sludges contain considerable available food energy for microorganisms and hence are readily putrescible. a

Treated sludges are normally more biologically- stable than raw (untreated) sludges. Usually, sludges are initially wet, having only a 4-7% solids content, however, in preparation for composting, sludges are thickened to around 24-27% solids, commonly using belt filter presses, which greatly improves the handling of the sludge, excessively wet sludges not being fit for composting as they will settle into a gelatinous mass with no porosity. Such a pressed sludge is commonly called sludge-cake.

Table 28 in a paper published by Toth, S.J. and Nocitra, N.J. (1982), "Sludge Composting and Utilization: Chemical Composition and Agricultural Value of Sewage Sludge Composts", New Jersey Agricultural Experiment Station, Cook College, Rutgers University,

New Brunswick, New Jersey, U.S.A., provides information on the mean composition of some raw sewage sludges. In the volatile solids fraction are contained proteins, carbohydrates, fats, and organic fibre. Sewage sludges as settled are 5 to 7% solids and behave as non-Newtonian fluids. Before composting, sludges have to be further dewatered to a moisture content of 24 to 27%, most commonly using belt filter presses.

Dewatering of the sludge gives the sludge a structure similar to wet cardboard, which allows it to be handled in a composting operation. Normally, sludge cake is then mixed with an external bulking agent, most commonly wood chips, in order to add further structure and porosity and also dilute the biologically available unit volume energy density. However, in the process of the present invention, such external bulking agent is dispensed with and replaced with what is hereinafter described as recycled compost bulking agent. Sewage sludges vary substantially in

'strength' ('strength' refers to the relative amount of putrescible material present) and also vary to a lesser extent in composition. Variations can occur between treatment facilities, or over time within the same facility. However, these variations do not have much effect on a sludge composting facility, except to the extent that any composting facility needs to be designed to handle the maximum 'strength' waste possible at any specific facility.

Similar waste matter is any waste material having characteristics similar to sewage sludge, with enough readily-available food energy for microorganisms to support a temperature increase characteristic of composting systems, yet, like sewage sludge, lacks sufficient structure or porosity to be composted without a bulking agent. Similar wastes include food and food processing wastes, manures, wet organic fractions separated from municipal refuse, fermentation wastes, and other materials with a high density of biologically available substrate and poor structural characteristics, such as organic wastes derived from municipal refuse, oily wastes, or materials such as soils contaminated with high levels of unstructured organic materials.

Biological treatment of waste materials such as those named above by the composting process of the invention is very similar in management and consequences to the composting of sewage sludge by the process of the invention. The bulk density of some waste materials, such as the organic fraction of refuse, may be less than that of sewage sludge, allowing for higher vertical filling of such materials in the enclosed reactor used in carrying out the composting process of the invention. Similar waste matter includes hazardous organic wastes which can be treated via a composting procedure such as the composting process of the invention, however, hazardous wastes can require more refined process control than other wastes, as the types of microorganisms which can degrade the waste may be restricted. For example, many organisms can decompose sugars, while a much more restricted group could decompose fuel oil. Some aromatic ring decompositions can be carried out only by specific fungi within a comparatively narrow optimal temperature range.

Compost is the product or residue of a composting process, which is the biological decomposition of organic substrates in an energy dense matrix phase, primarily aerobic and invariably resulting in a self-heating response. To be considered as compost, the organic substrate material must have undergone sufficient decomposition to be significantly stabilised in a biological sense and to have different characteristics from those of the original starting material. An immature compost may lack biological stability to the extent that it may still slowly reheat in a large stockpile, yet should be stable enough not to become an odorous, anaerobic, viscous mass. A fully matured compost exhibits nitrification, which indicates • a close carbon to nitrogen ratio and that the remaining carbon is in a relatively unavailable form.

Sewage sludge composting is extensively used in the United States of America and Europe, however, as indicated, a major problem affecting the composting of sewage sludges is that they lack the physical structure to maintain the porosity required for composting. In consequence, all currently used composting systems either employ a bulking agent, usually woodchips, or require mechanical mixing. Investigative work on the use of recycled compost as a bulking agent is described, for instance, in a publication by Miller, F.C., S.T. MacGregor, K.M.

Psarianos, and M.S. Finstein, 1982, "Static-pile sludge composting with recycled compost as the bulking agent",

Proceedings 14th Mid-Atlantic Industrial Waste Conf.,

Ann Arbor Science Pulb., An Arbor, Mich., U.S.A., pp.

35-44, the disclosures in which are incorporated herein by reference.

In the cited publication it was shown that recycled (finished) compost could be successfully used as a bulking agent in a ventilated, temperature feedback controlled, static pile composting system, and that such usage offered many significant potential advantages. However, a major problem was that the high substrate density caused a very strong temperature gradient to occur within the pile, this extreme temperature gradient precluding satisfactory temperature control in a pile over 1 meter high.

Further references include the following, some of which are concerned with composting process control based on temperature control via ventilation, utilizing the so-called "Rutgers Strategy" , whilst others are concerned with sewage sludge composition:

MacGregor, S.T., F.C. Miller, K.M. Psarianos, J.

Cirello, and M.S. Finstein. 1981, "Composting process control based on interaction between microbial heat output and temperature", Appl. Environ. Microbiol. 41(6) :1321-1330;

Toth, S.J., and N.J. Nocitra. 1982, "Sludge composting and utilization: chemical composition and agricultural value of sewage sludge composts", New Jersey Agricultural Experiment Station, Cook College, Rutgers University, New Brunswick, New Jersey, U.S.A.; Finstein, M.S., F.C. Miller, P.F. Strom, S.T. MacGregor, and K.M. Psarianos. 1983, "Management of the composting microbial ecosystem for waste management", Bio/

Technology 1(4) :347-353; Miller, F.C, and M.S. Finstein. 1985, "Materials balance in the composting of sewage sludge as affected by process control strategy", J. Water Pollut. Control

Fed. 57(2) :122-127; and

Finstein, M.S., F.C. Miller, and P.F. Strom. 1986, "Waste treatment composting as a controlled system",

Biotechnology: Volume 8, Microbial Degradations. Eds.

H.J. Rehm and G. Reed. VCH Verlagsgesallschaft,

Weinheimm, FRD. pp 363-398, the disclosures in each of which are incorporated herein by reference.

STATEMENT OF INVENTION We have now developed an aerobic batch composting process for the biological treatment of sewage sludge or similar waste material under controlled temperature conditions for management of the composting, which comprises subjecting a batch of the sewage sludge or similar waste material in intimate admixture with a batch of recycled composting bulking agent (previously composted sewage sludge or composted similar waste material) to aeration in an enclosed environmentally controlled reactor, by means of an air recirculation system supplying a recycling flow of air to the reactor for dispersal of waste heat generated by the composting sewage sludge or similar waste material, said admixture having sufficient porosity and aggregate structure for aeration dispersal of the excess waste heat generated by the composting and oxygenation of the composting material, the aeration controlling the temperature of the admixture so as to be below about 60°C and maintaining the vertical temperature gradients of the admixture so as to be within a range of about 45^β-60^βC for a period of time after peak composting activity is reached appropriate for the particular sewage sludge or similar waste material being composted, whereby a relatively dry product of about 25-50% wt. moisture content and biologically-stable enough not to self-heat or produce odours significantly when stockpiled and having an earthy smell and containing an acceptably low level of pathogens is obtained.

Recycled compost bulking agent has the general attributes of: adequate biological stability, that is, sufficiently stable that significant self-heating will not occur if the material is placed in a large stockpile, and a physical structure that is in the form of stable aggregate.

When used in the process of the invention, recycled compost bulking agent functions to form a relatively stable aggregate with the sewage sludge or other waste, that has sufficient porosity as to allow the sludge or similar waste to be processed in accordance with the invention.

Preferably, the recycled compost bulking agent is fibrous in nature and dry enough to remove water from wet sludge or waste being composted, if water is in abundance therein. Hence, the recycled compost bulking agent preferably is moderately dry in nature, that is, between 30-50% wt. moisture content.

Recycled compost bulking agent derived from raw sludge is much preferred to compost made from anaerobically digested sludge, since anaerobic digestion substantially reduces the amount of available substrate energy and tends to produce a fine-structured and muddy residue. The preferred bulking agent is a compost material which has been subjected to an appropriate composting process using the so-called "Rutgers Strategy" for process control as described in the first-mentioned Miller et al publication cited above , which, it has been found, produces the most suitable recycling compost bulking agent for the purpose of the invention.

Significant advantages of using recycled compost bulking agent are: (a) that it minimises components of the waste stream; (b) does not have to be recovered at the end of processing; (c) provides a pre¬ formed innoculum; (d) acts as an odour absorbent; (e) costs virtually nothing; and (f) is usually near at hand.

Although other relatively stable material, in particular, wastes having a stability/porous structure/ dryness as indicated above, might be used in association with previously composted sewage sludge as the recycled compost bulking agent, the adding of a further material into the waste stream would not normally be considered unless the added material is especially useful for the purpose of the invention, since the purpose of waste treatment is to minimise the amount of waste for disposal.

Thus, the invention^{^}involves the use of recycled compost bulking agent to give porosity and aggregate structure to the sludge or similar waste matter to be biologically treated by composting, coupled with the use of recirculating air to overcome the temperature gradient problems of static composting referred to above. Whereas recycled compost satisfies most of the needs required of a bulking agent, such as wood chips, recycled compost does not sufficiently dilute the unit volume substrate density to be used in a standard ventilated static pile composting system, however, the combination of recycled compost bulking agent in conjunction with recirculated/recycled air, provides a practical solution by solving many composting problems simultaneously.

Acceptable levels of vertical temperature gradient are determined by processing criteria, for instance, on the basis of United States Environmental Protection Agency recommendations, sewage sludge undergoing composting should achieve a minimum of 55^βC for three days to ensure adequate pathogen destruction. Above 60^βC, composting process performance deteriorates due to the temperature ranges of the microorganism populations responsible for composting being exceeded. Therefore, a temperature gradient of 5^βC, between about 55-60^βC, would be preferred as suitable for the purpose.

On the other hand, where wastes are to be processed in which pathogens are not a concern, a larger temperature gradient, say, from about 45-60"C, is adequate. In the case where a specific waste is being degraded by a specific organism, then a small gradient of a few degrees around the optimal temperature for the specific organism^'is desired. For example, some aromatic compounds are degraded best by fungi that prefer mildly thermophilic temperatures, so a range of about 45-47°C is most suitable.

The enclosed environmentally controlled reactor may consist of any form of physical containment in which the sewage sludge or similar waste material to be composted and processed according to the invention, can be placed 1.5 to 3 metres high, in association with an aeration system which can move air through the composting mass such that a large amount of such air flow is recirculated therethrough. Airflow can be in either an upward or downward direction, but an upward air flow system is much preferred, as air downflow systems tend to blow compost and water into the aeration ductwork, besides also tending undesirably to assist gravity in moving water downward through the compost.

PREFERRED EMBODIMENTS

Types and population structures of biological organisms present in compost used for admixture with the sewage sludge and similar wastes to be processed in accordance with the invention will vary, based upon the substrate composition and the processing conditions. In most composted waste, bacteria (especially Bacillus sp. ) are the primary degraders, although significant growth of thermophillic actinomycetes and fungi such as the basidiomycete class of fungi, can also be important, especially in later stages of processing. A mixed population structure is desirable for treating most wastes.

Process management is based on high rates of decomposition without particular concern as to the responsible organism populations. High rates of decomposition are achieved by selecting and controlling important physical parameters, such as temperature, oxygen, moisture content, as well as the organism populations which will best adapt to those conditions and prosper. In some situations, process management can be directed toward favouring general fungal decomposition by maintaining lower temperatures. I I

Utilising an enclosed reactor permits the recirculation of air therein and provides an environmentally controlled system, the enclosure having advantages in that exhaust gases can be controlled for release if further treatment is needed for ammonia or odour control. Enclosure of the composting and processing. system also has advantages in reference to public acceptability.

Recirculating/recycling of the air has the advantage that such air, having passed through the compost, will be almost as warm as the compost, and close to water saturation. This means that the heat removal capacity of the air will be reduced, and will therefore not excessively cool the lower areas of the composting mass. Oxygen levels are still high in air passed through a composting mass, as approximately 9 times more air is needed for heat removal than is needed to supply oxygen under normal composting conditions (60°C) .

Recirculating/recycling of the air also tends to conserve nitrogen in the form of ammonia, as well as volatile organic substances, since longer retention times of volatiles in the system increase the probability of reincorporation or further decomposition. Thus, recirculating/recycling of the air can decrease the potential for air pollution and odour emissions. A further advantage is that high volume air flow improves oxygen exchange at the microsite level, and fosters aerobic decomposition of the sewage sludge or similar waste.

Total/substantially total heat removal from the composted sewage sludge or similar waste decomposing in the reactor is achieved by the use of relatively /2 large volumes of recirculated air, concurrently with the addition of relatively small volumes of cooler ambient ventilation air, in consequence of which only relatively small volumes of hot wet air are wasted. With air that is already warm and moist, the capacity, for further heat removal per unit volume of air is reduced. The recirculation of large volumes of warm, moist air tend to distribute the heat released during composting in a uniform manner, permitting the minimisation of a vertical temperature gradient.

The required volume of recirculated air to be delivered to the composted sewage sludge or similar waste matter in the reactor is determined by the unit • volume heat output of the compost, the compost depth, and the range of temperature gradient which can be tolerated. The closed system conveniently controls the recycled air flow with an air exhaust port for release of excess recirculation air from the reactor, a single exhaust port improving the monitoring of the system and permitting exhaust air treatment to be carried out if required.

Air recirculation rates for any specific application are determined by the gradient to be achieved and the heat evolution per unit volume of the waste material being composted. As the heat evolution per unit volume varies with both the specific material being composted and the time course of composting, air recirculation rate capacity needs to be determined on a case by case basis.

An approximate guide for maintaining a small/few degrees gradient is based on the amount of air recirculated during peak heat production being 90% of the total air flow, the remaining 10% being fresh make-up air for cooling. Humidity can be ignored as a control factor, as, for practical purposes, the recirculating air will always be saturated or substantially saturated. Ventilation-based temperature control via the temperature of inlet air at an inlet air port after the mixing of ambient and recycled air, is the preferred form of temperature control, as control based on the exhaust air temperature is erratic.

Vertical temperature gradients with respect to air recirculation rates are independent of any specific bulking agent except with respect to how that bulking agent will affect substrate unit volume density. Bulking agents not only impart porosity to composting materials but also significantly decrease substrate density. The only important factor in temperature gradient and air recirculation rates is the rate of heat evolution per unit volume.

In the case of wastes of low energy density, rates of heat evolution per unit volume will remain low, and vertical temperature uniformity will not be a significant problem, within common constraints of reactor height based on any mechanical limitations associated therewith. In the case of substrates of high energy density, vertical gradients could become a significant problem with inadequate air recirculation. In general, with respect to substrate energy density, the greater the density the more advantage will be gained by utilising a recirculated air system as described above.

The accompanying drawing, identified as Figure 1, schematically illustrates a reactor suitable for operating the composting process of the invention, that is, using a recycled compost bulking agent and recirculated air for temperature gradient control. Referring to that figure of drawings, the stippled area represents the composting mass; the cross-hatching represents insulated reactor walls; the arrow heads represent the direction of ventilation air flow; the numeral 1 indicates insulated reactor walls; the numeral 2 indicates ventilation headspace above the composting mass; the numeral 3 indicates the ventilation plenum floor, which floor must be robust enough to support the composting mass and loading equipment with a minimal impediment to air flow; the numeral 4 indicates a sub-plenum ventilation conduit; the numeral 5 indicates a temperature control sensor for ventilation temperature control system, the temperature feedback control system controlling the amount of cooling via the fresh air inlet port in response to inlet air temperature, which in turn, provides control over the temperature of the composting mass; the numeral 6 indicates an in-duct fan; the numeral 7 indicates an inlet air damper controlled by the temperature control system; the numeral 8 indicates fresh air inlet port; the numeral 9 indicates the system exhaust air port; the numeral 10 indicates an exhaust pressure spill damper; the numeral 11 indicates the reactor ventilation exit; the numeral 12 indicates an air recirculation duct; the numeral 13 indicates a mixing area for recirculated and fresh air; and the numeral 14 indicates sealable doors for loading and unloading the composting process components.

Reactor dimensions are not critical in terms of width or depth, which can be determined by the amount of material to be composted and any construction constraints. Height of the reactor is a comparatively important parameter, although it is not likely that heights of sludge-compost mixture will exceed 2 metres, as mechanical compaction caused by gravity will collapse the porosity of the mixture. Dependent on the structure IS of the materials composted, the maximum height of fill generally will be less than 2 metres . Factors involved in fill-height are mixture physical structure and the maintenance of porosity, biologically available energy density per unit volume, mixture airflow resistance and subsequent fan economy, and the level of temperature gradient that can be tolerated.

PRACTICAL EXAMPLE

Composting of a dewatered sludge cake was initiated by mixing with recycled compost bulking agent, ratios of compost to sludge (wet weight to wet weight) being within the range of 1.5:1 to 7:1. Ratios are not critical, but should give a mixture moisture content between 50 to 60% moisture. After mixing, the sludge- compost mix was placed directly into the composting reactor, care being taken to ensure that the filling equipment filled the mixture in a manner that gives uniform heights and material densities without compacting the mixture.

Depending upon the biologically available energy in the compost and the sludge, the final mix can be made more dilute by the presence of more recycled compost, or stronger by the presence of more sludge. As the structure of the mixed sludge and compost can vary because of facility specific variations, care should be taken that more compost should be used if mixture porosity is not adequate. It is important that the sludge and compost be properly mixed, that is, the - mixture should be highly uniform, and the mixing operation should maintain an open structure. Pug mills work well for such mixing as they mix with much shear and tend to 'fluff-up' the mixture. Mixers that compress the mixture, such as auger mixers, form compacted balls that will not properly compost. Aeration should be controlled so that temperatures of the compost are restrained from exceeding 60^βC, otherwise composting will be greatly retarded and other operational problems such as the development of odours might occur. Interstitial oxygen concentrations should be maintained above 12-14% oxygen to prevent oxygen limitation and the possibility of anaerobic microsites developing. The air flow required to maintain process control will comprise two parts, a recycled air flow to reduce the temperature gradient between the top and bottom of the reactor, and a fresh air flow to provide oxygen and to remove excess heat from the compost to maintain the desired temperature range. The amount of fresh air capacity should be sufficient to meet peak heat evolution during the composting process.

For a raw primary sewage sludge, which is a substrate with a very high amount of available energy, a peak heat evolution of 600 joules per gram initial sludge volatile can be anticipated. Other sludges or other materials can be expected to have lower peak heat evolution. To maintain a small temperature gradient with a two metre high fill, recycled air capacity should be approximately 10 times the fresh air capacity. The total fresh air plus recycled air ventilation capacity per square metre surface area will vary with fill height and volumetric peak heat evolution.

Based on extensive experience with the composting of various substrates, the time course of actual composting process is predictable. Initially, a short lag period occurs for a few hours or even days, depending mainly on the temperature of the mix at filling. At 25^βC, composting activity with a subsequent temperature rise will occur in a few hours. With a substrate temperature of 5^βC, a lag of a 3 or 4 days can occur. After composting activity is initiated, temperatures will tend to increase rapidly with peak activity occurring within 30 to 60 hours of the initial temperature rise.

Activity then tapers off so that by 300 hours, activity rates will be less than 5-10% of peak rates. At this time, ceiling temperatures become difficult to maintain depending on heat losses in the reactor system. By 300 hours, or as short as 200 hours, composting will be completed, based on various finish characteristics, such as biological stability, dryness, odour, pathogen kill, appearance, and C/N ratio.

At the end of the active processing period, the composted product will be significantly different from that of the initial starting material. During processing up to 40% of the initial sludge volatile solids will have been decomposed to carbon dioxide and water, with a comparatively small additional loss of volatile nitrogen. The composted product: should be relatively dry (25-50% moisture); should have an earthy and not a strong or unpleasant smell; should be biologically stable enough not to self- heat or produce odours significantly when placed in a large stockpile; and should contain an acceptably low level of pathogens.

Processed compost colour will be a medium brown and somewhat fibrous and aggregated in the manner of a dried peat. At this stage, the compost will not be mature enough for many horticultural uses, and further storage to reach a higher state of maturation may be desirable. Within 2 to 3 months of proper curing, nitrification should occur, marking the ultimate completion of any composting process. At the end of active processing, it may be desirable to further mill and screen the compost if it tends to be in blocks or in the form of undesirably large aggregates. The finished compost takes on more of the characteristics of humus or peat and would not suggest sludge to the casual observer.

Specific chemical analysis of the organic fraction of the finished compost is not possible because of the wide diversity of compounds present and their large complex structures. For example, while lignin has chemical characteristics, it does not have a specific chemical structure. The ash fraction of the finish compost will generally be unchanged by the composting process. Availability of minerals in the ash fraction may be changed during composting because of changes in pH and the production of chelating organics. Properly composted, the compost produced at the end of the active phase should be suitable to be directly re-used as a recycled compost bulking agent.

GENERAL DISCUSSION

In general, the process of the invention is applicable to composted sewage sludge or similar waste matter having sufficient porosity that aeration/ ventilation can be used to provide oxygen and remove excess waste heat under conditions indicated above. A wet, structureless material, which cannot be given adequate structure with a bulking agent, is not adapted for processing in the composting system of the invention.

Thus, the process of the invention is able successfully to compost materials too energy-dense to be managed in currently-used composting systems, except for those employing mechanical mixing. Although very αry waste materials would not be handled well by the composting system of the invention, water could be added to such materials. Very dilute substrates, with a very low heat output per unit volume, would not compost well in the system of the invention, as high temperature achievement would be impeded by heat...losses caused by the air ventilation system.

Composting systems have been devised in the past which can handle very dense substrate mixtures and still maintain temperature control and overall processing uniformity, however, these systems are all based on mechanical mixing to achieve uniformity, mechanical mixing requires heavy and well designed equipment to mix sludge and similar waste materials. In contrast, the considerable advantage of the process according to the present invention is that recirculated air is used to maintain process uniformity for such materials.

Processing uniformity is an important concern in waste treatment, a lack of uniformity having proved to be of concern in composting systems. In aqueous phase systems, uniformity is easy to maintain. On the other hand, composting a physical matrix in which uniformity is required, can be difficult to achieve and maintain. Air is much easier to move than compost, and requires much simpler equipment, this simplicity making the aeration/ventilation system of the invention much more reliable. Thus, large flows of recirculated air can in effect be used to cause a matrix system to behave similarly to an aqueous system in reference to environmental uniformity.

Although atmospheric air might be pre-heated and humidified to achieve the same effect as the air recirculation of the present invention, the volume of air to be pre-conditioned would be huge, as would be the attendant cost. Furthermore, nitrogen losses to the atmosphere would be greatly increased as volatilized ammonia would be lost to the atmosphere, instead of being re-incorporated into the compost. Such ammonia losses would increase air pollution and decrease the value of the compost. Moreover, an air pretreatment system would require the construction of a significant physical plant, which would require considerable energy supply for operation. With a once-through system, the exhaust air volume would be increased 10-fold, causing a much more visible condensation plume, hence although possible, once-through air flow system is not feasible.

Overall advantages of the waste treatment system according to the invention include considerable monetary savings on bulking agents as well as the cost of separating the bulking agent after composting. The recycled compost serves as a good innoculum and can be added warm, which shortens processing time by as much as some days. Odour problems can be greatly reduced, in that the finished compost has a significant capacity to absorb odours, most composting odours being intermediate products which can be further degraded biologically if kept within the system. The very good temperature control possible with the air recirculation/ventilation system of the invention permits temperatures to be optimised for specific microbial communities, making the composting system of the invention particularly suitable for the treatment of hazardous organic wastes.

Claims

-P ICLAIMS :

1. An aerobic batch composting process for the biological treatment of sewage sludge or similar waste material under controlled temperature conditions for management of the composting, which comprises subjecting a batch of the sewage sludge or similar waste material in intimate admixture with a batch of recycled composting bulking agent (previously composted sewage sludge or composted similar waste material) to aeration in an enclosed environmentally controlled reactor, by means of an air recirculation system supplying a recycling flow of air to the reactor for dispersal of waste heat generated by the composting sewage sludge or similar waste material, said admixture having sufficient porosity and aggregate structure for aeration dispersal of the excess waste heat generated by the composting and oxygenation of the composting material, the aeration controlling the temperature of the admixture so as to be below about 60^βC and maintaining the vertical temperature gradients of the admixture so as to be within a range of about 45^β-60^βC for a period of time after peak composting activity is reached appropriate for the particular sewage sludge or similar waste material being composted, whereby a relatively dry product of about 25-50% wt. moisture content and biologically stable enough not to self-heat or produce odours significantly when stockpiled and having an earthy smell and containing an acceptably low level of pathogens is obtained.

2. An aerobic batch composting process according to claim 1 wherein the recycled compost bulking agent is fibrous in nature and has a moisture content between 30-50% wt. so as to be dry enough to remove water from the sludge or waste material to be composted, if water is in abundance therein.

3. An aerobic batch composting process according to claim 1 or 2 wherein said admixture has a moisture content between 50-60% wt. and the ratio of recycled compost bulking agent: sewage sludge or similar waste material constituting said admixture is within the range of 1.5:1 to 7:1 wet wt., respectively.

4. An aerobic batch composting process according to any one of claims 1 to 3 wherein the recycled compost bulking agent is derived from raw sludge and has been subjected to the so-called "Rutgers Strategy" composting process described by Miller, F.C, S.T. MacGRegor, K.M. Psarianos, and M.S. Finstein, 1982, "Static-pile sludge composting with recycled compost as the bulking agent", Proceedings 14th Mid-Atlantic Industrial Waste Conf. , Ann Arbor Science Pulb., An Arbor, Mich., U.S.A., pp. 35-44.

5. An aerobic batch composting process according to any one of claims 1 to 4 wherein sufficient air is supplied to said admixture by the aeration system to maintain interstitial oxygen concentrations in said admixture to above about 14% wt. oxygen.

6. An aerobic batch composting process according to any one of claims 1 to 5 wherein vertical temperature gradients between about 55°-60°C are maintained in the reactor for the degradation of sewage sludge or similar waste material containing pathogens which are subject to degradation by microorganism populations operative for the purpose within that temperature range.

7. An aerobic batch composting process according to any one of claims 1 to 5 wherein vertical temperature gradients between about 45^β-47^βC are maintained in the reactor for the degradation of sewage sludge or similar waste material which is subject to degradation by microorganism populations operative for the purpose within that temperature range.

8. An aerobic batch composting process according to any one of claims 1 to 7 wherein the recirculation system of aeration utilizes a relatively large volume(s) of recirculated air to minimize the vertical temperature gradients of said admixture between the top and bottom of the reactor, with the concurrent addition of a relatively small volume(s) of fresh make-up ventilation air to the aeration system to provide oxygen for the composting and to disperse excess heat from the compost.

9. An aerobic batch composting process according to claim 8 wherein the amount of recirculating air in the system during peak heat production of the composting admixture is about 90% of the total air flow, the remaining amount of about 10% being fresh make-up ventilation air.

10. An aerobic batch composting process according to claim 8 or 9 wherein any excess amount of recirculation air is released from the reactor via an air exhaust port in the reactor, concurrently with the introduction of fresh make-up ventilation air into the reactor via an air inlet port in the reactor.

11. An aerobic batch composting process according to any one of claims 1 to 10 which further comprises subjecting the product so obtained to storage for a 2.A- period of two (2) to three (3) months for maturation, curing and nitrification, optionally followed by milling and screening of the resultant product to produce a finished compost having the general characteristics of " humus or peat.