CA2138091C - Anaerobic treatment of waste at ambient temperatures - Google Patents

Abstract

A process to stabilize, deodorize, recover energy, reduce pollution potential, and add value to organic waste such as animal manure is described. The process involves the anaerobic digestion of animal manure at low temperatures in intermittently fed sequencing batch reactors . The process of the present application offers several advantages over the prior art processes including (1) the process works very well at low temperatures and therefore does not require pre-heating of the animal manure; (2) the process does not require continuous or daily feeding nor does it require continuous mixing; (3) the process makes use of existing handling and storage equipment at the farm and requires minimal supervision and skill by the operator; (4) the process is very efficient in retaining the slow growing microorganisms in the system and (5) the system is not affected by high concentrations of volatile acids and ammonia or nitrogen. Consequently, the process is low cost and does not interfere with regular farm operations.

Description

FIELD OF THE INVENTION
The present invention relates to a process for treat-ing animal manure on small and large farm operations in order to stabilize, deodorize, recover energy, arid add value to the animal manure. In particular, the process involves the psychro-philic anaerobic digestion of animal manure in intermittently fed sequencing batch reactors. -EACKGROUND OF THE INVENTION
Animal manure management practices, principally in regions Where there is a surplus of manure are often detrimental to the environment and also represent a potential hazard to human and animal health. Animal manures can produce strong odours, encourage fly breeding, induce weed problems and pollute air, soil and water. For example, in some areas of Canada, the drinking water source is polluted and water bodies cannot be used for recreational purposes due to manure contamination. The affected communities are expecting changes in manure management from the farm industry. The National Workshop on Land Applica-tion of Animal Manure, CARC (1991), recommended innovative research that would allow farmers to adopt sustainable and environmentally sound agricultural practices where animal manure is integrated into the overall production systems. It was further recommended that economical processes to stabilize, deodorize; recover energy and add value toanimal manure be developed:
Conventional anaerobic digestion 'of animal manure in farm scaledigesters was attempted at several locations across I
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Canada during 1975-1985. It was not successful for several reasons (Van Die,-1987) as follows: 1) The digesters were designed to operate at mesophilic (35oC) or thermophilic (60°C) temperatures. Because of prolonged sub-freezing winter tempera-tures in parts of North America,-digesters operating at these temperatures during the winter used not only most of the gas they produced but sometimes rerniired supplementary heating to maintain the digester temperature. Fox :example, in a full scale-anaerobic digester for cattle_ manure in Southern Ontario, more energy was required to run the digester in winter months than the energy generated in the biogas produced. 2) The anaerobic digesters were not cost effective because they were designed to produce electricity which made them even_more capital intensive.
3) The digesters were not practical for farm use because their control and maintenance required skilled operators, increased labour input, daily supervision and sometimes changes in farm operational procedures. 4) The digesters were difficult to control and had poor stability because they were pushed to the limit to achieve maximum gas production.
Anaerobic digestion of municipal waste water-and animal manures at low (psychrophilic) temperature has been.
reported in previous studies (O~Rourke, 1968; Stevens and Schulte, 1977; Ke-Xin and Nian-Gua, 1980; Wellinger and Kauf-mann, 1982; Chandler et al., 1983; Cullimore et al., 1985; Lo and Liao, 1986; Sutter and Wellinger, 1987; Balsari and Bozza, 1988; and Safley and Westerman, 1992, 1994). Most of these studies were aimed at biogas production while little considera-2138~9~

tion was given to odour reduction, waste stabilization or increases in fertilizer value or.plant nutrient availability.
There was a vaide variation in the reported experimental results.
Some studies were successful in producing methane at tempera-tures below 20oC while others were not.. The information pro-vided in the above reports is inadequate to provide possible reasons for these discrepancies. In most of these studies the solids were separated from the liquid or the slurry solids content was very low (less than 2~) compared to the typical solids content of manure slurry at Canadian farms. It is unlikely-that farmers would dilute manure slurry for anaerobic digestion because it would require larger storage facilities and increase substantially the volume of liquid manure to spread on the land. Furthermore-; farmers are not interested in separating the liquid and solid fraction of manure slurry as this necessi-tates two different types of manure handling equipment, storage and land application equipment, to handle both.the liquid and solid fractions. Dague et al. (1992) indicated that the sequencing batch reactor is highly suitable for anaerobic digestion because: 1) It provides quiescent settling condition for the anaerobic bacteria; and 2) The high food to micro-organism ratio (F/M) at the beginning of the feed period and the low (F/M) at the end of react period enhances the sludge settling characteristics.
SUMMARY OF THE INVENTION
In view of the above, there is a need to develop a process to treat animal manure that is low cost, is very stable, 2~~~Q9~.

simple, easy to operate, requires minimum skill and does not interfere With regular farm operations. Anaerobic digestion to-treat animal manure under North American conditions is a viable option as it would have a low capital and operational cost if it could: 1) Make use of existing handling- equipment and storage facilities at the farm; 2) Operate at relatively low tempera-ture; 3) Require minimum handling; and 4) Does not require daily maintenance and supervision.
The present inventors have developed a process for the anaerobic digestion of animal manure that overcomes the draw-backs ofthe prior art processes. In particular, the present inventors have determined that psychrophilic anaerobic digestion (PAD) of .animal manure in-intermittently fed sequencing batch reactors (SBR), sterilized, deodorized, reduced pollution potential, recovered energy, and increased plant nutrient -availability from swine manure slurry.
In accordance with the present.invention, there is provided a process for treating organic waste comprising:
(a) feeding the waste to a digester containing a layer of acclimatized anaerobic sludge wherein said digester is at a temperature from about 5°C to about 25°C; and (b) allowing the waste to react with the sludge.
Some of the advantages of the process according to the present invention include:

(1) The-process works at ambient temperatures ranging between 5 and 25°C. (Previous systems that were tried in Canada worked at temperatures of 25 to 65°C.) As a consequence, the. animal manure slurry does not need to be heated before it is fed to the digester.
(2) The process makes use of Sequencing Batch Reactors (SBRs) which were_not used previously with low temperature an-aerobic digestion processes.

(3) The-process does not require continuous or daily feeding.
It can be intermittently fed only 1 to 3 times a week or every two weeks. Because of intermittent feeding this process does not need an expensive calibrated pump. It will make use of existing handling equipment at the farm.

(4) The.process does not really require mixing. Although, it is preferable to provide a minimum level of mixing up to 30 minutes per day. Mixing can be provided by biogas recircu-lation (Previous systems were mixed continuously). Because the a lurry and digester content does not have to be heated and continuous mixing is not required, all the energy produced will be available for on farm use.

(5) The process works very well with either short or long feed-ing and reaction periods. Fill and react period lengths of up to two months did not affect the.process stability and -performance.

(6) The process is very stable when comparedto previous systems. It was not affected by high concentration of 2138~9~

volatile acids (6500 mg/L) and ammonia nitrogen (3700 mg/L).

(7) Because this process works at low temperature, does not require mixing and is not.affected by long fill and react periods it can make use of existing manure slurry storage at the farm.

(8) The process does not.require supervision because it is very stable and also it does not interfere with regular farm operations. This is because the digester is fed only dur-ing normal manure removal operations and the farmer will deal the effluent from the-digester: once a month or every two months or even less often. (Previous systems required daily supervision and farmers had to deal with the digesters effluent on a daily basis:) (9) The proces-s is-the only system that works satisfactorily with pig manure-under Canadian climatic conditions.

(10) The process does not require manure slurry dilution and solid/liquid separation. This process works well with manure slurry that has a solids concentration between 1 and 10~.

(11) The process will work well at organic loading rate ranging between 0.1 to 4.0 g COD per litre of digester volume per day.

(12) The process very efficiently retained a high concentration of slow growing microorganisms in the system.

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(13) The-process will have low capital and operational costs.
Furthermore, if the energy recovered is used at the farm the-process will be very cost effective.
BRIEF DESCP.IPTION OF THE DRAWINGS
Figure 1 is a schematic representation of a laboratory scale sequencing batch reactor_ Figure 2A is a graph that illustrates the cumulative methane production over time for various sequencing batch react-ors at different organic loading rates.
Figure 2B is a graph that illustrates the soluble COD
production over time for various sequencing batch reactors at different organic loading rates.
Figure 2C is a graph that illustrates the acetic. acid production over time for various sequencing batch reactors at different organic loading rates.
Figure 2D is a graph that illustrates the propionic acid production over time for various sequencing batch reactors at different organic loading rates.
Figure 3A-is a graph that illustrates the cumulative-methane production over time for various sequencing batch reactors -at different levels of mixing.
Figure 3B is a graph that illustrates the soluble COD
production over time for various sequencing batch reactors at different levels of mixing.
Figure 3C is a graph that illustrates the acetic. acid production over time for various sequencing batch reactors at different. levels of mixing.

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Figure 3D is a graph that illustrates the propionic -acid production over time for various sequencing batch reactors at different levels of mixing.
Figure 4A is a graph that illustrates the cumulative methane production over time for various sequencing batch react-ors at different organic loading rates and different levels of mixing.
Figure 4B is a graph that illustrates the soluble COD
production over time for various sequencing batch reactors at different organic loading rates and different levels of mixing Figure 4C is a graph that illustrates the acetic acid production over time for various sequencing batch reactors at different organic loading rates-and different levels of mixing.
Figure 4D is a graph that illustrates the propionic -acid production over time for various sequencing batch reactors at different.o~ganic loading rates and different levels of mixing.-Figure 5A is a graph that illustrates the cumulative methane production over time for different feeding frequencies at a cycle length of 28 days.
Figure 5B.is a graph that illustrates the soluble COD
production over time for different deeding frequencies at a cycle length of-28 days.
Figure-5C is a graph that illustrates the acetic acid production over time for different feeding frequencies at a cycle length of 28 days.
g -r ~1~~~91 Figure 5D is a graph that illustrates the propionic acid production over time for different.feeding frequencies at a cycle length of 28 days.
Figure 6A is a-graph that illustrates the cumulative methane production over-time for different feeding frequencies at a cycle length of 14 days.
Figure 6B is a graph that illustrates the soluble COD
production over time for different feeding frequencies at a cycle length of 14 days.
Figure 6C is a graph that illustrates the acetic acid production over time for different feeding frequencies at a cycle length of 14 days.
Figure 6D. is a graph that illustrates the propionic acid productionover time for different-feeding frequencies at a cycle length of 14 days.
Figure 7A is a graph that illustrates the cumulative methane production over time for different cycle lengths with the sequencing batch reactors fed three times a week.
Figure 7B is a-graph that illustrates the soluble COD
production over time for different cycle lengths with the sequencing batch reactors fed three times a week_ Figure 7C is a graph that illustrates the acetic acid -production over time for different cycle lengths with the sequencing batch reactors fed three times a week.
Figure 8A is a graph that illustrates the cumulative methane production over time for different cycle lengths with the sequencing batch reactors fed once a week.
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~ 238091 Figure 8B is a graph that illustrates the soluble COD
production over time for different cycle lengths with the sequencing batch reactors fed once a week.
Figure 8C is a graph that illustrates the acetic acid production over time for different cycle lengths with the sequencing batch reactors fed once a week.
Figure 9 is a graph illustrating the effect of cycle length on cumulative methane production.-Figure 10 is a graph illustrating the effect of cycle length on daily methane production.
Figure 11 is a graph illustrating total daily methane production from two sequencing batch reactors operated simul-taneously.
Figure 12A is a graph that illustrates the cumulative methane production over time in various sequencing batch reactors -for different acclimatization times.
Figure 12B is a graph that illustrates the soluble COD
production over time in various sequencing batch reactors at different acclimatization times.
Figure 12C is a graph that illustrates the acetic acid production over time at various sequencing batch reactors at different.acclimatization times Figure 12D is a graph that illustrates the propionic acid production over time at various sequencing batch reactors at different acclimatization times.

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213~Q91 Figure 13A is a graph illustrating the cumulative methane production over time for four-successive cycles in sequencing batch reactors-numbers 9-10.
Figure 13B is a graph illustrating the cumulative methane production over time for four successive cycles in sequencing batch reactors numbers 11-12.
DETAILED DESCRIPTION OF THE INVENTION
The present inventors conducted a thorough study into the feasibility of using psychrophilic anaerobic digestion to stabilize and deodorize swine manure and'to recover biogas for energy. The inventors conducted two test runs, each test run consisting of twelve sequencing batch reactors. Various para-meters of the process were altered such as organic loading rates, fill and react period lengths, mixing intensity, feed frequency and sludge age.
Manure slurry was obtained from gutters under a par- -dally slatted floor in a growing-finishing barn at a commercial swine operation. 'The manure was up to four days old at the time of collection. It was screened to remove particles larger than 3.5 mm toprepare SBR feed samples. These large particles tend to create. operational problems with small scale laboratory digesters. The feed samples were ~ tored in a freezer at -15°C
to prevent biological activity. Manure feed samples were heated to the digester operating design temperature (20°C) prior to feeding.

29184-2(S) Figure 1 is a schematic illustration of a laboratory scale sequencing batch reactor according to the present inven-tion. Each of the reference numerals refer to the following:
1. 300 mm diameter plexiglass digester 2. sludge bed zone, 8.0 L
3. variable volume zone, 28.0 L
4. head space zone, 6.0 L
5_ biogas recirculation line 6_ gas pump 7. influent line 8. effluent line 9. sludge sample port, also used for sludge wastage 10. mixed liquor or supernatant sampling port 11. gas outlet 12. gas meter 13. thermocouple 14. feeder tube 15. gas pump 16. hydrogen gas monitor 17. liquid pump 18. dissolved hydrogen gas monitor In the SBR shown in Figure 1, the manure is loaded into the feeder (14) and fed to the digester (1) through influ-ent line (7). The manure is fed through the bottom of the digester which has been pre-inoculated with an anaerobic sludge (2). In certain test runs the reactor was mixed by recirculat-23.38092 ing the biogas produced in the headspace (4) through the biogas recirculation line (5) using a gas pump (6).
The inventors altered various parameters of the process keeping in mind that for results to be applicable to farm conditions the laboratory tests should simulate as closely as possible the actual farm operation. At a typical farm, manure is-generally removed from the barn one to three times a week. Therefore the SBR should be intermittently fed one and three times a week. The fill cycle should not be longer than a month in. order to limit the volume of the SBR. The settling period should be long enough to provide complete solids/liquid separation. The react period should be long enough to produce almost odourless effluent with reduced pollution potential and increased fertilizer value. For the PAD in SBR to be cost effective, it is very important that the.operational cost is kept very low. The operation of SBR at ambient temperatures and the reduction or elimination of mechanical mixing would substan-tially reduce the energy input and increase. the energy effi-ciency of the SBR because all the energy produced will be available for on farm utilization.
Table 1 gives the SBRs operating conditions that were.
used in this study. Test run No. 1 and 2 investigated the effect ofloading rates, mixing intensity, fill-react period length, feeding frequency and sludge age on the performance of PAD of swine manure slurry in SBR.
Digesters 5 to 12 were inoculated in a previous run (Masse et. al. 1993). Digesters 1 to 4 were. also inoculated with 29184-2(S) the same mixture of anaerobic sludges. In particular, the digesters were started using 7.5 L of granulated anaerobic sludge obtained from the Agropur dairy wastewater treatment plant at Notre-Dame du Bon Conseil in Quebec province and an additional 2 L of anaerobic non-granulated sludge obtained from the municipal wastewater treatment plant of the City of Ottawa in Ontario. In general, the anaerobic sludge to manure ratio should be within 0.5 to 1Ø
A mixed liquor sample was withdrawn (through a sampl-ing port (10) from each SBR at the beginning of the experiment and once a week during the experimental run. At the end of the test, after the sedimentation period, additional samples were withdrawn from the supernatant (3) and sludge bed zones (2).
The samples were analysed for pH, alkalinity, solids, volatile acids, total Kjeldahl nitrogen (TKN), ammonia nitrogen, total chemical oxygen demand (TCOD) and soluble COD (SCOD). Some of the samples were further analyzed to determine concentration of C, H, N and other elements. The biogas production was monitored daily and its composition analyzed weekly. All the analytical tests carried out on the mixed liquor were also performed weekly on samples of swine manure slurry fed to the SBRs.
Soluble COD was determined by analyzing the super-natant of centrifuged slurry. The pH, alkalinity, and solids were determined using standard methods (APHA, 1992). TKN and ammonia nitrogen were determined using an auto-analyzer.
Volatile acids and biogas composition were determined by gas chromatography. Metal concentrations (K, Ca, Mg, Cu, Zn, Na, 2~38~9~

Hg) weredetermined by the inductively coupled plasma (ICP) methods (APHA, 1992).
Table 2 gives compositions of the swine manure and inoculum sludges used in the experimental runs. The total solids content of the manure slurry was high. It was around 4.1~ (weight basis). The fresh slurry had a neutral pH and very high concentrations of TCOD, SCOD, TKN, NH3-N and volatile acids and alkalinity. The concentration of inorganic elements such as calcium, magnesium, potassium, sodium, zinc and copper were also quite high.
The main characteristics of the Agropur granulated sludge was that it had a vary high TS, TCOD, SCOD, TKN and cal-cium content. The municipal sludge is less concentrated than the granulated Agropur-sludge but it has a higher fibre content on a dry weight basis and also has a lower alkalinity than dairy sludge. Both of these sludges came from digester operated at 35°C.
All the SBRs maintained an alkalinity around 12000 mg (CaC03)/L and a pH between 7.5 and 8.0 during experimental runs 1 -and 2. Both the pH and alkalinity decreased slightly during the feed period due to volatile acids-(VA) accumulation and they both increased slightly during the react period due to the VA
utilization.
Figure 2-shows the typical response of the SBR fed with different organic loading rates. During the four-week fill period the cumulative biogas production was identical for the three organic loading rate. T7~e reason for this might be that 2I38~~~.

the three set of digesters had about the same population of methane formers at the start of the test and the methane produc-tion rate was not limited by the substrate availability but was rather controlledby the growth rate of methane formers_ During the subsequent four-week react period the digesters with the lowest organic loading rate (0.81 g COD/1-d) stopped producing methane. This was because most of the soluble COD and volatile acids were consumed during the fill period. The digesters with the intermediate organic loading rate (1.22 g COD/1-d) stopped to producegas midway through the react period for the same reasons.
Figure 2 also illustrates the soluble COD, acetic and propionic acids concentrations as a function of time. As ex-pected, the concentration of volatile acids and SCOD in the SBR
increased with an increase in organic loading rate. This in-creased in volatile acids indicates that hydrolysis and acidi-fication-were occurring and that utilization of acetic acid by the acetoclastic methane formers was the rate limiting step.
This accumulation in volatile acids and-SCOD was typical of all experimental run_ For the lowest loading rate (0.81 g COD/1-d) there was no propi.onic acid accumulation in the SBR while acetic acid con-centration stayed below S00 mg/L. For the SBRs with the highest loading rate (1.63 g COD/L-d) acetic and propionic acids were both present and-their respective concentrations reached maximum values o~3000 and 900 mg/L at the end of the fill period. For each loading rate the volatile acids were completely utilized at the end of the react period. From these results it can be con-chided that the SBRs were very stable at these loading rates.
The lowest loading rate would not be recommended because no treatment occurs during the react period. A loading rate of 1.63 g COD/L-d-should be recommended. As shown in Figure 2 at this loading rate the react period is utilized to its maximum.
Complete=utilization of both volatile acids and soluble COD
occurred at the end of this period.
Figures 3 and 4 compare the SBR performance for different intensity of mixing at loading ratesof 1.22 and 1.63 gCOD/L-d, respectively, Figure 3 shows that intermittent mixing slightly increased the production rate of methane and the util-ization rate of volatile acids, but did not have an effect on soluble COD. Because the SCOD and VA concentration were the same at the beginning and end of the cycle for the intermittent-ly mixed and non-mixed SBRS, the methane production and VA
utilization should have been the same. These differences in methane production and VA utilization could be due to a slightly different organic loading rate.
Figure 4 shows that for digesters fed a higher organic loading rate (1.63 g COD/L-d) there was-rio difference in process performance between the intermittently mixed and non-mixed digesters. Mixing of a full scale digester-consumes large amounts of energy, and based on these experimental results, SBR
mixing may not necessary for full-scale farm digesters. This would simplify the operation of the SBR, reduce maintenance cost as well as possible mechanical problems.

Figures 5 and 6 compare the typical response of SBR to feeding frequency of 1 to 3 times a week with the same total weekly organic loading for all digesters. Figure 5 shows that frequency of feeding had no significant effect on SBRs with a feed-react cycle length of 28 days. The SBRs fed once a Week produced 13~ more gas and had about the same effluent soluble COD concentration as reac'tbrs fed 3 times/week. These results indicate-that the SBRs fed once a week were also very stable and treated the swine manure slurry adequately.
For the SBRS with a cycle length of 14 days the feed-ing fre-quency had no effect on SCOD, acetic and propionic acids accumulation. Only the cumulative methane production was 14~
higher. These experimental results indicate that both one and three times a week feeding frequency may be acceptable for farm scale SBRs.
Cycle length is an important parameter in the design of SBR because it controls the size of the digester, the treat-ment efficiency as well as the frequency that the farmer has to deal with SBR effluent removal.
Figures 7 and 8 show that the cycle length has only a small effect on the distribution of SCOD and acetic acid concen- .
trations.- The SBR with the shorter feed-react period had twice the number of cycles compared to the. longer feed react period over the 56 days of operation. As a result, there was more fluctuation in the SCOD and acetic acid concentration. Figure 7 and 8 also show that the cycle length has no effect on process performance. Final concentration of SCOD and acetic acid were the same: after 56 days of SBR operation using either the two-week or four-week cycle-Figures 9 and 10-show the effect of cycle length on cumulative and daily methane production. For both cycle lengths the maximum daily methane production occv.rred at the end of the fill period and the=minimum at the end of the react period (Fig. 10). As expected. the SBRs with the shorter cycle length (14 days) showed more variation in weekly methane production:
Figure 9 shows that the total cumulative methane production after 56 days was the same for both cycle lengths. Therefore the total amount of-energy recovered by PAD in SBR was not affected by the cycle length investigated in this study.
At the farm a steady and constant production of methane gas would be preferable in order to develop an adequate biogas utilization strategy. A minimum of two SBRs would be required to process the swine manure slurry at a farm.
Figure 11 shows the total daily methane production from two SBRs operated simultaneously at fill/react cycle-lengths of 14 and 28 days. By comparing Figure lI with Figure 1D, it is obvious that a pair of SBRs provides a more-constant supply of methane than a single SBR. Figure 11-also shows that with a pair of SBRs the cycle length did net have a significant effect on the total daily methane production. Therefore at the farm, SBRS with cycle length of either 14 or 28 days would be acceptable.
In the start-up run, digesters-were inoculated with fresh anaerobic sludge at 35°C and either fed with milk pro-cessing plant wastewater sludge or municipal sludge. It was y 2~~~~~~

expected-that the inoculum sludge in the SBR would acclimatize -with operating time.
Figure 12 compares the SBRs response to sludge age.
Anaerobic sludge in digesters 7 and 8 in the start-up run (Masse et al. 1993) were exposed to swine manure slurry and low temperature for thefirst time. During this run there was a long lag phase in the biogas production during the.feeding period. In test run number one, the same sludge that had already been exposed to swine manure slurry and low temperature for a period of three months. Both were fed about the same organic loading rate. The SBRs with an older acclimatized sludge had: 1) a shorter lag phase and a substantially higher methane production rate; and 2) substantially lower concentra-tion of soluble COD, acetic and propionic acids at the end of the react period. These experimental results indicate that sludge age has a significanti.nfluence on the process response.
Figure 13 compares the cumulative methane production for each.consecutive cycle during test run number two. These figures clearly indicate that the initial methane production rate and the total cumulative methane production for each cycle increased after each successive cycle and the lag phase at the beginning of the cycle decreased as the test progressed. These results clearly indicatethat micro-organisms' acclimatization to low temperatures and swine manure slurry was taking place.
The biogas produced in test runs one and two was of high quality with a methane concentration between 75 and 80~.
Table 3 gives the methane production as a function of unit mass ~~.380~~

of volatile solids fed to the digester_ The CHa production ranged from 0.48 to0.66 L/g VS'for most of the experimental run. Methane productions obtained in this study were substan-tially higher than methane production from swine manure obtained by digestion at 35°C in continuous flow digesters by Kroecker et al. (1979) who reported methane production of 0.45 L CH,/g VS
added for a loading rate of 2.5 kg VS/m'-day, and by Hashimoto (1983) who reported 0.42 L CH4/g VS added for a loading rate of 2.5 kg VS/m'-day.
The higher methane production per gram of volatile solids -fed to the SBRs obtained in this study could be due to the lower organic loading rate and longer hydraulic residence time. Another possible reason could be the lower operating temperature and the absence of. mixing which maintain a higher concentration of hydrogen and carbon dioxide gases in the liquid phase. As a result more carbon dioxide can be reduced to methane by the hydrogen-utilizing methanogens. A high rate of methane production was not the main objective of this work but it is very useful to asses the system performance and stability.
The steady production of methaneper unit mass of volatile -solids fed indicates that anaerobic digestion of swine manure at 20°C in the laboratory-scale SBR digesters was a stable process.
Table 3 also gives the level of removal of TCOD, SCOD
and volatile solids for all runs. The total COD removal ranged from 41 to 83~ and the volatile solids removal ranged from 46 to 84~. Results for volatile solids and total COD were highly - 21 _ 2~.38Q91 variable due to sampling variation caused by rapid settling of heavy particulates_ Some samples had less solids than others.
This affected the VS and TCOD determination as well as the calculated methane production per gram of VS.
The soluble COD test results were consistent. High SCOD renmoval was achieved during most of:the experimental runs.
Its removal ranged from 798> to 9.3~ except in a few runs dis-cussed below. Experimental runs that achieved 70 to 93~ SCOD
removal and complete utilization of VA's, produced treated manure that was relatively odourless compared to the raw manure.
SBRS-1, 2, 3 and 4 in test run.No. 2 had very low energy recovery and reduction in SCOD. These SBRs were started-up in test run 1 and their organic loading rate was doubled in test run-2. This rapid increase in organic loading rate caused their total failure.
Theanaerobic sludge had excellent settling charact-eristics: In the SBR that were not mixed, there was a clear interface between the liquid and sludge bed zones. A thick layer of-sludge was observable at the bottom of the digester.
At the end of the react period where the.biogas production was very low, the demarcation between the liquid and solids was even more evident. In the SBRs that were mixed there were no dis-tinguishable supernatant and sludge zones. For these SBRs, when mixing was stopped at the end of react period, It would take about 2 to 6 hours for a zone settling or liquidlsolids inter-face to form and another 24 to 48 hrs for the sludge blanket to completely settle at the bottom of the SBR. Therefore the SBR

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provides excellent settling conditions to retain the slow grow-ing-microorganisms when enough time-is allowed for the settling period.
Another very important feature of a SBR is that it does not required continuous feeding. Therefore, in farm applications, PAD in SBR will not interfere with regular farm operations as previous systems did. It could be loaded during normal manure removal operations and thefarmer would not have to deal daily with the digester effluent. At the farm the SBR
effluent will need to be handled once every one or two months, depending on the operating conditions. Because of intermittent feeding the SBR could make use of existing manure handling equipment at the farm and also because SBR will not interfere with farm operation, it will increase substantially the interest in anaerobic digestion to treat animal manure on small and large farm operations.
It will be appreciated to one skilled in the art that various modifications can be made to the above described system without departing from the scope and spirit of the invention.
For example, the size of the SBR and the fill and react periods will depend largely on the size of the farm as well as the personal choice of the operator. The process can also be used -to treat other ypes oforganic waste such as slaughter house waste water, food processing plant waste-water and high strength waste water produced by other types of industries.

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REFERENCES
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2. Balsari, P. and E. Bozza, (1988). Fertilizers and Biogas Recovery Installation in a Slurry Lagoon. In Agricultural Waste Management and Environmental Protection. Proceedings of the 4th International Symposium of CIEC, ed. E. White and I. Szabolcs, 71-80.

3. CARC, 1991. Proceedings of the National Workshop on Land -Application of Animal Manure. Eds. Leger,D.A.,--Patni,N.K., and Ho,S.K., Canadian Agricultural Research Council, Agriculture Canada, Ottawa, ON, 176 pp.

4. Chandler, J.A., S.K. Hermes, and K.D. Smith, (1983). A Low Cost 75 kW Covered Lagoon-Biogas System. Presented at Energy from Biomass and Waste VII, Lake Buena Vista, FL. -23 pp.

5. Cullimore, R.R.,- A. Maule, and N. Mansui, (1985). Ambient _ Temperature Methanogenesis from Pig Manure Waste Lagoons.

Thermal Gradient Incubator Studies, Agricultural Waste, 12:147-157.
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Initial Studies on the Anaerobic Sequencing Batch Reactor,-Water Science and Technology, 26: Nb. 9-11, 2429-2432 7. Hashimoto, A.G. (1983), Thermophilic and Mesophilic An-aerobic Fermentation of Swine Manure, Agricultural Wastes, Vol. 6, 175-191.

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8. Ke-Xin, I. and L. Nian-Guo, (I980). Fermentation Tech-nology for Rural Digesters in China. Proceedings Bioenergy 80, Bio-Energy Council, New York, 440-442.

9. Kroeker, E.J., D.D., Schulte, A.B., Spading and H.M. Lapp, (1979), Anaerobic Treatment Process Stability. Journal Water Pollution Control Federation, Vol. 51, 718-27.

10. Lo, K.V. and P.H., Liao, (1986), Psychrophilic Anaerobic Digestion of Screened Dairy Manure. Energy in Agriculture, 5:339-345 11. Mas s, D.I., R.L. Droste, K. Kennedy and N.K. Patni, 1993.

Psychrophilic Anaerobic Treatment of Swine Manure in Inter-mittently Fed Sequencing Batch Reactors. Presented at the 1993 International Winter Meeting of the American Society of Agricultural Engineers, ASAE Paper No. 93-4569, St-Joseph, MI, 49085-9659-.

12. O'Rourke, J.T. (1968), Kinetics of Anaerobic Waste Treat-ment at Reduced Temperature. Ph_D. Thesis, Stanford University, Cali-forma, US.

13. Safley, L.M., and P.W. Westerman, (1992). Performance of a Dairy Manure Anaerobic Lagoon, Bioresource Technology, 42:43-52 -14. Safley, L.M., and P.W. Westerman, (1994). Low Temperature Digestionof-Daizy and Swine Manure Bioresource Technology, 47:165-171 15. Stevens, M.A., and D.D. Schulte, (1977). Low Temperature Anaerobic Digestion of Swine Manure. American Society 21~~~~~

Agricultural Engineers, Paper 77-1013, St-Joseph, MI_ 19 pp_ 16. Sutter, K:, and A., Wellinger, (1987). ACF-System: A New Low Temperature-Biogas Digester. Tn Proceedings of the 4th International Symposium ofCIEF, 11-14 March 1987, Braun-schweig-Volkenrode, Germany.
17. VanDie, P: (1987). An Assessment of Agriculture Canada's Anaerobic Digestion Program. Engineering and Statistical Research Centre_ Contribution No. I-933, Agriculture and Agri-Food-Canada, Ottawa, Ontario, K1A OC6.
18. Wellinger; A., and R., Kaufmann, (1982). Psychrophilic Methane Production from Pig Manure.' Process Biochemistry, 17:26-30 2~3809~

Table 1. SBR Operating Conditions RUN DIGESTERLOADING FEEDING MIXING""FILL. REACT N0.
N0. NO.. RATE FREQUENCY PERIODPERIOD of CYCLE

g COD/feedg COD"/L-d(Per (WEEK)(WEEK) week) 1 1-2 14.25 0.81 3 N 4 4 1 3-4 14.25 0.81 3 Y 4 4 1 5-6 21.40 1.22 3 N 4 4 1 7-8 21.40 1.22 3 Y 4 4 1 9-10 28.50 1.63 3 N 4 4 1 11-12 28.50 1.63 3 Y 4 4 1 2 1-2 28.50 1.63 3 N 4 4 1 3-4 85.50 1.63 1 N 4 4 1 5-6 28.50 1.63 3 N 2 2 2 7-8 85.50 1.63 1 N 2 2 2 9-10 28.50 1.63 3 N 1 1 4 11-12 85.50 1.63 1 N 1 1 4 * Equivalent loading rate if the swine manure would have been fed continuously.
** SBR was intermittently mixed by biogas recirculation.
Mixing lasted 10 minutes every thirty minutes.

y 213~~91 Table 2 Compositioa of S~wiae Manure Slurry (Substrate) and Inoculum Anaerobic Sludges CONSTITUENT SWINE AGROPUR MUNICIPAL
MANURE SLUDGE SLUDGE

Total Solids (TS), ~ 4.1 11 2.6 Volatile Solids (VS), ~ 2.7 5.6 1.26 Soluble COD (SCOD), g/L 28 10 3 Total COD (TCOD), g/L 57 73 8.2 TKN, g/L 6.8 7.9 1.8 NH4-N, g/L 5.0 1.3 1.0 pH 7.3 7.6 7.3 Alkalinity, g 13.5 16 6 CaC03 /L

Acetic Acid, g/L 5.3 0.0 0.0 Propionic Acid, g/L 1.7 0.0 0.0 Butyric Acid, g/L 2.2 0.0 0.0 Cellulose, ~ TS 2.43 0.70 0.84 Hemicellulose, ~ TS 4.15 0.73 3.98 Lignin, ~ TS 1.31 1.56 2.88 Total Carbon, ~ VS 38.18 48.4 55.9 Total Nitrogen, ~ VS 4.69 9.64 10.6 Hydrogen, ~ VS 6.10 7.54 8.48 Calcium, mg/kg TS 54800 84720 46800 Copper,- mg/kg TS 960 80 630 Magnesium, mg/kg TS 8600 1770 2600 Mercury, mg/kg TS NA NA 2420 Potassium, mg/kg TS 42800 6160 10000 Sodium, mg/kg TS 13900 7060 400 Zinc, mg/kg TS 450D 1240 600 M H
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Claims (18)

1. A process for the psychrophilic anaerobic digestion of organic waste comprising the steps of:
(a) intermittently feeding the waste to a digester containing a layer of acclimatized anaerobic sludge; and (b) allowing the waste to react with the sludge at a temperature from about 5°C to about 25°C.

2. A process according to claim 1 wherein the waste is allowed to react with the sludge in step (b) for a react period of time that is approximately equal to a feed period of time that the waste is fed to the digester in step (a).

3. A process according to claim 1 further comprising intermittently mixing said waste and said sludge.

4. A process according to claim 3 wherein said mixing is carried out for approximately thirty minutes daily.

5. A process according to claim 1 wherein said waste is fed at a rate from about 0.1 to about 4.0 g COD per litre of digester volume per day.

6. A process according to claim 1 wherein said waste is fed at a rate from about 0.7 to about 1.7 g COD per litre of digester volume per day.

7. A process according to claim 2 wherein said react period and said feed period is approximately one month.

8. A process according to claim 2 wherein said react period and said feed period is approximately two weeks.

9. A process according to claim 2 wherein said react period and said feed period is approximately one week.

10. A process according to any one of claims 1 to 9 wherein said temperature is from about 15°C to about 25°C.

11. A process according to any one of claims 1 to 9 wherein said temperature is about 20°C.

12. A process according to any one of claims 1 to 11 wherein said waste is animal manure.

13. A process according to claim 12 wherein said manure is fed in a form containing between 0.1% and 10% solids.

14. A process according to claim 3 or 4 wherein the waste and the sludge are allowed to settle after mixing for a period of time between 4 hours and 2 days to form a supernatant and a sludge bed zone.

15. A process according to claim 12 wherein the ratio of anaerobic sludge to manure in the digester is approximately 0.5 to 1Ø

16. A process according to claim 14 further comprising removing the supernatant from the digester.

17. A process according to any one of claims 1 to 16 wherein the waste has an ammonia nitrogen concentration of 3700 mg/L or greater.

18. A process according to any one of claims 1 to 17 wherein the waste has a volatile acid concentration of 6500 mg/L or greater.