Operation strategies for psychrophilic anaerobic digestion of swine manure slurry in sequencing batch reactors

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  • Operation strategies for psychrophilic anaerobic digestion of swine manure slurry in sequencing batch reactors1

    D.I. Masse, N.K. Patni, R.L. Droste, and K.J. Kennedy

    Abstract: The objective of this study was to evaluate the performance of psychrophilic anaerobic digestion in sequencing batch reactors (SBRs) under operating strategies that would optimize process performance and stability while minimizing the interference of the bioreactor operation with regular farm activities. Process performance was evaluated on (i) reduction in pollution potential; (ii) energy recovery: and (iii) odour reduction. Experiments were carried out in twelve 40-L SBRs. Experimental results indicated that psychrophilic anaerobic digestion of swine manure slurry at 20C in an intermittently fed SBR (i) reduced the pollution potential of swine manure slurry by removing 84-93% of the soluble chemical oxygen demand and 41 -83% of total chemical oxygen demand; (ii) produced biogas at rates exceeding 0.48 L of CH, per gram of volatile solids fed; and (iii) successfully reduced odours. Other findings were that (i) for all experimental runs, psychrophilic anaerobic digestion of swine manure slurry in SBRs was very stable; (ii) the process generally performed well without external mixing; and (iii) intermittent feeding of once or three times a week did not affect process stability and performance. As a result, this process requires little energy input and most of the energy produced will be available for farm use. Since this process is very stable, process feeding could be integrated with the routine operation of manure removal from the barn, thereby minimizing interference with other farm operations.

    Key words: anaerobic, anaerobic treatment, psychrophilic, animal manure, methane production, process control, manure treatment.

    Rbsumb : L'objectif de cette t tude Ctait d'Cvaluer la performance de la digestion anakrobie en milieu psychrophile dans des rCacteurs biologiques siquentiels (RBS) sous des stratCgies d'opkration pouvant optimiser la performance et la stabilite du procCdC tout en minimisant les interfkrences avec les activitCs rCgulii3res B la ferme. La performance du procCdC a CtC CvaluCe d'aprks : (i) la rCduction du niveau de pollution; (ii) la rtcupCration d'knergie; et (iii) la reduction d'odeur. Des expCriences ont Ctt effectuCes dans 12 RBS de 40 litres. Les rCsultats expCrimentaux indiquent que la digestion anaCrobie en milieu psychrophile du lisier de porc h 20C dans des RBS alimentCs de facon intermittente a : (i) rCduit le niveau de pollution du lisier de porc en rCduisant la demande chimique en oxygene soluble d e 84 B 93%, e t la demande chimique en oxygkne total de 41 i? 83%; (ii) produit du biogaz B des taux exctdant 0,48 L d e CH, par gramme de solide volatile; et (iii) reduit les odeurs avec succks. Les risultats indiquent aussi que : (i) pour toutes stratkgies d'opkration expirimentale, la digestion anakrobie en milieu psychrophile du lisier de porc dans les RBS Ctait trks stable; (ii) le proctdC a gCnCralement bien perform6 sans agitation externe; et (iii) une alirnentation intermittente d'une B trois fois par semaine n'a pas affect6 la performance et la stabilitC du procCdC. I1 en rCsulte que c e procCdC nicessite un minimum d'apport Cnergetique externe, et la plupart de I'Cnergie produite va Ctre disponible pour l'usage interne de la ferme. Puisque ce procCdC est trks stable, son alimentation peut @tre intCgrCe avec I'enlkvement routinier du fumier, minimisant ainsi I'interfkrence avec d'autres opCrations B la ferme.

    Mots cl4.y : anakrobie, traitement anakrobie, psychrophile, dejection animal, production de mCthane, contr8le de procCdt, traitement de fumier. [Traduit par la rCdaction]

    Received September 25, 1995. Revised manuscript accepted June 20, 1996

    D.I. Massb. Dairy and Swine Research and Development Centre, Research Branch, Agriculture and Agri-Food Canada, Lennoxville, QC J I M 123, Canada. N.K. Patni. Centre for Food and Animal Research, Research Branch, Agriculture and Agri-Food Canada, Ottawa, O N K I A OC6, Canada. R.L. Droste and K.J. Kennedy. Department of Civil Engineering, University of Ottawa, Ottawa, ON KIN 6N5, Canada.

    Written discussion of this paper is welcomed and will be received by the Editor until April 30, 1997 (address inside front cover).

    ' This paper is a modified version of a paper published in the Proceedings of the Seventh International Symposium o n Agricultural and Food Processing Wastes, Chicago, Illinois, June 18-20, 1995.

    Can. J. Civ. Eng. 23: 1285- 1294 (1996). Printed in Canada 1 lmprimC au Canada

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  • Can. J. Civ. Eng. Vol. 23, 1996

    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. Occasionally, in some areas of Canada, the drinking water source is polluted and water bodies cannot be used for recreational purposes owing to manure contamination (Leger et al. 1991). The affected communities are expecting changes in manure management from the farm industry. The National Workshop on Land Application of Animal Manure 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 (Leger et al. 199 1). It was further recommended that pro- cesses to stabilize, deodorize, and add value to animal manure be developed.

    Conventional anaerobic digestion of animal manure in farm-scale digesters was tried at several locations across Canada during 1975-1985 and found to be unsuccessful for several reasons (Van Die 1987). (i) Digesters were designed to operate at mesophilic or thermophilic tempera- tures. Because of subfreezing winter temperatures in Canada, digesters operated during the winter used not only most of the gas they produced but sometimes required supplementary heating to maintain the digester temperature. (ii) Anaerobic digesters were not cost effective because they were designed to produce electricity which made them capital intensive. (iii) They were not practical for farm use because their con- trol and maintenance required skilled operators, increased labour input, and sometimes changes in farm operational procedures. (iv) Digesters were unstable and difficult to con- trol because they were pushed to the limit to achieve maxi- mum gas production.

    The farm industry will be interested in animal manure treatment only if the process can be achieved at low cost, is very stable and easy to operate, requires minimum skill, and does not interfere with regular farm operations. Anaerobic digestion to treat animal manure under Canadian conditions would have a low capital and operational cost if it could (i) make use of existing manure handling equipment and storage facilities at the farm, (ii) operate at a relatively low temperature, and (iii) require only a low level of agitation.

    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-Guo 1980; Wellinger and Kaufmann 1982; Chandler et al. 1983; Cullimore et al. 1985; Lo and Liao 1986; Sutter and Wellinger 1987; Balsari and Bozza 1988; Safley and Westerman 1992, 1994). Most of these studies were aimed at biogas production while little consider- ation was given to odour reduction, waste stabilization, or increases in fertilizer value or plant nutrient availability. There was wide variation in the reported experimental results. Some studies were successful in producing methane at tem- Deratures below 20C while others were not. The informa- tion provided in the above reports is inadequate to provide possible reasons for these discrepancies. In most of these studies, the slurry solids contents was very low (less than 2%) compared with 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 and treatment facilities and substan- tially increase the volume of liquid manure to spread on the land. Dague et al. (1992) indicated that a sequencing batch reactor (SBR) is highly suitable for anaerobic digestion because (i) it provides quiescent settling conditions for the anaerobic bacteria and (ii) the high food-to-microorganism ratio (FIM) at the beginning of the feed period and the low FIM at the end of the react period enhance sludge settling characteristics.

    Mass6 et al. (1993) evaluated the feasibility of using psychrophilic anaerobic digestion at 20C in nonmixed and intermittently fed SBRs2 to stabilize and deodorize swine manure slurry and also to recover biogas for energy. In that study, (i) the SBR fill and react periods lasted 1 month each; (ii) the SBRs were intermittently fed three times a week; and (iii) the swine manure slurry fed to the SBRs had a high solids content (5 %). ~ x ~ e r i m e n t a l results from the start-up run indicated that psychrophilic anaerobic digestion of swine manure slurry in SBRs is technically feasible: (i) it stabilized and deodorized swine manure slurry; (ii) the process was very stable and not affected by high concentration of volatile acids (VA), ammonia nitrogen, intermittent feeding, and absence of agitation; and (iii) it produced high quality biogas (0.3-0.66 L CH41g volatile solids added). Also, the SBRs provided excellent settling conditions and retained a high concentration of anaerobic bacteria in the svstem.

    Because of these very promising results, psychrophilic anaerobic digestion of swine manure in SBRs was studied further (Mass6 1995). This paper reports on the effect of environmental and operational factors such as organic load- ing rates, fill and react period lengths, mixing intensity, feeding frequency, and sludge acclimation on the perfor- mance of psychrophilic anaerobic digestion in a SBR. This research was conducted to develop operational strategies to optimize the stability of the SBR while minimizing the inter- ference of the SBR operation with regular farm activities.

    Experimental procedures

    Figure 1 is a schematic of the bench-scale SBRs used in this study. Twelve 42-L plexiglass digesters were located in a temperature-controlled room (20C). Each operating condi- tion was run in duplicate reactors. The sludge volume at the beginning of a cycle was 7.5 L. For the experiments when mixing was applied, slurry in the SBRs was mixed by recir- culating the biogas. Wet tip gas meters were used to measure the daily biogas production. Manure slurry was obtained from gutters under a slatted floor in a growing-finishing barn at a commercial swine operation. The manure was up to 4 days old at the time of collection. It was screened to remove particles larger than 3.5 mm to prepare SBR feed samples. These large particles tend to create operational problems with small-scale laboratory digesters. The feed samples were stored in a freezer at - 15 "C to prevent biolog- ical activity. They were heated to the digester operating temperature (20C) prior to feeding.

    The experimental plan was designed to investigate the fol- lowing performance features. For results to be applicable to farm conditions, laboratory tests should simulate as closely

    ' U.S. and Canadian patents applied for.

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    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 to three times a week. The SBR settling period should be long enough to provide complete solids and liquid separa- tion. The SBR react period should be long enough to produce almost odourless effluent with reduced pollution potential and increased fertilizer value. For PAD SBR to be cost effec- tive, it is very important that the operational cost is low. The operation of SBR at ambient temperatures and the reduction or elimination of mechanical mixing would substantially reduce the energy input and increase the energy efficiency of the SBR because all the energy produced would be available for on farm utilization.

    Based on the above considerations and also on informa- tion gathered in preliminary runs, the operating conditions in Table 1 were used in this study. In the first phase (run I), the primary variables investigated were organic loading rate and mixing. This phase was designed to find the maximum organic loading rate that could be handled effectively in the reactor. Inadequate mixing has often been cited as the reason for poor performance in anaerobic reactors (Monteith and Stephenson 1981; Speece 1981). Therefore two sets of reac- tors were run at each loading: one set was mixed and one set was unmixed. In the mixed reactors, mixing was applied intermittently by recirculating biogas for 10 min during each 30 min period. Cycles with feed and react periods of 4 weeks each (total of 8 weeks) had been shown to be feasible in preliminary work and they were used in this run. All reactors were fed at the same frequency.

    In the second phase (run 2), the maximum organic loading rate treated effectively from the first phase was used. NO mixing was applied, as discussed later. Feeding frequency and cycle length were varied to examine their effects on per- formance.

    Sampling A mixed liquor sample was withdrawn from each SBR at the beginning of the experiment and once a week during each experimental run. At the end of the test, after the sedimenta- tion period, additional samples were withdrawn from the supernatant and sludge bed zones. The samples were ana- lysed for pH, alkalinity, solids, VA, total Kjeldahl nitrogen (TKN), ammonia nitrogen, total chemical oxygen demand (TCOD), and soluble COD (SCOD). Biogas production was monitored daily and its composition was analysed weekly. All analytical tests carried out on the mixed liquor were also performed weekly on samples of swine manure slurry fed to the SBRs.

    SCOD was determined by analysing the supernatant of centrifuged slurry. The pH, alkalinity, and solids were deter- mined using standard methods (APHA 1992). TKN and ammonia nitrogen were determined using an auto-analyser. VA and biogas composition were determined by gas chroma- tography .

    Organic loading rates and cycle operation Organic loading rates, which are given in Table 1, are based on the amount of COD fed to the volume of sludge present at the start of a cycle (7.5 L). Measuring the concentration of sludge in the reactors was difficult because the density of

    Fig. 1. Schematic of laboratory-scale sequencing batch reactor. I . I

    Biogos Recirculation Line

    sludge would vary in the reactor throughout the cycle and especially after settling. Also, some heavy sludge particles would settle very quickly after any mixing was terminated. It was not possible to mix the reactor contents uniformly and obtain enough samples to give a representative result without removing more sludge than desired from a reactor. There- fore loading was based on the volume of sludge present in the reactor at the start of a cycle. The loading equation is as follows:

    where Ls,, is the loading rate based on the initial sludge volume and the feed time; Vf is the volume of feed; C i s the COD concentration in the feed; Vs is the volume of sludge at the beginning of a cycle; and tf is the fill time.

    In the literature there is no consistent definition of loading for SBR systems. Loading has been expressed on the total reactor volume utilized and the total cycle time. Table 2 presents loadings calculated according to each of the defini- tions to clearly define the operating conditions in this study. The equations for each of the loading calculations are as follows:

    where LS,, is the loading rate based on the initial sludge vol- ume and the total cycle time, and where t, is the cycle time.

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    Table 1. SBR operating conditions.

    Can. J. Civ. Eng. Vol. 23, 1996

    Loading rate Feeding Fill React Run Digester frequency period period No. of No. No. g CODIfeed g COD/(L.d)* (per week) Mixing (week) (week) cycles

    *Based on feed time and initial sludge volume in the reactor.

    Table 2. Loading rates.

    COD per Feed Volume at Loading rate feed frequency Fil1:react end of fill* (g) (No.1week) (week: week) (L) Ls,f Ls,, Lv.f Lv., LsI,,

    14.25 3 4:4 10.5 0.81 0.40 0.58 0.29 0.016 21.40 3 4:4 12.0 1.22 0.61 0.76 0.38 0.024 28.50 3 4:4 13.5 1.63 0.81 0.90 0.45 0.036 28.50 3 2:2 10.5 1.63 0.81 1.16 0.58 0.036 28.50 3 1:l 9.0 1.63 0.81 1.36 0.68 0.036 85.50 1 4:4 13.5 1.63 0.81 0.90 0.45 0.036 85.50 1 2:2 10.5 1.63 0.81 1.16 0.58 0.036 85.50 1 1:l 9.0 1.63 0.81 1.36 0.68 0.036

    *Initial volume in the reactor was 7.5 L.

    where L,,, is the loading rate based on the total reactor volume and the feed time, and V, is the liquid volume in the reactor after fill is completed.

    where Lv,, is the loading rate based on the total reactor volume and the cycle time.

    Furthermore, in Table 2 , the specific COD loadings are presented based on a volatile suspended solids (VSS) concen- tration of 25 000 mg/L, which is approximately the average concentration of VSS in the settled sludge in these reactors. The loading equation is

    where Lsl,, is the specific sludge loading rate based on the cycle time.

    VSS loadings, in particular, must be interpreted with care for systems treating a waste of this nature. Refractory organic components such feed particles can be present in large amounts in swine manure slurry and they may accumu-

    late in the reactor constituting a significant portion of VSS. Other performance parameters evaluated were TCOD

    removal based on the total amount of COD fed and all COD removed from the reactors through sampling and operation. SCOD removal is based on the effluent SCOD compared to influent SCOD. Volatile solids (VS) removal is also reported but because of sampling difficulties of VSS in the reactor discussed above, it is the least reliable parameter.

    The initial sludge volume (V,) in all reactors was adjusted to 7.5 L for the runs reported in this paper. The cycle time is defined by

    where t , is the duration of the react period and tSd is the duration of the settle and draw periods. When external mix- ing was applied (biogas recirculation), the biogas recircula- tion was terminated before the end of the react period. Final reaction and settling occurred at the end of the react period. Draw times were less than one-half hour. There was no formal settle period in reactors that did not have external mixing. As reaction subsided, settling occurred. Therefore reaction times given in this paper include the settling and draw periods for both the mixed and unmixed systems.

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    Table 3. Composition of swine manure slurry.

    Constituent Swine manure

    Total solids, TS (%) Volatile solids, VS (%) Soluble COD, SCOB (g/L) Total COD, TCOD (g/L) TKN (g/L) NH,-N (g/L) pH Alkalinity (g CaCO,/L) Acetic acid (g/L) Propionic acid (g/L) Butyric acid (g/L)

    After feed, react, and settle phases were completed, the volume of supernatant withdrawn returned the volume in the reactor containing the settled sludge to 7.5 L.

    Results and discussion

    Table 3 gives the composition of the swine manure slurry. The total solids content of the manure slurry was high at approximately 4.1 % (weight basis). The fresh slurry had a neutral pH and very high concentrations of TCOD, SCOD, TKN, NH,-N, VA, and alkalinity.

    All SBRs maintained an alkalinity around 12 000 mg as CaC03/L and a pH between 7.5 and 8.0 during runs 1 and 2. Both pH and alkalinity decreased slightly during the feed period owing to VA accumulation and they both increased slightly during the react period owing to VA utili- zation.

    Phase 1 Figure 2 shows the response of SBRs to which intermittent mixing was applied in run 1. The reactors were fed at differ- ent organic loading rates. The data in this figure (and data in all figures that describe reactor output) are the average response of two reactors. The performance of the reactors to which no mixing was applied is discussed below. During the 4-week fill period, the cumulative biogas production was identical for the three organic loading rates. The reason for this may be that the SBRs fed with different organic loading rates had about the same population of methane formers at the start of the test, and methane production rate was not limited by the substrate availability but was rather controlled by the growth rate of methane formers. During the subse- quent 4-week react period, methane production decreased to negligible rates at about days 35 and 50 in the cycle for the digesters with the lowest organic loading rate (0.81 g COD/ (L - d)) and intermediate loading rate (1.22 g COD/(L. d)), respectively. Available substrate was largely consumed dur- ing the fill period as observed from the drop in the concentra- tions of SCOD and acetic and propionic acids at about the same times in these reactors. As expected, the maximum concentrations of SCOD and acetic and propionic acids increased with an increase in organic loading rate. The accu- mulation pattern of VA along with the methane production curves indicate that hydrolysis and acidification were occur-

    Fig. 2. Comparison of sequencing batch reactor (SBR) operating performance in test run 1, for different organic loading rates (OLR). (---) SBRs 3 and 4, OLR = 0.81 g COD/(L.d); (. . .) SBRs 7 and 8, OLR = 1.22 g COD/(L.d); (- --) SBRs 11 and 12, OLR = 1.63 g COD/(L.d).

    Time (d)

    ring and that utilization of acetic acid by the acetoclastic methane formers was the rate-limiting step. The accumula- tion patterns for VA and SCOD were typical of all experi- mental runs.

    For the lowest loading rate (0.81 g COD/(L-d)), there was no propionic acid accumulation in the SBR while acetic acid concentrations stayed below 500 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 concentrations reached maximum values of 3000 and 900 mg/L, respec- tively, during the fill period. For each loading rate, the VA were completely utilized at the end of the react period. From these results it can be concluded that the SBRs were very stable at these loading rates. The lowest and intermediate loading rates tested should not be recommended because degradable substrate was essentially exhausted early during the react period. A loading rate of 1.63 g COD/(L. d) was recommended from the results of run 1. As shown in Fig. 2, at this loading rate, the react period is utilized to its maxi- mum extent. Complete utilization of both VA and SCOD had occurred at the end of this period.

    Figure 3 compares the SBR performance for different intensities of mixing at a loading rate of 1.63 g COD/(L. d). There was little difference in process performance between the intermittently mixed and nonmixed digesters at this load- ing. Table 4 summarizes the measures of treatment per-

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    Fig. 3. Comparison of sequencing batch reactor (SBR) operating performance in test run 1, for different levels of mixing. Organic load rate = 1.63 g COD/(L. d). (---) SBRs 9 and 10, no mixing; ( . . .) SBRs 11 and 12, mixed for 10 minutes each half hour. - ,

    Time (d)

    formance; methane production; and TCOD, SCOD, and VS removal for all runs. Methane production and SCOD removal were nearly the same, regardless of whether mixing was applied or not. Mixing did have a positive effect on TCOD and VS removal, but the effect was generally not large. Intense biogas production occurred after each feeding in the reactors.

    Mixing of a full-scale digester is an energy expenditure, and based on these experimental results, SBR mixing may not be necessary for full-scale farm digesters. This would simplify the operation of the SBR and reduce the main- tenance cost as well as possible mechanical problems. Based on the results from run 1, mixing was not applied to the digesters in run 2.

    Phase 2 The results for run 2 experiments are also given in Table 4. The performance of reactors 1-4 in this run was signifi- cantly lower than for all reactors in this study, regardless of the operating conditions. One reason for the poor perform- ance of these reactors may have been that the organic loading rate was doubled from the previous run. The high organic loading on a sustained basis may have shocked the systems which had experienced a lower organic loading rate for a sig- nificant time. The other reason may have been due to lack of external mixing. Although the reactors that were not mixed in both runs exhibited good treatment, inadequate mixing is still considered a common cause of anaerobic reactor failure.

    Fig. 4. Comparison of sequencing batch reactor (SBR) operating performance in test run 2, for different feeding frequencies. Cycle length of 14 days. (---) SBRs 9 and 10 fed 28.5 g COD three times a week; ( . . .) SBRs 11 and 12 fed 85.5 g COD once a week. 3 100 1 7- I

    Time (d)

    We remain guarded in the conclusion that mixing is not required for these systems and further experimentation is in progress on this point. Beyond these two reasons, there was no other obvious reason for the poor performance of these four reactors.

    Feed frequency Run 2 results are presented in the remaining figures on this paper. Figure 4 compares the typical response of SBRs to feeding frequencies of 1 and 3 times a week for digesters with the same total organic loading. Refer to Table 4 for treatment performance of all reactors fed at different fre- quencies. From the treatment performance parameters, feeding frequency did not have any discernible effect on treatment efficiency. If anything there is a slight trend toward improved performance with less frequent feeding. The sys- tem was not shocked by delivering the total quantity of weekly feed in one dose as long as the organic loading rate is maintained in an acceptable range. The ability of the sys- tem to handle single large feed doses is of practical impor- tance to a farm operation; manure will not need to be stored for an equalized feeding rate. It can be conveniently fed to the reactors at the time of barn cleaning.

    Cycle length Cycle length is an important parameter in the design of SBRs because it controls the size of the digester (compare volumet- ric loading rates based on the total cycle time in Table 2) as

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    Table 4. Average treatment performance of the reactors.

    Loading rate CH4 production Removal (d) Run Digester Feeding Fill React No. of (L CH4/g VS No. No. g COD/feed g COD/(L. d) frequency Mixing period period cycles after 56 days) TCOD SCOD VS

    well as the frequency that the farmer has to deal with SBR effluent removal. Figure 5 is a typical comparison of the per- formance of reactors fed at different cycle times but at the same organic loading rate. Figure 6 shows the effect of cycle length on cumulative and daily methane production. Cycle length did not have a significant effect on reactor perform- ance (Table 4). Fluctuations in any liquid constituent or gas production were wider as the cycle time increased, but over- all removals or cumulative methane production were quite close among the different cycles. Since cumulative methane production was nearly equal, psychrophilic anaerobic diges- tion extracted the same amount of energy from the substrate at each cycle time.

    Sludge acclimation In the start-up run, digesters were inoculated with fresh anae- robic sludge obtained from a milk processing plant or muni- cipal sludge digester operated at 35C. It was expected that the inoculum sludge in the SBR would acclimatize to swine manure slurry with operating time. Figure 7 compares the SBR responses as sludge acclimatizes. Anaerobic sludge in digesters 7 and 8 in the start-up run (Mass6 et al. 1993) were exposed to swine manure slurry and low temperature for the first time. During this run there was a long lag phase in the biogas production during the feeding period. In run 1, the same sludge had been exposed to swine manure slurry and low temperature for a period of 3 months. Reactors 5 and 6 in run 1 were fed about the same organic loading rate as reactors 7 and 8 of the start-up run. The SBRs with an older acclimatized sludge had (i) a shorter lag phase and a substan- tially higher methane production rate and ( i i ) substantially lower concentrations of SCOD and acetic and propionic acids at the end of the react period. These experimental results indicated that sludge acclimation has a significant influence on the process response. Figure 8 compares the cumulative methane production for each consecutive cycle during run 2 for reactors 9 and 10. This figure indicates that the initial methane production rate and the total cumulative methane production for each cycle increased after each suc- cessive cycle, and the lag phase at the beginning of the cycle diminished as the test progressed. These results clearly indi-

    Fig. 5. Comparison of sequencing batch reactor (SBR) operating performance in test run 2, for different cycle lengths. SBRs were fed three times a week. (---) SBRs 5 and 6 were a 28-d cycle; ( . . .) SBRs 9 and 10 were a 14-d cycle.

    0 10 20 30 40 50 Time (d)

    cate that microorganisms' acclimation to low temperatures and swine manure slurry was taking place.

    Biogas production and overall performance The biogas produced in test runs 1 and 2 was of high quality with a methane concentration between 75 % and 80%. Table 4

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    Fig. 6. Effect of cycle length on cumulative and daily methane productions. Sequencing batch reactors (SBRs) were fed once a week. (---) SBRs 9 and 10, cycle length of 14 days; ( . . .) SBRs 11 and 12, cycle length of 28 days.

    Fig. 7. Comparison of sequencing batch reactor (SBR) performance for different acclimatization times (no mixing). (---) SBRs 7 and 8, with a loading rate of 1.20 g COD/(L. d), (Mass6 et al. 1993); ( . . .) test 1, SBRs 5 and 6, with a loading rate of 1.22 g COD/(L .d).

    0 10 20 30 40 50 60

    Time (d)

    gives the methane production as a function of unit mass of VS fed to the digester. The CH4 production exceeded 0.48 L/g VS for most of the reactors. Methane productions obtained in this study were substantially higher than methane production from swine manure obtained by digestion at 35C in continuous flow digesters by Kroecker et al. (1979), who reported methane production of 0.45 L CH4/g VS added for a loading rate of 2.5 kg VS/(m3. d), and by Hashimoto (1983), who reported 0.42 L CHJg VS added for a loading rate of 2.5 kg VS/(m3 . d).

    The higher methane production per gram of VS fed to the SBRs obtained in this study could be due to (i) the lower organic loading rate and longer residence time of the manure slurry in the bioreactor; (ii) the measured VS in the influent being lower than the actual VS concentration (this inaccuracy is due to the volatilization of some VA and other soluble organics during the VS determination); and (iii) the mea- sured methane flow rate including the methane produced from microorganism decay. Another possible reason could be that the lower operating temperature and the absence of mixing maintain higher concentrations of hydrogen and car- bon dioxide in the liquid phase. As a result, more carbon dioxide can be converted to methane by the hydrogen utiliz- ing methanogens. Also, with the continuous flow anaerobic

    Time (d)

    processes previously studied, some C02 , H2, and CH4 were lost in the digester effluent. The production of methane was not the main objective of this work, but it is very useful to compare the performance and stability of the bioreactors under different operating strategies. The steady production of methane per unit mass of VS fed indicates that anaerobic digestion of swine manure at 20C in the laboratory-scale SBR digesters was a stable process.

    TCOD removal ranged from 41% to 83 % and the VS removal ranged from 46 % to 84 % (Table 4). Results for V S and TCOD were highly variable due to sampling variation caused by rapid settling of heavy particulates as noted earlier. Some samples had less solids than others. This affected the VS and TCOD determinations as well as the cal- culated methane production per gram of VS. The SCOD test results were consistent. High SCOD removal was achieved during most of the runs. Its removal ranged from 84 % to 93 % .

    Reduction in swine manure slurry odours was one of the objectives of this study. The major volatile compounds that produce odours in animal manure slurries are VA, amines, carbonyls, esters, hydrogen sulphide, and ammonia. Labora- tory staff observed that test runs that achieved nearly com- plete removal of VA and 70-96 % removal of SCOD produced treated manure that was relatively odourless compared to raw manure. A large reduction in SCOD may result in complete utilization of amines, carbonyls, and esters. The

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    Fig. 8. Comparison of cumulative methane production for four successive cycles in test run 6, sequencing batch reactors (SBRs) 9 and 10, and 11 and 12. Each cycle lasted 14 days. (---) cycle 1; ( . . .) cycle 2; (---) cycle 3; (-) cycle 4.

    SBRs 9 and 10 25 o I

    Conclusions

    Psychrophilic anaerobic digestion in sequencing batch reac- tors is able to successfully treat highly concentrated swine manure wastes. The following conclusions may be drawn from this work. 1. The process is stable with organic loading rates rnain-

    tained at o r below 1.6 g C O D per litre of sludge volume at the beginning of a cycle.

    2. Methane production of at least 0.48 L CH, per gram o f volatile solids will occur.

    3. Soluble C O D removals of 8 4 % or better w e r e achieved. 4 . Volatile solids destructions of 64% or better were achieved

    in well-acclimatized reactors. 5. Cycle times from two to eight weeks had n o effect on the

    performance of the reactors. 6 . Intermittent feeding frequencies from one to three times

    per week had no effect o n the performance o f the reactors.

    Acknowledgements

    Financial support by the FtdCration des producteurs de porcs du Quebec, the technical support by M. Lemieux, C . Defelice, A. Olson, D . Lu , and G. Vitali, the artwork by R. Pella, and construction of the SBRs by the Engineering Laboratory staff of the Centre for Food and Animal Research a r e appreciated.

    References

    Time (d)

    actual degree of reduction in odour intensity was not deter- mined because the techniques recommended to measure odour intensity are complex, subjective, and time consum- ing, and could not feasibly be used within the time frame of this study. Quanitification of odours will be addressed in future studies.

    The SBR sludge had excellent settling characteristics. In the SBRs that were not mixed, there was a clear interface between the liquid and sludge bed zones. A thick layer of sludge was observed at the bottom of the digester. At the end of the react period when the biogas production was very low, the demarcation between the liquid and solids was even more evident. In the SBRs that were mixed, there was no dis- tinguishable supernatant and sludge zones. F o r these SBRs, when mixing was stopped at the end of the react period, the sludge blanket completely setled to the bottom of the SBR. Therefore the SBR provide excellent settling conditions to retain the slow growing microorganisms.

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