Evaluation of Two-stage Anaerobic Sequencing Batch

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Anaerobic digestion is a controlled biodegradationprocess that converts organic matter inwastewater into biogas. Conventional digestersused for animal wastewater treatment include

continuously stirred-tank reactors (CSTR) and plug-flowreactors (Varel et al., 1977; Rorick et al., 1980; Hills andMehlschan, 1984; Lo et al., 1984 and 1985; Lo and Liao,1985; Wohlt et al., 1990). In these two types of digesters,hydraulic retention time (HRT) equals solid retention time(SRT) and active biomass is removed from the digester in

the effluent on a daily basis. The HRT needs to be longenough to ensure a sufficient SRT in the digester so that aviable bacteria population necessary to complete theanaerobic digestion process is maintained. The minimumSRT varies with the digester temperature and generallydecreases with the increase of temperature (Pfeffer et al.,1967). The Anaerobic Sequencing Batch Reactor (ASBR)is a suspended-growth, biomass-retaining reactor that hasbeen found to be more cost-effective for treating diluteanimal wastewater than conventional digesters (Dague andPidaparti, 1992; Schmidt and Dague, 1993; Zhang et al.,1997a). The ASBR treats wastewater in small batches and

operates in four phases (feed, react, settle, and decant) ineach treatment cycle. The biomass-retaining capabilityallows the ASBR to treat wastewater in a short HRT whilemaintaining a long SRT. Zhang et al. (1997b) have foundthat the ASBR is effective in treating dilute swine manureat 35°C with a HRT as short as three days.

Anaerobic digestion systems consisting of the ASBRs intwo-stage configurations were studied by Dugba andZhang (1999) for treating dairy wastewater. In their study,two thermophilic (55°C) mesophilic (35°C) systems were

evaluated against one mesophilic (35°C) mesophilic (35°C)system at two HRTs (three and six days) and five volatilesolids (VS) loading rates (2, 3, 4, 6, and 8 g/L/d). The VSreduction of the three systems tested varied from 26.1% to44.2%. With the same design configurations, compared tothe mesophilic-mesophilic system, the thermophilic-mesophilic system removed 8 to 12% more VS at thesix-day HRT and 8 to 14.6% at the three-day HRT. Dugbaet al. (1999) also developed a dual-substrate computersimulation model for predicting the performance of two-stage ASBR systems for animal wastewater treatmentunder different operating conditions. The model providesan effective tool to aid in the study and design of suchanaerobic treatment systems. In the study presented in thisarticle, further evaluation of the two-stage ASBR systemsin treating screened dairy and swine manure was conducted

to compare the digestion properties of these two types of manure and generate more experimental data for validatingthe computer model developed for the ASBR systems.Furthermore, the effects of anaerobic treatment on thegeneration of odorous sulfur gases in the effluent storagewere also evaluated in a quantitative manner. Theoreticallyspeaking, a well-controlled digestion process degrades the

EVALUATION OF TWO-STAGE ANAEROBIC SEQUENCING BATCH

REACTOR SYSTEMS FOR ANIMAL WASTEWATER TREATMENT

R. H. Zhang, J. Tao, P. N. Dugba

ABSTRACT. Anaerobic treatment of screened swine and dairy manure was studied in the laboratory with two-stageanaerobic sequencing batch reactor (ASBR) systems. The effects of anaerobic treatment on odor control in subsequent manure storage units were evaluated. One thermophilic (55°C) mesophilic (35°C) system (II) was evaluated against onemesophilic (35°C) mesophilic (35°C) system (I) at a system hydraulic retention time (HRT) of six days and four volatilesolid (VS) loading rates (1, 2, 3, 4 g/L/day). Generally, anaerobic digestion under all the test conditions resulted inhigher solids reduction in swine manure than in dairy manure. The thermophilic-mesophilic system had a better

performance in treating dairy and swine manure with 6 to 15% more VS removal than the mesophilic-mesophilic system.The headspace gas analysis results using manure storage jars showed that both systems were effective in reducing thegeneration of odorous sulfur gases during storage. The untreated dairy and swine manure exhibited strong offensiveodors with high hydrogen sulfide (H 2S) and mercaptan concentrations detected in the headspaces of storage jars. Theanaerobically treated manure, however, showed minimal residual odors while in many cases, H 2S and mercaptans werenot detectable. With the consideration of its better capability for destructing fecal bacteria in animal manure, thethermophilic-mesophilic ASBR system is more advantageous than the mesophilic-mesophilic ASBR system for treatinganimal manure. However, the higher energy requirement for heating the reactors in the former system needs to be

considered when selecting thermophilic vs. mesophilic anaerobic digestion systems. Keywords. Anaerobic digestion, Anaerobic sequencing batch reactors, Odor control, Dairy, Swine, Manure.

Article was submitted for publication in February 1999; reviewed andapproved for publication by the Structures & Environment Division of ASAE in August 2000. Presented as ASAE Paper No. 98-4107.

The authors are Ruihong Zhang, ASAE Member, Associate Professor,Jun Tao, Research Assistant, Biological and Agricultural EngineeringDepartment, University of California, Davis, California; and Prince N.Dugba, ASAE Member, Engineer, Murphy Family Farms, Inc. (formergraduate student, University of California, Davis, Calif.). Correspondingauthor: R. Zhang, University of California, Biological and Agricultural

Engineering Dept., Davis, CA 95616, phone: 530.754.9530, fax:530.752.2640, e-mail: <[email protected]>.

Transactions of the ASAE

© 2000 American Society of Agricultural Engineers 0001-2351 / 00 / 4306-1795 1795VOL. 43(6): 1795-1801

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vast majority of compounds that contribute to odors.Therefore, the completely digested manure is supposedlystabilized and should undergo little further decompositionin subsequent storage units. Human panels have indicatedthat anaerobically digested manure is less offensive thanundigested manure (Welsh et al., 1977; Pain et al., 1990).There will be residual odor in the digested manure, butwith regard to the volatile odors, there will be a significant

reduction via this anaerobic digestion process (Vetter,1994). However, quantitative information on the effect of anaerobic digestion on odor reduction of animal manureduring storage is scarce in the literature. Research isneeded to quantify such effects so that livestock producerscan have a scientific basis to make decisions withappropriate expectations when using anaerobic digestion asan odor control method.

Quantitative odor measurement has been long regardedas a difficult task. Observed manure odor is made up of many individual odorants interacting with one another.Most odorants occur as a result of incomplete anaerobicdecomposition of organic matter. These include ammonia(NH3), hydrogen sulfide (H2S), and volatile organiccompounds, such as volatile fatty acids (VFA), aldehydes,alcohols, amines, mercaptans (R-SH), sulfides, indoles, and

skatoles (ASAE, 1998). It is difficult to accurately describeodor quantity or quality on the basis of one or several of the primary constituents. However, several researchershave reported that the presence and concentrations of several principal manure odor components are indicative of total odor quantity and quality (Burnett et al., 1969; Barthet al., 1974). Concentrations of headspace gases, such asH2S and mercaptans, and VFA in the manure can be usedto indicate the odor intensity of manure undergoinganaerobic decomposition (Zhang et al., 1997b).

The objectives of this study were to: (1) evaluate theperformance of two-stage ASBR systems with differenttemperature configurations in treating both dairy and swinemanure; and (2) quantify the odor reduction in dairy andswine manure during storage as a result of anaerobic

digestion.

MATERIALS AND METHODSWASTE COLLECTION AND PREPARATION

The dairy and swine manure used as digester feed in thisstudy were collected from dairy and swine research farmsof the University of California at Davis. The dairy cowswere fed with four parts alfalfa cubes, three parts grainpellet (14% protein) and two parts beet bulbs. The swinewere fed with a corn-based ration. The manure collectedfrom the farm was screened through a No. 10 screen with2-mm openings. The screened manure was stored inairtight five-gallon buckets in a freezer at –18°C untilready for use. Each bucket of screened manure was thenthawed at room temperature (22 ± 2°C) for two days and

diluted with tap water to yield a desirable VS content. Thediluted manure was kept in a refrigerator at 4°C for nomore than six days while being used to fill the holdingtanks for the anaerobic reactors. The holding tanks werekept at room temperature.

ANAEROBIC DIGESTION SYSTEMS

Two parallel anaerobic digestion systems (Systems I andII), each consisting of two ASBRs in series, were tested.The two reactors (R1 and R2) in the first system wereoperated at two different temperatures, 55°C and 35°C,respectively, and the two reactors in the second systemwere at the same temperature of 35°C. The reactors are thesame reactors used by Dugba and Zhang (1999). Each

system had a total liquid volume of 15 L divided betweenthe first- and second-stage reactors with 1:4 volume ratio,which was selected based on the previous research findingsof Dugba and Zhang (1999) for different volume ratios. Athree dimensional view of the experimental system isshown in figure 1. The well-mixed feed in the holding tank was pumped into the first-stage reactors while theireffluents were pumped into the second-stage reactors. Eachreactor was intermittently mixed for 3 min each hour. Eachbatch treatment cycle was four hours during which 0.5 hwas used for feeding and decanting (15 min for each), 2.5 hfor reacting, and 1.0 h for settling. The operation of theentire system was automated using a computer-basedcontrol system. A custom-made level sensor was used ineach reactor to ensure precise reactor feeding anddecanting. The computer control and sensor systems for the

anaerobic reactor systems were described in detail byDugba and Zhang (1996).

Four system VS loading rates of 1, 2, 3, and 4 g/L/daywere tested with each system at a six-day HRT. Bothsystems were fed and decanted six times per day. At eachfeeding, 416 ± 2 mL substrate was pumped in after thesame amount was decanted from the system. The samplesfrom the influent, effluent, and mixed liquor of each reactorwere analyzed for total solids (TS), volatile solids (VS),and volatile fatty acids (VFA) using Standard Methods(APHA, 1992). Ammonia-nitrogen and pH were measuredwith an Accumet gas-sensing electrode and an Accumet pHmeter, respectively (Fisher Scientific, Pittsburgh, Pa.). Thebiogas production rate of each reactor was measured dailyusing a wet tip gas meter. The biogas samples were taken

from the gas collection line of each reactor once a week and analyzed with a gas chromatograph (GC) (HP5890A,Hewlett Packard, Avondale, Pa.) for the contents of methane (CH4) and carbon dioxide (CO2). A thermalconductivity detector (TCD) was used with the GC. The TSand VS reductions and pH of each reactor were measuredtwice a week to monitor the performance of the reactors.After all the reactors had reached a quasi steady-statewhich was characterized by a less than 5% variation indaily biogas production during a one-week period, themeasurements of all parameters were then repeated threetimes for each VS loading rate by performing themeasurements on three consecutive days. On average, ittook three to four HRTs to obtain the steady state for eachexperimental treatment. The data reported in this article onthe solids reduction and biogas production rate are the

average of three repetitive measurements. The reduction of solids (TS or VS) in the manure after treatment wascalculated as the difference of solid concentrations (TS orVS) in influent and effluent minus the accumulation of thesolids (TS or VS) in the reactors.

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EXPERIMENTAL SETUP FOR HEADSPACE

GAS ANALYSIS OF STORED MANURE

The influent and effluent of each anaerobic systemoperated at the VS loading rate of 2 g/L/day was collected

as the untreated and treated manure for headspace gasanalysis during a four-week storage period. Twenty liters of untreated (raw) manure and 20 L of treated manure werethen stored in 12 L jars (10 L in each jar, two jars for eachmanure) at room temperature (22 ± 2°C). All storage jarswere sealed with screw caps to assure anaerobic conditionsduring storage. Gas samples were taken weekly from theheadspaces of the jars and analyzed for odorous sulfurgases.

For the dairy manure experiments, the gas samples wereanalyzed for hydrogen sulfide (H2S) and methyl mercaptan(CH3SH) using a gas chromatograph equipped with asulfur chemiluminescence detector (SCD). The gas in theheadspace was sampled with a syringe through a rubberseptum placed on the top of each jar. During the four-week storage period, all the jars were kept sealed. The measured

gas concentrations were therefore cumulativeconcentrations. The GC went out of order right before thestorage test for swine manure was started. Gas detectortubes for H2S and mercaptans (R-SH) had to be usedinstead. The detection ranges were 1 to 150 ppm and 100 to2000 ppm for H2S, and 1 to 10 ppm for mercaptans. Eachmeasurement was performed by inserting a detection tubethrough a gas sampling port on the top of the jar and

placing the inlet of the detection tube about 4 cm above theliquid surface. The measurement took about 1 min. Due tothe air exchange between the headspace of each jar and theambient environment when the sampling port was opened,

the headspace gas concentration measured only representsthe cumulative concentration during one week when the jarwas closed between the gas measurements. At each gasmeasurement, 2 min were allowed for each jar to vent allthe gases in the headspace before the jar was closed againby sealing the gas sampling port. For both dairy and swinemanure, in addition to headspace gas analysis, a 100-mLliquid sample was taken before and after the storage, fromeach jar through a liquid sampling port located near thebottom of the jar. The liquid samples were analyzed for pHand volatile fatty acids (VFA). The liquid in the jar wasmixed rigorously by manually shaking the jar before aliquid sample was drawn in order to obtain a homogeneoussample.

RESULTS AND DISCUSSIONPERFORMANCE OF THE ANAEROBIC DIGESTER SYSTEMS

The characteristics of the screened swine and dairymanure are shown in table 1. There are large variations inthe characteristics of the raw manure. The VS and TS ratioof both types of manure are in the range of 70 to 80%.

The TS and VS reduction and biogas production rates of individual reactors and systems at six-day HRT for the two

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Figure 1–Two-stage ASBR systems for treating animal manure in the laboratory.

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types of manure are shown in tables 2 and 3. The VSreduction and biogas production rates of the two systems

are shown in figures 2 and 3 for comparison purposes.With the dairy manure at the VS loading rates of 1, 2, 3,and 4 g/L /day, system I (mesophilic-mesophilic ASBRsystem) achieved 30.0%, 41.5%, 30.9%, and 26.1% VSreduction, respectively; system II (thermophilic-mesophilicASBR system) achieved 40.6%, 47.5%, 41.7%, and 38.9%VS reduction, respectively. With all four VS loading rates,system II yielded 6.0 to 12.8% more VS reduction thansystem I. Both systems achieved the highest VS reductionat the VS loading rate of 2 g/L/day. With the swine manureat the same four loading rates, system I achieved 73.0%,71.0%, 56.5%, and 54.7% VS reduction, respectively;system II achieved 84.0%, 86.0%, 68.7%, and 65.5% VSreduction, respectively. Again, system II performed betterthan system I with 10.6 to 15.0% more VS reduction. Asshown in table 2 and figure 2, the VS reduction for both

swine and dairy manure showed decreasing trends when

higher loading rates were applied. Also, the two anaerobicsystems removed 25.6 to 43.4% more VS from swinemanure than from dairy manure. This result agrees with theprevious research findings reported in the literature. TheVS reduction of 20 to 50% from dairy manure by anaerobicdigestion was reported by Hill (1982). Swine manure has

higher biodegradability than dairy manure (Hill, 1982).


Table 1. Manure characteristics of screened dairy and swine manure

TS (%) VS (%) VS/TS (%) pH

Dairy manure 3.5- 4.5 2.5-3.5 70-79 6.1-6.8

Swine manure 2.0-3.9 1.7-3.1 72-80 5.5-6.5

Table 2. Total and volatile solids reduction of two anaerobic digestion systems at six-day HRT

Reactor System Reactor System Reactor System Reactor System

I* II† I II I II I II

TS Reduction (%)

Reactor 1 gVS/L/day 2 gVS/L/day 3 gVS/L/day 4 gVS/L/day

Dairy R1 14.2 18.5 17.9 18.3 9.8 14.4 11.3 13.4

Manure R2 9.1 12.3 14.0 17.4 14.4 18.2 8.9 16.6System 23.3 30.8 31.9 35.7 24.2 32.6 20.1 30.0

Swine R1 40 43.3 42.6 52.5 26.0 33.8 24.1 30.7

Manure R2 17.1 19.5 13.7 15.0 18.6 21.0 19.3 20.7System 57.1 62.8 56.3 67.5 44.6 54.8 43.4 51.4

VS Reduction (%)


Dairy R1 18.4 24.8 23.4 24.3 12.7 18.4 14.6 17.4Manure R2 11.6 16.2 18.1 23.2 18.4 23.3 11.5 21.5

System 30 40.6 41.5 47.5 30.9 41.7 26.1 38.9Swine R1 51.0 58.7 53.1 65.4 33.0 42.4 30.4 39.1Manure R2 22.1 25.3 18.1 20.5 26.5 26.3 24.3 26.4

System 73.1 84.0 71.2 85.9 56.5 68.7 54.7 65.5

* 1:4 volume ratio, two-stage mesophilic-mesophilic ASBR system.† 1:4 volume ratio, two-stage thermophilic-mesophilic ASBR system.

Table 3. Biogas production rates of two anaerobic digestion systems at six-day HRT

Reactor System Reactor System Reactor System Reactor System

I* II† I II I II I II

Biogas Production Rate (L/L/day)


Dairy R1 0.83 0.97 1.55 1.64 1.41 2.06 2.19 2.6Manure R2 0.13 0.16 0.30 0.39 0.52 0.65 0.43 0.81

System 0.27 0.32 0.55 0.64 0.7 0.94 0.78 1.17

Swine R1 1.82 2.27 3.72 4.48 4.67 5.18 5.61 6.57Manure R2 0.20 0.25 0.30 0.32 0.83 0.81 1.12 1.11

System 0.52 0.65 0.984 1.152 1.60 1.68 2.02 2.2

* 1:4 volume ratio, two-stage mesophilic-mesophilic ASBR system.† 1:4 volume ratio, two-stage thermophilic-mesophilic ASBR system.

Figure 2–The VS Reduction of two anaerobic treatment systems fortreating swine and dairy manure (System I, 35°C to 35°C; System II,

55°C to 35°C).

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Safley and Westerman (1990) reported that swine manurehad higher methane yield (0.36-0.52 m3 /kgVS) than dairymanure (0.17-0.24 m3 /kgVS).

The data on biogas production rate (table 3 and fig. 3)also showed that system II (thermophilic-mesophilic ASBRsystem) outperformed system I (mesophilic-mesophilicASBR system) and both systems had much better overallperformance with the swine manure than with the dairymanure. The biogas production rate was calculated as thetotal volume of biogas produced per unit total reactorvolume in the system per day (L/L/day). System IIproduced significantly more biogas than system I. Also, thesystem fed with swine manure produced twice as muchbiogas as the system fed with dairy manure. Thecompositions of biogas produced from all the reactors weresimilar, containing 62 to 66% CH4 and 28 to 31% CO2.

HEADSPACE GAS ANALYSIS OF UNTREATED AND TREATED

MANURE DURING STORAGEThe headspace concentrations of H2S and CH3SH in theuntreated and treated dairy manure during the four-week storage period are shown in figures 4 and 5. The pH and

VFA concentrations in the dairy manure before and after

the storage are shown in table 4. Each value presented isthe average of two determinations. The untreated dairymanure had initial VS of 12.1 g/L. The treated dairymanure from systems I and II had initial VS of 7.1 and6.2 g/L , respectively. For the untreated manure, very highconcentrations of H2S and CH3SH were detected duringthe storage. Both gases showed a strong upward trend overthe storage period with the highest concentrations reachedin the fourth week (5,632 ppm and 176 ppm, respectively).In contrast, the H2S in the headspace of the treated dairymanure was below 3 ppm except for the H2S of system IIeffluent in the second week, which was 8.3 ppm (fig. 4),still very low compared to the H2S of the untreated dairymanure on the same day of measurement (435 ppm). TheCH3SH was not detected during the entire storage period

for the treated dairy manure as shown in figure 5. About99% reduction in the headspace H2S and CH3SHconcentrations was achieved.

The untreated swine manure had initial VS of 12.5g/L.The treated swine manure from systems I and II had initialVS of 3.5 and 1.6 g/L, respectively. Figure 6 shows theheadspace H2S concentrations of the untreated and treatedswine manure. The untreated swine manure had strongoffensive odors characterized by its high concentrations of H2S and mercaptans in the headspaces of the jars. Theconcentrations of mercaptans in the headspaces of theuntreated swine manure exceeded the detection limit of 10 ppm except on the first day of storage. The H2Sincreased from 5 ppm at the beginning of the storage to ashigh as 500 ppm in the third week (fig. 6). In contrast, noH2S and mercaptans were detected in the headspace of the

treated swine manure during the entire storage period. Bothanaerobic treatment systems I and II effectively reducedthe generation of sulfur gases in the manure, and thereappeared to be no significant difference in the overalleffects on the reduction of sulfur gases between these twotreatment systems. The reduction of headspace H2Sconcentration was over 95% after anaerobic treatment.

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Figure 3–Biogas production of two anaerobic treatment systems fortreating swine and dairy manure (System I, 35°C to 35°C; System II,55°C to 35°C).

Figure 4–The H2S concentration in the headspaces of storage jarscontaining treated and untreated dairy manure.

Figure 5–The CH3SH concentration in the headspaces of storage jarscontaining untreated and treated dairy manure.

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The VFA concentrations in the untreated manureexceeded the VFA concentrations in the treated manure bya wide margin as shown in table 4. Higher VFA

concentrations corresponded to the higher concentrationsof the odorous sulfur gases measured in the headspace of storage jars. This agrees with the past research findings(Williams, 1984) that the VFA could be used as anindicator of offensive odors. During the storage period, thepH of all the manures decreased slightly as shown intable 4.

As shown clearly in this study, anaerobic digestiongreatly reduced the generation of odorous gases in thestored animal manure. A study by Pain et al. (1990) alsoshowed that anaerobic digestion reduced the odor emissionrate from land application by 91% when the digested swinemanure was compared with pit-stored swine manure. Ourstudy further quantitatively verified the beneficial effects of anaerobic digestion on odor control. In particular, ourresults showed that both dairy and swine manure could be

effectively treated with the two-stage ASBR systems forodor reduction.

CONCLUSIONSThe thermophilic-mesophilic ASBR system (II) had a

better performance, with higher solids (TS and VS)reduction and biogas production rate than the mesophilic-

mesophilic ASBR system (I) at the same VS loading rates.Compared with system I, system II removed 6 to 15%more VS for dairy and swine manure. With theconsideration of its better capability for destructing fecalbacteria in animal manure, the thermophilic-mesophilicASBR system is advantageous than the mesophilic-mesophilic ASBR system for treating animal manure.However, the higher energy requirement for heating the

reactors in the former system needs to be considered whenselecting thermophilic vs. mesophilic anaerobic digestionsystems. Swine manure had higher digestibility than dairymanure. Under all the conditions tested in this study, VSreduction was 54.7 to 86.0% for swine manure and 26.1 to47.5% for dairy manure.

The gas analysis results from the subsequent storageexperiments showed that anaerobic treatment using bothanaerobic treatment systems tested effectively reduced thegeneration of odorous sulfur gases during the manurestorage. The raw dairy and swine manure exhibited strongoffensive odors with high H2S and mercaptanconcentrations detected in the headspaces of storage jars.The treated manure, however, showed little residual odorswhile in many cases, H2S and mercaptans were notdetectable. This study verified the beneficial effect of

anaerobic digestion on odor reduction of animal manure.

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Figure 6–The H2S concentration in the headspaces of storage jarscontaining untreated and treated swine manure.

Table 4. VFA and pH in dairy and swine manure during storage

VFA (mg/L) pH

Week 0 Week 4 Week 0 Week 4

Dairy Manure

Untreated 1,443 1,800 7.5 7.2Treated (System I) 270 348 7.5 7.3Treated (System II) 192 336 7.2 6.7

Swine Manure

Untreated 1,030 4,092 5.9 5.8Treated (System I) 282 495 7.5 7.4

Treated (System II) 297 385 7.8 7.7

Note: The VS loading rates of both anaerobic digestion systems are2 g/L/day.

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Evaluation of Two-stage Anaerobic Sequencing Batch