Agricuhural Wastes 8 (1983) 155-165

The Effect of Solids-Separation Pretreatment on Biogas Prodaction from Dairy Manure

K. V. Lo, N. R. Bulley, P. H. Liao & A. J, Whitehead

Bio-Rcsource Engineering Department, University of Briti~;h Columbia.

2357 Main Mall, Vancouver. BC. V6T I W5, Canada

A BSTRA C T

The t(ff~'et o/ liqtdd solids separation pretreatment on methane prmhtetion ./i'om mesophilie (30~C) anaerobic digestion o f dai O' cattle manttre was htrestigated it7 the laboratory. Sereenhtg otH the coarse solids /i'om the waste he/ore digestion had a re#ritual eff~,ct on the rate q / hiogas pr¢,,duction .[or equal roiatih, solids Ioadhtg rates at a 16-~hty h.rth'aulic retention time ( t tRT) . At a 12-. 10-. 8., and 6-day HRT, a sign([ieant im'rease was,/ound in the biogas prothtetion rate per litre qf digester or pet" grant o./ rolati/e solids athh'djor the screened tnamtre orer the unsereened tnanure, 7;tie nlethtttle prodttcliOtl rate./rom ,~ert'ened waste (0,50 litre Clla/litre in digester per day, at 6 tht.lW H R T ) wa:; al~proximate D' double that uhta#led./)'om unscreened shtrt:v Jor similar retention times, l)igesler t:ffieien~T, measured ht terms oj'lu'reentage re~htetiot; o f COD, I S arid I,:~', was greatest at I# atul ,S' days t lRT.

The resu/ls SUl)llort the eom'ept that a liquid solhh s~7~aration pretre~ttment st~T eotth[ sign([ieant[y re~ha'e the rohttne o/ digester reqttired./or a.fitrm, without decreasittg /ffogas prodt;clion.

1 NTR O D U CTi ON

In a world of dwindling !bssil fuel reserves, the concept of converting a rer;ewable resource into either a liquid fuel or a combust ible gas has become an attractiv,~ one. Biogas production from animal manure is one

155 Agricultural Wastes 014~..~607/83~$03'00 ~ Applied Science Publishers Ltd, Eqgland. 1983, Printed in Great Britain

156 K. V. Lo, N. R. Bulley, P. H. Liao, A. J. Whitehead

of the energy conversion alternatives that has received increased attention in recent years.

The present state of the art of biogas production has ~'en reviewed by several researchers (Hashimoto et aL, 1979; Hashimoto et al., 1980; Lehman & Wellinger, 1980; Smith, 1980). Most of the existing on-farm anaerobic digesters are being operated using cattle-waste feeds with the total solids (TS) content in the range of 8-12 ~,,. Many of the digesters operated under these conditions are having materials-handling problems typified by difficulties in manure-slurry pumping and solids settling, out in the digesters (Abeles et al.. 1978: Bartlett eta/ . , 1977: Barllctt e/al . , 1980; Ecotope Group. 1977). Additional water is sometimes added when attempting to solve these problems, but this results in an increase in digester wflume requirements and hence higher capital cost,

In the convcntional anaerobic digestion process the volatile solids (VS) breakdown in cattle waste is usually in the range of 15 30"~i (Hills, 19~.0: I;.S. National Academy of Sciences. 1977). It is logical to assume that the principal organic fractions being metabolized consist of tile orgaMc molecules in solution and the very fine particles having a high surface ~o xolttme ratio, which are readily attacked by bacterial enzymes.

if it is hypothesized that a very high percentage of the VS fraction, which is destroyed in most digesters, is contained in this liquid t'ractiom then a liquid solids separation pretreatmenl of the wa:;te should produce a filtrate which on digestion will still yield a high w)lume of biogas but which will not have the :¢alnc materials-handling problems. It would also be ,:xpected that tile size of digester needed to treat the screened waste t'r~m~ a given number of animals would be smaller than that lot unsci'ecned waste, with only a small decrease in total biogas production.

l h e objective of this project wits to determine the feasibility of mcrc:tsing the biogas output capability of laboratory-scale digesters by ttsing a solMs-separation pretrealment to remove the less biodegradable portion of the VS in tl.c waste.

M 1:'I t I OI)S

l"eed material

Manure fr~,m a confined IIolstem dairy heM at Blair Farms. Aldergrove. BC, was used for this study. The cows were fed a ration of dairy concentrate (I 6 ",, protein), alfalfit hay, grass and/or con~ silage, salt all(.{

Separation ~JJ'dairy waste for biogas production 157

minerals. No antiobiotics were incorporated into the feed. Manure from the free stall barn was scraped twice daily to the end of the alley. Samples of this manure were collected in plastic pails every four to six weeks and stored in the University Bio-Res,~urce Engineering cold storage room at 4 °C. Before being placed in the cold room, the fresh manure (about 15 '~ TS wet-weight basis) was mixed with an equal volume of water to give a final feed material of about 7.5 ~,,, TS (6 % VS), a level typical of those being recommended in the literature for anaerobic digesters (Lapp et al., 1978).

Experimental design

The study compared biogas production and mesophilic anaerobic dig~ stion efficiency in four bench-scale reactors over a raJ~ge of hydraulic retention times (HRT) from 16 to 4 days. Reactors A and B were loaded with slurry prepared from the i :1 manure-water mixture filtered through a 2.0mm ~No. 10 mesh) and a 2.44mm (No. 8 mesh) vibr~lting ,;creen, respectively. The choice of No. 10 mesh was based on preliminary screening trials aimed at obtaining the desired volatile solids levels for Iced materials. The No. 8 mesh was chosen to match the aperture of a commercially available, rotating screen, liquid-solids separator (Roto- strainer, Hycor Corporation, Lake Bluff, Illinois 60044). Reactor C received unscreened slurry which was diluted to obtain a solids content equal to the feed for Reactor B. Reactor D received unscreened 1:i manure-water slurry and served as the control. Average TS and VS content of the feed to Reactors A through D, respectively, was 4, ! ",~,/TS

" V ./, ,,. and 3. I/,, S, 4.4 ~,, TS and 3.4 VS, 4.4 Y, o TS and 3.4 ",', VS, and 7.3 '), TS • 'rod 5.8 '!,i VS.

Digesters A and B compared the effect of screen size on tiltrate VS content and biogas production rate. Digester'; C and D compared two dilutions ot" unscreened slurry. Digesters B and C compared pretreated and untreated slurries of equal nutrient concentration.

Anaerobic digesters

The reactors were constructed of a,:rylic plastic and had a ~orkil~g capacity of 4 litres. Complete mixing was achieved in each digester by using a DC motor-driven impeller tbr 15 minutes every hour. The digester tempe~'ature was maintained at 30 "C using a thermostatically controlled internal electric heater.

158 K. V. Lo, N. R. t,,,[h.y, P. H. Liao, A. J. Whitehead

Gas collection was accomplished with a fluid displacement system consisting of a graduated cylinder which was connected to a reservoir lilled with saline water. Daily gas yield was measured by equilibrating the gas to atmospheric pressure, as indicated by a mercury manometer, and reading the liquid level in the cylinder.

Digester operation

At start up, two litrcs of the thoroughly mixed eltluents from the anaerobic digesters used in our preliminary sludies and two litres of the prepared fccd m~rterials were added to eacil digester, After a one-week period, daily l~eding began and the loading rates were gradually increased tu the desired level within a four-week period. Prior to daily feeding, each d,gester was thoroughly mixed for 15 minutes. A lixed volume of effluent ~,:ts then v, ithdrawn and prepared feed material of equal volume: added. The HRT was set at ! 6 days for four volume turnovers, then chan.~;ed to 12 days for four cycles, and subsequently to eight and six days for at least three cycles, Four-day HRT operation was ,'Jttemptcd.

Chemical analysis

Chenaical analyses were pertbrmed weekly lbr the feeds and the effluents of all digesters. Analyses tbr TS, VS and pH wcrc carried out according to Stamhn'd ?,lethods (A PItA. 1975). Total Kjeldahl nitrogen (TKN) and ;,.mmonia-N were determined using a block digester and a Te,:hnicon Auto Analyscr !1 according to the method of Schumann et al. ,~1973).

Thc biogas production rates were monitored dail~ and the gas was ;malysed tbr methane content every lwo weeks during each phrase. Gas ~;amples, collected in gas-tigilt syringes, were analysed with a Fisiaer Gas Partitioner equipped with ~, Chromosorb W column. All gas n~easure- mcnts arc expressed at 0'~C and standard pressure (760 mm of ttg). No correction was mz~d,: for water vapor since it was believed to be present in very small qtlanli!ics.

RI/SULTS AND DISCUSSION

The composition of tile dairy manure remained stable over the ten-month period of tile experiment, The average values of ,,he chemical

Separation of dairy iraste.[or biogas produ,'tion 159

compositions of the feed materials and ettluent for each experimental run are presented in Table 1.

All digesters reached a relatively constant 19iogas production rate within 40 days of start up. A new constant rate of biogas production was reached within 10 days of any change in the HR'I-. The biogas production rates reported in Table 2 are the arithmetic mean values for at least thr,:e cycles of operation at each HRT.

The biogas composition from all digesters was 63.5 + !.5 7,, CH4 with the balance beiog CO,. and trace impurities. The methane production rate is presented as litres CH4/litre in digester per day. litres CH,/g VS added per day and litres CH.~/g VS destroyed per day.

Effect of liqtfid-solids separation on gas production

Digester performance was measured in terms of gas production and VS destruction (Tables ! and 2). LNuid- solids separation using the 2.44 mm and 2.00 mm screens removed 39 '% and ,~4 "/,,J ol" the TS and 41 .... ,.,, and 46" ,, of the VS. respectively, from thc I : I manure-water mixture. As expected. the larger screen a')erture retained less solid material. At 16 days HRT, pretreatment of the wastes by screening did not improve gas production relative to unfiltered slurry. The digesters receiving approximately the same VS loading rate (Digesters A, B, C) had the same rate of methane production (I).18 litres CH4/litre/day). At this longer itR'I', the higher coarse-solids content of Digester C did not affect the rate of methane production relative to the screened slurries (l)igestms A and B). The higher VS loading rate of unscreened waste in Digester i, approximately double that of Digesters A and B resulted in only a slightly greater gas production (0.20 litres CH4/litre/day). in terms of biodegradability of the screened versus untreated slurry, methane production per gram VS added and per gram VS destroyed was greater from Reactors A, B and C (having similar loading rates) than t'rom the Control, D.

As the hydraulic retention time was reduced (lind the loa0ing rate thus increased), the effect of liqukt solids separation on gas production became more pronounced. The daily gas yields from Digesters A and B increased from 0-18 to 0.49 litres CH4/litre/day as the itRT decreased from 16 to 6 days and the loading rates increased from 1.75 to 5.5 g VS/litre/day. At 8-day and 6-day HRT the screened waste produced 80 % and 50 % more methane per digester litre, respectively, than the unscreened material having equivalent VS loading (Digester C). Digester 1), receiving it VS

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162 K. Y. Lo, N. R. Bulley, P. H. Liao, A, ,I. Whitehead

loading rate almost twice that of the other reactors, produced less methane less efficiently than Digesters A, B and C at all retention times of less than 16 days.

It is interesting :,o note that with unscreened slurry (Digesters C and D), as the HRT was reduced from 16 to 6 days, tile maximum methane production rate was obtained at about 8 days HRT and remained relatively constant at the shorter retention times. Varel et al, (1980) obtaincd similar results with digestion of beef cattle waste at 30°C, rcporting maximum gas production rate at 9 days HRT, By comparison. the trend in methane production from screened slurry (Digester A) showed a steady increase in response to reduced HRT (or increased loading rate). These data suggest that in unscreened slurry the rclatit:nship between bacterial population and nutrient availability reaches optimal ellicicncy at an HRT of about eight days, whereas in scrccnt'd slurry this condition is achieved at an HRT of less than six days.

Attt mpts w,sre made to I urthcr reduce the l-iRq to four days. However. within Iw~ days of testing, evidence of a wash-out was observed, as gas l~r~tlucti~n drt~pped sharply in all digesters. In order to prevent further depletion of the bacterial biom:lss from the digesters, the tbur-day HRT runs wclc stupped after three days. "l'hi~ marked reduction in methane production was expected, and agrees with estimations based on the cqu;ttion p.er~cntcd by ( 'hen & | tashimot~ (1978).

It ~lpp,';;~ that ~lt a shorter JtRT. anaerobic digestion using liquid liltratc (tkzcds A and B). has a dctinitc advantage ow;r that using unscreened liquid n~a,lure (feeds (7 and D). The data arc comparable tt) tho~c rcp~rted by ( 'hen et al. 1198(I).

l h c rckllivcly high variation in C O l ) values ~as expected as a very I;trgc dilution factor was in~'olved in the chemical ~tnalysis. Average C O l ) reducti~m oxcr all I tRT runs (n~ d~lta available for 16 days I-IRT) was hi!~hcst in the screcJlcd slurries (approximately 20,,,). followed by the ttn:;crccncd v.';lstc of cquiv:dcnt VS content (I 8 ,',~) and the control slurry ( 13 ",,). (ircatcr ( ' ( )D removal clticicncies w crc achieved at the 8-d~:y and IOtk;y l lRT. regardless of screening or loading rate.

i h e :mma()nizt and TKN data sh()w that. within all digesters, tt)tal nitrogen was c()nscrved Z|lltt the prt)porti()n of amnlonia increa:,ed. Nt) trcatmcnt-spccitic trends ~cre observed. The pH remained very stable throughout the study with influcnt at 7.2 and cl]lucnt at 6.9.

"l'hc rcductit)n ctlicicncics of total s()lid~, and volatile s(>lids w crc

Separation of' dairy waste Jor bit,gas production 163

greatest at 8 to 10 days HRT regardless of treatment. The effect of screening was more evident in the VS conversion data than in the corresponding TS values (Table l), with a trend of decreasing VS conversion with increasing loading rate, For example, at i0 days HRT, VS conversion ranged from 26.8 ~,, for pretreated waste (Digester A) to 18.2~ for diluted unscreened waste (Digester C) to 10.3'~, for the control. The difference in TS conversion between treatments A, B, C and D is less clear, with B and D showing anomalous, very low, cfficiencies at i 6 days and 8 days HRT, respectively. The slight difference in TS removal between screened (Digester A) and unscreened (Digester C) slurries of similar nutrient concentration reflected the difference in biodegradability attributable to the relative proportions of coarse solids in the influents. It appears that even a 16-day HRT of unscreened dairy manure slurry does not provide enough time to allow the bacterial decomposition of the carbon bound up in the coarse-solids fraction, and that the matter which is capable of passing through the No, 10 or 12 screen is the principal biomass fraction converted to methane. Evidently, at briefer retention times, methane production from the liquid filtrate can be facilitated by the more favourable ratio of bacterial mass to nutrients, in the experiments reported here, the limiting factor proved to be the loss of bacterial biomass incurred due to digester operation at an HRT of less than eight days, The results suggest that a further reduction in HRT (with similar daily gas production) could be achieved by means of bacteria',~tnomass retention.

Assuming that the data obtained in this study could be successfully scaled-up to a farm-sized operation, then the results could have a significant impact on the design of full-scale biogas productioxa :,ystems for dairy manure. In our liquid--solid separation step, the total mass of the liquid filtrate was about equal to that of the s,.~lid residue. For any given qt, antity of dairy manure, if liquid solids separation ~ L/S) pretreatmcnt is carried out on the feed to an anaerobic digester, the amount of liquid manure entering the digester could be halved and this would result in a 50 ~0 reduction in the required digester v~lume even if the same retention times were used. Due to the more rapid breakdown of the filtered waste, it is possible to reduce the hydraulic retention fiom the conventional 16 days down to 10, 8 or even 6 days. The corresponding further reduction of the digester volume for 6-, 8- and 10-day HRT would be 63.5, 50 and 37.5'~.ii. Combining the L/S pretrcatment and the shorter HRT would result in .'t digester volume of 18.75'~,, 25';i, and 31.25','~,i of the

164 K, V, Lo, N. R. Bulley, P, H. Liao, A. J. Whitehead

conventional anaerobic digester (for an HRT of 6, 8 and 10 days re:~pectively).

This represents a reduction in digester volume of more than 60'~,,, and would certainly result in a significant decrease in the capital cost of building anaerobic digesters.

We believe that this is an important concept in both animal waste rnanagement and biogas production. Further investigation is currently tmderway on the effects of VS loading rate, HRT, and particle size on the r.,te of biogas production, The impact of liquid-solids separation prctrcatmcnt on the economics of the whole manure management system remains to be investigated.

ACK N OW L [:. D G I:- M E N T

The authors acknowledge the tinat~.cial support provided by the Natural Sciences and linginccring Research Cotmcil of ( ' anada ,

i", EF i 'R E NCt-S

Ancle~. T, P.. Freedman. I). F.. l)¢baer¢. L. A, & l'~llswortil, 1). A. (1978), /:)wrg)' uml economic assc,,,'smenl q/ an,cwrohic dige,',ler,,; aml hiofuels.fi.~r rural WUSlC mwmgemenl, Oasis 2000. University. of Wisconsin Center -Barron ('ounty. Rice l,akc. Wisconsin 54868,

A Incric~.Ill Public ]-]ealth Association CA PHA), (I 975). Stuudard metho~L~lor the cxaminatit,I q/n 'awr end wu,~tcwutcr ( 14th l!dn). A I>ttA. A WWA, W PCF. Washingt,m D('.

Bartlett. I1, i).. Persson, S.. Regan. R, W. & Branding. A, !:., (I 977), Experience.s from oprrating a full size anaerobic digester, l>aper No, 77-4053 presented at tilt: 1977 Anrmal Meeting. ASAE, North Carolina State University. Raleigh. NC, June 26 29. 1977.

Bartlett. Ii. t)., Pcrsson. S., Shin, II. S. & Rcg;11~. R. W. (1980). Biogas generation and ttscs on livestock I,trms. l>aper No. NARS(}-41 I presented at the 198(I Annual Mectiqg. No,'th Atlarttic Reyitm. ASAE. Willimantic. ( 'T, August 3 6. 1980.

('hen. Y. R. & Hashimoto. A. G. (1978). Kinetics of methane fermelmltion, Prec. BiolechnohJgy and Bioenghwcrillg ,S~l'mposium, No. 8. Johll Witty & Sons, Inc., New York. pp. 269 82.

('hen. Y. R.. Varcl. V. II, & tlashimoto. A, G, (1980). Effect of temperature on ll]t2ih;.llle J'~l'll'lentlit Jell k inctics of bee!'c',Jtt!e tll,'llltll'C, Paper presented at the Second Symposium on Biotechnology in I~ll¢l'gy Production ,111(I ('ouservation, ( ia l l inburg. TN.

Separation of dairy waste for biogas production 165

Ecotope Group (1977). Operation of a 50,000 galhm anaerobic digester, Monroe State Dairy Farm, Monroe, Washington, Second quarterly technical status report, December 31, 1977. U.S. Dept. of Energy, Seattle

Hashimoto, A, G., Chert, Y. R., Varel, V. H. & Prior, R. L, (1979). Anaerobic fermentation of animal manure. Paper No. 79.4066, Proc. 1979 Summer Meeting of ASAE and CSAE, June 24-27, 1979. University of Manitoba, Winnipeg, Canada.

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