Agricultural Wastes 8 (1983) 233-249

Performance, Operation and Benefits of an Anaerobic Digestion System on a Closed Piggery Farm

J, Poels, G. Neukermans, P. van Assehe, M. Debruyekere & W. Verstraete

Faculty of Agricultural Sciences, State University of Gent, Coupurc 653, 9~00 Gent, Belgium

ABSTRACT

In February, 1981, a clo.ved piggeryfarm with forty-five productiL~e sows and 300fattening pigs started to use biogas produced in an anaerobic digester treating pig manure. The blogas plant was installed by the ,farmer himself under the supervision of the Faculty of Agricultural Sciences, State University of Gent.

Raw pig manure (5-6 % dry matter) is pumped fr~m the holding pits to a cylindrical reactor (73m 3 working volume) which operates in the mesophilic temperature range with hydraulic residence times of about 40-$0 days and low loading rates, Completely automatedp!ant operation ensures a continuous biogas production, The steady-state production corresponds to I8~20m 3 biogas (65-70%CH,)per cubic metre pig slurry. The biogas replaces fuel oil for heating farrowing units.

During the first year of operalion, the 8600m s of biogas produced replaced half of the fuel oil consumption. After the expansion of the piggery houses to 600 fattening pigs, the farm will be sell:supporting in its fuel ~upply. This will produce a saving of 140000BF a ye:~l'. The investment costs of the digestion system were low (40000PBF or ca. 8000 US dollars) because the farmer constructed the ptant by himself

INTRODUCTION

Anaerobic digestion of piggery wastes for methane production recently started to be practised in Belgium. Escalating energy prices motivate

233 Agricultural Wastes 0t41-4607{$~.,: ~03,00 (~ Applied Science Publishers Ltd, Eniland, 1983. Printed in Great Britain

234

farmer++ to l~g ' fo~ :~. meh can replY' ~nventional fuels. Production Ol':SiO~+:Oni~ fa+mi:~nds a teehnoiogy which assures a continuous bio~s~ pt~uef ion with0ut additional labour, Tile plant operation must only require skills which are easily learned and which integiatc into farm routine.

This paper evaluates an anaerobic digestion system installed on the Deeapmaeker's farm+ near leper in Flanders+ It has been in operntion sioce February+ 1981. The purpose of this paper i,+ to describe the system +is it has been operated in the farm setting. First+ the system is described in terms of construction and process flow. Secondly+ the start up, the biological perl~.~rmanc¢ and the energy balance are discussed,

|)t:iSCI~<J P' l l ON OF T H I). PI,AN']"

Dive, let ¢oJi~trlJ~th)l!

The ~:ylindrical digest~zr, partly dug in. is constructed in brickwork with hollow concrete blocks, 3''he construction of the digester tank comprises three parts: (i) a rcil~f,.~rced concrete bot tom plate (sole); (ii) a cylindrical wall run up with h~fliow concrete blocks with circular formed bar-iron betv;,cen the h~yers and vertical bar-iron in the cavities, the latter filled with :+ light pl;+st,ic concrete m<>rt, r: (iii)a tixed roof consisting uf p:.~fa brJ¢.,~t~d vaullings rcsling on one or two concrete supporting beam.~. In tt~c digester walland root" are a manhtfle+ an inlet fbr the pig slurry, a +iphtm t,irain+ofl' of pig slurry), two passages tk~r the heat exchanger and two, metal l'ramewurk~ in the roof a central one to hang up the mixing me,.qmm~m and a second one in which control apparatus can be installed.

I~; orde; Io be sure lhat lhe inside wall is gastight and to protect the insi~!c a~amst corrosion. ~. coating is necessary, A polyuretharie solution wa,, chosen, it is a I~rushablc thin solutkm :tnd the polymerisatiori is ~:au~.M by luw hunlidity in the air.

The bottom of the reactor is conically shatx:d to peevent the build up of ~,udi'nei]l+ To insul:tle lhe rea,:lor, extruded polyslyrelle-pla~es 16 cm~ with u cki~ed cell sttttcttirc tire used. The .ioiJlis hetwccn the insulation pl~es v+ur~) b lilled with glass wool aim stopped up with g~.ilt], theii tile whole {nsui~ttion was ct+v.'rapI~cd with a wire netting, lightened at difl;erent .t~l'acc~,+ Tke in~,ulaiion is protected by a cement piaster, treated with an e!a,,;lic waterproof paint {(1"5 ml'~] thickness} which rmrticuhtrly prevcnt.,~ vvat.cr lwnetr;ilitPni ill w]iltCi" i>+riods,

An anaerobtc digestion system on a closed piggery ]arm 235

Gas storage

The biogas is collected under low pressure (10-20mbar) in an aluminium tank floating in water, The three components of tire gasholder are: a cylindrical tank, run up in brickwork, filled with water; the gasholder (14m a) floating in water; the guide framing for the holder. The cylindrical open tank is partly underground, The construction is similar to the digester. A layer (5 era) of oil prevents the water from freezing during winter periods. The gasholder is treated with a polyurethane coating as mentioned above. To monitor the amount of gas produced, a dry gasmeter is installed in the house, just before the burner.

Heating system

The digester is heated with hot water circulating in a spiral shaped steel pipe painted with a protective coating against corrosion. The inside diameter of the ~piral is about two-thirds of the reactor diameter, The lowest winding of the spiral is installed 0.5 m from the bottom with a minimum distance of 0.2 m between the windings in order to allow the circulation and/or mixing of the pig slurry. Under normal conditions, the heat exchanger has to compensate the daily heat losses and the daily supply of heat for the fresh pig slurry. The spiral heat exchanger has three windings and a total length of about 30 m.

Mixipg nJechanism

The mixing mechanism is a vertical submersed Archimedean screw revolving at 600rpm in a steel pipe and driven by a motor (l.SkW) in:~talled on the roof of the digester.

The formation of a crust layer of organic material at the liquid surlhce was prevented by mixing 10 15 rain a day. Three days operating without mixing gave rise to a crust layer of 0.5 m and indicated that regular mixing was required.

Feeding and drain off of the piggery manure

A ~;ubmersibte pump (3 kW) supplies the piggery manure to the reactor while a PVC overflow pipe (0,25 m diameter) is used to drain off the effluent. The digested pig slurry is stored in a pit of 160m "~ volume.

236 J. Poeb et al.

S a f e ~ m e a s u r ~

A siphon filled with oil is insta!!ed on the metal framework of the digester roof as a safety precaution against gas overpressureo The gasholder is provided with a special pipe so that if the holder reaches its maximum height, excess gas flows to th+~ atmosphere.

In the gas pipelines, between gasholder and gas burner and between gasholder and digester, wads of steelwool serve as check valves to prevent a blow back of the flame. All the gas pipelines have a suitable slope to drain off the condensate at the lowest points. Metal constructions as, for e:;ample, motors and gasholder, are provided with lightning arresters, The gasburners come withir, the prescribed installation requirements. Pipelines are installed in frost-proof underground channels.

P L A N T LAYOi_IT A N D P R O C E S S I N G

Figure I shows the flow scheme of the biogas plant. The digested pig slurry is stored before spreading on the land at regular times. The supply of the piggery manure and the mixing of the digester works autom.atically t~y means of timers, which can be changed according to the desired plant operation, For example, the daily supply of piggery manure t~kes 2 mJn

~. Digester 6 Fe~:~Jmp |l. Mpxing i~BOhoJili~im ,~lge~i~r • Oor~holdaf "7 Bio~ls. outl,.~t ~:l, Oa$ h~g,d bO~Sr

3 Henl e~r~hanget 8 Gosmeter 1~.0i[ ~ated bo4er l, Mc, nure dralr~ 9 Ma~re d4ch 'J..Mix~g ¢~ve r~ Storngedigeoted 'nonure 10 MJzJn 9 mechanism |S, Piggery rouses heating c~r¢~Jt

. ~ ................ ~ %*'% ~ II ;

~1~l"d: ,':ir"t " : ~ ~ i!~:..~ ,i ~ . , .~ i~:r~ ~:r:: - r - l ~ ~ , ~ / ...... ~. . . . . ~ i.-.< ,,,,,,.. ':

.t ; " " . . . . ~, '> ..,:.i / , . ,~ ' i li.

Fig. I , Flow scheme of the biogas plani.

An anaerobic digestion system on a closed piggery farm 237

and the mixing mechanism works during 15 rain once a day. The water heating the digester is circulated by a pump controlled by a thermostat. The heat exchanger with the circulation pump and the thermostat form a separate circuit connected with the central hea~ing system. The thermostat is placed on the metal apparatus plate in the roof of the digester, together with a thermometer. The sensors of both these instruments are contained in a submersed pipe filled with oil. This pipe projects 2-3 m under the surface of the digester liquid. The biogas is used in heating the digester and the pig houses. An electro-mechanical system controls, with priority to biogas, either a gas or an in-parallel oil burner. The switch-over from oil to biogas occurs automatically. A micro-switch fixed on the guide frame of the gasholder is turned on when the holder reaches the highest position and a magnetic switch excites the electric igniter of the gasburner. When biogas is consumed, the gasholder falls and another micro-switch breaksthe circuit in the lowest position. At the same time, the above-mentioned magnetic switch turns off and another magnetic switch turns on the oil burner. The installation is such that at any time switch-over from manual to automatic operation, and vice versa, is possible.

TESTS OF THE PROCESS AND PLANT

The amount of biogas available for the replacement of conventional farm energy sources depends o , the net biogas production, In order to examine the importance of different management factors on the operation and yield of the digestion, the energy balance of the plant was studied.

The energy required for the operation ofa biogas plant i-~ defined as the process energy (PE). The gross energy production (GEP) minus the process energy i.~ the amount of energy which is effectively available to replace other energy sources; it is the net energy production ( N E P ) o f the plant.

The process energy is composed of thermal process energy (Qtot) and the mechanical or electrical process energy (Eel). The thermal process energy is the result of different components:

Q,o, = Qo + Qv ~" Qz

where: Qo is the energy needed to heat the influent to mesophilic temperatures (W), Q,, is the heat transmission lo~ses through the digester

238 J, Poets et al.

walls (W), Qt; is the heat loss in the hot water pipes between the burner of the central heating system and the digester heat exchanger (W'). In the experiments Qto, is measured by means of a calorimeter. The above- mentioned components are calculated wkh the following formulae:

Qo = pg~p(to -- 6) (Sprenger, 1979)

where: p is the volumetric density of pi~, slurry (1030kgm -3) (Tunney, 1979); V is the volume of inftuent pig slurry (m3); cp is specific heat at constant pressure (4.18 kJ kg'" 1 K" ~) and to, t~ are the temperatures, respectively, of the digested and fresh pig slurry (K).

Q, = ~ kA Lit with k = I ~ d !

where: k is the overall heat transmission coefficient (Wm -2 K-'r 1); ~j is the hea~ transmission coefficient for the inner medium (W m - ZK-1);% is the heat transmission coefficient for the outer medium (W m z K- J); d is the thickness of the wall material (m); 2 is the thermal conductivity coefficient of the particular wall material (W m "~ t K - ~); A is the surface of the digester wall zone under consideration and At is the temperature difference between the two materials on either side of the wall (K).

Figure 2 shows the different zones into which the digester walls are divided according to lhe different materials composing the digester wall and to the media on both sides of the wail. Figure 2 also gives the positions of the ten copper-constaatan thermocouples. They ,serve to define the temperature intervals between the two sides of the different wall zones. To exclude temporary heating from sunshine, the thermocouples 2, 9 and I 0 were not taken into account but ambient temperatures were recorded as the more exact values of the local meteorological services.

with:

Qt, = k~,L(t~ - to)0.8

k,, = . ._L_ + + ..r.! .... %D,. 2), % 0 ,

(Sprenger, 1979)

An ana: robic digestion system on a closed piggery ]arm

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zone |

~ r T T ' T 2

~ , ~ . ' ~ ~ ~ i ~ ~' ~ " ~ ' " ' " ~" ~"

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239

product io~q {~EP)

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/.i m / ,z

! !

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net ~nergy p~oduction (;-,IEP)

Xr--i i electr ical or mechanlcaEenerg~, (Eel)

I I energy to he~t up process ~,Ix I I fhe, infLuent O-v en~[gy

",, ] - -J tran~.m~sSion Losses qto! P'E x~, U " of ¢,he digesterQ v

\transmr.ssfOn losses o! p~pes Q t

Energy flows in a biogas plant.

.240 J. Poeb et al.

where: kp is the linear overall heat transmission coefficient (Win- 1 K); a;, ~ are the heat transmission coefficients of the inner and outer media (Wm-'2 K-t); 2 is the thermal conductivity coefficient (Win- ' K~t); D~, D, are, respectively, the inner and outer diameters of the pipe (m); L is the length of the pipe (in) and t~, t o are the temperatures on the inner and outer side of the pipe (K). The mechanical and electrical process energy (E,i) was measured by means of kwh meters.

Figure 3 shows the different energy components. The determination of these energy flows ~,esults in the energy balance. In these experiments, the energy balance was calculated for two test periods:

Test period A (1 September-15 October, 1981) with ambient temperatures of about 14.0°C. Test period B (5 January-5 February, 1982) with ambient temperatures of about 1"9°C.

RESULTS AND DISCUSSION

Process chemistry and microbiology

The digester was started up in February, 1981. No 'seed-culture' was added. The reactor ~vas filled with normal pig manure (6-7 ~ DM) and was heated up to 30--32°C in about 10 days. In order to allow the

TABLE ! Mean Biogas Production During the Start Up Pe:iod

Period m'* biogas per m 3 pJg manure '/,~ CH a

21 Mar¢~h- 30 March 8.0 67 31 Mart:h- 13 April 8.3 66 14 April. 21 April 7'9 ...... 22 April--26 April 8'5 ..... 27 April 2 May 9'0 .... 3 May-..l I May 9'5 .... 12 May-24 May 9'8 68 25 May 30 May 11.5 .... I June--18 June 12~3 73 t9 June--29 June 11,5 . . . . . .

30 Jut~e -8 July ~56 ..... O July- 17 July 18'9 67

An anaerobic digestion sysWm on a closed piggery farm 241

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methane-forming bacteria to build up, the digester was not fed during the first montt~. The feeding was then started with a loading rate of 1.5 kg COD per cubic metre reactor per day and a hydraulic residence time of about 35 days. Gas production was noted from the first days after the heating up, but increased very slowly during the following weeks, as shown in Table 1.

The reacter was operated with low loading rates and long hydraulic retention times during this first winter period. Indeed, the farmer was in the process of expanding his livestock and only ca, 1.5 m 3 manure a day was produced and available for digestion at that period. The process parameters and results are summarised in Tables 2 and 3.

Table 2 shows COD reductions of 45- 52 %--values in accordance with those reported by Hobson et al. (1979) and Van Velsen (198I). The VFA reductior, s of 88-3 % and 92'7 Yo are lower than the 96,9 '~ reported by Van Velsen (1981), but similar to the 92,6'y,, found by Summers & Bousfield (1980). Table 3 indicates that the digested pig slurry contains only small amounts of residual acetic acid and that the other Volatile Fatty Acids are ao longer detectable.

Microscopic examination (Polyvar microscope, equipped with a 420nm fluorescence module) of the digested slurry revealed that the methanogenic association consisted largely of small non-motile rods. Cells were cylindrical to coccoid with blunt rounded ends, and often irregularly crooked. They belonged to at least two difli~.,ent species, one

TABLE 3 VFA Al~:lly~is ~f ltflluel~l and |~tlllicnt of a l)igcst~r Runrling at 4 0 51) I)ay~ ].)t~telltit)n

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168.1t2 7"83

An anaerobic digestion system on a close l piggery farm 243

probably being Methanobacteriumforraicium, while the other should be an acetate degrading methanogen. This observation is in accordance with the results obtained b~/Hobson & Shaw (! 974). Although differetlt studies showed the importance of acetate in methane formation, Methanosarcina could only be detected in minor numbers. The fermentative bacterial populations were investigated by the methods described by Van Assche (1978). The bacterial count of the inttuent was found to be a hundred times higher than that in the digester. This corresponds with the findings of Shaw (1971). Isolates from the highest diluttons of the effluent (10- ~ and 10 -s) were predominantly Gram-positive anaerobes. Tenta- tive identification led to the following genera: Peptostreprococcus, Peptococcus, Propionibacterium and Actinomyce,~. These results support, to a certain extent, the findings of Iannotti etal. (1982). They also found Peptostreptococcus and Peptococcus to be an important group. However, in the present study predominantly Actinomyces and Propionibacterium were isolated from the fermentation liquor.

The major fermentation products formed by these organisms in pure cultures included acetate, propionate~ lactate and sueeinate. Acetate was produced by all strains and was the sole fermentation end product of some of the isolates. Although found m lower concentrations (10 s-. 10 7

per millilitre) the genera Clo.~.tridium, Eubacwrium, Bacteroides and Sarcina seemed also to be perr;lanently present in the digested slurry.

Energy balance

Table 4 gives specifications for the wail-compositions in the different digester zones. In Table 5, the heat losses through the walls are summarised during the two test periods considered. The ambient temperatures were registered by the local meteorological services. The mean temtc~erature of the influent was 17.0°C in period A and 12.0°C in period B.

The Q,, values obtained tbr zones I V and V suggest that it is advisable to insulate the underground part of the digester: 65--70 % of the total heat losses were noted in 41-5 % of the total w~-.ll surface, i.e. zone IV and zone V.

Table 6 presents the total heat balance of ~.he plant during the two test periods, while Table 7 gives the total energy balance. Calculatiol~.~ are also made for periods A' and B', They extrapolate to the near future when the livestock will be expanded and the digester will receive 3'3 m 3 :nanure

244 J, Poels et al.

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per day. Based on the results of Van Velsen (1981), it is assumed for these calculations that gas production and COD removal values win not be affected by decreasing the hyfraulic residence time fi'om 40 to 22 days.

About 40--50 % of the Q,, total is used for the heating up of the fresh pig manure to mesophilic temperature ranges. A simple heat exchanger is able to transfer about 40 % of the heat in the effluent to the influent (Smith et aL, 1979). Our own trials indicate that about 45-50 % of Qo can be recovered by pre-heating the influent with the effluent. The energy needed for the feed pump, the hot water circulation and the mixing mechanism is 2-3 ~ of the total process energy and about 1.2 ~ of the gross energy production. Schellenbaeh (1982) reported about 4.6 ~ of the gross energy production for the feed pump, the rccirculation pump, the hot water circulation and the compressor of a 680m ~ digester, processing the manure from 1000 head of beef cattle.

The respective net biogas productions of 311 "2 MJ day- J and 992.4 MJ clay i during periods B and B' with ambient temperatule of 1.9°C are in accordance with those reported by Hawkes &Horton (!981). They found about 320 MJ day-L for 400 pigs and 5% TS manure, a situation comparable with period B (Table 2), and about 1000 MJ day- : for 600 pigs, and 7-5 '~ TS manure, a situation comparable with period B'.

A preliminary financial analysis suggests that, under the conditions represented by periods A' and B', i.e. when the digester will operate at a loading of ca. 3-4kg COD per cubic metre reactor per day and will produce ca. 20 000 m 3 biogas a year, the payback period will correspond to 3 years (Table 7}.

The digester described is simple to construct and operate. Its relatively i~v investment costs facilitate its acceptance; on the level of a common Flemis~ family farm.

In terms of net energy production from piggery wastes (7~DM), detention times of maximum 20.-25 days or loading rates of rrr~,~mum 3.01:g. of COD per cubic metre reactor per day are advisable. Furthdi re.ore, adequate insulation and preheating of the influent with the ettluent ar~'imperative.

ACKN OWLEDGEM ENTS

Thanks ztre expressed to the Board of Administrators and the Direction of the Belgian Institute for the Encouragement of Scientific Research in

An anaerobic digestion system on a elosed piggery farm 249

Industry and Agriculture (IWONL, Brussels) which subsidised this research. The friendly and skilful co-operation with Mr M. Deeapmaeker, farmer and cvnztructor of the plant, is also acknowledged.

REFERENCES

ltawkes, D. L. & Horton, R. (198I). Anaerobic digestion design fundamentals, Part ili. Process Biochemistry, 16, I0-12.

Hobson, P. N. & Shaw, B. G. (1974). The bacterial population of piggery waste anaerobic digesters, Water Res., 8, 507-18.

Hobson, P. N., Bousfield, S., Summers, R. & Mills, P, J, (1979). Anaerobic digestion of farm animal wastes, in Engineering problems with effluents from livestock, EEC, Brusse]s-Luxembourg, Eur 6249 EN.

lannotti, E. L., Fischer, J, R. & Sievers, D. M. {1982) Characterisation o~ bacteria from a swine manure digester. Appl. Erz, h'on. Microbiol., 43, 136-43.

Sehellenbach, S. (1982). Case study of a farmer owned and operated 1000 head feedlot anaerobic digester. In Energy from Biomass and Waste, VI. Symposium Papers, Institute of Gas Technology, ITT Center, 3424 South State Street, Chicago, Illinois 60616,

Shaw, B, G. (1971). A practical and bacteriological study of the anaerobic digestion of waste from an intensive pig unit. PhD Thesis, University of Aberdeen.

Smith, R. J., Hein, M. E. & Greiner, T. H. (1979). Experimental methane production from animal excretain pilot-scale and farm-size units. Journal of Animal Science, 48, 202-17.

Sprenger, E. (1979). Tasehenbuch flit Heizung und Klimatechntk (Oldenbourg, R. (Ed.)). Miinchen, Wien, 1516pp.

Summers, R. & Bousfield, S. (1980). A detailed study of piggery-waste anaerobic digestion. Agricultural Wastes, 3, 61-78,

Tunney, N. (1979). Dry matter, specific gravity and nutrient relationship of cattle and pig slurry. In Engineering problems with effluents from iivestocL'. EEC, Brussels- Luxembourg, Eur 6249 EN.

Van Assche, P. (1978). Taxonomische studte van de bacteroideceae en van de dominante ,¢pecies bij de big. PhD Thesis, State University Gent, Belgium, 237pp.

Van Velsen, A. F. M. (1981). Anaerobic digestion ofpiggery wastes. Agricultural University Wageningen, The Netherlands, 103 pp.