J. agric. Engng Res. (1986)34, 229-243

Mesophilic Anaerobic Digestion of Dairy Cow Slurry on aFarm Scale: Economic Considerations

B. OLIVER*t; B. F. PAINt; v. R. PHILLIPS*

A computer program wasdevelopedto model the economicsofenergyproduction from anaerobicdigestion of slurry on UK dairy farms. Data incorporated into the model were based on resultsand experience from operating two identical 125m' continuously stirred tank reactor (CSTR)digesters for over two years using whole or mechanically separated slurry as feedstocks. Themain aim of the model was to compare the effects of changing design, operational and financialparameters on the annual profit or loss from the process. Energy, as electricity, or as surplus hotwates from a boiler or a combined heat and power (c.h.p.) unit was taken as a source of profit andthe sale of separated slurry solids as a horticultural compost was also considered.

Initially the model was used to predict that digesting slurry from a 200 cow herd under typicalconditions could incur an annual loss of approximately £50/cow. Increasing slurry total solidscontent (TS%) from 5to 8% and period of digesteroperation from 180to 350days whilstdecreasingretention time from 20 to 15days and operating temperature from 35 to 30°C reduced this loss by47 and 40% for wholeand separated slurry respectively. The inclusion of a c.h.p. unit gave furtherreductions of22 and 10%.

Further savings from a reduction in the capital cost and interest rate or from increases in energyprices were assessed together with the effects of an increase in the size of the herd or of sellingseparated slurry solids. When each parameter was considered individually, relatively largechanges were needed, for example a 40% reduction in capital cost or a 30% increase in energyprices, to ensure the process did not operate at a loss over a 10 year plant life. In practice, it ismore likely that smaller changes in several parameters would occur simultaneously. Under suchcircumstances it is possible to operate the process at a small profit in energy terms. Despite thegenerallylowercosts of digestingseparated slurry, digestingwhole slurry was much more attractivewhen energy production was the only credit to the process.

1. Introduction

The potential benefits of anaerobic digestion include conversion of semi-liquid slurry to a lesspolluting effluent that is easier to pump and store with reduced or less offensive odour andimproved fertiliser value. In addition, biogas is produced for use as a source of energy on thefarm. Despite these benefits, it is still difficult to justify the process economically on all but verylarge farms.' The importance of pollution and odour reduction will depend upon individualfarm circumstances and it is difficult to assign a generally applicable monetary value to thesebenefits. Similarly, although the breakdown oforganic nitrogen compounds to forms more readilyavailable to plants could increase the fertilizer value of slurry, this has not been accuratelyquantified. Currently therefore, it is only the value of the biogas that can be accurately quantifiedand energy production is often regarded as the only source of income from digestion of dairywaste. Studies in North America indicate that, under these circumstances, the cost of gas orelectricity produced from anaerobic digestion is several times greater than that produced fromnational utilities. For example, in Canada it has been shown that for a 60 cow herd, the cost ofproducing biogas was at least five times the commercial rate." However, using a low cost plugflow digester design in the U.S.A., methane could be produced on farms with 100cows or more at

*Nationallnstitute of Agricultural Engineering. Silsoe, Bedford MK45 4HS. England

tPresent address: Water Research Centre, Elder Way, Stevenage, Herts sa 11TH, England

[National Institute for Research in Dairying. Shinfield, Reading RG2 9AT_England

NIRD closed on 31 March 1985. Present address for correspondence: Animal and Grassland Research Institute, Hurley, Maidenhead, Berks, SL65LR, England

Received II March 1985;accepted in revised form 24 August 1985

229oo21~8634/86/070229+ 15$03.00/0 © 1986The British Society for Research in Agricultural Engineering

230 ANAEROBIC DIGESTION OF DAIRY COW SLURRY

a cost of 50 to 60% ofliquid fuel prices." In another study in the U.S.A.,4 using a mathematicalmodel to study economics of conventional digesters, it was concluded that the process could notbe justified on farms with less than 1000 head of cattle unless the process was credited with otherbenefits in addition to energy production.

In the U.K. economic assessments of digesters have tended to be based on pig enterprises buthave also demonstrated the difficulty in justifying the process solely as an energy producer. It hasbeen concluded that a 4000pig place digester could be economic only if a value (£2'50/pig place a inthis example) could be placed on odour reduction." Other workers have compared the relativemerits of a digester and of other investment opportunities on a dairy farm requiring the samecapital expenditure." The digester proved to be potentially the least attractive although the auth­ors pointed out that economic evaluation is very dependent on individual farm circumstances.Whether or not the process was profitable depended on the efficiency ofgas production, the levelofgas utilization and the value placed on pollution control.

Economic evaluation of dairy farm digesters in the U.K. is hampered by lack of reliable datafrom farm scale installations. Performance values have generally been based on those obtainedfrom laboratory or pilot scale digesters assuming that optimum operating conditions (retentiontime and temperature, for example) for laboratory digesters are the same for full size plant.

More recent experiments with two 125m' digesters? at the National Institute for Research inDairying (NIRD) have provided an opportunity to collect design and performance data relevant todigester operation on a dairy farm. The information, together with assumptions regarding certainfinancial aspects of digester purchase and operation has been used to develop a mathematicalmodel of the economics of the process on dairy farms. The main aim was to assess the sensitivityof the economics to changes in design, operational or financial parameters rather than to predictabsolute costs.

2. Mathematical model

A model describing the economics of anaerobic digestion was developed using a BBC "B"microcomputer. A flow-chart of the model is given in Fig 1.

2.1. Input variablesInput variables fall into three categories, viz. farm circumstances, digester design specifications

and financial aspects.

2.1.1. FarmcircumstancesThe daily volume of slurry is the volume to be fed to the digester each day and would normally

be directly related to herd size. For example, assuming the average dairy cow produces 701 ofslurry/day including parlour and collecting yard washings, a 200 cow herd would produce14m3/day, at approximately 8% TS.

Normally, a dairy farm digester can be operated only during the winter months when slurry isavailable from housed cattle. A period of 180days of digester operation is, therefore, included inthe model but with provision to increase this when the digester is run for a longer period. It hasproved possible to operate a digester at NIRD on poultry slurry during the summer period whencattle were out at grass, without needing to shut down the digester. Poultry slurry at 30% TS,collected once a week from laying hens kept in a battery house, was diluted to 5 to 7% TS withdairy collecting yard washings prior to feeding to the digester each day.

The mean ambient temperature is chosen to suit the particular location and later used tocalculate the heating requirements of the digester. In the Reading area, the value is 5°C for 180day winter operation increasing to 9°C for year round operation.

The volume and TS% of the liquid fraction produced by separation prior to digestion using acommercial (Farrow Ltd) two-stage roller press type machine" are calculated from the chosen

':fJor'....-<rr1';Q

tTl'""l:l>

t"'

lin1enllnceand reg,;, cost

in t:api1at costs% redUCtion

~ Dig.6o'S'Vol

M&Bnam"bianttemP6rttture

Hot IN«tltr prod

vee

Vah,Jlt of separatedsolids

Fig. 1. Simplifiedflowchart ofmathematical model ofanaerobic digestion ofdairy cow slurry.\ I . input data; I I , output data

Nw-

232 ANAEROBIC DIGESTION OF DAIRY COW SLURRY

Table I

Biogas yields (m3jkg TS fed) from whole and separated slurry atdifferent retention times and operating temperatures

Operating Retention time, dSlurry type temperature,

°C 10 15 20

Whole 35 0·150 0·200 0·230Whole 30 0·139 0·186 0·214Whole 25 0·09 0·\20 0·138

Separated 35 0·210 0·270 0·290Separated 30 0·195 0·251 0·270Separated 25 0·126 0·162 0·174

volume and TS% of whole slurry. With whole slurry at 7% TS, for example, 82% is converted toa liquid fraction at 4·5% TS.

2.1.2. Digester design specificationsThe digester design specifications and performance data incorporated in the model are based

on two 125 m' digesters at NIRD. These are ofcontinuously stirred tank reactor (CSTR) design,one having been operated primarily on whole and the other on separated dairy cow slurry forover two years. The design has been previously described in more detail and comparisons madebetween digestion of whole and separated slurry at different retention times." Biogas yields at 10,15 and 20 d retention times are given in Table 1. Since the farm scale experiments were carried outwithin a limited temperature range (30 to 35°q, data from laboratory experiments" at 25, 30 and35°C were incorporated into the model to see the effects of changing operating temperature(Table 1). The same feedstock was used for both types of experiments and experience has shownthat results from the farm scale digesters are generally in good agreement with those from laboratoryscale. 7

The NIRD installation has provision either to burn biogas in a modified domestic boiler or in acombined heat and power unit (c.h.p), hot water from either source being used for digester heatingby a series of internal heat exchangers. An efficiency of 80% is ascribed to the boiler option. Thec.h.p. unit (Serck Heat Transfer Ltd) is based on a 21 Land Rover engine downrated to 1500 rev/min producing electricity by means of a 15 kVA asynchronous generator and producing hotwater at about 85°C from waste heat recovery. The unit consumed about 10rrr' of biogas/h andis designed to convert 61% of the gross energy input to hot water and 24% to electricity.

2.1.3. Financial aspectsThe capital cost of the digester is based on manufacturers' estimates (see Section 2.2 below) but

the model allows the calculated value to be reduced by a chosen percentage in order to study theeffect on net annual costs. Similarly, a value of 11% for annual interest rate on capital is includedbut this can also be varied.

Slurry separation produces a stackable, fibrous fraction in addition to the liquid fraction fed tothe digester. There is interest from professional and amateur horticulture in the composted solidfraction for use in plant growing media or as a soil conditioner or mulch. It is, therefore, ofinterest to see to what extent placing value on this additional "product" affects the economics ofthe process. Hence, there is provision to vary the value ascribed to the separated solids in themodel.

B. OLI VER ET A L. 233

2.2. Capital costsManufacturers' estimates at 1984 prices are used as a basis for the capital cost of the digester

including pipework, insulation and internal heating equipment. This was calculatedl" from:

Cost (£) = 738(vol m3) O.6 8 2

where volume is based on the retention time and daily feed rate. A 10% gas space above theliquid is allowed.

Digester mixing on the NIRD installation is by gas re-circulation, the capital cost?" of acompressor being calculated from:

Cost (£) = kW required x 243·5

assuming a specific mixing power of 20 Wjm3 of digester volume .Pumps for both feeding and discharging from the digesters are of the flexible scroll and stator

type (Mono Ltd) and costed at £1500 each for whole slurry and £300 each for separated slurry:much smaller pumps can be used for the latter application assuming the feed rate does not exceed18 m3 jd.

A 15kW biogas boileriscosted at£1000installed, a second boiler being required ifgas productionexceeds 250m' jday. Similarly, the c.h.p. unit costs £6500with a second unit being included if gasproduction exceeds 170m' jd.

A roller press separator is priced at £8000and includes the cost of a mounting gantry and feedpump. An additional machine is included when cow numbers exceed 300 (20 m3 jd of slurry) at acost of £7000since a complete second gantry and pump would not be essential.

2.3. Maintenance and repairThe annual cost ofmaintenance and repair for the digester installation is taken as I% ofcapital

costs plus £3jman hour for labour and is based on operating experience from the NIRD digesters.In addition a cost of IO pjh of operation is included for the c.h.p. unit.

2.4. Heat balance2.4.1. Heat produced

Daily biogas production is calculated from the appropriate biogas yield, the daily volume ofslurry fed to the digester and the TS% of slurry. The amount of heat produced from either theboiler or c.h.p. unit is calculated from the volume assuming a biogas calorific value? of20 MJjm3

.

2.4.2. Heat requiredHeat is required to raise the temperature of the cold feed slurry to the digester operating

temperature and to counteract losses through the digester walls, roof and floor. The former iscalculated from:

Q=M.CpAT

where Q=heat input (W), M=mass flow rate (kgjs), Cp=hcat capacity (Jkg- 1 K- 1) andLI T = temperature change (K) , and assuming a heat capacity for slurry similar to that for water(4'12 x 103 J kg - 1 K - 1). The rate of heat loss from the digester walls and roof is estimated fromthe temperature differences between the ambient air and the slurry inside the digester. A similarrate of loss per unit area is assumed from the floor. The main resistance to heat transfer is thepolyurethane foam insulation, for which a thermal conductivity of 0·033 W jmK was taken . Fulldetails of the heat loss calculations have been described previously' '

234 ANAEROBI C DIGE STION OF DAIRY C O W SL URRY

Table 2

"Baseline 1" conditions

Parameter

N urnber of cowsTS % fed to digesterNumber of days operation/aDigester temperatureResidence time (d)Gas boiler or c.h.p. un itValu e of separated solidsAmbient tempera ture

Whole slurry

2005

18035°C20

Gas boilerNot applicable

5°C

Separated slurry

2003

18035°C20

Gas boilerO£/t

5°C

2.5. Electrical balance2.5.1. Electricity produced

Electricity is required to power a pump (5 kW) for mixing slurry in the reception pit and feedingthe separator (l kW), for pumps for feeding and discharging from the digester (3 kW for wholeslurry and 1 kW for separated slurry) and for the gas compressor (2 kW) for digester mixing. It isassumed that when the gas boiler opt ion is chosen or when production from the c.h.p. unit isinsufficient for these duties, electricity would be purchased from an electricity board for4·5 p/kWh.

2.6. Annual returnsFor the purposes of this investigation, three sources of income from the digester installation

are included . Surplus electricity is priced at 4·5 p/kWh from the c.h.p. unit and surplus hot waterat 2 p/kWh from c.h.p . or boiler to reflect commercial prices, once the operational demand s ofthe installation are satisfied as in Sections 2.4 and 2.5 above .

The annual return from the sale of separated solids depends upon the chosen value/t, using thevolume ofslurry and its TS content to calculate the weight produced from a roller press sepa rator.No additional costs are included for composting or marketing, owing to the lack of reliableinformation.

2.7. Annual costsThe major annual cost is repa yment of capital and this is calculated using a simple mortgage

type calculation.P assuming an annual interest rate of 11 % and a 6% annual rate of inflation.Additional costs are electricity requirement (see Section 2.5.2) and maintenance and repair (seeSection 2.3).

Finally , the model calculates the net annual loss or profit , assuming a plant life of 5, 10, 15 or20 years.

3. Analysis and results

3.1. Baseline 1 conditionsInitially a set of baseline cond itions (Table 2) was selected to represent a relatively unattractive,

but not atypical, situation for anaerobic digestion on a 200 cow dairy farm. Under these circum­stances the process would incur an annual cost per cow ofapproximately £50/a since the repaymentofcapital over 10 years plus maintenance and repair charges would exceed the value of the energyproduced as hot water. The model was then used to showthe percentage decrease in the annualcost which could be achieved by changing the conditions.

B. OLIVER ET AL. 235

25 c

330

0~

~ 35

~ Du

40 B

"::>cc« 45

A

50 A 5 6 7 8 9B 150 250 350C 10 15 20D 25 30 35

Fig. 2. Effects ofchanging digester operating conditions on the annual cost ofdigesting whole slurry from 200dairy cows

A = whole slurry solids content (TS%); Bv- period of digester operation (d]a); Cw retention time (d);D = digester operating temperature (0C)

25

C3 300

~

~ 35if>

D0u

B

" 40::>cc« 45

A

50 A 5 6 7 8 9B 150 250 350C 10 15 20D 25 30 35

Fig. 3 Effects of changing digester operating conditions on the annual cost of digesting separated slurry from200 dairy cows

Ae whole slurry solids content (TS%); B=period of digester operation (d]a}; Cv-retention time (d);D = digester operating temperature (OC)

3.2. Slurry TS%, retention time, temperature and period ofoperationFig. 2 shows for whole slurry the decrease in annual cost as a function of the TS% of the feed

slurry, the period of operation, the retention time and the digester operating temperature. Fig. 3gives the equivalent information for separated slurry. In both Figs 2 and 3, the four operatingconditions are ranked in decreasing order of their influence on annual cost reduction. Thus, forboth whole and separated slurry, increasing TS content of the whole slurry had the greatest effectand decreasing operating temperature the least. In producing the graphs, an optimum value waschosen for each operating condition in turn, starting with slurry TS%, and used in all the followingcalculations. Although the relationship between annual cost reduction and whole slurry TS%appeared linear, an optimum value of 8% was chosen since thicker slurries are difficult to pumpand so decrease both the speed and efficiency of separation. Similarly, increasing the period ofoperation gives a linear improvement, but 350 d was selected to allow 15dja downtime for main­tenance and repair etc. Decreasing retention time from 20 to 15d decreased annual cost but from15 to 10d less so, especially for whole slurry. Hence 15d was chosen for whole slurry and 10d forseparated slurry, the former tending to show evidence of operational instability at lower retention

236 ANAEROBIC DIGESTION OF DAIRY COW SLURRY

Table3

"Baseline 2" conditions

Parameter Whole slurry Separated slurry

Number of cowsTS% fed to digesterNumber of days operation/aDigester temperatureResidence time (d)Gas boiler or c.h.p, unitValue of separated solidsAmbient temperature

2008

35030°C15

c.h.p. unitNot applicable

9°C

2005

35030°C10

c.h.p. unito£/t

9°C

30

20

~10

.!("ll 0u;.Q5 -10

~a. -200:JCc;

-30<l

-40

5 20Plant life, a

Fig. 4. Effects of reducing capital costs of a digester installation on the annual cost of digesting whole slurryfrom 200 dairy cows

30

20

~ 1010~

0~

"tl 02

ur e'" 15 iii.Q

5 -10e'"- .S0;:::

K -20 gc

-0 c:J <lcc -30<l

-40

-505

Fig. 5. Effect of annual interest rateontheannual costof digesting whole slurry from 200dairy cows

B. OLIVER ET AL.

40

30

s 200

-"2".J

10:i.S!

" 0'='2 -10Q.

0"c:

-20c:-r

-30

-40

50

Plant life. a

237

Fig. 6. Effects of increasing electricity and hot water priceson the annual cost of digesting wholeslurryfrom200 dairy cows

times. Finally, the fourth curves show the effect of changing operating temperature for digestersassuming whole slurry at 8% TS, 3S0 d/a operation, and a 15 and 10 d retention time for wholeand separated slurry respectively. In both situations there was a well defined maximum at 30°C.

The procedure described above allowed stepwise optimization of the four main operatingconditions for digestion of whole and separated slurry. The overall effect was to reduce theannual cost from about £SO/cow to £27 and nl/cow for whole and separated slurry respectively.

Using the gas to fuel a c.h.p. unit rather than burning in a boiler achieves a further reduction inannual costs of22% for whole slurry and 10% for separated slurry respectively.

3.3. Baseline2 conditionsAt this stage, a second set of conditions was defined at Baseline 2 (Table 3). This incorporates

the optimum values established in Section 3.2 above and also includes a c.h.p. unit instead of abiogas boiler. Baseline 2 was then used as a basis for assessing the effects ofchanging the remaininginput variables on the economics of the process. This was done by considering each variableindependently of the others and calculating the annual profit or loss/cow with a plant life of 5,10, 15 or 20 years.

3.4. Capital cost, interest rate and energy pricesFigs4 to 6 respectively show the effects of reducing capital cost, of changing interest rate or of

increasing electricity and hot water prices. A 40% reduction in capital cost (Fig. 4) would berequired to break even in energy terms with a 10 year plant life, unless interest (Fig. 5) was ratedat 0%. A 30% reduction would reduce the cost/cow from £14 to £2ja over the same period.Increasing energy prices (Fig. 6) had a greater effect, an increase of about 30% being required toavoid incurring a loss.

3.5. Cow numbersFig. 7 is based on the conditions included in Baseline 2 except that data are included for 100

200300 and 400 cows, or cow equivalents where slurry is available from other livestock. Underthese conditions, a 100 cow digester would clearly not be profitable and even with a 400 cow herda plant life of 12 years would be required.

238 ANAEROBIC DIGESTION OF DAIRY COW SLURRY

30

20

10

3 00u $?<, c:'tl

-10.J!1

.,; 0>

'" :;E cr6 -20

Q)

~

~c

a. -30 '0C Q;:> ..cc: Ec -40 :><l Z

-50

-60

-7020

Plant life, a

Fig. 7. Effect ofcow numbers on the annual cost ofdigesting whole slurry

0

-103 '"0 c.!( -20 Q)

"ll c.,; .2:

'" :>.Q -30 cr~

Q)

0 30

.;: -40o

0 -is. 0

Q;C ..c:> -50 Ec:c: :><l Z

-60

-70

-8020

Plant life. a

Fig. 8. Effect ofcow numbers on the annual cost ofdigesting separated slurry

3.6. Separated slurry digesterData for separated slurry digestion comparable to those presented above for whole slurry were

not included since under all circumstances the former was much less attractive in economicterms. For example, Fig. 8 illustrates that even with a very large digester and long plant life thecost/cow would be about £6/a. However, the inclusion of a separator in the installation opens up

B. OLIVER ET AL.

90

80

70

60

50

~ 400~... 30Ii".Q

5 20-g& 10a~

§ 0<l

-10

-20

-30

-40

-50

10

5 10 15 20Plan! life, Q

239

Fig. 9. Effect of value ofseparated solids on the annual cost ofdigesting separated slurry from 200 dairy cows

the possibility of selling the solid, fibrous fraction as a horticultural compost or soil conditioner.The very large effects of placing a value on this material are shown in Fig. 9. With a 10year plantlife, increasing the value from zero to £IO/t converted an annual loss on a 200 cow digester of£20/year to a profit of £45/cow.

3.7. Combined effectsWhen capital cost, interest rate, energy prices, cow numbers and the value of separated solids

are considered individually as in Fig. 4 to 9, relatively large changes are needed to at least breakeven with an acceptable plant life. In practice, it is more likely that several of these factors willchange simultaneously but to a lesser extent than the maxima included in the figures. An exampleis given in Table 4. The conditions are as in Baseline 2 except cow numbers are increased from200 to 300 and the separated fibre is valued at either £3/t or £O/t. In addition, electricity and hotwater prices are increased by 25% and capital cost reduced by 15%.

The capital cost of the separated slurry digester was lower than that for whole slurry despitethe inclusion of a separating machine in the former. This is because less volume is available fordigestion after separation and the retention time is shorter so the digestion vessel is smaller andless expensive. Also smaller, cheaper pumps and mixers are specified for separated slurry. Thelarger volume of gas produced from whole slurry requires two c.h.p. units. Although the heatingand electrical requirements are lower for separated slurry, the net production is much greater forwhole slurry. Running costs (maintenance and repair) are also greater for the latter, primarilydue to the inclusion of two c.h.p. units. The main source of profit from the separated slurrydigester is from the sale of separated solids. In this example, both whole and separated slurryinstallations would make a small annual profit assuming a 10 year plant life, assuming solidsfrom the latter were valued at £3/t.

240 ANAEROBIC DIGESTION OF DAIRY COW SLURRY

Table 4

Digestion of whole andseparated slurry froma 300cowdairy herd

SeparatedslurryWholeslurry

(I) (2)

1. Capital costs £ £ £Digester 39800 25600 25600Gas mixer 1700 900 900Slurry pumps (2) 3000 600 600C.h.p. unit 1300 6500 6500Slurry separator - 8000 8000

TOTAL: 48900 35400 35400

2. Heat balance(kWh/d)Raise temperature of feed slurry 515 403 403Loss through digester walls 47 36 36

Total heat required 562 439 439

Hot water from c.h.p. unites) 1059 600 600Surplus heat 497 161 161

3. Electricalbalance(kWh/d)Operating requirements 187 109 109Produced from c.h. p..unit 417 236 236Surpl us electricity 229 127 127

4. Annual return (£)Surplus electricity (5'7 p/kWh) 4710 2610 2610Surplus hot water (2'5 p/kWh) 4470 1450 1450Separated solids - 4910 -Running costs -2060 -1400 -1400

TOTAL: 7120 7570 2660

5. Net annualprofitPlant life (a) £/cow

5 -16 -3 -2210 5 14 -915 15 24 -320 26 33 I

(I) Separated solids valued at B/t(2) Separated solids valued at £O/t

4. Discussion

This investigation illustrates the sensitivity of the economics of anaerobic digestion of dairycow slurry to a range of design, operational, and financial parameters. Wherever possible, infor­mation collected from two farmscale digesters in operation at NIRD was used in the calculations,although some estimates were included, for example, with regard to the capital costof digestersand labour charges, and some assumptions were made. Full utilization of the net energy producedis assumed although it is recognized that this could be difficult on many farms due to problems ofmatching energy supply with demand. Lack of data on this aspect precludes the inclusion ofrealistic values in the model. The emphasis, therefore is on comparing the effects of changing theparameters considered rather than on the absolute costs reported: the latter should be treated

B. 0 LI VER ET AL. 241

with caution. Similarly, apart from considering the financial implications of marketing separatedfibre, the sole credit to the process is net energy as electricity or hot water. Other authors haveascribed a value to odour control" and to the feed value of the digested waste."

Of the four operational parameters examined , the TS% of the whole slurry had the greatestinfluence on the economics followed by operation time (d/a) , retention time and operatingtemperature in that order. A higher solids content than the 8% chosen as optimum couldtheoretically produce more gas but limitations are imposed by the pumping and, more especially,slurry separation machinery where this is included. As for slurry TS%, the relationship betweenreduction in annual costs and days of digester operation per year was linear and illustrates thedesirabil ity of running a dairy farm digester longer than the housed winter period. The feasibilityof achieving year round operation, allowing a few days each year for maintenance and repair etc,was demonstrated by importing poultry manure a short distance on to the farm and diluting thiswith collecting yard washings to produce a pumpable slurry and to reduce NH~-N concentrationto non-inhibitory levels.13 The aim here was to change feedstocks rapidly as the cattle were movedout to grass, without significant loss in gas production. In this example, no allowance wasincluded for the extra costs involved in importing poultry manure since, depending upon individuallocation and circumstances, other feedstocks may be more convenient and economic.

Previous laboratory experiments'" have shown the optimum retention time and operatingtemperature for cow slurry digestion to be 20 d and 35°C respectively. In economic terms, thereappears to be advantage in operating at lower values where net energy production is of primeimportance. The main effectof decreasing retention time is to reduce the size, and hence the capitalcost , of the digester required for a given number of stock. Decreasing operating temperaturefrom 35 to 30°C may slightly reduce gross gas production but increases net production becauseless gas is required to warm the feed slurry to the lower temperature.

Applying the optimum values for all four parameters in the model still does not make the processeconomic in energy terms alone but when a combined heat and power unit was included in thedesign it reduced the annual cost by 69% for whole slurry and by 50% for separated slurry.Thus , the cost of other benefits such as water pollution or odour control, could be substantiallyreduced .

Increasing cow numbers, or cow equivalents where slurry was available from dairy followers orother stock , did not dramatically improve the annual return/cow. Four hundred cow equivalentswould be required to break even with a repayment time of about 12years for a whole slurry system.In the model, increasing cow numbers above 200 to 300 head involves the purchase of both anadditional combined heat and power unit and, for separated slurry, an additional separator,which significantly increases capital costs . Alternatively, reducing capital cost by about 33%, orannual interest rate from 11 to 5%, or increasing electricity and hot water prices by about 40%would enable a 200 cow digester to break even with a 10 year plant life. Applying these changessimultaneously at lower levels than stated above would enable reasonably profitable situations tobe envisaged. Increasing gas yields would also have a significant effect on the economics. Thispossibility is not considered here since it was the intention to include values so far achieved fromthe NIRD digesters over relatively long periods of operation. Reducing capital costs, whilst atleast maintaining current gas yield, is likely to bring about the greatest improvements. This haspreviously been demonstrated by designing low cost digesters" or by subsidizing installationsfrom public funds.!"

Although separating prior to digestion appears the least attractive option in terms of energyproduction, previous work? suggests that a greater degree of pollution reduction can be achievedthan by digesting whole slurry. Marketing the separated fibre after stabilization by compostinggreatly improves the economics ofdigesting separated liquid but it may also, ofcourse , be feasibleto add value to the whole slurry system by separation after digestion.

A further potential advantage of using separated slurry is the inclusion of a heat exchanger inthe installation for recovering heat from the slurry discharged from the digester and using this topre-warm the cold feedstock . All except expensive specialized heat exchangers would readily

242 ANAEROBIC DIGESTION OF DAIR Y COW SLURR Y

block with whole slurry but if a simpler heat exchanger could be made to work with separatedslurry, this would significantly reduce the heating requirements of a separated slurry digester."

5. Conclusions

I. When financial returns are based solely on energy production and no credit is allowed for otherbenefits of the process, an anaerobic digestion system treating slurry from 200 dairy cows couldincur an annual loss ofapproximately £50/cow.

2. This annual loss can be reduced to approximately £28/cow by increasing feed slurry TS%,operating the digester throughout the year, decreasing retention time and digester operatingtemperature. Further reductions can be achieved by replacing the biogas boiler with acombined heat and power unit.

3. Relatively large reductions in capital costs, increases in energy prices or in cow numbers arerequired for the process to break even with a lO year plant life. If smaller changes in allparameters occurred simultaneously, the process would operate at a small profit in energyterms. Profitability could be increased by improving biogas yields.

4. For energy production, digesting whole slurry was more attractive than digesting separatedslurry despite the generally lower costs of the latter.

5. Ascribing a value to other benefits of the process, pollution or odour reduction for examplewould improve the economics as would marketing separated slurry solids to horticulture.

Acknowledgements

We thank MAFF for funding the project and Mr I. D. Fearnley for help with the development of thecomputer program. Both NIRD, now AGRI, and NIAE are institutes of the Agricultural and FoodResearch Council.

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Biogas op Veebedrijven, 1982