Download - The economics of the treatment of dairy cow slurry by anaerobic digestion

J. agric. Engng Res. (1986) 35, 259-276

The Economics of the Treatment of Dairy CowSlurry by Anaerobic Digestion

D. J. PARSONS·

The cost of treating dairy cow slurry by anaerobic digestion on 50, 100 and 200 cow farms iscalculated using the value of the energy produced to offset the running costs. The impact oftechnical changes is evaluated, including: four retention times, pre-treatment separation,reduction in water entering the slurry, use of an effluent-influent heat exchanger, three operatingtemperatures and improved gas yield. The effects of changes in energy prices and componentcosts are considered. The results are presented as average annual costs for operating the systemsover a ten-year period at given levels of energy utilization and price.

The annual cost of anaerobic digestion with present technology ranges from £9/cow to£70/cow making it uneconomic when considered solely as a source of energy. Lower costs maybe achieved by combining several technical and operational improvements. The importance ofkeeping the digester in operation for the full year is shown. The best methods of improving theeconomics of anaerobic digestion are good waste management to maintain a high slurry drymatter and increasing the gas yield at short retention times.

1. Introduction

Anaerobic digestion is the degradation of organic matter by micro-organisms in theabsence of oxygen. One of the groups of bacteria involved produces a mixture of gases,known as biogas, consisting mainly of methane and carbon dioxide. If offers a m~ans ?freducing pollution and odour from organic wastes, such as animal slurries, and the blogas IS

potentially useful as a fuel. Anaerobic digestion takes place very slowly in septic tanks. andmore quickly in specially designed reactors, which have formed part of municipal wastetreatment systems for about one hundred years. Diogas is used to supply energy for thetreatment process, either directly through heating, or by electricity generation and has alsobeen used for lighting, space heating and vehicle fuel.

Various types of digesters have been used for treating livestock wastes, including batch,continuous stirred tank (CSTR) and plug-flow reactors.' The most common is the CSTR,which is an insulated tank, usually cylindrical, which is fed continuously, or at frequentintervals, with a flow which is small relative to its volume. Effluent is removed at the samerate to maintain a constant volume. It is designed so that the mean retention time formaterial in the digester is several days. typically 10 to 20. The reactor contents arc mixedand maintained at a constant temperature, usually in the mesophilic range (30 to 40°C) orthe thermophilic range (SO to 60°C) to suit a particular population of micro-organisms. Theresults in this paper are given for CSTRs, but most physical and financial parameters arechanged. so that some results for other types of digesters may be deduced.

Although anaerobic digestion is sometimes claimed to be a profitable system, it should beemphasized that treating waste will usually cost more than simply disposing of it byreturning it to the land. However environmental considerations such as pollution and odourmay make some form of treatment essential and in order to provide a comparison withanother method of waste treatment, Appendix 1 calculates the annual costs for sample

• Operational Research Group. NIAE. Wrest Park, Silsoe. Bedford MK454I1S

Received 12 September 1985; accepted in revised form 15 March 1986

2590021-8634/86/120259+ 18 $03.00/0 iO 1986 The British Society for Research in Agricultural Engineering

260 ECONOMICS OF ANAEROBIC DIGESTION OF SLURRY

Overall heat transfer coefficientfor roof. W m- 2 K- 1

Volume of digester. m3

Slurry input rate, m3/dGas yield from volatile solids,m3jkgYield parameter in Contoismodel of gas production, m3jkgRetention time, dMaximum specific growth rateof bacteria, d - I

Yo

o~

VVdY

Notation

V,Area of digester base, m2

Area of digester roof, m2

Area of digester wall, m2

Specific heat capacity of slurry,kJ K-l kg- 1

Parameter in Contois model ofgas productionInfluent volatile solidsconcentration, kg/m3

Operating temperature ofdigester,OCAmbient temperature, °COverall heat transfer coefficientfor base, Wm- 2 K- 1

K

TombVb

T

So

Ab

Ar

A.Cp

aeration systems. None of the systems considered includes the cost of spreading the slurryafter treatment. This is not likely to be greatly different than for untreated slurry, so thecosts presented are in fact relative to land spreading with no treatment.

Some of the variations in the digestion systems considered, such as different temperaturesand retention times, are possible with present technology. Others, such as increased gasyields and effiuent-influent heat exchangers represent potential improvements which havenot yet been achieved in practice.

Unless otherwise stated, most of the information on equipment and equipment costs wasprovided by the NIAE Farm Buildings Division, and refers to 1984 prices.

2. Costs and benefits of anaerobic digestion

2.1. Costs

The main cost of anaerobic digestion is the high capital outlay, typically over £20000 on a100 cow farm, which leads to high annual repayments. In addition there are maintenanceand running costs and the cost of electricity, where this is not being generated from thebiogas.

The cost in pounds (£) of the main digester components is estimated by

746Vo·682 •

This is derived from a survey of prices conducted in 1981 by Teesside Polytechnic.2 It includesthe reaction vessel and fittings, but not gas storage and utilization or slurry storage andhandling equipment. This estimate lies in the range given by ADASjBABA' except for thelargest digesters, where it is lower. However the ADAS prices included items which arecosted separately here. Two slurry pumps are required, costing £1500 each for whole slurry,or £500 each if the larger slurry particles are removed by mechanical separation. Acompressor for gas recirculation mixing costs £4·9V and a boiler for up to 250 m3 gas perday costs £1000. Storage ror the digested slurry is not included, since some form of storageforms a part of most waste handling systems.

Information about the labour and maintenance requirements ror digesters is not readilyavailable. On the basis of several American studies (see Parsons4

) and the ADAS/BABA

D. J. PARSONS 261

report, this study assumes maintenance costs of 3% per year of capital cost for static itemsand 10% per year for moving ones. The whole system has a life of 10 years, with movingequipment being replaced after five years. The most costly item of moving equipmentconsidered in this study is an internal combustion engine plus generator for producingelectricity from biogas. Comparing this with the costs given by Audsley and Wheeler' (orsimilar items, assuming 1800 hours use per year, showed the chosen costs to be slightly low.The annual costs of capital are calculated using the discounted cash flow method describedby Audsley and Wheeler,' which includes interest rate (12%), inflation rate (5%),maintenance costs and loan repayments.

A later section explains how the energy balance is calculated. The only energy inputconsidered in any of the cases is electricity for mixing at 4·5 p/kWh, since other uses o(electricity are negligible by comparison.

2.2. Benefits

The benefit which is easiest to assess financially is the energy produced. Biogas and engineheat are both valued at 2 p/kWh, as an equivalent to liquified petroleum gas (LPG), andelectricity at 4·5 p/kWh, as a replacement for mains electricity. If electricity was to be resoldto the generating board the price would probably be lower than this.

The other reason for installing a digester is to reduce odour and pollution problems. It isdifficult to put a value on such benefits, so Appendix I evaluates the cost of an aerationsystem intended to produce a reduction in chemical oxygen demand similar to digestion, as abasis for comparison.

Two further effects which are difficult to quantify are the changes in fertilizer value andhandling characteristics of the slurry. In monitoring a digester, ADAS' found decreases of11%,9% and 18% in nitrogen, phosphorus and potassium respectively. However it has beensuggested that some of the nitrogen is converted to ammonium nitrogen during digestion,making it more readily available to plants. In the absence of firm evidence, this studyassumes that the fertilizer value is unchanged.

An improvement in slurry handling characteristics would be expected to produce a savingthrough reducing the time required for slurry disposal. However this saving is likely to besmall. ADAS7 estimate that for a 4·5 m3 tanker spreading slurry in a field 800 m from thefarm, only 5 min of the total time of 20 min per load, is spent in filling the tanker. Assumingthat this can be reduced by 2 min saves 19 h/a for 100 cows. At a contractor's rate' of£12·7S/h this is £240/a. If farm labour was being used for spreading, the saving would bemuch lower, so this benefit is not included in the rest of the study.

3. Digester design

To determine the dimensions of the digester, the gas output and so on requires the dailyslurry production, volatile solids concentration, retention time and operating temperature tobe known. For most of this study it is assumed that the total slurry produced when dilutionby rainwater and washwater is included is equivalent to 70 lid for each cow, at 8% drymatter and that volatile solids form 75% of the dry matter. The working volume is calculated(rom the slurry volume and retention time and the volume of the vessel is taken to b~ lOrolarger than this to allow (or a head space, with a 2 : 1 height to diameter ratio, which ISwithin the range used in practice.

Heat losses are calculated for an above-ground insulated steel tank. Most of the resultspresented are for a particular level of insulation, however the effect of a different ~evel ofinsulation is considered in section 4.7. Such a change could be achieved by changang thethickness of the insulation, or the material used.

262 ECONOMICS OF ANAEROBIC DIGESTION OF SLURR Y

3.1. Calculation of energy output

The gas output is calculated from the expression given by Hashimoto and Chen:'

Y = Yo(l- K/(0.u+ K -I», (1)

where K is a constant for the type and concentration of waste. The biogas yields reporled byPain et al.'o for farm scale digesters at the National Institute for Research in Dairying(NIRD) are lower than those usually found in lab scale experiments. There is insufficientdata on farm scale digesters of this type to make detailed changes to the parameters, so Yoalone is changed from the value used by Hashimoto and Chen. The value used for Yo is0·30 m3jkg volatile solids for whole slurry and 0·42 m3jkg for separated slurry. They do notrepresent the results of batch fermentations.

The maximum specific growth rate (,u) depends on the operating temperature T eq andis calculated from the following expression given by Chen and Hashimoto:"

~ =0-013T-0'129 (20 < T < 60). (2)

The value of K is also calculated from an expression given by Chen and Hashimoto for cattleslurry:

K =0·8 +0-0016 exp (0-06So)' (3)

A typical graph of gas yield against retention time is shown in Fig. 1. The gas is assumedto contain 55% methane and to have a calorific value of 20 MJ/m3 (5,6 kWh/m3) (see Painet al.'o).

Fischer et al.12 show that raising the temperature of influent slurry and replacing lossesthrough the vessel walls are the main heating requirements, so these are the only onesconsidered.

The energy needed to heat the influent is

Cp(T - T.mb)V.J3-6 kWh/day. (4)

Chen and Hashimoto" give the specific heat capacity of cattle slurry with 10% dry matter as3·92 kJ K-1 kg -1 and Fischer et al. remark that it changes only slightly with the dry-matter

0'30..,.......:!2 0·25'0..~0 0·20'0>

'""'",;;.. 0·15E~..>. 0'10..0goiii 0-05

0·000 5 10 15 20 25 30

Retention time, d

Fig. 1. Biogas yield vs retention time for cattle slurry, (8 % dry matter) dIgested at 35°C

D. 1. PARSONS 263

content, so this figure is used throughout. The energy required to replace losses is

(T- T.mb)(A,U,+AbUb+A.U.) x 0·024 kWh/day. (5)

The values for the heat transfer coefficients are based on a number of sources includingFischer et al.,1Z Hil113 and Hawkes.'4 For the walls, Hill gives 0·55 W m- 2 K-1 for 51 mm ofpolyurethane insulation, while Hawkes gives values from 0·20 to 0·81 for glasswoolinsulation from 200 to SO mm. Hill also gives 0·48 W m- 2 K-I for the roof and Hawkesgives 0·36 to 0·70 for 100 to SO mm insulation. Hill's values are used here. For the base, Hillgives 1W m- 2 K-I, whereas Fischer et al. use larger values. The value used here is1·5 W m-2 K-I. The effect of a change in these values on the economics of the system isconsidered later.

The heat is supplied either by a biogas boiler with an efficiency of 80% or from enginewaste heat. Where a heat-exchanger is used to return heat from the emuent slurry to theinfluent, its efficiency is taken to mean the fraction of the emuent heat which is returned tothe influent; that is, the fraction by which the influent heating requirement is reduced.

Fischer et al. also show that most of the electricity consumed is used for mixing of thereactor contents so that slurry pumping etc. may be neglected. Gas recirculation mixing isassumed, since the capital cost is lower than for mechanical mixing. A continuous input of20 W/m3 is used.

3.2. Utilization of biogasThe simplest method of using the biogas, and the one requiring the least capital outlay is

to burn it in a boiler to produce hot water with a conversion efficiency of 80%. In this casethe gas is valued at 2 p/kWh (11,1 p/m3

) as an LPG equivalent.It is also possible to use the gas to fuel an internal combustion engine coupled to an

electricity generator. This combination is assumed to convert 25% of the available energy toelectricity. It is still necessary to have a boiler to provide healing for the digester. Thismethod values the remaining gas at 6 p/m3 (assuming electricity is worth 4·5 p/kWh) and asit always costs more than using the gas in a boiler, especially when the extra capital cost istaken into account-approximately £3300 for a 10 kW generator,' it will not be consideredfurther.

In order to use more of the energy of the gas it is possible to recover waste heat from theengine. It is assumed that 62% of the energy in the gas can be converted to heat in this way.This is based on the performance of a combined heat and power (CHP) unit (Serck HealTransfer Ltd) which has an electrical output of 15 kW and costs £6500. The heat recovered isvalued at 2 p/kWh. This values the gas at 12·7 p/m l , only slightly more than gas burnt in theboiler. Assuming a life of 5 years for the CHP unit and retaining the boiler for digesterheating, a net gas output of 3800 m3/a would be required to cover the cost of the CUP unit.This cannot be achieved with digesters of the sizes considered here.

It is however possible to reduce the cost by omitting the boiler and using heat from theCHP unit for the digester. This would usually mean that heat was only available for fairlyshort periods, but, since the digesters considered cool at a rate of less than I°C/d, thismethod of operation might be feasible if heat was supplied when cold slurry was enteringthe digester. This method saves the cost of the boiler and produces more electricity than anyof the others. Only this use of the CHP unit, and the combustion of all the gas in a boilerwill be considered in the following sections.

4. Economics of anaerobic digestion systems

Three sizes of dairy farms are considered: 50, 100 and 200 cows. Data from theGovernment Statistical Service" show that 45% of British dairy farms have fewer than SO


cows, 82% have fewer than 100 and only 2·4% have more than 200. The mean size of holdingis 67 cows.

Each system is evaluated for 6 month-per-year operation and year-round operation. Gasoutput is assumed to be steady throughout the period of operation, though in practice astart-up period of several weeks during which output would fluctuate could be expected. Asthe expressions for heat loss, Eqns (4) and (5), given in Section 3.1 are linear, an averageambient temperature can be used. The mean winter temperature (October to March) forSilsoc from 1960 to 1979 was S·soC, so SoC is used in this study. Each system is also checkedto see whether it is a net energy producer at ambient temperatures down to -SoC.

Table 1 lists the standard values of the features varied, which are described and discussedin detail in the following sections. Each of these is varied independently to assess its effect onthe overall cost. Details of the energy balance and costs for a 100 cow, 20 day retentiondigestion systems are shown in Tables 2 to 4.

4.1. The effect of scale

Most of the tables in later sections include all three farm sizes. Generally the annual costincreases as the size of the farm increases, but less rapidly, so that the cost per cow falls. Inmost cases the annual cost doubles from 50 cows to 200. In the cases where most energy isbeing produced, that is for 10% dry matter slurry at 10 day retention time and for 8% DMslurry with improved yield, the cost is lower for 200 cows than for 100.

4.2. Retention time

The digesters using slurry with 8% dry matter are evaluated at retention times of 20, 15, 10and 7 days (see Table 5). Twenty days is the one most often recommended for cattle slurry,though IS and 10 days have been used. The shortest, 7 days, is chosen as the time whichgives the lowest cost when using a gas boiler. Beyond this the declining yield leads to anincrease in cost. However at the 7 day retention time none of the digesters is able to produceenough gas to meet its own heating requirements when the ambient temperature is - SoC. Afurther problem at short retentior': times is the loss of process stability. Small changes inslurry characteristics can have a large effect on loading rate, possibly leading to failure of thedigester. Hill" showed the importance of maintaining a constant loading rate when usingshort retention times, but this would be difficult to achieve under farm conditions. Ittherefore seems unlikely that retention times less than 10 days would be used in practice.

Table 1Features or digestion system considered

Variable

Retention timeSeparationMethod or utilizationHeat exchangerOperating temperatureSlurry dry mailerGas yieldLevel of energy utilizationCapital costGas priceElectricity price

Standard value

20dNoGas boilerNoWC8%(}26 mJ/kg VS100";'

2p/kWh4'5p/kWh

Range

7. 10. 15. 20 dNo/yesBoiler/CHP unitNo/yes2S,3S,40·C8%.10%Q'I6-0'26mJ/kg VS50";', 75%. l00''!.

2-3'2 p/k.Wh4'5-1'2p/kWh

D. J. PARSONS

Table 2Energy balance for a 100 cow digester

265

Influent slurry:Retention time:Working volume:Working temperature:Gas yield:Gross gas:Ambient temperature:Heat losses:1nfluent heating:Net gas:Net energy Crom gas:Elemicity Cor mixing:CHP unit

Output power:Run Cor:Heat output:Electrical output:

7 m3/d at 8% dry matter20d140m'WC0·26 mJ/kg volatile solids109 mJ/dS·C77 kWh/d229kWh/d41 mJ/d229 kWh/d67kWh/d

ISkW10-2 hId382 kWh/d153 kWh/d

Table 3Capital cosls ofeompollt'nts, 100 cows,

retention

Iyltem20 day

[um I Cost, L-

Digester 23154Compressor 754Pumps 3000Boiler 1000Generator 3300CHP unit 6500

Table 4Annual cost breakdown, 100 cows, 20 days retention

Period of Netoperation Annual cost Maintenance Net fuel annualper year, afcapital, costs, l-a/ue. cost,months Utilitarian £Ia i/a Ila Ila

6 Boiler 4294 /100 280 51146 CHP unit S72S 1720 967 6478

12 Boiler 4294 1100 S60 483412 CHP unit 572S 1720 1934 5511

For retention times above 7 days, reducing retention time always produces a reduction ~ncost, usually about IS to 25% (or a 5 day change. Two (actors contribute to this. The mamone is the reduction in capital costs, since servicing the capital Corms the largest part of theannual cost. The other is the reduced energy requirement (or mixing the reactor contents andreplacing heat losses. Set against this is the loss of gas production. However, Fig. 1 shows


Table 5Effect or retention time on annual cost, £

Period 01operarlon Retenllon lime, dper year, Numbermonth3 01,"ow3 20 15 10 7

6 SO 3685 3618 2658 237S-6 100 S114 4269 3458 3081-6 200 7331 5943 4660 4134-

12 SO 3590 3026 2525 2355*12 100 4834 3910 3136 2951*12 200 6629 5108 3926 3803*

• Unable to maintain digester temperature at -5°C ambient

that gas output changes relatively little for retention times above 10 days compared with thereduction in energy required.

4.3. Period ojoperation

Most of the tables show the costs when the full volume of slurry is available for 6 or 12months of the year. Provided that the value of the energy produced is greater than that ofthe energy consumed, the advantage of year-round operation is clear. However the resultsshow that the dilTerence for most systems is small. For systems with a boiler this is due to thelow value of the gas compared with the electricity required for mixing. With a CHP unit it issimply the result of the small quantity of energy produced. The difference in cost is largestwhere a CHP unit is used since this yields the largest quantity of high-value energy. Otherthan this the period of use has the largest impact where most energy is produced. Forexample, using a 10 day retention time to treat slurry of 10% dry matter from 200 cows,using gas in a boiler, the cost is £3431 for six month operation, compared with £2167 forwhole year use. This is in contrast with aeration where the cost increases with period of use.

Treatment costs for full year operation with 20 day retention time and whole slurry are£71.8/cow for 50 cows, £48,3/cow for 100 cows and £33.1/cow for 200 cows (with boileronly).

Where the cattle are housed for half the year, the digester could be kept in constantoperation by storing the slurry. The savings produced by operating at half the volume forthe full year, excluding the storage cost, with the gas used in a boiler and assuming that thereis no change in the slurry during storage are shown in Table 6. :ro store half the slurryproduced in 6 months by SO, 100 and 200 cows would require tanks with volumes of 315,630 and 1260 m3 respectively. The costs of steel tanks of these volumes estimated by

2928+8·543 x volume

(derived from manufacturers' prices) are £5620, £8310 and £13 690 respectively, givingannual costs of £792, £1172 and £1930. In practice other costs would be incurred, forexample mixing the stored slurry to prevent crusting, and odour problems could also arise.

This shows that there is some advantage in storing slurry when a 20 day retention time isbeing used, but not with 10 day retention time. The difference arises from the greaterreduction in capital cost through halving the volume of lhe larger reactor rather than thesmall one. Storing slurry in this way also has the advantage of reducing the frequency withwhich the digester has to be shut down and restarted.

O. 1. PARSONS

T.ble 6Saving due 10 operaling half-slze di&ester for full year

No.o/cows IRtttrtlion limt, I Annual saving,

d i-50 20 933so 10 57J

100 20 1524100 10 933200 20 2497200 10 1524

267

4.4. Method of utilization

Results for the two methods of gas utilization are shown in Table 7. They show that theextra value of the energy produced by the CHP unit is insufficient to offset the increasedcapital and running costs. The only cases examined in which the annual cost was reducedwere when the slurry had a dry matter content of 10% or the gas yield was increased. Forthese systems, with full year operation, the cost with the CHP unit was up to £400 lowerthan with the boiler.

4.5. The effect of slurry separation

Removing the solid fibrous fraction before digestion by mechanical slurry separation olTersseveral advantages for anaerobic digestion. The solid fibrous fraction is more slowlydegraded than the liquid and smaller particles, so digesters can be smaller without aproportionate loss of output, and it is possible to use less powerful pumps and smallerpipework. In th,is study this is assumed to reduce by two-thirds the cost of the pumps.

Results from a brushed-screen, roller-press separator used at NIRD, have shown that theslurry volume is reduced by 20% and the volatile solids concentration by 40%. However, theparameter Yc. in Eqn (1) is increased from 0·30 to 0·42 mJ/kg to represent the greater gasyield obtained at normal retention times.

The results shown in Table 7 do not include the cost of the separator since it is useful toconsider this separately. This allows for the possibility of a digester being installed where aseparator is already in use. Assuming a capital cost or £8000 ror separator and associated

Table 7Elfect of separalion and melhod of utilization on annual cosl txcluding ",paralion cOliI. 20 day rtttntlon

Annual cosl, £Period ofofX'raliol/ Whole slurry S,paraltd slurryptr ytar. Nwnbt,months o/cows Boile, CliP un;t Boiler CliP unit

6 SO 3685 5384 2744 45266 100 5114 6478 4065 55936 200 7331 8037 6156 7185

12 SO 3590 4936 2764 427712 100 4834

,5511 4027 5032

12 200 6629 5989 6958 5966


equipment, with a lifetime of 10 years gives an annual cost of £1362. This may be added tothe cost of the digester to give a complete system cost.

Introducing separation always reduces the net gas output from the digester. Despite this,separation reduces the annual cost of digestion on the 50 and 100 cow farms substantially(by £800 to £1100), as servicing the capital is the largest item in the costing. However, for the200 cow farm the reduction is sometimes less marked and in certain cases, there is anincrease in cost due to the loss of gas production. Including the separator cost alwaysproduces a higher annual cost than for whole-slurry digestion.

Once the slurry has been separated, some use must be found for the solid fraction.Provided that the dry matter content is high enough, this will compost readily and it is oftensuggested that it could be sold. The total mass produced from the 100 cow farm is 1·4 t/d or252 t per half year. Thus a value of £6.24/t for half-year operation, or £3.12/t for whole yearoperation is required to offset the cost of the separator. These values are proportionallyhigher for 50 cows and lower for 200. Estimates of the actual value vary from £5/t upwards.

4.6. The use ofheat exchangers

The use of a heat exchanger to return heat from the effluent slurry to the influentpotentially offers a method of reducing the amount of energy consumed by the digester, thusincreasing the net gas production." Unfortunately, designs which are efficient when usingliquids, such as plate heat exchangers, block when used with cattle slurry, even with theseparated liquid. It is possible that relatively inefficient designs, such as pipe-in-pipe, mightbe usable.

Table 8 compares the effects of different efficiencies on various sizes of digesters assumingthat a heat exchanger costs £900, and gives break-even capital costs for the installation ofheat-exchangers. Twenty days retention time and biogas utilization in a boiler are assumed.

Treatment costs for whole-year operation at 20 days retention time with whole slurry arereduced by £3.6/cow for 50 cows, £3.9/cow for 100 cows and £5.5/cow for 200 cows. In everycase the extra energy produced by using the 30% efficient heat exchanger is sufficient tojustify its inclusion, provided of course that the energy can be used.

Table 8Impact or tbe Installation or. beat exchanger on the ec:ooomic:s or whole-slurry digestion

Half year operation Full year operation

Ileat Annual cost Value of Breakeven Annual cost Value of Breakevenexchanger of energy capital cost of energy capilal cos1

Number efficiency whole plant, saved. ofheal whole plant. saved, afheatof cows % £ £/a exchanger, £ £ £/a exchanger, £

50 O· 3685 - - 3590 - -SO 20 3709 103 729 3m 206 1464SO 30 3657 155 1100 3408 309 220050 40 3606 206 1464 3305 411 2936

100 O· Sl14 - - 4834 - -100 20 5034 208 1471 4549 411 2936100 30 4932 309 2200 4343 617 4407100 40 4829 412 2936 4137 822 5879200 O· 7331 - - 6629 - -200 20 7046 412 2936 5932 822 5879200 30 6840 617 4407 5521 1234 8814200 40 6634 822 5879 5109 1645 It 757

• No heat exchanger

D. 1. PARSONS 269

4.7. Reduced heat Joss

Another method of increasing the net energy output fiom the digester is to reduce the heatloss by improving the insulation of the shell. Since the heat losses are far smaller than therequirement for influent heating, this cannot be expected to have as large an impact as a heatexchanger. Clearly the effect is largest where the surface area is large and the digester is inoperation for the whole year. Table 9 shows the heat loss and the saving produced by areduction in heat losses of 25% at 2 pfkWh. It also shows the breakeven capital cost belowwhich such an improvement is profitable. All the digesters shown operate for the whole yearat 20 day retention time.

The saving is so small that only a very small increase in digester cost can be justified. Thesavings are less at short retention times because the surface area is smaller.

4.8. Other operating temperatures

Lowering the digester temperature provides a third method of reducing the heatingrequirement of a digester. However, it also reduces the rate of gas production, as shown byEqn (2). The effect of reducing the temperature to 25°C is considered for retention times of20 days and 10 days. All show the same pattern of results (Table 10). At 20 days there is asmall reduction in the annual cost, but at 10 days the loss of yield leads to a rise in cost.

Since the reduction in cost at 20 days is small, and since waste is less effectively treatedthan at 35°C there is little reason for operating digesters at lower temperatures. On the otherhand, raising the digester temperature increases the gas yield and increases the need forheating. The effect of increasing the temperature to 40°C is considered for retention times of20 days and 10 days. Under all circumstances the cost is higher than for operation at 35°C.

4.9. Increased slurry dry matter

Dairy cow slurry always contains water from sources other than the animal excreta, suchas washing water and rainfall. This increases the size of the digester and the mass to beheated without contributing to gas production. The effect of raising the dry matter contentto 10% by reducing the water in the slurry by 22% is considered. This means that 5·4 m3/d ofslurry is available on the 100 cow farm at a volatile solids concentration of 78 kgjm3. Itshould be noted that the gross gas production is slightly lower with the more concentratedslurry, due to its greater inhibiting effect on the micro-organisms [see Eqn (3)]. Again, tworetention times are considered.

As expected the costs are lower than for treatment of 8% dry matter slurry (Table tl). Forexample, on the 100 cow farm with 6 month operation, there is a reduction of £1080 at 20days retention time and £700 at 10 days. For fulI-year operation the savings are highe.r.These results show that substantial costs can be justified for improved waste management 10

order to reduce the amount of water entering the slurry.

Table 9Savings due to impro~ed digester Insulation

No. oJ Reactor cost. Heat loss, Annual saving. Breakeven capitalcows £ kWh/d £ cosl,£

50 15432 48 86 610100 24154 77 139 990200 38148 122 220 1560


Table 10[(feet of operatinll temperature on annual cost, £

Period of Operating lemperalure. ·Coperation. Number Retention

months/year ofcows lime, d 25 35 40

6 SO 20 3559 3685 37796 SO 10 2691· 2658 27066 100 20 4891 5114 52856 100 10 3543 3458 35456 200 20 6933 7331 76506 200 10 4858 4660 8247

12 SO 20 3338 3590 377712 SO 10 2591" . 2525 262212 100 20 4389 4834 5t7712 tOO 10 3S05 3136 331012 200 20 5833 6629 726712 200 10 4323 3926 4244

"Unable to maintain digester temperature at -S·C ambient

4.10. Increased gas production

Clearly, an increase in gas production from a particular digestion system will reduce thenet cost of running it, provided that the additional energy produced can be utilized. It maybe possible to increase gas production by increasing the micro-organism population in thedigester, possibly by extracing bacteria from the effluent and recycling them, or byintroducing media on which bacteria can grow and thus be retained within the reactor.However, such methods will add to the cost of the system.

Increasing the yield at 10 days retention time to that previously achieved at 20 days givesapproximately 20% more gas. Considering the increase in this way ensures that the yield iswithin that which is possible from the type of waste being used.

Table 11Effect of dry matter conlent on annual cost

Period ofoperalion Annual cosl, £

Number per year. Retentionofcows months lime, d 8%DM JO%DM

SO 6 20 3685 305950 6 to 2658 2254

100 6 20 5114 4034tOO 6 10 3458 2758200 6 20 7331 5446200 6 10 4660 3431

SO 12 20 3590 2766SO 12 10 2525 1981

100 12 20 4834 3372100 12 10 3136 2164200 12 20 6629 4003200 12 10 3926 2167

D. J. PARSONS

Table 11Effect of improved biogas yield at 10 days compared with

normal yield at JO days

Number of I Annual cos/ Breake~en

cows reduction. £ capital cost. £

SO

I357 2532

100 715 5071200 1430 10142

Table J3Ellect of improved bio&as yield at JO days compared with

normal yield at 20 days

Number of I Annual cos/ I Breakevencows reduclion. £ capilal cosl. £---

50

I1422 I 10085

100 2413 17113200 4133 29312

271

Savings compared with the normal yield at 10 days and 20 days retention time for fullyear operation are shown in Tables 12 and 13, together with a breakeven capital cost forachieving the yield increase.

The savings compared with 10 day retention and normal yield are substantial. The largesavings compared with 20 days retention (which should give the same standard of treatment)show that improving the gas yield at short retention times has good potential for reducingthe cost of anaerobic treatment.

4.11. Level of biogas utilization

The results given in the previous sections assume that all of the energy produced can beused. While this gives an indication of the potential of anaerobic digestion, it would bedifficult to achieve in practice. Included in Table 14 are results for utilization levels of 75%and 50%. Naturally, the costs are higher at low-levels of utilization, and the differences aregreater for whole-year operation and for larger farms. An interesting feature is that the costsare often higher for whole-year operation than for half-year, when the gas is being used in aboiler. This is because the cost of the electricity used for mixing is higher than the value ofthe heat produced.

4.12. Reduced capital cost

Since servicing the capital is the major cost of most digestion systems, changes in capitalcost have a marked effect on the annual cost. With the conditions used, every £1000reduction in the cost of an item with a 10 year life produces a £141 reduction in the repaymentcosts. In addition, maintenance costs are reduced. In previous sections, various changes havebeen compared with capital cost changes. Here capital cost changes alone are considered.

A reduction of 20% in the cost of the main· components is considered. The results appearin Table 15. The savings are of course larger for the larger systems. They are the same for all


Table 14Effed of level of utilization on annual cost, t

Periodo/use Level 0/utilization. %

per year. Number Retentionmonths o/cows time, d 100 75 50

6 50 20 3685 3777 38696 50 10 2658 2725 27926 100 20 5114 5320 55266 100 10 3458 3607 37556 200 20 7331 7779 82276 200 10 4660 4979 5299

12 SO 20 3590 3774 395812 SO 10 2525 2660 279412 100 20 4834 5246 565812 100 10 3136 3443 373012 200 20 6629 7525 842012 200 10 3926 4565 5204

periods of use and methods of utilization, since only main digester components areconsidered.

4.13. Increased lifetime

In all the preceding sections the static digester components were given a lifetime of 10years. Results are also given in Table 15 using 15 years. By extending the period over whichrepayments are made, the annual cost is lowered. The effect is larger with the boiler, wherethere is less moving equipment being replaced at 5 year intervals.

4.14. Energy price rises

The results presented so far assume that energy prices rise at the same rate as other prices.In recent years however this has not been the case. Two other situations are considered:

Table 15Effect of pricn and costing period on annual cost, £

Effect on annual cost 0/Periodo/use Retention Basic Reduced Increased Rising

prr year, Number time. annual capital energy energy 15 yearmonths o/cow.!' d cosl cosls prices prices lifelime

6 50 20 3685 3185 3628 3658 32336 50 10 2658 3246 2578 2620 23816 100 20 5114 4311 4946 5034 43046 100 10 3458 2958 3265 3366 30066 200 20 7331 6043 6910 7130 61506 200 10 4660 3857 4219 4449 3928

12 50 20 3590 3090 3476 3536 313812 50 10 2525 2213 2366 2449 224812 100 20 4834 4031 4499 4674 413012 100 10 3136 2636 2749 2951 268412 200 20 6629 5341 5787 6227 545212 200 10 3926 3123 3044 3504 3194

D. J, PARSONS 273

energy prices rising 5% per year faster than inflation; and initial costs of 3-2 p/kWh for gasand 7·2 p/kWh for electricity, rising at the rate of inflation. The latter represents the relativeprices for capital equipment and energy at the end of a 10 year period, in which energyprices rose by 5% per year more than the general rate of inflation. Results are shown inTable 15. Since most of the systems are producers of energy their costs are reduced.

4.15. Combinations offeatures

The single change considered in the preceding sections which caused the greatest reductionin cost was the use of slurry having a dry matter content of 10%. If this is combined with theuse of a heat exchanger or with increased gas production, the costs can be reduced further.Results are shown in Table 16 where the efficiency used for the heat exchanger is 30% andthe increase in gas yield is chosen to raise the output with 10 days retention time to thatpreviously used for 20 days. The annual costs are reduced substantially in most cases andthere is a small surplus for the 200 cow farm with full·year operation when all threeimprovements are combined.

4.16. DiscussionThe results of the preceding sections are summarized in Table 17, with the annual costs

given per cow, rather than for the whole system. The costs exclude subsequent landspreading of the slurry. Since the treatment process is likely to make only a small differenceto the overall cost of spreading, the costs presented here are effectively given relative to landspreading alone, which is the most common present practice. The "standard" cost for thebasic digester, fed with whole slurry, at 20 d retention time and 35°C, with the gas used in aboiler, is £33/cow to £74/cow depending on herd size. The "present range" of costs whichare feasible with present technology, including four retention times, three temperatures, twolevels of dry matter, separation and the effects of energy price changes, shows that for largeherds digestion costs ill/cow to £47/cow more than land spreading, but for small herds£40/cow to £119/cow more. The "possible minimum" is that obtained by combiningincreased dry matter with two potential improvements. namely, an emuent-influent heatexchanger and improved gas yield at 10 day retention time. The requirements for anaerobicdigestion to be economic are large herds and full-year operation. Even then only the largest

Table 16

Annual cost for treating 10% dry mailer slurry at 10 day retention with modilled .ystems

Annual cost. £

Number Period of use With heal With increased Withofcows per year, months Utilization exchanger gas output both

SO 6 Boiler 2261 2062 2069SO 6 CHP unit 4067 3807 303650 12 Boiler 1869 1596 1484SO 12 CHP unit 3429 3838 2972

100 6 Boiler 2646 2373 2261100 6 CHP unit 4216 3823 3159100 12 Boiler 1814 1394 I 1044100 12 CHP unit 2901 2242 1987200 6 Boiler 3081 2662 2312200 6 CHPunit 4184 3525 3270200 12 Boiler 1341 628 -198200 12 CHPunit 1495 303 -333


Table 17Summary or trealment tosls per toW

Period of use Standard Present Possible AerobicNumber per year. cost,· rO/lge.t minimum.~ treatment cost.ofcoll's months £/0 £/0 £/0 £10

so 6 74 4S-119 41 32-37SO 12 72 40-113 30 39-S0

100 6 SI 27-72 23 23-28100 12 48 22-68 10 30-40200 6 37 17-47 It 18-23200 12 33 11-41 -2 26-36

• 20 days retention, gas used in boilert including energy price changes, two levels of dry mattert With heal exchanger and improved gas yield

herd size is actually profitable. So, without restrictions on land spreading which require theslurry to be treated, digestion will not be profitable in the majority of cases.

The aerobic treatment costs are calculated in Appendix A. The aeration system is assumedto use a continuous reactor prior to storage. At present aeration is rarely used for cattleslurries, which may present problems due to their high solids contents. It is thereforenecessary to make several assumptions in order to calculate these costs, which should betaken as indicative of the likely magnitude, rather than as absolute values.

On smaller farms aeration is generally cheaper than anaerobic digestion. Although theminimum digestion cost is lower than aeration for whole-year operation, this takes noaccount of possible technical improvements in aeration, so the two are not derived on acomparable basis. The situation is more favourable for anaerobic digestion on the largestfarm. Generally there is an indication that aeration is more favourable for 6 month/a operation, reinforcing the impression that all year operation is essential for economicanaerobic digestion.

S. Conclusions

(I) Anaerobic digestion of cattle slurry on dairy farms with present technology is noteconomic when considered only as a source of energy.

(2) Costs after deducting the value of energy produced are £0 to £70 per cow per year.Low costs (below £20) can only be achieved on large farms (200 cow), or on smaller farms(100 cow) with careful control of water in the slurry. Costs below £10 can only be achievedon large farms with control of water and improved gas yields.

(3) The cost is always lower where the slurry supply is constant throughout the year. Thedifference in cost between running the digester with slurry from cattle housed throughout theyear, and from the same number housed in winter and storing slurry for the summer isgenerally small.

(4) Feeding all the gas produced to a CHP unit and using heat from the unit to providedigester heating, provides excess electricity in addition to that required by the digester.However, even when all of the electricity can be used, the annual cost is higher than for aboiler in most cases.

(5) With present technology the most effective way to reduce the cost of digestion is toreduce the amount of water in the slurry. Pre-treatment separation also reduces the cost, butusually by less than the cost of owning and running the separator.

(6) Of the technical improvements, increasing the gas yield at short retention times wouldhave the most significant effect on the cost. Using an effluent-influent heat exchanger wouldhave a smaller one.

D. J. PARSONS 275

(7) Combining higher dry matter with improved biogas yield and the use of a heatexchanger would produce a system capable of low cost treatment or economic energyproduction on larger farms.

Acknowledgement

The author would like to thank Mr B. Oliver, formerly of NIAE Farm Buildings Division, forsupplying much of the numerical information used in this study.

An interactive anaerobic digestion costing program for MS·DOS and CP/M 80 microcomputers isavailable for sale from the author.

References

, Hayes, T. D.; Jewell, W. J.; DeIl'Orto, S.; Fanfoni, K. J.; Leuschner, A. P.; Sherman, P. F.Anaerobic digestion of cattle manure. In: Stafford, D. A.; Wheatley, D. J.; Hughes, D. E.Anaerobic Digestion. Applied Science Publishers Ltd, London 1980, pp. 255-285

2 Street, G.; Oliver, D.; Watson, D. A. The economic assessment of the 'Anaerobic Contact' process.Proceedings of the Second World Congress on Chemical Engineering, Montreal, 4-9 October1981, Vol. I, p. 207

S ADAS/BABA. An economic assessment of anaerobic digestion. Discussion document ofADASjBABA Working Party, November 1982

• Parsons, D. J. A survey of literature relevant to the economics of anaerobic digestion of farmanimal waste. Divisional Note DN 1225, National Institute of Agricultural Engineering, Silsoe,March 1984

• Audsley, E.; Wheeler, J. The annual cost of machinery calculated using actual cash flows. Journal ofAgricultural Engineering Research 1978, 13: 189-20 I .

• ADAS. Anaerobic digestion of farm wastes. Monitoring of a farm digester: Bore Place, Kent,August 1979 to May 1980. MAFF Farm Waste Unit

7 ADAS. Equipment for handling farmyard manure and slurry. MAFF, Booklet 2126, 1983• Agro Business Consultants Ltd. The Agricultural Budgeting and Costing Book No. 18. Agro

Business Consultants Ltd, May ]984• Hashimoto, A. G.; Chen, V.oR. Theoretical aspects of methane production. Proceedings on the 4th

International Symposium on livestock waste, 'Livestock waste: a renewable resource', 1980,pp. 86-91. American Society of Agricultural Engineers, St. Joseph, 1981

'0 Pain, B. F.; West, R.; Oliver, D.; Hawkes, D. L. Mesophilic anaerobic digestion of dairy cow slurryon a farm scale: first comparison between digestion before and after solids separation. Journal ofAgricultural Engineering Research 1984, 29 (3): 249-256

11 Chen, Y.oR.; Hashimoto, A. G. A microcomputer program for design of anaerobic digestionsystems. 1982, American Society of Agricultural Engineers, Paper No. 82-4021

11 Fischer, J. R.; Sievers, D. M.; Fulhage, C. D. Energy consumption from animal manure methanegeneration. Transactions of the American Society of Agricultural Engineers 1983, 26: 223-227

13 Hill, D. T. Energy consumption relationships for mesophilic and thennophilic digestion of animalmanures. Transactions of the American Society of Agricultural Engineers, 1983.13: 841-848

,. Hawkes, D. L. Digester design for high solids materials. Paper to ADAS special interest courscMaterials Handling: Livestock, November 1981 (unpublished)

tI Agricultural Statistics U.K. 1982. Government Statistical Service, 198311 Hill, D. T. Maximizing methane production in stressed fermentation systems for swine production

units. Agricultural Wastes 1984,9: 189-203" Olinr, D. Heat recovery to reduce the energy requirements of anaerobic digestion. Part I:

Predictions. Divisional Note ON 1232, National Institute of Agricultural Engineering, Silsoe,May]984

U Enns, M. R.; Misset, R.; Smith, M. P. W.; Thacker, F. W.; Williams, A. G. Aerobic treatment ofbeef cattle and poultry waste compared with the treatment of piggery waste. Agricultural Wastes1980,2: 93-101

,. Hughes, D. F. Extraction of energy from an aerobic farm waste lagoon. Journal of AgriculturalEngineering Research 1984,29: 133-139.


Appendix. The cost of aerobic treatment (UK, 1985)

The systems whose costs are calculated here are intended to be as similar as possible to theanaerobic digestion systems considered in this report in their mode of operation andchemical oxygen demand reduction. These may not be the most likely to be installed inpractice and it should be noted that odour reduction and the changes which will take placein subsequent storage are different from those obtained by anaerobic digestion. The costsobtained here should therefore be treated only as an indication of the cost of wastetreatment, not as an appraisal of aeration. Most of the information was supplied byDr A. G. Williams of NIAE Farm Buildings Division.

The slurry is treated continuously before storage, as for anaerobic digestion. Pain et al.'ofound that anaerobic digestion for 20 days retention produced a COD reduction fromC. = 75·5 kg/m3 to Co = 56·0 kg/m3• Using a relationship derived by A. G. Williams(personal communication) from Evans et at.," the retention time required to produce thisresult by aeration is

3·7/(Co/C.-0·571)-7·14 d,

i.e. 15 days.The mass of oxygen to be transferred into the slurry equals the COD removed, that is

19·5 kgfm 3• Assuming that an aerator transfers 1·5 kgfkWh and that it runs for 12 hid, theaerator power may be calculated.

To treat 7 m3{d (100 cows) the volume of the reactor is 126 m3 (including 20% freeboard)and the aerator power is approximately 8 kW. Using the formula given in Section 5.2 for thecapital cost of the tank, plus 20% for pipework and other equipment gives £4200, £4800 and£6100 for SO, 100 and 200 cows. The capital cost of the aerator is estimated by

£910+ 150 x power.

The method used to calculate the annual cost is the same as that used for anaerobicdigestion, and the changes in energy prices are as described in Section 5.14. No account istaken of the energy released as heat in the process, which could be used to offset the cost oftreatment." The results arc shown in Table AI. In calculating the cost for increased drymatter (10%) it is assumed that the same aerator power is required, but the volume of thetank is reduced.

rable AIAnnual COlit 01 aerobic treatment

Annual cosl, £Period ofoperation Current Rising Increased

Number of per year, energy Increased energy energycows mon/hs prices dry mailer prices prices

SO 6 1620 1586 1131 1853SO 12 2006 1974 2232 241S

100 6 2316 2264 2S39 2782100 12 J09J 3042 3539 4026200 6 3724 3622 4171 4658200 12 5280 5178 6172 7146

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