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Centrifugation for separating piggery slurry 3. Economic effects on aerobic methods of odour control
J. agric. Engng Res. (1988) 39, 199-208
Ccntrifugation for Separating Piggery Slurry 3. Economic Effects on Aerobic Methods of Odour Control R. W. SNEATH* The costs of reducing the odours from piggery slurries to an inoffensive level and storing that slurry in an inoffensive state are discussed. Five different combinations of aerobic treatment and separation for slurries containing initially between 1.5 and 8% d.m., from 2000 and 80@0 pigs, and with storage for five days or 30 days are considered. The five combinations are aeration of raw slurry; separation then aeration; centrifugation then aeration; aeration of raw slurry then centrifugation; and separation then aeration then removal of more solids by centrifugation. The costs of storing the effluents from these processes are also included. For slurry from the smaller herd and for a short storage period, the cost of aerobic treatment of raw slurry is lower than other treatments when the slurry is less than 4.5% d.m. Using a brushed screen/roller press separator to remove solids before aeration produces the cheapest control of odours for slurries above 4.5% d.m. When 30 days’ storage or more is required, separation before aeration is cheaper for slurries with more than 3% d.m. and the costs of using a decanting centrifuge (costing t38000) before aeration is similar to aerating raw slurry at 6% d.m. With a large (8000 pig) herd, using the centrifuge before aeration to produce slurry that is inoffensive for 30 days is the cheapest method at all dry matter contents. When storage is required for only five days, separation or centrifugation before aeration have similar costs, these costs are similar to the cost of aerating raw slurry when it contains less than 3% dry matter, but as dry matter content of the raw slurry increases the cost rises steeply. Removing solids from slurry by centrifugation after aeration increases the overall costs compared to aerating raw slurry.
1. Introduction
In 1984/85, Environmental Health Officers received 1469 justifiable complaints about odours from slurry stores and slurry spreading.’ With the increased incidence of odour nuisance arising on farms, there is a need to develop least-cost methods of controlling odours from piggery slurries so that more enterprises can afford to implement odour control measures. Aerobic treatment is an effective process for controlling slurry odours, but treatment times need to be minimised for economy. One problem with short treatment times is that the treated slurry does not necessarily remain stable during storage before land spreading. Previous work’ comparing the effects of the same period of aerobic treatment on slurries of 1.5 to 4.5% dry matter content (d.m.) showed that the slurry at 1.5% d.m. could, after treatment, be stored indefinitely without significant return of odour. In contrast at 4.5% d.m. the period of odour free storage was only a matter of days. Low dry matter contents can be achieved in two ways, either by dilution with water or by removing solids. Dilution with water, however, is unlikely to be practical and is expensive because of the cost of water, storage, transport and spreading. Removal of solids on the other hand reduces the volume to * AFRC Institute of Engineering Research, Silsoe, Bedford MK45 4HS, UK Received 14 November 1986; accepted in revised form 8 October 1987
199 0021-8634/88/030199 + IO %03.00/O
0 1988
The British Society for Research in Agricultural Engineering
200
CENTRIFUGATION FOR SEPARATING PIGGERY SLURRY, 3
Notation A
COD COD,, COD,, COD, M,
aerator power, kW, or the COD removal rate, kg COD/h Chemical oxygen demand of raw slurry, g/l residual biodegradable COD, g/l biodegradable COD in the feed, g/l feed COD, g/l raw slurry, dry matter content, %
M, centrifuged liquid dry matter content, % N number of pigs SRT solids retention time, d ST storage time, d V, mass of slurry aerated per pig each day, litre pig-place-’ day-’
be stored and reduces the spreading and transport costs. Removal of solids also reduces the liquid viscosity and because the solids are partly degradable the chemical oxygen demand (COD) is also reduced? but there will be an added cost4 for the process. Removal of solids from slurry by using mechanical slurry separators before any biological treatment is done has been advocated for many years. 5,6 Removing solids after biological treatment is less common and is usually only done if irrigation of the liquid is intended. Solids removal after biological treatment is arguably easier since the growth of large bacterial floes increases the numbers of large particles. In addition, degradation of soluble and colloidal compounds reduces the viscosity of the liquid. This makes centrifugation an appropriate process to consider. Removal of solids after aeration extends the stable storage period7 by about one third and reduces the storage volume needed. The solids (or fibre) removed should also be odour-free at the time of removal unlike those removed before treatment, which require composting to become odour free. This paper describes calculations to assess the cost of various combinations of solids removal and aerobic treatment. The cost of using combinations and the benefits obtained are compared. The object was to determine the effect of removing solids from slurry on the running costs of a treatment and storage installation designed to provide slurry for spreading on land without causing an odour nuisance. 2. The treatments considered Five combinations of aeration with centrifugation and/or separation were considered. The costs of using these combinations to treat the slurry from finishing herds of 2000 and 8000 pigs, and the cost of storing it for five and 30 days were calculated. The five combinations were: (1) aeration of raw slurry then storage; (2) separation of the raw slurry, aeration of the separated slurry then storage, (3) centrifugation of the raw slurry then aeration and storage, (4) aeration of raw slurry followed by centrifugation then storage, (5) separation of the raw slurry using a BSRP, aeration of the separated slurry, centrifugation of the aerated slurry then storage.
R. W. SNEATH
201 3. Calculation of the costs of slurry treatments
A model was developed on a BBC microcomputer with a 280 second processor running SuperCalc 2 spread sheet package to calculate the costs of the various treatment and separator options. 3.1. Input slurry properties The quantities and quality of the slurry which needs treating depends on farm circumstances. A value was derived from data on quantities of excreta from pigs,* equivalent to 4.1 kg d-’ pig-’ of slurry at 10% dry matter content. Where slurries of lower dry matter contents are produced, then dilution with water from washing or leaking drinkers or excessive water consumption by the pigs is assumed. The slurry COD value has been taken to vary with dry matter content,’ such that COD = 13.3M,.
(1) For separated slurry the COD is assumed to vary in the same way as for raw slurry CEqn (111. 3.2. The separator and centrifuge performance The performance of a brushed screen/roller press (BSRP) separator has been assessed previously with raw piggery slurry and the efficiency, derived from that work”-” is described in a related paper.’ The separator performance is assumed to be the same when slurry is separated after aerobic treatment for odour control. The performance characteristics of a decanting centrifuge in piggery slurry have been reported previously.3 Again, the performance was assumed to be the same with raw and aerated slurry. The capital costs of the BSRP separator installation which includes the separator, a gantry on which to mount the separator, a submersible electric pump to feed raw slurry to it, a reception pit and concrete pad, electrical equipment and wiring, and installation of all the equipment on site, was estimated at &15 000. Running costs were calculated from the estimated running times of the separator. Maintenance costs were charged at 2% of the capital cost but pro rata to the running time of the separator. The capital cost of the centrifuge installation included the same items as for the BSRP separator but the cost of the BSRP separator of g7000 was replaced by that of the centrifuge at &3OOOO making a total of 638 000. Running costs were calculated from measurements of power consumption3 and the running time of the centrifuge. Maintenance costs were calculated in the same way as for the BSRP separator. 3.3. Biological treatment process design The criteria adopted for the end point of the aerobic treatment process was that, after the required period of storage, the VFA level should be less than 0.23 g/l. This level has been related to the biodegradable COD remaining in the slurry at the end of treatment that is the residual biodegradable COD (COD,), by Williams et al.13 thus: _ST-36 _ _ ‘ODb = _ 2.685
(2)
To calculate the aerobic treatment time required to achieve this value of COD, a relationship derived experimentally by Williams et al.” is used. Evans et al.“,” developed
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CENTRIFUGATION FOR SEPARATING PIGGERY SLURRY, 3
equations based on Monod kinetics describing changes of COD during the continuous flow treatment of piggery slurry of the form: COD in treated slurry =
A + C x COD in feed slurry, 1 + (SRT Kd) >
where K, is the specific decay time, C is the non-biodegradable fraction, SRT is the residence time in the aerobic treatment and A is a constant, Williams established the values of the coefficients K,, C and A for COD changes as 0.4, 0.535 and 0.33 respectively in aerobic treatments of separated piggery slurry, with SRT between 1 day and 4 days and at a temperature of 33°C f 2°C. These were the same as those found by Evans et al.” for whole slurry in the range 25°C to 40°C. Since the return of offensive odour to the slurry (indicated by the VFA level) has been related only to the residual biodegradable COD in the treated slurry, then the term C (the non-biodegradable fraction) can be omitted from the equation. Thus Eqn (3) becomes 0.33 COD, = x COD,. 1 + (0.4SRT) Rearranging, to calculate the SRT to give the required CODb calculated from Eqn (2) gives
A minimum value of 0.7d SRT is used because aerobic treatment plants tend to become unstable with shorter SRTs. The aerator size required for the plant was estimated from the COD reduction which would occur. This was calculated from the difference between COD,, and COD,, so V
COD removed/d = (COD,, - COD,) x a 1ooo XNkd-’ or COD removed/h =
(COD,, - COD,)6 N 24000 ’
(6)
(7)
which, since the efficiency of the aerator was assumed to be 1 kg COD removed/kWh of electricity used, is numerically equal to the aerator power in kW. The aeration vessel volume was calculated from the product of SRT and daily quantity of slurry to be treated and, to allow for the inevitable intermittent foaming that occurs during aerobic treatment, then multiplied by 1.6. Where aerobic treatment was followed by centrifugation, the stable storage time was increased’ by 33%. Consequently, for the same storage time, the required biodegradable COD level was reduced by 33%.
3.4. Aeration vessel and storage costs The 1986 costs of circular glass coated steel above ground tanks were obtained from a major British manufacturer.l6 These included the costs of the tank, the concrete base and all erection and labour costs. The storage volume required was calculated from the mass of slurry produced by the pigs and the mass reductions caused by removal of solids before or after aerobic treatment and the reduction in the total solid content due to aerobic treatment. The density of the slurry was taken to be 1000 kg/m3.
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3.5. Electrical cost
All the calculations assumed that adequate mains electric power capacity was already available on site. A unit cost of electricity of iO.OS/kWh was used and no allowance was made for standing charges. However, the reduction in electricity costs available for night time and off-peak running will offset the standing charge in England. 3.6. Annual costs The annual costs of running the various systems was made up of the power costs (see Section 3.9, maintenance costs (see Section 3.2) and the capital charges on the equipment and tanks. These annual capital charges were calculated using a discounted cash flow method.” The interest rate on borrowed money was taken as 15% pa. An annual inflation rate of 5% was assumed, The cost of the separator, centrifuge and tanks were written off over 10 years and the aerator over 5 years. 3.1. Items excluded from calculations The treatment processes considered here were costed to illustrate the differences between various ways of achieving specific stable storage times. Therefore the costs of common items such as feed pumps, discharge pumps, electrical control equipment, wiring, etc., have not been included. The site costs and site preparation are also excluded together with the possible loss of income from the site. The treatment regimes also affect the fertilizer value of the treated slurry, but this has not been included. Aerobic treatment produces large quantities of heat which it is possible to extract and use; the quantity varies with each of the five treatments but their value has not been evaluated. Also, the methods of spreading the treated slurries on land and the costs of that process have not been considered. 4. The economics of separation combined with aerobic treatment 4.1. Relative costs of the treatment methods The calculations revealed that the total costs of treating and storing slurry so that it will not cause an offensive odour varies depending on the initial dry matter content and the
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Fig. I. Cost of treatment, and storage for 5 days. Slurry from 2000 pigs. -? ?? -, Aeration then centrifugation; H ‘. ‘, centrzjiigation then aeration; -v-, raw slurry; . -+- ., separation, aeration then centrzjiigation; . -A- ., separation then aeration
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CENTRIFUGATION FOR SEPARATING PIGGERY SLURRY, 3
Fig. 2. Cost of treatment, and storage for 30 days. Slurry from 2000 pigs. Key as Fig. 1
method of treatment chosen; economies of scale make the cost per pig lower in the larger herd. 4.1.1. For the 2000 pig herd If the odour from slurry is to be made inoffensive and the slurry stored for 5 days after treatment then, for slurry with an initial dry matter content of more than 4.5% coming from a 2000 pig herd, a BSRP separator used before aeration produces the lowest running costs (Fig. I). Aeration of raw slurry has the next lowest cost. When the initial dry matter content is less than 4*5x, aerobic treatment of raw unseparated slurry and then storage is the cheapest. Removing solids with the decanting centrifuge prior to aerobic treatment is considerably cheaper than aerating first and then using the centrifuge to remove the solids. Using the centrifuge after aeration of separated slurry adds to the total cost even though less aerobic treatment of the slurry and less storage capacity is required. When the slurry from 2000 pigs must be stored, with an inoffensive odour for 30 days after treatment, overall treatment costs, naturally, are higher but the relative costs of the processes change (see Fig. 2). For higher dry matter slurries separation using a BSRP remains the cheapest option; this time only slurries of less than 3.0’4 dry matter content are treated more cheaply if separation is not used. It is interesting to observe that the cost of treating raw
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8 %
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8
dry matter
Fig. 3. Cost of treatment, and storage for 5 days. Slurry from 8000 pigs. Key as Fig. 1
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R. W. SNEATH
2
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1
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o/o dry matter
Fig. 4. Cost qf‘treatment and storage for 30 days. Slurry from 8000 pigs. Key as Fig. 1
separated slurry and storing for 30 days remains almost constant regardless of the initial slurry dry matter content, whereas when a separator or centrifuge is used before aeration the cost decreases as the initial dry matter increases. With centrifugation after aerobic treatment, costs increase with slurry dry matter. Using the centrifuge after aeration is again more costly than removing the solids before aeration, also centrifuging already separated and aerated slurry is not worthwhile. 4.1.2. For the 8000 pig herd When the 8000 pig herd is considered and slurry is stored for five days (Fig. 3), overall costs per pig of each option are much lower than for the 2000 pig herd with the exception of aerating raw slurry; the costs in this case are reduced by only about EO.l2/pig. When slurry is stored for 5 days, centrifuging before aeration and separating before aeration have similar costs for slurries between 3% and 5% dry matter (Fig. 3). The centrifuge system becomes more expensive to run at lower dry matter content but cheaper at higher initial dry matter content. Aeration of raw slurry provides the lowest cost for 1.5% and 2% slurries but the second most expensive above 4.5% dry matter. Again, centrifuging after aerating raw slurry is the most expensive system. Centrifuging slurry after separation and aeration adds about &0.4O/pig to the cost where the centrifuge is not included as a final stage.
storoqe
Power
: ! Dry matter content of row slurry, TO
Fig. 5. Cost components for aerating raw slurry, 8000 pigs and 5 days’storage
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CENTRIFUGATION FOR SEPARATING PIGGERY SLURRY, 3
2
.P < w s- I u”
Aerotor power
Aerator +VSSSd
Centrifuge
C Dry matter content of row slurry, V.
Fig. 6. Cost components for centrifuging after aeration, 8000 pigs and 5 days’ storage
If slurry from 8000 pigs must be stored for 30 days (Fig. 4) for all dry matter contents, centrifugation before aerobic treatment is the cheapest system to run. Separation, using the BSRP system, before aeration and separation, aeration and centrifugation are about equal in cost. The cost of treatment and storage using each of these three systems decreases as the slurry dry matter content increases. The cost of aerating raw slurry is again independent of dry matter content and the cost of centrifuging after aeration increases with dry matter content and is not worthwhile at any dry matter content. As an alternative to using the decanting centrifuge, with low dry matter slurries, some simpler less expensive separation devices may be more cost effective. The results of the laboratory sieving experiments7 indicate that a O-075 mm hole size sieve removed a similar quantity and range of particles similar to a decanting centrifuge. Vibrating screen separators therefore have the potential for reducing the cost of the aeration process to a similar extent to that which a decanting centrifuge does when used after aerobic treatment, but for a lower capital and running cost. (The main disadvantage of vibrating screens, for separating raw slurry is the low dry matter content of the solids removed.) The performance of a commercial machine fitted with this, very fine, mesh size would need to be measured to determine the possible running costs and capital requirements.
0
4
a
Fig. 7. Cost components for centrifuging followed by aeration, 8000 pigs and 5 days’ storage
R.
W.
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SNEATH
Storcge
L Aerator power
i.:” Aerator c:.:: +vessel
Centrifuge
0
4
J
0
Fig. 8. Cost components,for centrifugation followed by aeration, 8000 pigs and 30 days’storage
4.2. Changes in cost components The total costs of treatment and storage have been divided into four components for comparison: the cost of storage; the cost of electrical power for aeration; the capital cost of the aerator and aeration vessel; and the capital and running cost of the centrifuge. The costs of these four components are illustrated in Figs 5 to 8. For the 8000 pig herd and 5 days’ storage (Fig. .5), it can be seen that overall costs of aerating raw slurry and storage increase with dry matter content mainly because of the increase in electricity costs for the aerator, the reduction in storage costs is countered by the increase in the capital costs of the aerator and aeration vessel. When the slurry is centrifuged after aeration total costs increase with higher dry matter slurry, again because of electricity costs for the aeration (Fig. 6). When the slurry is less than 3% dry matter running costs of the centrifuge increase and the larger stores required cost more. The aerator’s power consumption is much less when the centrifuge is used before aeration (Fig. 7), and remains almost constant with slurry dry matter, whereas the storage costs and the centrifuge running costs increase with the higher volumes of low dry matter slurry. In contrast, if the slurry from 8000 pigs must be stored for 30 days, using the centrifuge before aeration (Fig. 8), the aeration power costs increase with the slurry dry matter content. Total costs, however, are reduced with increasing dry matter content and reducing volume because the cost of storage is a lesser part of the total cost now and the cost of centrifuging decreases because the power consumed is lower since a smaller volume is processed. 5. Conclusions
(1) In order to minimize costs of treating piggery slurry to control odours for a herd of 2000 pigs, high dry matter slurry should be separated with a BSRP separator before aeration, but low dry matter slurry should be aerated whole. (2) When controlling odour from low dry matter slurry for a large pig herd to give a storage time of 5 days, aeration of whole slurry and separation before aeration have similar costs per pig. (31 If the slurry from the large herd is of a high dry matter content and must be stored for 5 days, then centrifuging the slurry before aeration is the lowest cost treatment.
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CENTRIFUGATION FOR SEPARATING PIGGERY SLURRY,
3
(4) If slurry must be aerated to remove odours and stored in an inoffensive state for 30 days, then centrifuging before aeration is the lowest cost treatment irrespective of dry matter content. (5) In all cases, except when slurry from a small herd needs storing for only 5 days, the cost of treating and storing piggery slurry is lower when the dry matter content is high. Acknowledgements I thank Dr A. G. Williams, Dr V. R. Phillips and Dr J. M. Randall of the Buildings and Livestock Division for their help in preparing this paper. References 1 2
3
4 5 6
7
a 9 10 11
12 13
14 16 16
l7
Environmental Health Officers Report 1984/85 IEHO, Chadwick House, Rushworth Street, London SE1 Williams, A. G.; Shaw, M.; Adams, S. J. The biological stability of aerobically-treated pig slurry during storage. Journal of Agricultural Engineering Research 1984, 29: 231-239 Sneath, R. W.; Shaw, W.; Williams, A. G. Centrifugation for separating piggery slurry. I. The performance of a decanting centrifuge. Journal of Agricultural Engineering Research 1988, 39: 181-190 Sneath, R. W. Centrifugation for separating piggery slurry. 2. Economic effects of slurry storage. Journal of Agricultural Engineering Research 1988, 39: 191-197 Hawkins, J. C. The handling of animal wastes. Veterinary Record 1978, 102: 162-165 Osborne, L. E.; Hepherd, R. Q.; Sneath, R. W. An integrated separation, aerobic treatment and sludge dewatering system for pig slurry. Journal of Agricultural Engineering Research 1976, 21: 109-120 Sneath, R. W. The effects of sieving and centrifugation on the VFA levels in aerobically treated slurry. Divisional Note DN/l370, National Institute of Agricultural Engineering, Silsoe Studies on farm livestock wastes. Agricultural Research Council, London 1976, pp. 39-69 Evans, M. R.; Hissett, R.; Smith, M. P. W.; Ellam, D. F. Characteristics of slurry from fattening pigs, and comparison with slurry from laying hens. Agriculture and Environment 1978,4: 77-83 Pain, B. F.; Hepherd, R. Q.; Pittman, R. J. Factors affecting the performance of four slurry separating machines. Journal of Agricultural Engineering Research 1978, 23: 231-242 Sneath, R. W.; Widdows, C. F. The slurry treatment plant at the Integrated Piggery. Part III Relationship between feed, water and slurry. Divisional Note DN/ll88, National Institute of Agricultural Engineering, Silsoe 1983 (unpubl.) Redman, P. L.; Brewer, A. J.; Balls, R. C. The separators for slurry MAFF. ADAS Mechanisation (private communication), 1983 Williams, A. G.; Shaw, M.; Selviah, C. M.; Cumby, R. J. Oxygen requirements for controlling odours from pig slurry by aeration. In: Odour Prevention and Control of Organic Sludge and Livestock Farming (Nielsen, V. C.; Voorburg, J. H.; L’Hermite, D., eds). Elsevier Applied Science Publishers 1986, pp. 258-272 Evans, M. R.; His&t, R. S.; Smith, M. P. W.; Ellam, D. F.; Baines, S. The effect of micro-organism residence time on aerobic treatment of piggery waste. Agricultural Wastes 1979, 1: 67-85 Evans, M. R.; Svoboda, 1. F.; Baines, S. Heat from aerobic treatment of piggery slurry. Journal of Agricultural Engineering Research 1983, 27: 45-50 Boythorpe Ltd. Personal communication Audsley, E.; Wheeler, J. A. The annual cost of machinery calculated using actual cash flows. Journal of Agricultural Engineering Research 1979, 23: 189-201
Ccntrifugation for Separating Piggery Slurry 3. Economic Effects on Aerobic Methods of Odour Control R. W. SNEATH* The costs of reducing the odours from piggery slurries to an inoffensive level and storing that slurry in an inoffensive state are discussed. Five different combinations of aerobic treatment and separation for slurries containing initially between 1.5 and 8% d.m., from 2000 and 80@0 pigs, and with storage for five days or 30 days are considered. The five combinations are aeration of raw slurry; separation then aeration; centrifugation then aeration; aeration of raw slurry then centrifugation; and separation then aeration then removal of more solids by centrifugation. The costs of storing the effluents from these processes are also included. For slurry from the smaller herd and for a short storage period, the cost of aerobic treatment of raw slurry is lower than other treatments when the slurry is less than 4.5% d.m. Using a brushed screen/roller press separator to remove solids before aeration produces the cheapest control of odours for slurries above 4.5% d.m. When 30 days’ storage or more is required, separation before aeration is cheaper for slurries with more than 3% d.m. and the costs of using a decanting centrifuge (costing t38000) before aeration is similar to aerating raw slurry at 6% d.m. With a large (8000 pig) herd, using the centrifuge before aeration to produce slurry that is inoffensive for 30 days is the cheapest method at all dry matter contents. When storage is required for only five days, separation or centrifugation before aeration have similar costs, these costs are similar to the cost of aerating raw slurry when it contains less than 3% dry matter, but as dry matter content of the raw slurry increases the cost rises steeply. Removing solids from slurry by centrifugation after aeration increases the overall costs compared to aerating raw slurry.
1. Introduction
In 1984/85, Environmental Health Officers received 1469 justifiable complaints about odours from slurry stores and slurry spreading.’ With the increased incidence of odour nuisance arising on farms, there is a need to develop least-cost methods of controlling odours from piggery slurries so that more enterprises can afford to implement odour control measures. Aerobic treatment is an effective process for controlling slurry odours, but treatment times need to be minimised for economy. One problem with short treatment times is that the treated slurry does not necessarily remain stable during storage before land spreading. Previous work’ comparing the effects of the same period of aerobic treatment on slurries of 1.5 to 4.5% dry matter content (d.m.) showed that the slurry at 1.5% d.m. could, after treatment, be stored indefinitely without significant return of odour. In contrast at 4.5% d.m. the period of odour free storage was only a matter of days. Low dry matter contents can be achieved in two ways, either by dilution with water or by removing solids. Dilution with water, however, is unlikely to be practical and is expensive because of the cost of water, storage, transport and spreading. Removal of solids on the other hand reduces the volume to * AFRC Institute of Engineering Research, Silsoe, Bedford MK45 4HS, UK Received 14 November 1986; accepted in revised form 8 October 1987
199 0021-8634/88/030199 + IO %03.00/O
0 1988
The British Society for Research in Agricultural Engineering
200
CENTRIFUGATION FOR SEPARATING PIGGERY SLURRY, 3
Notation A
COD COD,, COD,, COD, M,
aerator power, kW, or the COD removal rate, kg COD/h Chemical oxygen demand of raw slurry, g/l residual biodegradable COD, g/l biodegradable COD in the feed, g/l feed COD, g/l raw slurry, dry matter content, %
M, centrifuged liquid dry matter content, % N number of pigs SRT solids retention time, d ST storage time, d V, mass of slurry aerated per pig each day, litre pig-place-’ day-’
be stored and reduces the spreading and transport costs. Removal of solids also reduces the liquid viscosity and because the solids are partly degradable the chemical oxygen demand (COD) is also reduced? but there will be an added cost4 for the process. Removal of solids from slurry by using mechanical slurry separators before any biological treatment is done has been advocated for many years. 5,6 Removing solids after biological treatment is less common and is usually only done if irrigation of the liquid is intended. Solids removal after biological treatment is arguably easier since the growth of large bacterial floes increases the numbers of large particles. In addition, degradation of soluble and colloidal compounds reduces the viscosity of the liquid. This makes centrifugation an appropriate process to consider. Removal of solids after aeration extends the stable storage period7 by about one third and reduces the storage volume needed. The solids (or fibre) removed should also be odour-free at the time of removal unlike those removed before treatment, which require composting to become odour free. This paper describes calculations to assess the cost of various combinations of solids removal and aerobic treatment. The cost of using combinations and the benefits obtained are compared. The object was to determine the effect of removing solids from slurry on the running costs of a treatment and storage installation designed to provide slurry for spreading on land without causing an odour nuisance. 2. The treatments considered Five combinations of aeration with centrifugation and/or separation were considered. The costs of using these combinations to treat the slurry from finishing herds of 2000 and 8000 pigs, and the cost of storing it for five and 30 days were calculated. The five combinations were: (1) aeration of raw slurry then storage; (2) separation of the raw slurry, aeration of the separated slurry then storage, (3) centrifugation of the raw slurry then aeration and storage, (4) aeration of raw slurry followed by centrifugation then storage, (5) separation of the raw slurry using a BSRP, aeration of the separated slurry, centrifugation of the aerated slurry then storage.
R. W. SNEATH
201 3. Calculation of the costs of slurry treatments
A model was developed on a BBC microcomputer with a 280 second processor running SuperCalc 2 spread sheet package to calculate the costs of the various treatment and separator options. 3.1. Input slurry properties The quantities and quality of the slurry which needs treating depends on farm circumstances. A value was derived from data on quantities of excreta from pigs,* equivalent to 4.1 kg d-’ pig-’ of slurry at 10% dry matter content. Where slurries of lower dry matter contents are produced, then dilution with water from washing or leaking drinkers or excessive water consumption by the pigs is assumed. The slurry COD value has been taken to vary with dry matter content,’ such that COD = 13.3M,.
(1) For separated slurry the COD is assumed to vary in the same way as for raw slurry CEqn (111. 3.2. The separator and centrifuge performance The performance of a brushed screen/roller press (BSRP) separator has been assessed previously with raw piggery slurry and the efficiency, derived from that work”-” is described in a related paper.’ The separator performance is assumed to be the same when slurry is separated after aerobic treatment for odour control. The performance characteristics of a decanting centrifuge in piggery slurry have been reported previously.3 Again, the performance was assumed to be the same with raw and aerated slurry. The capital costs of the BSRP separator installation which includes the separator, a gantry on which to mount the separator, a submersible electric pump to feed raw slurry to it, a reception pit and concrete pad, electrical equipment and wiring, and installation of all the equipment on site, was estimated at &15 000. Running costs were calculated from the estimated running times of the separator. Maintenance costs were charged at 2% of the capital cost but pro rata to the running time of the separator. The capital cost of the centrifuge installation included the same items as for the BSRP separator but the cost of the BSRP separator of g7000 was replaced by that of the centrifuge at &3OOOO making a total of 638 000. Running costs were calculated from measurements of power consumption3 and the running time of the centrifuge. Maintenance costs were calculated in the same way as for the BSRP separator. 3.3. Biological treatment process design The criteria adopted for the end point of the aerobic treatment process was that, after the required period of storage, the VFA level should be less than 0.23 g/l. This level has been related to the biodegradable COD remaining in the slurry at the end of treatment that is the residual biodegradable COD (COD,), by Williams et al.13 thus: _ST-36 _ _ ‘ODb = _ 2.685
(2)
To calculate the aerobic treatment time required to achieve this value of COD, a relationship derived experimentally by Williams et al.” is used. Evans et al.“,” developed
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CENTRIFUGATION FOR SEPARATING PIGGERY SLURRY, 3
equations based on Monod kinetics describing changes of COD during the continuous flow treatment of piggery slurry of the form: COD in treated slurry =
A + C x COD in feed slurry, 1 + (SRT Kd) >
where K, is the specific decay time, C is the non-biodegradable fraction, SRT is the residence time in the aerobic treatment and A is a constant, Williams established the values of the coefficients K,, C and A for COD changes as 0.4, 0.535 and 0.33 respectively in aerobic treatments of separated piggery slurry, with SRT between 1 day and 4 days and at a temperature of 33°C f 2°C. These were the same as those found by Evans et al.” for whole slurry in the range 25°C to 40°C. Since the return of offensive odour to the slurry (indicated by the VFA level) has been related only to the residual biodegradable COD in the treated slurry, then the term C (the non-biodegradable fraction) can be omitted from the equation. Thus Eqn (3) becomes 0.33 COD, = x COD,. 1 + (0.4SRT) Rearranging, to calculate the SRT to give the required CODb calculated from Eqn (2) gives
A minimum value of 0.7d SRT is used because aerobic treatment plants tend to become unstable with shorter SRTs. The aerator size required for the plant was estimated from the COD reduction which would occur. This was calculated from the difference between COD,, and COD,, so V
COD removed/d = (COD,, - COD,) x a 1ooo XNkd-’ or COD removed/h =
(COD,, - COD,)6 N 24000 ’
(6)
(7)
which, since the efficiency of the aerator was assumed to be 1 kg COD removed/kWh of electricity used, is numerically equal to the aerator power in kW. The aeration vessel volume was calculated from the product of SRT and daily quantity of slurry to be treated and, to allow for the inevitable intermittent foaming that occurs during aerobic treatment, then multiplied by 1.6. Where aerobic treatment was followed by centrifugation, the stable storage time was increased’ by 33%. Consequently, for the same storage time, the required biodegradable COD level was reduced by 33%.
3.4. Aeration vessel and storage costs The 1986 costs of circular glass coated steel above ground tanks were obtained from a major British manufacturer.l6 These included the costs of the tank, the concrete base and all erection and labour costs. The storage volume required was calculated from the mass of slurry produced by the pigs and the mass reductions caused by removal of solids before or after aerobic treatment and the reduction in the total solid content due to aerobic treatment. The density of the slurry was taken to be 1000 kg/m3.
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3.5. Electrical cost
All the calculations assumed that adequate mains electric power capacity was already available on site. A unit cost of electricity of iO.OS/kWh was used and no allowance was made for standing charges. However, the reduction in electricity costs available for night time and off-peak running will offset the standing charge in England. 3.6. Annual costs The annual costs of running the various systems was made up of the power costs (see Section 3.9, maintenance costs (see Section 3.2) and the capital charges on the equipment and tanks. These annual capital charges were calculated using a discounted cash flow method.” The interest rate on borrowed money was taken as 15% pa. An annual inflation rate of 5% was assumed, The cost of the separator, centrifuge and tanks were written off over 10 years and the aerator over 5 years. 3.1. Items excluded from calculations The treatment processes considered here were costed to illustrate the differences between various ways of achieving specific stable storage times. Therefore the costs of common items such as feed pumps, discharge pumps, electrical control equipment, wiring, etc., have not been included. The site costs and site preparation are also excluded together with the possible loss of income from the site. The treatment regimes also affect the fertilizer value of the treated slurry, but this has not been included. Aerobic treatment produces large quantities of heat which it is possible to extract and use; the quantity varies with each of the five treatments but their value has not been evaluated. Also, the methods of spreading the treated slurries on land and the costs of that process have not been considered. 4. The economics of separation combined with aerobic treatment 4.1. Relative costs of the treatment methods The calculations revealed that the total costs of treating and storing slurry so that it will not cause an offensive odour varies depending on the initial dry matter content and the
2
4
6
El
2
4
6
0
% dry matter
Fig. I. Cost of treatment, and storage for 5 days. Slurry from 2000 pigs. -? ?? -, Aeration then centrifugation; H ‘. ‘, centrzjiigation then aeration; -v-, raw slurry; . -+- ., separation, aeration then centrzjiigation; . -A- ., separation then aeration
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CENTRIFUGATION FOR SEPARATING PIGGERY SLURRY, 3
Fig. 2. Cost of treatment, and storage for 30 days. Slurry from 2000 pigs. Key as Fig. 1
method of treatment chosen; economies of scale make the cost per pig lower in the larger herd. 4.1.1. For the 2000 pig herd If the odour from slurry is to be made inoffensive and the slurry stored for 5 days after treatment then, for slurry with an initial dry matter content of more than 4.5% coming from a 2000 pig herd, a BSRP separator used before aeration produces the lowest running costs (Fig. I). Aeration of raw slurry has the next lowest cost. When the initial dry matter content is less than 4*5x, aerobic treatment of raw unseparated slurry and then storage is the cheapest. Removing solids with the decanting centrifuge prior to aerobic treatment is considerably cheaper than aerating first and then using the centrifuge to remove the solids. Using the centrifuge after aeration of separated slurry adds to the total cost even though less aerobic treatment of the slurry and less storage capacity is required. When the slurry from 2000 pigs must be stored, with an inoffensive odour for 30 days after treatment, overall treatment costs, naturally, are higher but the relative costs of the processes change (see Fig. 2). For higher dry matter slurries separation using a BSRP remains the cheapest option; this time only slurries of less than 3.0’4 dry matter content are treated more cheaply if separation is not used. It is interesting to observe that the cost of treating raw
I
I
1
1
1
2
4
6
8 %
2
4
6
8
dry matter
Fig. 3. Cost of treatment, and storage for 5 days. Slurry from 8000 pigs. Key as Fig. 1
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2
4
6
I
I
I
1
2
4
6
6
o/o dry matter
Fig. 4. Cost qf‘treatment and storage for 30 days. Slurry from 8000 pigs. Key as Fig. 1
separated slurry and storing for 30 days remains almost constant regardless of the initial slurry dry matter content, whereas when a separator or centrifuge is used before aeration the cost decreases as the initial dry matter increases. With centrifugation after aerobic treatment, costs increase with slurry dry matter. Using the centrifuge after aeration is again more costly than removing the solids before aeration, also centrifuging already separated and aerated slurry is not worthwhile. 4.1.2. For the 8000 pig herd When the 8000 pig herd is considered and slurry is stored for five days (Fig. 3), overall costs per pig of each option are much lower than for the 2000 pig herd with the exception of aerating raw slurry; the costs in this case are reduced by only about EO.l2/pig. When slurry is stored for 5 days, centrifuging before aeration and separating before aeration have similar costs for slurries between 3% and 5% dry matter (Fig. 3). The centrifuge system becomes more expensive to run at lower dry matter content but cheaper at higher initial dry matter content. Aeration of raw slurry provides the lowest cost for 1.5% and 2% slurries but the second most expensive above 4.5% dry matter. Again, centrifuging after aerating raw slurry is the most expensive system. Centrifuging slurry after separation and aeration adds about &0.4O/pig to the cost where the centrifuge is not included as a final stage.
storoqe
Power
: ! Dry matter content of row slurry, TO
Fig. 5. Cost components for aerating raw slurry, 8000 pigs and 5 days’storage
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CENTRIFUGATION FOR SEPARATING PIGGERY SLURRY, 3
2
.P < w s- I u”
Aerotor power
Aerator +VSSSd
Centrifuge
C Dry matter content of row slurry, V.
Fig. 6. Cost components for centrifuging after aeration, 8000 pigs and 5 days’ storage
If slurry from 8000 pigs must be stored for 30 days (Fig. 4) for all dry matter contents, centrifugation before aerobic treatment is the cheapest system to run. Separation, using the BSRP system, before aeration and separation, aeration and centrifugation are about equal in cost. The cost of treatment and storage using each of these three systems decreases as the slurry dry matter content increases. The cost of aerating raw slurry is again independent of dry matter content and the cost of centrifuging after aeration increases with dry matter content and is not worthwhile at any dry matter content. As an alternative to using the decanting centrifuge, with low dry matter slurries, some simpler less expensive separation devices may be more cost effective. The results of the laboratory sieving experiments7 indicate that a O-075 mm hole size sieve removed a similar quantity and range of particles similar to a decanting centrifuge. Vibrating screen separators therefore have the potential for reducing the cost of the aeration process to a similar extent to that which a decanting centrifuge does when used after aerobic treatment, but for a lower capital and running cost. (The main disadvantage of vibrating screens, for separating raw slurry is the low dry matter content of the solids removed.) The performance of a commercial machine fitted with this, very fine, mesh size would need to be measured to determine the possible running costs and capital requirements.
0
4
a
Fig. 7. Cost components for centrifuging followed by aeration, 8000 pigs and 5 days’ storage
R.
W.
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Storcge
L Aerator power
i.:” Aerator c:.:: +vessel
Centrifuge
0
4
J
0
Fig. 8. Cost components,for centrifugation followed by aeration, 8000 pigs and 30 days’storage
4.2. Changes in cost components The total costs of treatment and storage have been divided into four components for comparison: the cost of storage; the cost of electrical power for aeration; the capital cost of the aerator and aeration vessel; and the capital and running cost of the centrifuge. The costs of these four components are illustrated in Figs 5 to 8. For the 8000 pig herd and 5 days’ storage (Fig. .5), it can be seen that overall costs of aerating raw slurry and storage increase with dry matter content mainly because of the increase in electricity costs for the aerator, the reduction in storage costs is countered by the increase in the capital costs of the aerator and aeration vessel. When the slurry is centrifuged after aeration total costs increase with higher dry matter slurry, again because of electricity costs for the aeration (Fig. 6). When the slurry is less than 3% dry matter running costs of the centrifuge increase and the larger stores required cost more. The aerator’s power consumption is much less when the centrifuge is used before aeration (Fig. 7), and remains almost constant with slurry dry matter, whereas the storage costs and the centrifuge running costs increase with the higher volumes of low dry matter slurry. In contrast, if the slurry from 8000 pigs must be stored for 30 days, using the centrifuge before aeration (Fig. 8), the aeration power costs increase with the slurry dry matter content. Total costs, however, are reduced with increasing dry matter content and reducing volume because the cost of storage is a lesser part of the total cost now and the cost of centrifuging decreases because the power consumed is lower since a smaller volume is processed. 5. Conclusions
(1) In order to minimize costs of treating piggery slurry to control odours for a herd of 2000 pigs, high dry matter slurry should be separated with a BSRP separator before aeration, but low dry matter slurry should be aerated whole. (2) When controlling odour from low dry matter slurry for a large pig herd to give a storage time of 5 days, aeration of whole slurry and separation before aeration have similar costs per pig. (31 If the slurry from the large herd is of a high dry matter content and must be stored for 5 days, then centrifuging the slurry before aeration is the lowest cost treatment.
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CENTRIFUGATION FOR SEPARATING PIGGERY SLURRY,
3
(4) If slurry must be aerated to remove odours and stored in an inoffensive state for 30 days, then centrifuging before aeration is the lowest cost treatment irrespective of dry matter content. (5) In all cases, except when slurry from a small herd needs storing for only 5 days, the cost of treating and storing piggery slurry is lower when the dry matter content is high. Acknowledgements I thank Dr A. G. Williams, Dr V. R. Phillips and Dr J. M. Randall of the Buildings and Livestock Division for their help in preparing this paper. References 1 2
3
4 5 6
7
a 9 10 11
12 13
14 16 16
l7
Environmental Health Officers Report 1984/85 IEHO, Chadwick House, Rushworth Street, London SE1 Williams, A. G.; Shaw, M.; Adams, S. J. The biological stability of aerobically-treated pig slurry during storage. Journal of Agricultural Engineering Research 1984, 29: 231-239 Sneath, R. W.; Shaw, W.; Williams, A. G. Centrifugation for separating piggery slurry. I. The performance of a decanting centrifuge. Journal of Agricultural Engineering Research 1988, 39: 181-190 Sneath, R. W. Centrifugation for separating piggery slurry. 2. Economic effects of slurry storage. Journal of Agricultural Engineering Research 1988, 39: 191-197 Hawkins, J. C. The handling of animal wastes. Veterinary Record 1978, 102: 162-165 Osborne, L. E.; Hepherd, R. Q.; Sneath, R. W. An integrated separation, aerobic treatment and sludge dewatering system for pig slurry. Journal of Agricultural Engineering Research 1976, 21: 109-120 Sneath, R. W. The effects of sieving and centrifugation on the VFA levels in aerobically treated slurry. Divisional Note DN/l370, National Institute of Agricultural Engineering, Silsoe Studies on farm livestock wastes. Agricultural Research Council, London 1976, pp. 39-69 Evans, M. R.; Hissett, R.; Smith, M. P. W.; Ellam, D. F. Characteristics of slurry from fattening pigs, and comparison with slurry from laying hens. Agriculture and Environment 1978,4: 77-83 Pain, B. F.; Hepherd, R. Q.; Pittman, R. J. Factors affecting the performance of four slurry separating machines. Journal of Agricultural Engineering Research 1978, 23: 231-242 Sneath, R. W.; Widdows, C. F. The slurry treatment plant at the Integrated Piggery. Part III Relationship between feed, water and slurry. Divisional Note DN/ll88, National Institute of Agricultural Engineering, Silsoe 1983 (unpubl.) Redman, P. L.; Brewer, A. J.; Balls, R. C. The separators for slurry MAFF. ADAS Mechanisation (private communication), 1983 Williams, A. G.; Shaw, M.; Selviah, C. M.; Cumby, R. J. Oxygen requirements for controlling odours from pig slurry by aeration. In: Odour Prevention and Control of Organic Sludge and Livestock Farming (Nielsen, V. C.; Voorburg, J. H.; L’Hermite, D., eds). Elsevier Applied Science Publishers 1986, pp. 258-272 Evans, M. R.; His&t, R. S.; Smith, M. P. W.; Ellam, D. F.; Baines, S. The effect of micro-organism residence time on aerobic treatment of piggery waste. Agricultural Wastes 1979, 1: 67-85 Evans, M. R.; Svoboda, 1. F.; Baines, S. Heat from aerobic treatment of piggery slurry. Journal of Agricultural Engineering Research 1983, 27: 45-50 Boythorpe Ltd. Personal communication Audsley, E.; Wheeler, J. A. The annual cost of machinery calculated using actual cash flows. Journal of Agricultural Engineering Research 1979, 23: 189-201