Minimum-Cost Biofilters for Reducing Odours and Other Aerial Emissions from Livestock Buildings: Part 2, A Model to Analyse the Influence of Design Parameters on Annual Costs

J. agric. Engng Res. (1996) 64, 155 – 164

Minimum-Cost Biofilters for Reducing Odours and Other Aerial Emissions from Livestock Buildings: Part 2, A Model to Analyse the Influence of Design Parameters on Annual Costs I. M. Scotford; C. H. Burton; V. R. Phillips Silsoe Research Institute, Wrest Park, Silsoe, Bedford MK45 4HS, UK (Received 2 August 1995; accepted in revised form 12 February 1996)

A model to optimize the design of biofilters for livestock buildings is presented. This was developed from four sub-models covering biofilter size, biological performance, pressure drop and costs. The sub-model for biological performance predicts that for a specified duty, residence time multiplied by effective surface area of packing is constant. Hence direct comparison of different packing media can be made without the need to quantify the difficult parameter of odour. Five different packing media were compared, based on given inputs. The model predicts that peat and heather, a common packing medium, is the cheapest packing when fewer than 5000 broilers are housed.

emissions are not controlled, while if too high the biofilter will be larger and more costly than necessary. Wright4 reports residence times of between 10 and 60 s, Ottengraf and Diks5 10 to 30 s and Zeisig6 less than 10 s. These results suggest that residence time need not be more than about 1 min.

÷ 1996 Silsoe Research Institute

1. Introduction When designing biofilters for livestock buildings, the most important factors are the maximum specific airflow rate that the biofilter must treat and the residence time to give the required level of treatment. The former is usually the maximum ventilation rate of the livestock building. If residence time and depth of bed are known, the cross-sectional area of the biofilter can then be calculated. However, in practice biofilters are rarely sized in this way. Rodhe et al. ,1 for example, determined the size by specifying a maximum specific airflow rate per unit area of bed (m3 s21 m22) with a bed depth of 0?5 m. Scholtens and Demmers2 specified maximum airflow rate per unit area of bed (m3 s21 m22), depth of bed (m) and required plan area of bed per pig (m2 / pig). Scholtens et al.3 determined the size by specifying maximum airflow rate per unit area of bed (m3 s21 m22), depth of bed (m) and maximum ammonia (NH3) concentration (g / m3 of air). Any of these methods can be used, depending on the data available, but the correct residence time must be used for efficient biofiltration. If too low, the odours and 0021-8634 / 96 / 060155 1 09 $18.00 / 0

155

Notation

a a, b A Ca Cc Ce Cm Cf Cp Cr Ct D E h H i Kr Kw k1 k2 Lm M n N DP Q R t

effective surface area of packing, m2 / m3 constants cross-sectional area of biofilter, m2 annual construction cost, £ total construction cost, £ annual power consumption cost, £ total packing medium cost, £ packing medium cost, £ / m3 annual filter medium cost, £ annual maintenance cost, £ cost of treatment per animal, £ diffusivity, m2 / s average ventilation rate over a period of one year, % of maximum fan efficiency, % packing depth, m interest rate on loan, % zero order kinetic rate, kg / m3 s cost of electricity, £ / kWh constant, 4400 m2 s / m3 constant, 430 m life expectancy of filter medium, yr Henry’s constant number of animals housed throughput of animals per year pressure drop, Pa maximum ventilation rate, m3 / s residence time, s term of loan, yr ÷ 1996 Silsoe Research Institute

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u

y V w W X ge X go

superficial air velocity or ‘‘empty pipe’’ velocity, m / s maximum ventilation rate per unit live weight of animal, m3 / s kg biofilter volume, m3 average weight of animals housed, kg width of biofilter, m concentration of component in exit gas stream, kg / m3 concentration of component in feed gas stream, kg / m3

The depth of the bed is also important, since this and the type of packing used determine the pressure drop across the biofilter for any given airflow rate. If the bed is too shallow the overall cross-sectional area of the biofilter will need to be large to meet the residence time requirements. If too deep, the crosssectional area will be less but the pressure drop will increase, thus increasing the power costs. Williams and Miller7 report depths of 0?5 to 2?5 m while Rodhe et al.1 and Scholtens and Demmers2 both indicate that 0?5 m is a sufficient depth. This suggests that the required depth is unlikely to be less than 0?5 m or to exceed 2?5 m. Packing type is relevant to the performance and costs of biofilters. Typically, mixtures of peat and heather in various proportions have been used.1,2,4 Alternatively, materials including garbage compost, bark compost,6 woodchips,4 compost / bark mixtures,2 peat / spruce branch mixtures8 and heather4,9 can be used. Results from experimental biofilters are summarized in Table 1, showing their performance at controlling odour and other emissions. These studies

suggest that biofilters are able to reduce odours and emissions by up to 95%, but the lack of a design base and formal evaluation means that the optimal design is difficult to achieve. A design base is required that offers a framework for comparisons of the different packing media, in order that the optimal design can be found to reduce the cost of biofiltration. 2. Development of the model 2.1 . Oy erall scheme Drawing on previous research, a model was developed using a spreadsheet software package10 to predict the costs of building and operating biofilters, with different packing media, on livestock units. The approach chosen was to compare biofilters achieving similar treatment, which allows identification of the best system. To achieve this comparison, a test case was required. Pearson et al.9 detail the packing type, residence time, size, performance and building costs of a biofilter, and this was chosen as the test case. The model was divided into four sub-models which reflect the main aspects of biofilter design: (1) biofilter size, (2) biological performance, (3) pressure drop and (4) annual cost. 2.2 . Sub -model of biofilter size The sub-model of biofilter size is constructed using calculations for maximum ventilation rate and superficial air velocity.

Table 1 Packing media and their relative performance

Reference

Packing medium

Rodhe et al.1

10% heather / 90% peat, by wet weight 30% heather / 70% peat woodchips heather peat / heather compost / bark peat / spruce branches bark / compost heather

Wright4

Scholtens and Demmers2 VDI Guideline 34778 Pearson et al.9 * No data available.

Oy erall pressure drop , DP , Pa

Residence time , R, s

Emission remoy al efficiency , %

6

,50

p

50

87

p p 120 250 ,120 500 ,55

50 55 6 p 6 9 20

30 65 75 75 p 95 72

200

MINIMUM-COST BIOFILTERS FOR REDUCING ODOURS

Maximum ventilation rate, Q , is calculated using a formula developed by Randall11 of the form Q 5 nwy

(1)

where n is number of animals housed, w average weight of animal housed, kg, and y is the maximum ventilation rate per unit live weight of animals, m3 / s kg. The empirical values of y used are those quoted by Randall.11 Maximum superficial air velocity, u , (that is, the ‘‘empty pipe’’ velocity with no allowance for the partial obstruction of the cross-section by the packing medium) is calculated using u5

Q A

(2)

where A is the cross-sectional area of the biofilter, m2.

2.3. Sub -model of biological performance The measurement of biofilter performance is often difficult owing to the very low concentrations of pollutant present in large volumes of air. Odour presents a special problem in that perception, rather than the actual concentration of component gases, becomes important. Evaluation of the effect of biofilter treatment then becomes based on olfactometry measurements as reported by Rodhe et al.1 Reliable

157

prediction of the quality of an effluent gas for a given biofilter will probably be impossible without a great deal of raw data. Even then, the likely result would still be a highly empirical correlation limited to the tested filter material types. In the approach taken in this study, the need to measure or predict the absolute quality of the effluent gas was avoided. An arbitrary duty is defined; that is, a few flow rate of ventilation air with a concentration of pollutant, X go, which is reduced to an output strength, X ge, as in Fig. 1 . Biofilters with different packing materials achieving the same duty are thus compared on a relative scale (data from a unit performing a known function can be inserted to enable actual costing for that duty as done in the example provided in Section 3). The consequence of this is that physical properties of the liquid films within the biofilter along with transport properties and reaction kinetics will be the same thus simplifying the modelling procedure. The reaction kinetics within a biofilter have been observed12,13 as zero order, that is, the reaction rate is independent of the component concentration. This is a likely consequence of a steady microbial population in which individual microbes have a definite capacity for consuming the pollutant. A fully ‘‘mature’’ biofilter would likewise be expected to have a maximum capacity for consumption of the pollutants based on the effective surface area of the packing (assuming

Exit concentration, Xge, kg/m3

Cross-sectional area, A, m2 Effective surface area of packing, α, m2/m3

Packing depth, H, m

Feed concentration, Xgo, kg/m3 Volumetric flow rate, Q, m3/s

Fig. 1. Arbitary giy en duty of a biofilter

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I . M . S CO T F O R D ; C. H . B U R TO N ; V. R . PH I L LI P S

constant biofilm thickness). As well as the prevailing reaction kinetics, the decomposition of pollutant can also depend on mass transfer effects, i.e. the movement of pollutant from the gas phase to the liquid film and then diffusion rate through it. Ottengraf and van den Oever12 show that the capacity of a biofilter can be limited by either reaction kinetics or mass transfer. However for the low concentrations present in the effluent air from livestock buildings, and the relatively high degradability of the pollutants involved, the process is likely to be diffusion controlled. For this situation, Ottengraf and van den Oever12 derive the equation X ge aH 512 X go u

–

Kr D 2X go M

(3)

In the modelling approach being applied, the conditions presented to the biofilter are fixed. Hence the feed and exit air concentrations, X ge and X go will be constant. Likewise, diffusivity, D , reaction rate, K r and Henry’s constant M will not vary. Hence Equation (3) simplifies to

aH 5 constant u

(4)

where a is effective surface area of the packing, m2 / m3, H is packing depth, m, and u is superficial air velocity, m / s. Replacing u by Q / A [Eqn (2)] gives

a HA a V 5 5 a R 5 constant k 1 Q Q

(5)

where V is a biofilter volume, m3 and R is residence time, s. This similar biological performance can be expected if the product of the effective surface area of the packing (a ) and the residence time of the air in the packing (R ) remain constant. Use of packing with a higher effective surface area will thus enable a pro-rata reduction in the volume of the biofilter. However, this is a purely theoretical approach and certain limitations should be considered for the practical situation as follows. 1. Concentration of pollutant. The modelling approach is based on the assumption that low concentrations would be expected from the ventilation air from a livestock building. High concentrations would lead to biomass growth and the risk of clogging, especially for packing materials of large surface area. 2. Degradation of pollutant. Most of the carbon based compounds that characterize odour are readily degraded in an aerobic environment. Methane and hydrogen sulphide are also readily oxidized. However

ammonia in large concentrations can poison the biomass because of its toxicity or from the fall of pH if nitrification occurs. 3. Eveness of airflow through the bed. A minimum depth is implied which will vary according to the packing material. Packing made from finer particles can be used in a shallower bed. In order to apply the derived model, data taken from Pearson et al.9 is used. Pearson et al.9 chose heather with a residence time of 20 s, so constant k 1 can be determined for that duty. Phillips et al.14 estimated the surface area per unit bulk volume, a , of heather to be 220 m2 / m3. Thus, k 1 5 4400 m2 s / m3. Hence residence time, R , can now be determined for any given packing using R5

4400 a

(6)

The implication of this is that the ratio A / H can change without affecting the performance of the biofilter. When assessing the effect of different packing media on the costs of the biofilter, it is useful that the A / H remains fixed in order to enable an objective comparison to be made. As an example, data from Pearson et al. ,9 may be used which considers four biofilters to be necessary for a 10 000 place broiler unit, each biofilter treating an airflow rate of 10 m3 / s. At the stated depth, 1?5 m, each m3 of air per second would require a cross-sectional area of 16 m2, thus the dimensions of the biofilter would be 640 m2 by 1?5 m deep, hence for this example A / H could be fixed at 430 m and given the symbol k 2 . Hence A is calculated from A 5 4430 V

(7)

and H H5

V A

(8)

2.4. Sub -model of y ariation of pressure drop The pressure drop through a filter medium can be determined from the superficial velocity of air flowing through the filter by the general formula DP 5 au bH

(9)

where DP is pressure drop, Pa, and a and b are constants depending on the type of packing media used. The values of constants a and b used within the

159

MINIMUM-COST BIOFILTERS FOR REDUCING ODOURS

Table 2 Packing media characteristics (after Phillips et al 14)

Packing medium

Effectiy e surface area , a , m 2 / m 3 of packing

Constant , a

Constant , b

Cost of buying packing medium including deliy ery , £ / m 3 as used

Heather Coir Wood chips Heather and coir Heather and peat

220 180 160 250 1100

344 698 1390 730 6025

1?38 1?20 1?69 1?26 1?35

17 24 12 20 18

model are those developed by Phillips et al.13 as reported in Table 2.

2.5. Sub -model of cost The costs of building and operating a biofilter can be split into four parts: construction costs, maintenance cost, packing medium cost and running costs. During 1991, Pearson et al.9 carried out a detailed costing exercise on the construction and running costs of a biofilter. Using their information, which is summarized in Table 3, the construction cost of any size of biofilter can be determined. The dimensions are determined as follows. Cross-sectional area, the crosssectional area from the biological model; length, plan area divided by the width, W ; perimeter, twice the (length plus width); wall height, packing depth, H , plus 1 m to allow for the plenum chamber under the filter medium; wall area, perimeter multiplied by wall height.

The total construction cost is thus the sum of the individual costs. Construction costs are represented as an annual figure assuming the cost of construction is funded using a fixed term loan paid back in equal instalments during the term of the loan. From Audsley and Wheeler15 this can be calculated using Ca 5

C c i (1 1 i )t (1 1 i )t 2 1

where Ca is the annual construction cost, £; C c is the total construction cost, £; i is the interest rate on the loan and t is the term of the loan, yr. The annual maintenance cost, Cr, which includes structure maintenance, filter maintenance and dust filter cleaning, is taken to be 10% of Ca, the annual construction cost.16 Total packing medium cost, Cm, is simply the volume of the biofilter multiplied by the unit cost of the packing, Cf. Hence packing medium cost C m 5 AHC f

Table 3 Construction costs of a biofilter in 1991 (after Pearson et al .9) Cost calculation based on Basework Cross-sectional area Perimeter Cross-sectional area Superstructure Cross-sectional area 1 wall area Wall area Cross-sectional area Sundries Length Cross-sectional area Cross-sectional area 1 wall area

(10)

Construction unit

Unit cost , including labour and materials

Excavate Foundation Concrete

£6.50 / m2 £8.25 / m £7.90 / m2

Timber frame

£4.10 / m2

Plywood cladding Packing support

£12.00 / m2 £5.80 / m2

Dust filter Irrigation Miscellaneous

£1.70 / m £11.70 / m2 £8.10 / m2

(11)

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can be calculated. Then if the fan efficiency, h , (pneumatic power / electrical power) and the electrical cost, £, per kWh are known, the annual power consumption, Ce, can be calculated from

However this assumes that the packing medium lasts for the term of the loan. If the life expectancy of the packing medium is only 5 yr and the loan is over 10 yr, the cost of the packing medium would double. Hence Equation (11) can be re-written Cm 5

AHC f t L

Ce 5

(12)

where L is the life expectancy of the packing medium, in yr and t is the term of the loan. The annual packing medium cost, Cp, is then calculated using the same criteria as the annual construction cost. Hence Cp 5

C m i (1 1 i )t (1 1 i )t 2 1

8760Q DPEK w 1000h

(14)

The predicted cost per animal produced is the sum of the four individual annual costs divided by the throughput of animals per year. The throughput of animals per year depends greatly on the farming practices used. However three batches of pigs, six crops of broilers and one flock of layers is the typical throughput per year, thus the model has been set at these values. Hence if the building is designed for n birds in a broiler unit, the throughput per yr, N , would be 6n birds. Thus the cost of the biofilter per animal produced, Ct is given by

(13)

When a biofilter is fitted to a livestock building, the additional power required is mainly due to the increased pressure drop which the ventilation fans must overcome. The pneumatic power consumption of the fan is given by flow multiplied by pressure drop. Working on the assumption that ventilation is continuous, throughout the year (i.e. 8760 h) and the fan works at an average ventilation rate, E (percentage of maximum rate), the pneumatic power used per year

Ct 5

Ca 1 Cr 1 Cp 1 Ce N

(15)

A flow chart of the overall model with the relevant sub-models is shown in Fig. 2 .

Inputs 1. Type of animals 2. Number of animals housed, n 3. Average weight of animals, w 4. Width of biofilter, W 5. Interest rate, i

Inputs used 1, 2

6. Term of loan, t 7. Fan efficiency, n 8. Average ventilation rate, E 9. Electricity cost 10. Biofilter packing media

Inputs used 10 Inputs used 1, 2, 3

Inputs used 4, 5, 6

Inputs used 10

Inputs used 5, 6, 10 Inputs used 7, 8, 9

Biological aspects

Biofilter size

Pressure drop

Cost model

Residence time, R

Ventilation rate, Q

Pressure drop, ∆P

Construction cost, Cc

Volume of packing, V

Superficial air velocity, u

Maintenance cost, Cr

Cross-sectional area, A

Packing cost, Cm

Power cost, Ce

Depth of packing, H

Cost per animal, Ct

Fig. 2 . Biofilter economic cost model

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MINIMUM-COST BIOFILTERS FOR REDUCING ODOURS

3. Application of the model

3.1 . Running the model For the duty given by Pearson et al. ,9 the model was run by varying input values to determine their effect on the cost per animal. For all the examples shown, the following inputs were constant: type of animals, broilers; average weight of animals, 2 kg; width of biofilter, 10 m; interest rate on loan, 10%; term of loan, 10 yr; fan efficiency, 25%; average ventilation rate, 50% of maximum; electricity cost per kWh, £0?0742 / kWh and life expectancy of packing, 10 yr. Values of a , b and a are available only for five different packing media: heather, heather / peat mixture, heather / coir mixture, coir and woodchips, hence the model is used to compare these media only. These values are shown in Table 2.

3.2. Effect of number of animals housed on operating costs

3.3 . Effect of A / H ratio on operating costs When comparing the packing media tested by Phillips et al. ,14 we see that the heather and peat mixture costs the most when in excess of 5000 broilers are kept in the house. However, this is based on the assumption that A / H 5 k 2 5 430. If k 2 is increased, A increases and H decreases so the pressure drop through the biofilter decreases. Due to the high pressure drop of the heather and peat mixture the extra cost of building a larger biofilter is not exceeded by the savings made on the annual power costs until

0·6

50000

0·5 40000

0·4

Annual cost, £

Cost of treatment, Ct, £ / broiler produced

The model was set up to predict the cost for the heather and peat mixture. The size of the house was increased through a range from 1000 to 40 000 to observe the effect on the cost per animal. This exercise was repeated for heather, coir, wood chips and heather / coir mixture. The results are shown in Fig. 3 . For up to 5000 broilers, the heather and peat packing media is the cheapest option owing to the high surface area of packing, at 1100 m2 / m3, compared with less than 260 m2 / m3 for the other four packings.

Thus the actual size and therefore the construction costs for the heather and peat biofilter will be less than the others. Once the house capacity has increased beyond 5000 broilers, the cost of the heather and peat biofilter increases rapidly. For all of the other packings, cost levels out at between £0?11 and £0?20 per bird produced but increases slowly at more than 35 000 broilers. The reason for the rapid increase in the cost per bird for the heather and peat biofilter is the high annual power costs due to the high pressure drop associated with the heather and peat mixture. This high pressure drop would also probably necessitate fitting new ventilation fans to the building, thus increasing the overall cost even further. If the annual costs are broken down into construction, packing and power costs (Fig. 4 ) , the rapid increase in the power cost compared with the other two costs is clear. However, if the same exercise is carried out for the heather packing (Fig. 5 ) the power costs are less than both the construction cost and packing costs.

0·3 0·2 0·1 0

30000

20000 10000

0

10000

20000

30000

40000

No. of broilers housed, n

Fig. 3. Cost of treatment per broiler y ersus number of broilers housed for the fiy e different packing media. e , heather and peat; h , heather; s , coir; n , wood chips ; x , heather and coir

0

0

5000

10000

15000

20000

No. of broilers housed, n

Fig. 4. Annual costs for the heather and peat packing y ersus number of broilers housed. s , annual power costs , Ce; h , annual packing costs , Cp; e , annual construction costs , Ca

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I . M . S CO T F O R D ; C. H . B U R TO N ; V. R . PH I L LI P S

8000

that the biofilter model does not include a cost for land which must be considered if land is of premium value.

Annual cost, £

6000

4. Discussion 4000

Cost of treatment, Ct, £ / broiler produced

The modelling approach set out in this paper is an attempt to bring an objective evaluation to the complex system represented by a biofilter. The main 2000 variable is the packing material itself; the ventilation air to be treated will dictate size of unit and from this running and capital costs are calculated. Choice of a finer packing implies a smaller unit but this must be 0 0 5 000 10 000 15 000 20 000 balanced against the cost of a higher pressure drop; No. of broilers housed, n hence an optimal packing material can be sought. Fig. 5. Annual costs for the heather packing y erses number However, use of the best packing will not in itself of broilers housed. s , annual power costs , Ce ; h , annual guarantee that treatment targets will be met or packing costs , Cp; e , annual construction costs , Ca maintained. Verification of biofilter performance remains a difficult area not least because of uncertainty k2 reaches around 3000. However with the other four over what standard is required. The subsequent monipackings, if k2 is increased to beyond 450 the extra toring can also be a difficult task owing to the high cost of building the larger biofilter outweighs the cost of frequent analysis of gas samples for odour or savings made on the annual power costs. Thus for a very low concentrations of specific chemicals. The model predicts that for small units, of less than given set of conditions the model can predict the most 5000 broilers, the heather and peat packing is the cost effective area to height ratio, i.e. when the overall cheapest but above 5000 broilers all of the other costs are at their lowest, that can be achieved. Fig. 6 packings of fer a cheaper option. However, it must be shows the effect of A / H ratio on the cost per bird remembered that this prediction is based on the set produced for a 15 000 bird broiler unit. Whatever inputs . If these changed the results could be different. value of k2 is selected, the heather and peat mixture For example , if the cost of power was reduced it is will always cost more at some point than the other likely that the peat and heather packing could prove materials due to its high pressure drop, but increasing to be the best option in most cases owing to its high k2 increases the number of birds that can be housed surface area per unit bulk volume. On the other hand, before the heather and peat biofilter costs more than if the price of peat increased it is likely that the the other packings. It must be remembered, however, heather and peat mixture would not be a feasilbe option for any size of house. Hence the model can be 0·75 used to determine the influence of many factors, thus determining the best packing media in a given situation. The application of biofilters in agriculture have 0·50 largely been confined to odour reduction. However, they might be expected also to remove a range of specific oxidizable substances such as methane or 0·25 hydrogen sulphide so long as the concentrations were of a similar order to the odour components. Ammonia however, presents a particular problem in that it can retard microbial activity by its toxic effect (or by the 0·00 0 250 500 750 1000 products of nitrification). If it is present in significant concentrations in the ventilation air, allowance for it A/H ratio, k2, m in the biofilter design will be necessary. This might, Fig. 6. The effect of the A/ H ratio on the cost of treatment per broiler for the fiy e different packing media. e , heather for example, be achieved by increasing the water and peat; h , heather; s , coir; n , wood chips ; 3 , heather and application to allow a drain water stream to remove coir ammonia and nitrates from the biofilter.

MINIMUM-COST BIOFILTERS FOR REDUCING ODOURS

5. Conclusions 1. A model of biofilter performance and cost has been developed which allows the objective comparison of different packing media. The approach has been to combine sub-models set up to predict pressure drop, biological performance, biofilter size and costs. 2. The difficulty in producing a reliable predictive model for the absolute removel of a specific pollutant was overcome by taking a comparative approach. For the same treatment, the theoretical model predicts that the product of the effective area of the packing and the biofilter residence time is constant. This assumes an even airflow distribution. 3. Comparisons of some existing packing materials have been made for use in deordorizing ventilation air from a broiler house and the model predicts peat / heather to be the cheapest option when fewer than 5000 animals are housed. The costs predicted, suggest that biofilters are still an expensive option.

4

5

6

7

8

9

Acknowledgements 10

We are pleased to thank Dr T. R. Cumby for many useful discussions and the Ministry of Agriculture, Fisheries and Food, for funding the project.

11

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References 1

2

3

Rodhe L; Thyselius L; Berglund U Biofilters for odour reduction-installation and evaluation. Jordbrukstekniska Institutet, Uppsala, Sweden, Report No. 76. (Joint JTI / AFRC Engineering, Silsoe, Translation No. 1, New Series, 1988). Scholtens R; Demmers T G M Biofilters and air scrubbers in the Netherlands. In: Odour and Ammonia Emissions from Livestock Farming. eds Nielsen V C et al. Elsevier Applied Science, 1991 Scholtens R; Klarenbeek J V; Bruins M A Control of ammonia emissions with biofilters and bioscrubbers. In: Nielsen V C et al. , eds. Volatile Emissions from Livestock Farming and Sewage Operations. Elsevier

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Applied Science, London, 1988 Wright R Biofiltration in odour control. Investigation of Air Pollution Standing Conference, Paper IAPSC 7 / 3 London 1989 Ottengraf S P P; Diks R M Promising technique, process technology of biotechniques. Lucht nummer 4, December 1991, 135 – 144 Zeisig H D Experiences with the use of biofilters to remove odours from piggeries and hen houses. In: Nielsen V C et al. , eds. Volatile Emissions from Livestock Farming and Sewage Operations. Elsevier Applied Science, London, 1988. Williams T O; Miller F C Odour control, biofilters and facility operations. Part 2, Design and operational factors related to the use of biofilters at composting facilities include moisture, loading rates, oxygen content, pH and pressure drop. Biocycle, November 1992, 75 – 79 Verein Deutscher Ingenieure (German Engineers Association). Biofilter Guideline NO 3477, December 1991 (in German and English). Available from Benth Verlag GmbH, Berlin Pearson C C; Phillips V R; Green G; Scotford I M A minimum cost biofilter for reducing aerial emissions from a broiler chicken house. In: Biotechniques for air pollution abatement and odour control policies. eds Dragt A J; van Ham J Elsevier Science, Amsterdam 1992 Microsoft Excel 5 Microsoft Ltd, Microsoft Place, Wharfedale Road, Winnersh Triangle, Wokingham, Berkshire RG11 5TP, UK Randall J M A handbook on the design of a ventilation system for livestock buildings using step control and automatic vents. NIAE Report No. 28, 1977 Ottengraf S P P; van den Oever A H C Kinetics of organic compound removal from waste gases with a biological filter. Biotechnology and Bioengineering, Volume XXV, p 3089 – 3102 1983 van Lith C; David S L; Marsh R Design criteria for biofilters. In: Effluent Treatment and Waste Disposal. I Chem E Symposium Series 116 pp. 127 – 138, 1990 Phillips V R; Scotford I M; White R P; Hartshorn R L Minimum-cost biofilters for reducing odours and other emissions from livestock buildings: Part 1, Basic Air Flow Aspects. Journal of Agricultural Engineering Research, 62, 203 – 214, 1995 Audsley E; Wheeler J The annual cost of machinery calculated using actual cash flows. Journal of Agricultural Engineering Research, 23, 1978 189 – 201 Perry R H; Chilton C H Chemical Engineers Handbook 5th Edition. McGraw-Hill Kogakusha Ltd, 1973