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Biopur®: Alternative Packaging for Biological Systems
Bwtechniques for Air Pollution Abatement and Odour Control Policies A J . Dragt and J. van Ham (Editors) 0 1992 Elsevier Science Publishers B.V. All rights reserved.
63
Biopurm: Alternative Packaging for Biological Systems R. Oosting, L.G.C.M. Urlings, P.H. van Riel,
C.
van Driel
T A W Infra Consult B . V . ,P.O. Box 4 7 9 , 7400 AL Deventer, The Netherlands
Abstract Due to more stringent regulations the need arises for "clean technology" in terms of newly developed techniques and inexpensive, but efficient, "end of the pipe" techniques. T A W Infra Consult B.V. has designed a biological treatment system called BIOPURS, which is based upon applying a new packaging material. The system can be modified in such a way that application to air treatment, combined air/water treatment or solely water treatment is possible. The performance of the system is described using three examples; soil remediation, removal of volatile organic compounds from gasses and NH3 treatment. In all three cases the BIOPURS system shows good treatment efficiencies and performance. 1. INTRODUCTION
The increasing awareness o f the effects of human activities on the environment, forces us to take protective measures. The legal demands regarding emission control and health concerns are becoming more and more stringent and force everyone to think within a new (environmental) frame. These recent developments act as a catalyst on the technological innovation; there is a demand for improved or new production techniques. The most ideal method would be to improve the processes in such a way that the emissions would fall within the legal framework or t o develop a completely new, alternative, process (clean technology). However, this is not always possible, therefore supplemental treatment techniques also have to be implemented. These so-called "end of the pipe" systems have to meet the following requirements:- low energy input; - low maintenance; - low cost; - high efficiency. Biological treatment can be a good alternative for a great number of contaminants. Compared with other treatment techniques the cost of biological treatment is relatively low. (Diks et. al., 1991; J . A . Don, 1984).
2. PACKAGING MATERIAL FOR BIOLOGICAL SYSTEMS
Compared with physical and chemical air treatment techniques, such as scrubbing, adsorption (van Dam and van Riel, 1990) or incineration, biological treatment can offer advantages such as lower energy demand and lower investment and operating costs. Biological treatment techniques have the advantage that they have a high reaction selectivity, therefore they can be efficiently applied to low concentration ranges. The products are mainly C02 and HZO, thus hardly any hazardous components are released into the environment. Furthermore the use of additional substances is minimal and energy consumption is low. The efficiency of the biological systems can be increased if the biomass is immobilized. The use of biofilters is very common in the air treatment field. The packaging material, used for supporting the biomass is usually compost. A great disadvantage of compost is the relatively high pressure drop. Furthermore the height of the compost filter bed tends to shrink. The use of a new, synthetic material offers the possibility to "construct" a tailor-made packaging which meets specific requirements. Some of the qualities o f the ideal packaging material are:good contact area (biofilm/liquid); chemical and mechanical resistant; light; durable; - low pressure drop; - inexpensive; - good support material for biomass; - no toxic effects on biomass. -
Furthermore, the choice of the material is strongly influenced by the need to: - minimize the volume necessary for the total reactor; - optimize the efficiency; - keeping energy consumption to a minimum. - minimize maintenance. In practice using a macro-reticulated synthetic material seems to meet most of the requirements mentioned above. This synthetic material is implemented in a newly developed treatment system, called BIOPURB. The system can be modified for air treatment ( A ) , combined air/water treatment (C), and water treatment (W) and can be used in:- soil remediation (C, organic compounds); - air treatment in the printing and paint industry (A, organic compounds); - livestock industry ( A , odour compounds, N H 3 ) ; - water treatment (W, organic compounds, (F. Spuij, 1991)).
65 3 . EXAMPLES
The performance of these systems will be discussed in the following three examples (one "full scale" project and "two pilot plant" scale projects) : Example 1 "Soil Vapour Extraction of Hydrocarbons, In-situ and On-site Biological Treatment". In terms of quality contamination of soil and groundwater by hydrocarbons are the most widespread form of pollution. Miller (1990) estimates that 90% of the transported hazardous waste in the U.S.A. consists o f gasoline, oil and jet fuel. Furthermore, it is estimated that 50-70% of the total in situ remediation costs are spent on air treatment, due to the volatilization of the organics. A combined soil vapour/groundwater treatment system has been developed by T A W (BIOPURB) , to help reduce these costs. In the case described, the system was used along with air based biodepradation (in situ) . Site characteristics Whilst carrying out soil remediation at a petrol station, contamination, by gasoline, was found below a provincial road. Excavation was not feasible due to financial and technical reasons. The most favourable remediation technique was to use soil vapour extraction, combined with biostimulation. Not only the volatile compounds had to be removed but also the biodegradation o f particularly non-volatile components had to be stimulated by the passive infiltration of air and nutrients. In order to achieve this the groundwater level was lowered by some 2.5 m. This increased the unsaturated zone which is favourable for soil vapour extraction and aerobe biodegradation. Groundwater and soil vapour were extracted on one side of the road, causing a continuous flow towards the extraction points. Air and nutrients were infiltrated at the other side, see Figure 1. Trealed 5011 vapur
Infinrationof
Grarndwaler level
Figure 1. Cross section o f remediation site
The extracted soil vapour and groundwater were treated in a combined soil vapour/groundwater treatment unit. The retention time was approximately 15 minutes for groundwater and less than 10 minutes for soil vapour extraction. Nitrogen and phosphorus nutrients were added to the groundwater influent stream. The system was operated in co-current flow, s e e Figure 2. Using the oxygen consumption and carbon dioxide production rates it was possible to calculate that the in-situ biodegradation progressed at a rate of 7 mg/kg soil/day. m S o i l vapour
~
(SO n3/hwr)
Groundwater
~
-
210 glh - ~ ~ 8 i o \ o g i c a l ~ 10 g l h - > I S Y S K M I
( 1 5 n3/hour1
ITreatment 1 -I
<
3
<
2 g/h
glh
, exhaust
~
ges
effluenr
>-
Figure 2 . Gasoline mass balance at a steady state It also seems to be a zero order process. Initial concentrations were approximately 80 g/m3. This dropped to 3 g/m31 after 20 weeks and 0 . 2 4 0 g/m3 after 60 weeks. In Figure 3 the removal results are compiled based on soil vapour analyses. It is clearly shown that the soil vapour extraction system was largely responsible for the total amount removed, later when most of the volatile compounds had been removed, this was largely attributed to biodegradation. Total amount
1
,000
20
40
(10
10
Time (weeks1
Figure 3 . Cumulative amounts of gasoline removed during soil vapour extraction. The treatment efficiency of the combined soil gas/groundwater treatment was greater than 98%. All the legal requirements towards effluent water streams were met (i.e. < 100 pg aromatics/l and < 1 mg mineral oil/l). For individual aromatic compounds the concentrations were < 0.5 pg/l. N o aromatics or other volatile organic compounds were detected, in the treated soil vapour, see Figure 4 , even with a detection limit of 0.1 ppm.
67
EXHAUST GAS I
SOIL VAPOUR
Figure 4 . Chromatogram of soil vapour (untreated) and exhaust gas (after biological treatment). Example 2 "Removal of Toluene from Gas Streams with a Biotrickling Filter" In the last years it has become clear that photo-chemical air pollution is a great problem for health and the environment. The reaction between NOx and volatile organic compounds causes the formation of ozone, which is hazardous to health and has an adverse effect on the environment. It is therefore imperative to reduce the emissions from NOx and volatile organic compounds. The reduction of emissions from volatile organic compounds is the aim of a Dutch governmental project called KWS-2000. The use o f biotrickling filters for the removal of organic compounds from gas streams has been described: Diks et. al., 1991; VDI-bericht 7 3 5 , 1989. T A W Infra Consult B.V. developed a biotrickling filter using a macro-reticulated synthetic packaging material (BIOPURB), see Figure 5. The system is operated in counter current mode. Active biomassa is present in both the packaging material and the recirculation liquid. Recirculation o f the process liquid has the advantage that the nutrient dosing and the pH can effectively be controlled (particularly essential for the removal of chlorinated volatile organic compounds).
68 Toluene was used as a reference component. The experiments were carried out using three different gas flow rates, i.e. 450 m/h, 700 m/h and 900 m/h. Gas phase concentrations were between 20 mg/m3 and 700 mg/m3. The liquid flow rate was 2.8 m/h. Figure 6 shows the elimination capacity as function of the component concentration at the gas inlet. During the experiments elimination capacities greater than 200 g/m3/h could easily be achieved. These experiments show that a good performance is possible using high gas flow rates: low pressure drop: < 4C Pa/meter bed, high elimination low maintenance, easy capacity, control. Initial experiments with multi-component systems show a good performance but its behaviour differs from that of mono-component systems. Further investigations are therefore aimed at describing the multi-component systems. Figure 5. Biotrickling filter with alternative packaging material.
250
ELIMINATION CAPACITY [g/m3*h]
0
200
0
150 0
0
100
0
750 m/h
0
900 m/h
200 300 400 500 TOLUENE CONCENTRATION [ mg/m3]
600
50
0' 0
I
100
I
I
Figure 6. Toluene concentration [mg/m3],
I
700
69 Example 3 In the livestock industry the emission of ammonia is a serious problem. The Dutch policy is to reduce the total emission by 70% before the year 2000. ,Structural changes in the accommodation design and new manure treatment techniques can help solve a great deal o f this problem. Still waste gas treatment also has to b e implemented at order to meet the government requirements and to avoid odour annoyance for the people living in the area. The use of new packaging materials with very low pressure drop (Figure 7) stimulates the coupling o f the treatment systems with the ventilation systems already installed. The removal efficiency of the system is high, 90% with 1 m packaging height. Gas velocity 1650 m/h ( ( 46 m/s) and NH3 concentration 90 rng/m3. LESSURE DROP [Pa]
400
300
P 200
100
0
0
0.5
1.5 2 2.5 3 3.5 4 SUPERFICIAL GAS FLOW RATE [m/h] *LO00
1
4.5
5
Figure 7. Pressure drop as function of the superficial gas flow rate [m/h] The concentrations found are given in Figure 8.
CONCENTRATION PROFILE O F AMMONIA gas veloclty: 1650 m/h. "H31
=
90 rng/rn3
RELATIVE CONCENTRATION [-]
0.6
-
0.4
-
0.2 -
01
0
0.2
0.4
0.6 0.8 COLUMN HEIGHT [HI
Figure 8. Concentration profile of ammonia.
I
1.2
70 Partial nitrification occurs in the liquid phase of the biotrickling filter. The additional nitrification takes place in a separate water treatment unit. 4 . CONCLUSIONS
The BIOPURm system for air treatment has great potential because the system can be operated in a "niche" where other systems were either very expensive or have failed. This is due to the application of a macroreticulated synthetic packaging material. Compared with a biofilter the BIOPURB system offers better process control (pH, nutrients), which is essential for the treatment of chlorinated organic compounds. Compared with bioscrubbers the BIOPURm system needs less space and uses less energy because absorption and treatment takes place in the same column. For combined air and water treatment or solely water treatment the application of BIOPURm system is advantageous in the field of soil remediation. The estimated costs for treatment in are low:f 0.50 per m3 water; f 1.00 - f 5.00 per 1000 m3 gas. The Biopurm seems to be a promising technique, however, more research is still required particularly in the field of waste gas treatment (multicomponent systems, chlorinated hydrocarbons). In the field o f NH3 removal at high gas velocities, the coupling of the removal system with the biological treatment system requires further research.
REFERENCES R.N. Miller, et. al. A Field Scale Investigation o f Enhanced Petrol Hydrocarbon Biodegradation in the Vadose Zone at Tyndall, AFB, Florida. Proceedings NATO/CCMS meeting France, December 1990. L.G.C.M. Urlings, et. al. Soil Vapour Extraction of Hydrocarbons. - In Situ and On Site Bioreclamation, an International Symposium, 19-21 March, 1991. San Diego, California, Proceedings: in press. F. Spuij, et. al. Biological Groundwater and Soil Vapour Treatment. Third Forum on Innovative Hazardous Waste Treatment Technologies, June 11-13 Dallas, Texas. P.H. van Riel. Aktieve koolvezeladsorptie. Proceedings Studiedag Luchtzuiveringstechnieken, 29 May, 1991, Arnhem, The Netherlands, Session 9. R.M. Diks et. al. Process Engineering Aspects of Biological Waste Gas Purification. International Symposium Environmental Biotechnology. April, 1991, Ostend, Belgium, Proceedings Part I, pp 353-367. VDI-Bericht 735. Biologische Abgasreiniging VDI-Verlag GmbH - Diisseldorf 1989. J.A. Don. Stroomversnelling bij de toepassing van biofiltratie. Proceedings Bestrijdingstechnieken luchtverontreiniging, December 1984, pp 59 - 6 3 .
63
Biopurm: Alternative Packaging for Biological Systems R. Oosting, L.G.C.M. Urlings, P.H. van Riel,
C.
van Driel
T A W Infra Consult B . V . ,P.O. Box 4 7 9 , 7400 AL Deventer, The Netherlands
Abstract Due to more stringent regulations the need arises for "clean technology" in terms of newly developed techniques and inexpensive, but efficient, "end of the pipe" techniques. T A W Infra Consult B.V. has designed a biological treatment system called BIOPURS, which is based upon applying a new packaging material. The system can be modified in such a way that application to air treatment, combined air/water treatment or solely water treatment is possible. The performance of the system is described using three examples; soil remediation, removal of volatile organic compounds from gasses and NH3 treatment. In all three cases the BIOPURS system shows good treatment efficiencies and performance. 1. INTRODUCTION
The increasing awareness o f the effects of human activities on the environment, forces us to take protective measures. The legal demands regarding emission control and health concerns are becoming more and more stringent and force everyone to think within a new (environmental) frame. These recent developments act as a catalyst on the technological innovation; there is a demand for improved or new production techniques. The most ideal method would be to improve the processes in such a way that the emissions would fall within the legal framework or t o develop a completely new, alternative, process (clean technology). However, this is not always possible, therefore supplemental treatment techniques also have to be implemented. These so-called "end of the pipe" systems have to meet the following requirements:- low energy input; - low maintenance; - low cost; - high efficiency. Biological treatment can be a good alternative for a great number of contaminants. Compared with other treatment techniques the cost of biological treatment is relatively low. (Diks et. al., 1991; J . A . Don, 1984).
2. PACKAGING MATERIAL FOR BIOLOGICAL SYSTEMS
Compared with physical and chemical air treatment techniques, such as scrubbing, adsorption (van Dam and van Riel, 1990) or incineration, biological treatment can offer advantages such as lower energy demand and lower investment and operating costs. Biological treatment techniques have the advantage that they have a high reaction selectivity, therefore they can be efficiently applied to low concentration ranges. The products are mainly C02 and HZO, thus hardly any hazardous components are released into the environment. Furthermore the use of additional substances is minimal and energy consumption is low. The efficiency of the biological systems can be increased if the biomass is immobilized. The use of biofilters is very common in the air treatment field. The packaging material, used for supporting the biomass is usually compost. A great disadvantage of compost is the relatively high pressure drop. Furthermore the height of the compost filter bed tends to shrink. The use of a new, synthetic material offers the possibility to "construct" a tailor-made packaging which meets specific requirements. Some of the qualities o f the ideal packaging material are:good contact area (biofilm/liquid); chemical and mechanical resistant; light; durable; - low pressure drop; - inexpensive; - good support material for biomass; - no toxic effects on biomass. -
Furthermore, the choice of the material is strongly influenced by the need to: - minimize the volume necessary for the total reactor; - optimize the efficiency; - keeping energy consumption to a minimum. - minimize maintenance. In practice using a macro-reticulated synthetic material seems to meet most of the requirements mentioned above. This synthetic material is implemented in a newly developed treatment system, called BIOPURB. The system can be modified for air treatment ( A ) , combined air/water treatment (C), and water treatment (W) and can be used in:- soil remediation (C, organic compounds); - air treatment in the printing and paint industry (A, organic compounds); - livestock industry ( A , odour compounds, N H 3 ) ; - water treatment (W, organic compounds, (F. Spuij, 1991)).
65 3 . EXAMPLES
The performance of these systems will be discussed in the following three examples (one "full scale" project and "two pilot plant" scale projects) : Example 1 "Soil Vapour Extraction of Hydrocarbons, In-situ and On-site Biological Treatment". In terms of quality contamination of soil and groundwater by hydrocarbons are the most widespread form of pollution. Miller (1990) estimates that 90% of the transported hazardous waste in the U.S.A. consists o f gasoline, oil and jet fuel. Furthermore, it is estimated that 50-70% of the total in situ remediation costs are spent on air treatment, due to the volatilization of the organics. A combined soil vapour/groundwater treatment system has been developed by T A W (BIOPURB) , to help reduce these costs. In the case described, the system was used along with air based biodepradation (in situ) . Site characteristics Whilst carrying out soil remediation at a petrol station, contamination, by gasoline, was found below a provincial road. Excavation was not feasible due to financial and technical reasons. The most favourable remediation technique was to use soil vapour extraction, combined with biostimulation. Not only the volatile compounds had to be removed but also the biodegradation o f particularly non-volatile components had to be stimulated by the passive infiltration of air and nutrients. In order to achieve this the groundwater level was lowered by some 2.5 m. This increased the unsaturated zone which is favourable for soil vapour extraction and aerobe biodegradation. Groundwater and soil vapour were extracted on one side of the road, causing a continuous flow towards the extraction points. Air and nutrients were infiltrated at the other side, see Figure 1. Trealed 5011 vapur
Infinrationof
Grarndwaler level
Figure 1. Cross section o f remediation site
The extracted soil vapour and groundwater were treated in a combined soil vapour/groundwater treatment unit. The retention time was approximately 15 minutes for groundwater and less than 10 minutes for soil vapour extraction. Nitrogen and phosphorus nutrients were added to the groundwater influent stream. The system was operated in co-current flow, s e e Figure 2. Using the oxygen consumption and carbon dioxide production rates it was possible to calculate that the in-situ biodegradation progressed at a rate of 7 mg/kg soil/day. m S o i l vapour
~
(SO n3/hwr)
Groundwater
~
-
210 glh - ~ ~ 8 i o \ o g i c a l ~ 10 g l h - > I S Y S K M I
( 1 5 n3/hour1
ITreatment 1 -I
<
3
<
2 g/h
glh
, exhaust
~
ges
effluenr
>-
Figure 2 . Gasoline mass balance at a steady state It also seems to be a zero order process. Initial concentrations were approximately 80 g/m3. This dropped to 3 g/m31 after 20 weeks and 0 . 2 4 0 g/m3 after 60 weeks. In Figure 3 the removal results are compiled based on soil vapour analyses. It is clearly shown that the soil vapour extraction system was largely responsible for the total amount removed, later when most of the volatile compounds had been removed, this was largely attributed to biodegradation. Total amount
1
,000
20
40
(10
10
Time (weeks1
Figure 3 . Cumulative amounts of gasoline removed during soil vapour extraction. The treatment efficiency of the combined soil gas/groundwater treatment was greater than 98%. All the legal requirements towards effluent water streams were met (i.e. < 100 pg aromatics/l and < 1 mg mineral oil/l). For individual aromatic compounds the concentrations were < 0.5 pg/l. N o aromatics or other volatile organic compounds were detected, in the treated soil vapour, see Figure 4 , even with a detection limit of 0.1 ppm.
67
EXHAUST GAS I
SOIL VAPOUR
Figure 4 . Chromatogram of soil vapour (untreated) and exhaust gas (after biological treatment). Example 2 "Removal of Toluene from Gas Streams with a Biotrickling Filter" In the last years it has become clear that photo-chemical air pollution is a great problem for health and the environment. The reaction between NOx and volatile organic compounds causes the formation of ozone, which is hazardous to health and has an adverse effect on the environment. It is therefore imperative to reduce the emissions from NOx and volatile organic compounds. The reduction of emissions from volatile organic compounds is the aim of a Dutch governmental project called KWS-2000. The use o f biotrickling filters for the removal of organic compounds from gas streams has been described: Diks et. al., 1991; VDI-bericht 7 3 5 , 1989. T A W Infra Consult B.V. developed a biotrickling filter using a macro-reticulated synthetic packaging material (BIOPURB), see Figure 5. The system is operated in counter current mode. Active biomassa is present in both the packaging material and the recirculation liquid. Recirculation o f the process liquid has the advantage that the nutrient dosing and the pH can effectively be controlled (particularly essential for the removal of chlorinated volatile organic compounds).
68 Toluene was used as a reference component. The experiments were carried out using three different gas flow rates, i.e. 450 m/h, 700 m/h and 900 m/h. Gas phase concentrations were between 20 mg/m3 and 700 mg/m3. The liquid flow rate was 2.8 m/h. Figure 6 shows the elimination capacity as function of the component concentration at the gas inlet. During the experiments elimination capacities greater than 200 g/m3/h could easily be achieved. These experiments show that a good performance is possible using high gas flow rates: low pressure drop: < 4C Pa/meter bed, high elimination low maintenance, easy capacity, control. Initial experiments with multi-component systems show a good performance but its behaviour differs from that of mono-component systems. Further investigations are therefore aimed at describing the multi-component systems. Figure 5. Biotrickling filter with alternative packaging material.
250
ELIMINATION CAPACITY [g/m3*h]
0
200
0
150 0
0
100
0
750 m/h
0
900 m/h
200 300 400 500 TOLUENE CONCENTRATION [ mg/m3]
600
50
0' 0
I
100
I
I
Figure 6. Toluene concentration [mg/m3],
I
700
69 Example 3 In the livestock industry the emission of ammonia is a serious problem. The Dutch policy is to reduce the total emission by 70% before the year 2000. ,Structural changes in the accommodation design and new manure treatment techniques can help solve a great deal o f this problem. Still waste gas treatment also has to b e implemented at order to meet the government requirements and to avoid odour annoyance for the people living in the area. The use of new packaging materials with very low pressure drop (Figure 7) stimulates the coupling o f the treatment systems with the ventilation systems already installed. The removal efficiency of the system is high, 90% with 1 m packaging height. Gas velocity 1650 m/h ( ( 46 m/s) and NH3 concentration 90 rng/m3. LESSURE DROP [Pa]
400
300
P 200
100
0
0
0.5
1.5 2 2.5 3 3.5 4 SUPERFICIAL GAS FLOW RATE [m/h] *LO00
1
4.5
5
Figure 7. Pressure drop as function of the superficial gas flow rate [m/h] The concentrations found are given in Figure 8.
CONCENTRATION PROFILE O F AMMONIA gas veloclty: 1650 m/h. "H31
=
90 rng/rn3
RELATIVE CONCENTRATION [-]
0.6
-
0.4
-
0.2 -
01
0
0.2
0.4
0.6 0.8 COLUMN HEIGHT [HI
Figure 8. Concentration profile of ammonia.
I
1.2
70 Partial nitrification occurs in the liquid phase of the biotrickling filter. The additional nitrification takes place in a separate water treatment unit. 4 . CONCLUSIONS
The BIOPURm system for air treatment has great potential because the system can be operated in a "niche" where other systems were either very expensive or have failed. This is due to the application of a macroreticulated synthetic packaging material. Compared with a biofilter the BIOPURB system offers better process control (pH, nutrients), which is essential for the treatment of chlorinated organic compounds. Compared with bioscrubbers the BIOPURm system needs less space and uses less energy because absorption and treatment takes place in the same column. For combined air and water treatment or solely water treatment the application of BIOPURm system is advantageous in the field of soil remediation. The estimated costs for treatment in are low:f 0.50 per m3 water; f 1.00 - f 5.00 per 1000 m3 gas. The Biopurm seems to be a promising technique, however, more research is still required particularly in the field of waste gas treatment (multicomponent systems, chlorinated hydrocarbons). In the field o f NH3 removal at high gas velocities, the coupling of the removal system with the biological treatment system requires further research.
REFERENCES R.N. Miller, et. al. A Field Scale Investigation o f Enhanced Petrol Hydrocarbon Biodegradation in the Vadose Zone at Tyndall, AFB, Florida. Proceedings NATO/CCMS meeting France, December 1990. L.G.C.M. Urlings, et. al. Soil Vapour Extraction of Hydrocarbons. - In Situ and On Site Bioreclamation, an International Symposium, 19-21 March, 1991. San Diego, California, Proceedings: in press. F. Spuij, et. al. Biological Groundwater and Soil Vapour Treatment. Third Forum on Innovative Hazardous Waste Treatment Technologies, June 11-13 Dallas, Texas. P.H. van Riel. Aktieve koolvezeladsorptie. Proceedings Studiedag Luchtzuiveringstechnieken, 29 May, 1991, Arnhem, The Netherlands, Session 9. R.M. Diks et. al. Process Engineering Aspects of Biological Waste Gas Purification. International Symposium Environmental Biotechnology. April, 1991, Ostend, Belgium, Proceedings Part I, pp 353-367. VDI-Bericht 735. Biologische Abgasreiniging VDI-Verlag GmbH - Diisseldorf 1989. J.A. Don. Stroomversnelling bij de toepassing van biofiltratie. Proceedings Bestrijdingstechnieken luchtverontreiniging, December 1984, pp 59 - 6 3 .