Biofiltration for VOCs and odours reduction: the green way for a friendly environment. My story with openTOP and container biofilters.

Introduction

Even if my education is catalysis oriented (for VOCs abatement and odours reduction out of coil-coating lines in the steel industry), I was involved in the selling and follow-up including maintenance of BIOFILTERs since 20 years.

My experience started with CLAIRTECH, a Dutch ? start-up ? company, what developed an innovative biomedia based on mushroom compost[1]. I always sold biofilters based on mesophilic (biodegradation temperatures between 15-40 °C) activity, even if I touched thermophilic one (biodegradation at temperatures up to 50 °C). If thermophilic was of ? bio-scrubber ? type, all of my related experience is fixed bed bioreactors.

Biofiltration is the cleaner process for a clean environment; it allows for the abatement of volatile organic, inorganic compounds and odour reduction.

It is used worldwide from the easiest to the most complex cases.

The most successful stories are for the treatment of odours in water treatment plants, but biofiltration may work for VOCs and another inorganic volatil compounds like H2S and ammonia. Such an unit with 4 bioreactors and a scrubber downstream are shown in figure 1 (waste water plant for sewage in food industry).

Biofiltration is an oxidation technology: it will destroy organic compounds to CO2 and H2O as well as heterogeneous ones to their oxides or elements. Mainly sulphured compounds will end like elementary sulphur whereas chlorinated (halogenated) will end like acid (HCl,…).

Degradation should allow the transformation of polluting compounds - some of which can be odorous - into odourless and atoxic compounds. The elimination of organic substrates by microorganisms is due to the fact that these organisms (mostly bacteria, mold, yeast, or microscopic algae,...) use these organic compounds as the only source of carbon for their own biosynthesis (anabolism) and as the essential energy source for the degradation of a substrate (catabolism). In the case of air treatment, only an aerobic metabolism occurs.

The Gas Hourly Space Velocity (GHSV) determining the biomass volume is a function of the application case and of the choosen biomass. The choice of the biomass being oriented by the application case. GHSVs are a property knowledge developed by experience.

Arrangement and types

Nature, temperature and concentration are limiting factors when choosing or operating a biofilter. Concentration and temperature are linked: a pure hydrocarbon (HC compounds) with a PCI (calorific inferior value) of @ 40 kJ/kg will increase the temperature of ambient air by 30 °C, such as 1 gram per Nm3 is a limit (see figure 3). Indeed this increase of temperature could increase the bed temperature to 50 °C – if the inlet temperature is ambient - what is the limit for mesophilic microorganisms operation. Nature of compounds to be treated will influence the choice of the biofilter technology (figure 4): if HCl is produced, the pH will have to be fitted such as a bioscrubber or percolating biofilter must be used. In both case, nutrifiants and another compounds like fungicide can be added.

Fig. 0 & 1 – Enclosed biofilter on agrofood industrial waste water plant : container with peripherical equipment (fan, scrubber feet and feeding – circulation and dosing pumps, electricals) supporting the stack, left picture and enclosed biofilters (bottom to top air circulation) right picture

Humidifier/scrubber

In every case saturated humidity must be ensured whereas it is better to maintain constant temperature, inlet concentrations and nature of pollutants. A humidifier is needed downstream the inlet; the best is to put a kind of scrubber like it is shown in figure 5 (the picture figure 6 is corresponding to this diagram).

My advice is that the humidifier must also be used like a scrubber. First rule is of course to ensure saturated wet conditions at the inlet of the BIOFILTER. Second is to protect the biofilter against dust. And last but not least, the humidifier can play like a chemical scrubber allowing to remove extra-charge for the biomass like desulphurization, removal of acid components, and improving the soluble products captation. In general way to improve the global efficiency of the system. Scrubbing liquor (water) can be recirculated in a loop including a water filter. For investment reason, the scrubber can be sometime by-passed and introduced air in the biofilter is conditioned only by simple nozzles for humidification. This way is not recommended but anyway it is better to use enclosed (container) biofilter configuration in what the raw air is introduced by the top (figure 15). Packed columns are to avoid because of possible plugging problems such as horizontal configuration is the best (figure 2); it can use sprayers and easy removable B-GON meshes can be used as packing and as efficient dust collector [the packing can be removed by side and cleaned with pressurized water (Karcher...)].

Fig. 2 – Horizontal scrubber using B-GON meshes

Fig. 3 – Selection diagram of a VOCs abatement technology

Fig. 4 – The different arrangement of biofilter

Fig. 5 – Two beds biofilter P&ID – 90 to 125 kNm3/h

Fig. 6 – Figure 5 biofilter (2 x 350 m3 biomass) with the scrubber at foreground

Enclosed biofilters

That is why enclosed biofilters are the preferred solution like in figures 1 and 6 to 9. Figure 6 biofilter is an on-site enclosed solution whereas figures 7&8 show customized solution by use of prefabricated containers.

Fig. 7 and 8 – Custom container enclosed biofilter with its PPH scrubber in front (left) and Bunk biofilter containers (right)

Containers are very useful because of their mechanical strength: they can be bunk as in figure 8, even to three stages (figure 9); furthermore the container can be sustained on work hall roof and can contain the machineries inside (circulating and dosing pumps, electrical cabinet, reactive storage,…). In general bunk biofilters are operating in parallel meaning that the flow must be spread equally.

Enclosed biofilter in container or/and covered ones are ideal in order to maintain constant the operating conditions of the biomass in regard to temperature and even humidity since the system is not subject to weather outdoor conditions.

A significant advantage of enclosed system is also the co-courant circulation of polluted air and water: when watering is done at the top, the top to down direction of the air leads the water to drain all the depth of the biomass bed.

Fig. 9 – Bunk biofilter containers

Open biofilter (OpenTop) (header picture)

Advantage of open biofilter is their lower cost at investment (CAPEX). Furthermore their maintenance cost could be lower (OPEX). But the pricing of the square meters area, and in general the available space into an industrial or another process plant will make the decision. We can see the draft fan, humidifier and piping. Here the flow is always going from the bottom to the top. No stack is required, the exhaust is done like a surface emission.

Fig. 10 – Maintenance : aeration/removal/replacement of biomass of an opentop biofilter (cement walls).

Containing the biomass can be done thanks to cement (low) walls (fig. 10 and 11) or synthetic ones (fig. 12). An intermediate solution is to put a roof over the biofilter (fig. 14) or to put it in a hall area (fig. 13).

Fig. 11 & 12 – Cement (left) and PPH (plus steel beams : right picture)

Design and biodegradability

Every exhaust has ist own specificity such as experience is of paramount importance in design of a biofilter.

Regarding chemical nature of the pollutants and viewing the operation mechanism of bioreaction, more the volatile compounds are soluble better it is such the following order of degradability is followed by decreasing kinetic:

• Alcohols
• Aldehydes
• Fatty acids
• Ketones and esthers
• Aromatics (BTX,…): dégradable at low concentrations
• Alkanes and cyclics (low degradability)

Fig. 13 & 14 – Open biofilter in a hall (wood bark with sprayers) and Covered biofilter (with watering pipes)

Fig. 15 – Inside container (with pierced flexible pipes)

If common VOCs are degraded to CO2 and H2O (and acids if halogenated), the odoriferous compounds like ammonia and hydrogen sulfur follow the path :

• NH3 to NO2 to NO3 = food for microorganisms
• H2S to S2 to H2SO4

Biofilters are also rejecting water. Ideally the water flow is recycled at the top, the rest being lost by vaporisation – what could increase the volumic flow at the exhaust of the unit. The elementary sulfur being solubilised to sulfuric acid, special attention, precautions and corrections must be kept.

Biomass and performances

Basic biomass is constituted by wood bark and compost. Activation can be helped thanks to local sludges (from a waste water treatment plant by example) or by specific inoculation. Hydroscopy, mechanical strength (resistance to settlement and crushing), porosity must be settled. Coarse porosity is controlling the electrical consumption, such as the full system including humidifier/scrubber will have a pressure drop of around 250 mmWC. The pressure drop could increase over the time due to settlement (decreasing of pressure drop is a sign of preferential paths into the bed).

Humidity is of paramount importance. Automatic watering system is used (figure 15); spring balances have been used in order to weight the biomass bed continuously. With ambient conditions, water consumption of the system will be around 6 grams per cubic meter of air (Nm3).

Homogeneous air distribution through the bed must be done.

Everybody can make his own biofilter such as each seller tries to differentiate themselves. Mushroom with styrene balls, heather to most complex stuffes with “semi-scientific” and trade names; composted pine peels mixed with sphagnum peat fiber (lignin content for strenghteness) density of 350 to 650 kg/m3 dry with 50 to 60% moisture, shellfishes…

We can also point out the:

• fungicide to prevent the occurence of mould set on the surface of biomedia beds can be added both in the scrubbing liquor and the humidification system ;
• addition of a dispersible aqueous liquid: coccoliths with an exchange area large enough to allow the formation of bioactive aggregates (VOCs metabolism)
• injection of thiobacilli strains (both in scrubber and biofilter itself)
• biofilter treatment with additives, nitrogen and phosphorus-based fertilizers and neutralizing carbonate refill

Fig. 16 – A 12000 Nm3/h unit treating a VOCs mixture of ethyl alcohol (main components) and ethyl acetate

 Regarding efficiency it is important to distinguish – dissociate - the efficiency of the scrubber/humidifier and the biofilter. Indeed the scrubber/humidifier (see fig. 17) – as well as the biofilter water film – can play like a buffer : absorbing and desorbing. Such as the efficiency may be negative sometime, depending how is loaded the water (saturation or not)… Regarding the unsoluble compounds, both scrubber and biofilter may display a certain efficiency of abatement but it is not only biodegradation activity : condensation, adsorption (in solid porosity of biomass materials), and extraction meaning diffusion of unsoluble hydrocarbons to soluble ones in the water phase. It must be pointed out that publication about sewing water chemical analysis from biofilter is rare.

The performance at the exhaust may be on odours units/ Nm3: less than 300 and on VOCs less than 50 mg/Nm3.

It will vary as a function of VOCs nature. The inlet limit concentration will remain less than 1,5 g/Nm3.

We have data about abatement efficiencies of a biofilter over 10 years of operation. Mixture was composed by sulfured, nitrogenated, aromatics, cyclics, aliphatics, alcohols, aldehydes, … with mainly aliphatics, average flow of 82000 Nm3/h, inlet concentrations respectively for volatil compounds and odours : 120000/33000 uo/Nm3 and 100/44 mg/Nm3 leading to efficiencies of around 70% on odours and 50 % on pollutants, respectively. It was not significative differences between VOC/hydrocarbons families.

Such as first conclusion it can be said that biofilter (fixed bed) is a right technology for big flows and relatively low concentrations. However the balance must be checked regarding all effluents (gas and liquid).

Biofilter construction

Except the scrubber, an open top biofilter is constructed/assembled on site.

For enclosed ones, the container and possible equipment inside are carried on site and biomass is putted into.

Fig.17 - Humidifier 65000 Nm3/h

Fig. 18 & 19 – Biofilter construction by Hobiofilter GmbH : membrane placement and biomass loading

 Grids for support can be in recycled PP material (fig. 20) or in wood, first type is preferred. The same support is used for enclosed biofilter.

Fig. 20 & 21 – Biofilter construction grading (left picture) and membrane placement for border tightness (right picture)

Biofilter maintenance

MPFilter-Phosphoris SAS proceeded to the replacement of biomass of the unit figure 16. Biomass : 50 m3 of Pinaceas, Pinus silvestris

Fig. 22 & 23 – Open biofilter maintenance by MPFilter/Phosphoris

Fig. 24 & 25 – Open biofilter maintenance by MPFilter/Phosphoris

 CONCLUSION

Biofiltration is today an half-century technology what emerged in industrial and waste management gaseous effluents.

It is a niche technology from what we cannot wait for an up to 98 % chemical abatement but the ratio chemical/odoriferous reduction is high regarding CAPEX and OPEX.

Mainly and whatever the chemical nature of pollutants, even with variation on their concentration, biofiltration can guarantee more than 50 % reduction. Contamination by pollutants by a few hondred mg/Nm3 is ideal. Life time of the biomass is up to 5 years. Our experience is showing 7 years operation before change… of a biological ecofriendly material.

Modern biofilters are more oriented to enclosed version or at least a covered ? openTop ? type configuration.

Biomass is taken into account in the OPEX but thanks to leasing, full enclosed container type biofilter can be included. Contact us.

Small enclosed BIOFILTERs are beautiful. Biofilter could be designed from 1 up to 2500 m2 area, going from the biggest to the smallest. We have also our MINI ClubBio system to treat 100 to 250 Nm3/h. Same GREAT concept than container enclosement.

Fig. 27 – MINI ClubBio

[1] media composition: 30% compost (from mushroom production) - 65% polystyrene - calcium carbonate buffer (CaCO3: 50 kg/m3)