Biofiltration Process

Technologies that have been employed to treat offgases from wastewater treatment facilities, food processing plants, organics processing facilities, and composting operations include acid/base scrubbers, incineration, bioscrubbers and biofilters. Biological abatement technologies have become increasingly popular because of low costs, operational simplicity and because they are intrinsically clean technologies. There are minimal requirements for energy and raw materials and minimal waste production. In particular, biofiltration has become an accepted and mature technique for air pollution control with substantial industrial confidence. This is demonstrated by the exponentially increasing number of biofilters around the world.

A biofilter is essentially a packed bed of particles in which microorganisms are immobilized on top of the packing particles called support medium. The contaminated air stream is humidified and passed through the bed. Figure 1 shows a schematic diagram of an open biofilter. The name secondary humidification system is a bit of a misnomer as, most of the time, this system is used to remove acid by-products (or excess alkali) as they form, by washing them out of the bed. A better name might be pH control system. It may also help to keep the packing materials at the required moisture content for optimum microbial activity. The secondary humidification water can be potable water or treated effluent from a wastewater treatment system. As shown in the Figure 1, leachate is collected at the bottom of the bed for further treatment.

Figure 1—Schematic diagram of an open biofilter

In the biofilter, contaminants (odorous compounds) are transferred to the interface of the gas and biofilm that is attached to the bed packing material, where they may be absorbed into the biofilm. The contaminants dissolve in the liquid phase and, in the biofilm may be degraded biologically. Within the biofilm, contaminants may be used as carbon and/or energy sources by the microorganisms. The uptake of contaminants by microorganisms creates a concentration gradient within the biofilm, which promotes molecular diffusion of substrate molecules from the gas-biofilm interface toward the biofilm-solid interface. Microorganisms convert the pollutants to CO2, water and other end products. Depending on the choice of the support media, there could be other means of pollutant removal such as adsorption in the packing media and absorption by the humidification water. In general more than one mechanism is involved in the removal of pollutants in a biofilter.

The treatment of air streams containing a mixture of inorganic and organic substances is made complex by the fact that there are a number of interacting chemical and microbial mechanisms that may be active. In addition, reactor design and operating conditions will impact performance. Mechanisms that are primarily physical or chemical in nature include gas-liquid mass transfer, adsorption to media and dissociation of ionic species. For some compounds (i.e., ammonia) these mechanisms are pH sensitive. However, the pH of a biofilter is influenced by microbial processes that generate inorganic acids (i.e., nitrification and oxidation of RSC), dissolution of CO2 gas as well as addition of buffer due to dissolution of weak bases such as ammonia.

Factors Affecting Biofilter Performance

Biofiltration is a process that utilizes microorganisms immobilized in the form of a biofilm layer on a porous filter substrate. The heart of a biofiltration system is the filter bed, which can provide a nutrient source and a surface for attachment of microorganisms. Desirable medium properties include optimal microbial environment, large specific surface area, structural integrity, high moisture retention, high porosity and low bulk density. Based on these properties, various types of porous materials have been employed as biofilter media, with compost and soil being the most common types. Other materials include wood bark, sand, sand and loam, a mixture of compost and diatomaceous earth, granular activated carbon and some synthetic materials. Some researchers demonstrated that organic media was superior to inorganic media when treating ammonia. They indicated that organic packing materials provide a more suitable environment for nitrifying bacteria than inorganic packing materials. Some media are likely to provide micro-nutrients to the microorganisms and may also provide a surface chemistry that promotes their growth.

Biofilter performance is generally affected by the proper selection of the medium. Many mixtures have been tested to develop optimal filter materials with high activities and a low flow resistance. The size of bed materials must be small enough to provide a large surface area; however, the bed may be plugged with excess biofilm or water logged by excess moisture in a small medium. The surface area to bed volume (specific surface area) for biofilter media ranges from 100 to 1100 m2/m3, with wood chips having a ratio of 160 m2/m3 while a mixture of heather and peat had an area to volume ratio of 1100 m2/m3.

For maximum compound removal, the bed material should have high moisture retention capacity in order to prevent drying, high porosity of filter bed to reduce head loss and to increase the distribution of incoming waste gas, available nutrients for optimal microbial growth, and a diverse microbial population. Structural stability and low density of the support medium reduces medium compaction potential which otherwise would cause excessive pressure drop. A void volume between 40-80% keeps the operating pressure drop low and ensures easy gas flow.

Absorption and adsorption are probably the first processes in the biofilter mechanism. Some filter materials such as GAC and wood chip mixtures have shown their potential adsorption during the start up periods of the biofilters.

The rate of water addition will impact the process chemistry and microbiology of a biofilter. Systems that have low rates of water supplementation will not generate a leachate stream and hence metabolic products such as nitrate will accumulate in the media. This has been found to result in inhibition of the microbial populations after some period of operation. Systems that employ higher water supplementation will tend to flush out metabolic byproducts. However, in these designs the media must be able to pass the extra water without becoming excessively saturated. High water content will reduce gas-liquid mass transfer and can result in the development of anaerobic regions in the biofilter. It also increases pressure drop and may cause irreversible structural damage in the media. The water content of wood bark below 60% of weight resulted in a severe loss of efficiency which could be reestablished by moistening the filter.

Most microorganisms capable of degrading pollutants show optimum growth in a certain pH range. Sulfur reducing microorganisms can survive even at very low pH. Most of the RSC biodegradation releases acidic substances (e.g., SO42-) in the biofilter leachate. The media pH dropped from 8.0 to 2.5 in 32 days in a compost biofilter receiving high H2S loading. To buffer acidic by-products in the media and maintain a neutral pH, most suppliers of biofilters add alkaline buffers, usually calcium carbonate or lime, to the filter material. Dissolution of NH3 from the offgases acted to buffer against pH decreases that would result from the biological transformation of RSC and NH3.

The removal efficiency of the reactor has a direct relationship to the bacteria population within the media. The bacteria population has direct relationship to moisture content of the support media. To prevent wide fluctuations in the moisture content of the support media and therefore the removal efficiencies, it is an established practice to saturate the gas stream before entering the reactor. Offgas pre-humidification is common in most recent full-scale biofilters to prevent bed drying. The pre-humidification system can also adjust the temperature of the gas entering the biofilter and remove any particulate matter that may clog the biofilter. The pre-humidification also absorbs NH3 gas from the offgases. Therefore pre-humidifiers function as a water scrubber (wet scrubber) as well.

For mesophilic microorganisms that are almost exclusively used in biofiltration, bed temperatures between 10 and 40 °C are acceptable, with an optimum range for biological activity of 20-35 °C. Most commercial gas streams do not automatically fit this requirement and must be conditioned. Furthermore, temperatures can affect the removal efficiency depending on the compound to be degraded. Biodegradation rates increase with increasing temperature and show an Arrhenius type of temperature dependence. Although non-optimal temperatures can reduce degradation rates, microorganisms often recover rapidly from temperature variations. Therefore, small temperature variations do not affect the overall biofilter performance.