Control of emission sources containing VOCs and HAPs with biofilters is a relatively new technology. Biofilters are composed of single or multiple beds of substrate, which may be porous inorganic materials like ceramics or plastics or, more commonly, compost, peat or soil-mulch mixes. Bacteria, fungi and various other organisms use these materials as attachment sites, growing to form large populations that ‘feed’ on the VOCs and HAPs from various industrial air emission sources. The air emissions are directed through the substrate in the biofiltration unit where the microbial community sorbs the airborne organics and uses them as a food source, oxidizing the pollutant into carbon dioxide and water; at the same time, using the various components for building added biomass and providing energy to the community of organisms. After the organic compound is broken down (metabolized), the sorption site is available to collect more organic materials in a relatively continuous cycle of biological activity. As temperatures increase to the optimum for specific communities of organisms (many function best at 85°–110°F (29–43°C), the metabolic reactions are very rapid, allowing sorption, metabolic degradation (oxidation) and re-sorption to occur almost instantaneously. All of this occurs with the generation of energy, new cell mass, some heat, carbon dioxide and water, and without the use of additional fossil fuels for oxidative combustion processes. Biofilters do not produce additional toxic or hazardous emissions (as can occur in incomplete combustion reactions) and do not create additional NOx or SOx as do thermal oxidizer technologies.
Biofiltration technology was experimented with in the 1950s in the United States and Europe, but was largely ignored in the United States while being successfully commercialized in Western Europe. Hundreds of biofilters (quite possibly nearing 1,000) are currently in operation in western Europe, while North America has fewer than 75. Applications vary, but have often been on odorous emission sources in Europe, where population density and proximity to residences created nuisance situations. Biofilters proved more than adequate to handle these odor applications, were lower in capital cost and more cost effective in operation and maintenance than conventional thermal oxidizers or chemical scrubbers. Europe was also less concerned with general control of VOCs, as ground level ozone was not a major issue. European countries did not require strict VOC control to the levels often mandated by the U.S. EPA. Therefore, the use of biofilters in Europe moved forward because they solved the odor problems, were not as costly as other controls (capital and operational), and generally did not have to meet exacting emission removal efficiencies that had been or were proposed in the United States.
In North America, the demands of relatively high (often exceeding 95% destruction efficiency, or “Dre”) removal efficiency regulations created difficulties for the use of biofilters as Best Available Control Technology (BACT) and Maximum Achievable Control Technology (MACT). Industries could not afford to chance a relatively unproven biological system for their main pollution-control device, as the established thermal oxidizers and chemical scrubbers gave them security from non-compliance with regulations, and their own emission limits and air-pollution control permits. In addition, the laws and regulations previously established for various industry emissions were not enacted with biofilter systems in mind. Energy was abundant and inexpensive, so conventional (available) thermal and chemical technologies were the only consideration. Control of the volatile organics was paramount, and the fact that thermal oxidizers produced additional NOx and considerable additional amounts of CO2 and some CO from the combustion of fossil fuels was not a significant issue.
Fast forward to the 21st century. With more emphasis on specific VOCs and HAPs, the threat of rolling blackouts, and with energy costs skyrocketing, there is a greater awareness of the viability of the biofilter as an adequate VOC and HAP control device. Additionally, progress in biofilter design has led to more compact sizing and greater VOC and HAP removal efficiency. All of these factors have directed the attention of certain state and federal regulatory agencies toward more sustainable technologies, such as biofilters. Add to this the increased emphasis on overall ‘facility emissions reductions’ instead of simply end of pipe controls and you have an even ‘friendlier’ biofilter application. Specifically, the coatings industry is an appropriate candidate for VOC and HAP emission controls with biofilters. The reason is quite simple — the MACT established for the industry allows for 75% removal of HAPs. This can be accomplished in a variety of ways, including source reduction (reducing the amount of HAPs in the product manufacturing process), in combination with end-of-pipe pollution controls. That means that biofilters (which may be able to achieve 90%+ Dre 90% of the time) can be used to satisfy emission reductions at reasonable capital and operating costs when used in combination with source reductions in order to achieve continuous compliance. With the added emphasis on pollution prevention, the biofilter is now a viable option for compliance in the coatings industry, while accomplishing the task at a much lower capital and operating cost. Biofilters are more energy efficient and environmentally friendly, and provide a sustainable control solution that addresses both VOC and HAP emissions.
Processes for Control
In the coatings industry, there are numerous applications for biofilters as VOC and HAP emission control devices. These include: paint manufacturing, aerosol paint can depressurization and recycling, paint booths, solvent-laden rags, and drying of paint solids and sludge for reuse or disposal. Bio•Reaction Industries LLC has installed biofilters in several of these operations with excellent operating results.