News | July 16, 1998

Biological Treatment of Air Pollution

The biological treatment of air pollution depends on aerobic microorganisms--mostly mesophilic bacteria--that feed on both organic and inorganic compounds in the waste gas. The microbes convert the pollutants into carbon dioxide, water, and salts.

The technology primarily has been implemented in Europe. Facilities there have been treating gas streams comprising hydrocarbons (as well as some chlorinated organics) at concentrations below 1000 mg/m3 and at gas temperatures of 10 to 43 ºC, with low dust loads.

Traditionally used for odor control (such as at livestock processing facilities), biofilters are now being used for ammonia control at composting facilities and for volatile organic carbon emissions control at wastewater treatment facilities, breweries, foundries, plastics, paper, and petrochemical industries. Biofilters also have been used to remove vapors from contaminated soil and groundwater.

There are two main types of biological treatment technologies:

1. Biofilter
This is the simplest and least expensive biological treatment method. Its main component is a bed of compost, tree bark, peat, heather, or soil, about 1m deep, through which the contaminant gas is blown (see Figure 1). The material in the biofilter bed provides a diverse culture of microorganisms that degrade the gaseous pollutants passing through.

Figure 1. Schematic of Biofilter System with air humidifying and water drainage

Most biofilter units resemble towers, being designed as sealed vertical containers. The design allows for better process control and exhaust monitoring than do open chambers (see Figure 2). The inlet gas flow distribution and humidity must be carefully controlled; otherwise the bed will behave simply as an adsorption filter and eventually clog. Typically the inlet gas should be relatively dust-free, saturated with water vapor, and be at a temperature of 10-40 ºC. Such units last between one and seven years, depending on the contaminant loading.

Figure 2. Tower design biofilter

2. Bioscrubber
A bioscrubber couples traditional air pollution control and wastewater treatment technologies and consists of two units: a scrubber and a biological treatment basin. The soluble waste gases and oxygen are continuously absorbed into water in the scrubber. Biological oxidation occurs in the basin unit, which often is the activated sludge basin of a wastewater treatment plant. Bioscrubbers are used where the biological degradation products (such as the acids produced during H2S and NH3 removal) would harm a biofilter bed. In addition to hydrocarbons, bioscrubbers are being used to remove chlorinated organics. Bioscrubbers come in two forms:

Activated-sludge scrubber. Gaseous pollutants are absorbed by a solvent in a countercurrent packed column tower. The absorption solution generally is a water and sludge mixture (1-10 g sludge per liter of water). After absorption in the column, the solution proceeds to a sedimentation tank where biodegradation takes place. Clean solution from the sedimentation tank recycles back to the packed column (see Figure 3).

Figure 3. Schematic of an activated sludge bioscrubber with sludge recycle

Trickling-filter scrubbers. As with the activated-sludge scrubber, gaseous contaminants are transferred into the liquid phase with a countercurrent scrubber. Instead of being fed into an activated sludge pond, however, the pollutant-laden scrubber wastewater is spread over a trickling filter. The technology offers greater control than does the activated sludge scrubber. .

The major advantage of biofiltration is its operating costs. Compared with biofiltration, the operating cost for scrubbing is on the order of 300% greater; for incineration, 1300% greater; and for carbon adsorption, 1800% greater. In addition, the installation cost of a biotower is comparable to these other treatment technologies. Also among its virtues, bioscrubbing produces no residues that require further treatment.

In some cases (for example, gas streams with high or highly variable contaminant concentrations), biofilters can be coupled with conventional scrubbing or activated carbon. The activated carbon (a regenerative adsorption technology) removes the majority of pollutant and precedes the biofilter to bring emissions within the biofilter's capability.

The preceding article is adapted from the lecture note supplements of Dr. Kurt Paterson, Michigan Technological University, Department of Civil and Environmental Engineering, 1400 Townsend Drive, Houghton, MI 49931-1295.