Composting odor control made easy

Odor control is the most significant problem facing composting facilities today and often is the largest hurdle to gaining public acceptance of a facility.

With careful development, odor can be managed successfully, provided the community and regulatory agencies have realistic expectations. Many technologies are available to treat compost-related odor.

To be effective, the planning and design of an odor control system must be an integral part of the composting facility. There are a number of alternatives from which a municipality can choose.

Biofiltration consists of directing the odorous air stream to a series of perforated pipes laid in a bed of gravel and covered with an organic media. As the air stream filters up through the media, odorous compounds are removed by a combination of physical, chemical and biological processes. The biofilter media typically consists of equal parts of wood chips, bark mulch and cured compost. Performance depends on numerous factors, including the type of the odorous compounds to be removed, characteristics of the filter media, moisture content of the bed, temperature, bed depth and time in service.

Some reported problems with biofilters include short circuiting, the tendency of the media to dry out and pH depression within the media due to the conversion of bound sulfur to sulfuric acid.

The unit capital cost of a biofiltration system typically is between $1,000 and $2,000 per daily cubic yard of compost. Annual operations and maintenance (O&M) costs, including power for the compost ventilation system and biofilter media replacement once every five years, is estimated at $5 to $10/cu. yd. of compost.

Due to the potential of achieving high removal rates at a moderate cost, biofiltration currently is one of the most commonly used odor control technologies.

Chemical wet scrubbers remove odorous compounds from the air stream through the absorptive and/or oxidative capacity of scrubbant solutions. There are two primary types of wet scrubbers: mist and packed towers.

The chemical wet scrubbing technology has made significant advances in the past five years. Today, the state-of-the-art method of scrubbing compost facility exhaust air consists of a minimum of two stages. A typical wet scrubbing system consists of centrifugal dust collectors followed by scrubbers, forming two parallel two-stage treatment trains. Treated air from the wet scrubbers is discharged through a dispersion rack.

The pH within the first stage is automatically maintained between 2.5 and 3. Exhaust air from the first stage then passes counter-currently through a second stage wet scrubber which uses sodium hypochlorite and sodium hydroxide as the scrubbant solutions.

Operational problems with wet scrubbers include difficulty in maintaining effective chemical feed rates, plugged nozzles and filters in mist-type scrubbers, plugged media in packed towers and the creation of chlorinous odors in the treated effluent from both mist and packed tower wet scrubbers.

Capital cost for two-stage scrubbing system is between $5,000 and $9,000 per cu. yd. per day of compost. The estimated annual O&M expense for the wet scrubbing system ranges from $10 to $15/cu. yd. of compost. Operating cost estimates include the power required to operate the entire building ventilation system as well as chemicals consumed by the wet scrubbers.

Thermal regenerative oxidation consists of exposing the malodorous air stream to temperatures of approximately 1,400[degrees] F for about one second. The removal efficiencies of pilot scale units treating compost odors have been reported to be similar to, but not better than, multi-stage wet scrubbing. However, capital and O&M costs for this system are significantly higher than those for wet scrubbing and biofiltration.

Activated sludge scrubbing consists of diffusing the odorous air stream through the mixed liquor in the aeration basin of the wastewater treatment plant. Typically, exhaust air from the composting facility is introduced into the inlet side of the aeration blowers.

Although data on employing activated sludge scrubbing to treat compost-related odor is somewhat limited, it seems likely that high removal efficiencies are achievable. The effectiveness of this odor control technology depends on the nature and concentration of the odor compounds, wastewater characteristics, volume of air treated and the area and depth of aeration basins.

No single technology can be predetermined to cost-effectively address all odor control problems at any composting site. Development of an efficient odor control facility for a given composting site requires detailed analysis and evaluation to account for the site-specific facility conditions and regulatory requirements.