OPPORTUNITIES FOR COMPOSTING BIOSOLIDS: CASE STUDIES OF FOUR FACILITIES

Biosolids composting has continuously increased over the past 13 years, with an estimated 280 facilities currently in operation. Economics, system flexibility, and the production of a Class A marketable product have been the key factors in this continuous expansion. The aerated static pile (ASP) method is the most prevalent, with over 121 facilities reported. Four case studies will be presented, which range from totally open to totally enclosed facilities. The four facilities are Abingdon, Virginia; Harrisonburg Rockingham Regional Sewer Authority (HRRSA), Mt. Crawford, Virginia; Hoosac Water Quality District (HWQD), Williamstown, Massachusetts; and Davenport, Iowa. The HWQD composts undigested biosolids, while the other three facilities compost digested residuals. For each of the facilities, the following information will be provided:Site and system designEconomicsProduct marketingThe Town of Abingdon, Virginia has been disposing of dewatered biosolids in a landfill. Based on a demonstration project and cost analysis, the Town opted to compost and produce an “Exceptional Quality” marketable product. The facility is an open facility. Capital costs are estimated at 314,000, and annual operating costs are estimated at 26,300.Composting at HRRSA is done under cover. The facility composts approximately 5.5 dry tons per day of 25 percent solids digested biosolids. The facility has been operating since January 1996 and has not had any odor complaints. Capital costs were approximately1,510,000, and net operating costs are approximately 120 per dry ton. The compost is principally sold to landscapers and the Virginia Department of Transportation for highway landscaping and wildflower production.The HWQD facility was initially constructed in 1983 and retrofitted and upgraded in 1997, thus increasing its capacity from 5.5 to 9 dry tons per day. It consists of totally enclosed bunkers with concrete walls and aeration trenches for the aerated static piles only. Odor control is achieved through biofilters. Aerated curing is accomplished under cover. The product is sold mostly to soil blenders and nurseries.Davenport, Iowa operates a totally enclosed composting facility and processes approximately 28 dry tons per day of 20 percent anaerobically digested biosolids cake. Composting occurs in a 66,000 square foot insulated building. All building and compost pile air is collected and vented directly to biofilters for odor control. In 1995, the facility cost 7,710,000, and current net operating costs are approximately 105 per dry ton. The operation and maintenance (O&M) costs are reduced by revenue from yard waste and product marketing. The product is marketed to landscapers, homeowners, nurseries, and other horticultural enterprises. A bagging operation has been installed, and the bagged product is marketed in local stores.This wide range of open, covered, and totally enclosed composting facilities illustrates the wide variety of alternatives for biosolids composting.