MOST HIGHLY MECHANIZED IN-VESSEL COMPOSTING FACILITIES ARE HISTORY: LESSONS LEARNED

Starting in the mid-1980s, many wastewater utilities interested in composting sludge turned to in-vessel composting technologies. These technologies consisted of a number of enclosed vessels with highly mechanized systems for mixing and conveying of materials, and air handling and odor control. Compared to other traditional methods of composting, such as static pile and windrow, the in-vessel composting technologies were thought to provide several advantages. These perceived advantages were small area requirements, fewer personnel for operation and maintenance, more effective odor control, and more consistent product quality.Nevertheless, many in-vessel composting facilities encountered serious problems in dealing with odors, achieving moisture reduction, maintaining reliable mechanical operation, and producing a consistent quality compost product that was marketable. The U.S. Environmental Protection Agency (EPA) commissioned CDM to conduct an in-depth survey of six in-vessel composting technologies and the resulting report was published in 1989 (EPA, 1989).In the time since the report was published, more than 50 additional in-vessel composting facilities have been constructed and operated. However, of the most highly mechanized technologies, a large percent have either substantially reduced throughput compared to design capacity or curtailed operations. This report summarizes the results of a survey of 25 highly mechanized composting facilities conducted by CDM in 2005. Many facilities reported problems with inconsistent product quality, reduced throughput, odors, unreliable materials handling, and, most importantly for those facilities now closed, lower costs for competing methods of sludge or biosolids management.