Biosolids Composting Technology: Where it Has come from and where it is Going

Since the first Journal (Water Pollution Control Federation) article entitled “A Forced Aeration System for Composting Wastewater Sludge” was published in April 1976, the number of biosolids composting facilities in North America has grown from a handful of operations to well over 260 such operations today (see Figure 1). Biosolids composting is currently practiced in 44 States, Puerto Rico and Canada. These facilities process over 3 million tons of dewatered biosolids cake producing over 7.5 million cubic yards of Class A biosolids product annually. The vast majority of these facilities are selling their compost product to generate revenue thereby offsetting a portion of operating costs.Over the past 40 years, several different composting technologies have developed including windrow, aerated windrow, aerated static pile (ASP), covered aerated static pile and a wide array of in-vessel or enclosed technologies (see Figure 2). Some of these technologies have come and gone while others have proliferated. This paper will summarize the technological development of composting operations and how the technology has evolved over this time, how facilities are coping with odor control issues, how compost is being marketed, and what technologies will continue into the future. Case studies of two different facilities (Davenport, Iowa; Spotsylvania, Virginia) are presented to provide information on various processing regimes, bulking agents used, operating costs and staffing needs. Generally, O&M costs for well developed composting facilities range between $35 and $50/ton of solids processed, with these costs being reduced by as much as a third through compost sales revenues. Details regarding the breakdown of O&M costs in terms of labor, utilities, fuel, bulking agent and maintenance will be provided.This paper and presentation will help planners, engineers and utilities to understand the differences in composting technologies available today. The information will aid in making the proper choice of composting technology to meet rigorous odor control needs demanded by today’s public as well as the demands of utilities to minimize labor and other operational costs.