Unsafe sewage sludge or beneficial biosolids?: Liability, planning, and management issues regarding the land application of sewage treatment residuals

Boston College Environmental Affairs Law Review, Summer 1999 by Goldfarb, William, Krogmann, Uta

A. Sewage Sludge Stabilization and Disinfection

During stabilization and disinfection, pathogens are either significantly reduced (Class B) or reduced below detectable levels (Class A).18 Class A treatment methods can be compared with pasteurization of milk or cooking of foods, where high temperatures significantly reduce or kill pathogens in order to prevent the risk of disease transmission.19 Class B stabilization methods reduce pathogens to higher levels than Class A stabilization, so site and crop restrictions are still necessary. Site and crop restrictions for Class B sewage sludge keep potential pathogens from human contact until environmental conditions, like sunlight, lower the pathogen density in the sludge so that pathogens are no longer a risk.20 Major stabilization methods include anaerobic digestion, aerobic digestion, lime stabilization, composting, advanced alkaline stabilization, and heat drying. The characteristics of the different end products and the levels of pathogen reduction are summarized in Table 1 of the Appendix.

1. Anaerobic Digestion, Aerobic Digestion, Lime Stabilization

Anaerobic digestion, aerobic digestion, and lime stabilization are the most common methods of sewage sludge stabilization. While anaerobic and aerobic digestion are biological methods, lime stabilization is a chemical method. Lime stabilization is a very simple and inexpensive method, where lime is added to the sewage sludge to raise the pH of the sewage sludge to twelve after two hours of contact.21 Both aerobic and anaerobic digestion reduce the volume of sewage sludge solids that require disposal. Another benefit of anaerobic digestion is the generation of methane, which can be used as an energy source. Anaerobic digestion is only economical for larger communities due to the high capital costs involved. Aerobic digestion, an energy-intensive process, is typically found at smaller POTWs. However, all three methods-anaerobic digestion, aerobic digestion, and lime stabilization-produce in most cases only a Class B sewage sludge. Since the use of Class B sewage sludge is more stringently regulated than Class A sewage sludge, there is an increasing interest in Class A stabilization methods, such as composting, alkaline stabilization, and heat drying.

2. Composting

Projects for composting sewage sludge, or co-composting with municipal solid waste (MSW), or yard waste, have increased in recent years. In 1998, there were 321 sewage sludge composting projects operating around the country.22 Composting is used mostly by communities that produce relatively low volumes of sewage sludge,23 and is often limited by the lack of long-term markets for the finished compost. The stigma attached to sewage sludge and sludge-derived products, stemming from public concern over potential increased concentrations of pollutants in soils and the resulting possible adverse environmental effects, continues to be the major obstacle to finding markets for compost.24 Furthermore, composting can be expensive and uncontrolled odors can expose sites to potential nuisance liability.25 In order to reduce nuisance liability, odor treatment at composting sites has improved substantially in recent years.26