Hazards from Pathogenic Microorganisms in Land-Disposed Sewage Sludge

Municipal sewage sludge is a complex mixture of organic and inorganic compounds of biological and mineral origin that are removed from wastewater and sewage in sewage treatment plants. Sludge is a by-product of physical (primary treatment), biological (activated sludge, trickling filters, or rotating biological contractors), and physiochemical (precipitation with lime, ferric chloride, or alum) treatment of wastewater. Many of the pathogenic microorganisms present in raw wastewaters will find their way into municipal sludges. Treatment of these sludges by anaerobic or aerobic digestion and/or dewatering will reduce the number of pathogens, but significant numbers will remain. The type of treatment will determine the concentrations and relative risk of disposal.