Sludge Composting

Sewage and Pollution

• Waste management systems release massive volumes of treated sewage into local waterways that eventually dump the materials into an ocean. Some of the water released every year goes untreated. This is primarily because most municipal facilities use combined sewage outflow (CSO) systems that carry both storm water runoff and raw sewage from residents and businesses to the same facility. When heavy storms cause an overflow, a series of gates release the excess runoff, along with some untreated sewage, into nearby waterways. Building additional compost treatment centers at these facilities could reduce these incidences of releasing untreated sewage and storm waters, as the facility's storage capacity would increase while creating a useable product.

Basic Composting

• Most wastewater facilities compost sewage sludge on a basic level. When raw sewage enters the treatment plant, large particles are separated and sent to the landfill, while the smaller solids are left behind. These suspended solids left behind are known as sludge. Bacteria colonies and oxygen are introduced into this sludge, which then digest the material. In this process, sunlight, heat and oxygen kill off 99 percent of pathogenic bacteria and 90 percent of enteroviruses, according to a report published by Colby College.

Additional Composting

• Most municipalities press the water out of the sludge using belt filter presses before directing it to a space for additional composting. The most common composting systems used at these facilities are in-vessel, windrow, aerated windrow and aerated static pile. Static piles heap the material in one location -- perhaps with the addition of microbial colonies -- and allow it to biodegrade on its own. Windrow systems pile the material in long containers that are turned often, and in-vessel systems heap the material in a bin and usually force air through the material via pipes at the bottom. Aerated systems describe any system that forces air into the compost environment and is used in combination with other methods.

Controversy Over Using Sludge

• Treated sewage sludge has been applied to land as fertilizer with mixed reviews. According to the Cornell Waste Management Institute, there has only been one significant study on the possible side effects to humans from using this product. In that study, half of those close to the site of application reported infections and half reported symptoms consistent with endotoxin exposure. In at least one of the sites in that study, symptoms decreased the farther the subjects were from the application site.