Sludges from wastewater and water treatment facilities are typically put in a landfill or applied to farmland reclamation sites, although some sewage sludge is heat treated and reused as compost. A little more than 50 percent of the sewage sludge produced by the more than 16,000 municipal wastewater treatment plants in the U.S. is applied to land, mainly to farmland, as a soil conditioner and fertilizer.
We anticipate increased innovative uses of sludge over time. Ten years ago, the majority of sewage sludge in the states within EPA Region 9 (Arizona, California, Hawaii, and Nevada) was either treated to "Class B" pathogen reduction levels (some but not all pathogens removed) and used as fertilizer, or put in a landfill. Counties began passing ordinances restricting use of "Class B" sludge, and there was a trend towards treating to "Class A" levels (more extensive treatment), mostly by composting, but also by heat drying, long-term air-drying, or thermophilic digestion.
Currently, about 40 percent of the sludge in Region 9 is treated to Class A levels and 20 percent treated to Class B levels for land application, and most of the rest is put into landfills, with small amounts incinerated, used as fuel, or used as construction material. In the near future a number of outdoor compost operations will have to close because of more stringent air quality requirements, and there has been a trend towards more heat drying, with the product useable either as soil amendment or fuel. New indoor compost operations and negative static aerated pile compost operations have started up. Costs for use or disposal have gone up from about $20 per ton to $50+ per ton.
Throughout the country, municipalities are beginning to look to their sludge as a source of energy rather than a waste. The City of Los Angeles has started an experimental project in which sludge will be injected into a deep well (4,000+ feet); the designers are hoping that methane can be captured to make energy. The city of Sanford, Florida has a long-term contract to have the city's wastewater sludge converted to green energy. Also, sludge from two wastewater treatment facilities in King County, Washington are being used to grow canola on Yakima County farms that will be refined into biodiesel to power Metro Transit buses. An estimated 2 million gallons of homegrown biodiesel will be produced through this partnership.
A recent survey coordinated by the North East Biosolids and Residuals Association (NEBRA) discusses this further. The July 2007 final survey report and detailed appendices are available online at http://www.nebiosolids.org.