<P> There is no process which completely eliminates the need to dispose of treated sewage sludge . </P> <P> Much sludge originating from commercial or industrial areas is contaminated with toxic materials that are released into the sewers from the industrial processes . Elevated concentrations of such materials may make the sludge unsuitable for agricultural use and it may then have to be incinerated or disposed of to landfill . </P> <P> The Edmonton Composting Facility, in Edmonton, Alberta, Canada, is the largest sewage sludge composting site in North America . </P> <P> Sewage sludge can be superheated and converted it into pelletized granules that are high in nitrogen and other organic materials . In New York City, for example, several sewage treatment plants have dewatering facilities that use large centrifuges along with the addition of chemicals such as polymer to further remove liquid from the sludge . The product which is left is called "cake," and that is picked up by companies which turn it into fertilizer pellets . This product is then sold to local farmers and turf farms as a soil amendment or fertilizer, reducing the amount of space required to dispose of sludge in landfills . </P>

The sludge digester can either be an anaerobic or aerobic treatment system