In the first post discussing Class A biosolids, we outlined the standards required by sludge to be used in the lime stabilization process. The sludge must not exceed the allowable concentration of pollutants set by the EPA (to view this list and allowable concentrations see March 19, 2010 post). If elevated levels of any pollutants are found the sludge is no longer eligible for land application and must be either land filled or incinerated. Incineration is the preferred disposal method in areas with high population density and limited access to landfills.
The two primary types of incineration are Multiple Hearth Furnaces (MHF) and Fluidized Bed Incinerators (FBI). MHF consist of a series of zones that material is passed thru that first dries and then incinerates biosolid sludge. FBI use a single chamber area that flash dries and then incinerates the sludge in a single step. Both operate at temperatures between 1400 to 1800 degrees Fahrenheit and utilize forced air systems for combustion and waste gas removal. The two primary byproducts of the processes are heat and ash, some of these byproducts can be beneficially reused. Waste heat can be used to heat incoming air for improved combustion efficiency and fly ash is sold to batch plants and used as an additive for concrete. Waste gasses are passed through scrubbers in order to prevent pollutants from being released into the atmosphere. MHF have become obsolete due to there space requirements and poor efficiency in comparison to FBI.