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The Anatomy of Pumping Biosolids Into Incenerators (part 1)


            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. 

Tags: Wastewater Treatment, Biosolids Handling, Biosolids Drying

The Anatomy of Class A Bisosolids


Wastewater is not considered sludge until it has been de-watered to at least 7% solids content, according to the EPA. To create an end product that is considered Class A grade acceptable, sludge must be used. Acceptable sludge must be free of pollutants, shown below is the EPA specified amount of pollutants allowed:

Pollutant  Must not exceed concentration [Milligrams / kilogram]
Arsenic  41
Cadmium   39
Chromium  1,200
Copper  1,500 
Lead  300
Mercury  17
Nickel  420
Selenium   36

If any of these pollutants are found to be above the ceiling set by the EPA, the sludge is not safe and must be land filled or incinerated. Providing the sludge is acceptable, a range of processes (drying, alkaline stabilization, etc.) can be used to create Class A product. Although no specific process is outlined and approved by the EPA over another, the restrictions for the end product are what sets the bar and defines a Class A product.            

Class A product must have "less than 1000 Most Probable Number per gram of total solids on a dry weight basis" (EPA regulation 503) of fecal coliform & salmonella sp. bacteria. These pathogens can only be tested for in the lab environment where a sample is put into a Petri dish and the bacteria is allowed to grow so it can be accounted for. Then the amount of organisms per gram of material must be below 1000. Meeting this level of pathogen kill is accomplished in most class cases by keeping the sludge in an elevated temperature environment for an extended period of time.

In addition to acceptable levels of pathogen kill the end product must also have "sufficiently reduced vector attraction of the material" (EPA regulation 503). Vectors are bugs such as flies or mosquitoes that carry diseases. Vector reduction can be accomplished in several ways. By increasing material PH or drying the material to greater than 90% the attractiveness of the material to sustain or breed for the bugs is eliminated. At a minimum, each batch needs to be tested and pass these standards before they can be considered Class A biosolids and land applied.                                  

Click here to learn more about Class A technologies

Tags: Biosolids Process, Bisolids Handling, EPA, Sewage Sludge, Biosolids Drying