The Anatomy of Class A Biosolids


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.

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