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What should we do with our biosolids?

As the children’s book has it, everyone poops. By and large, in the developed world, this means the toilet flush is our last contact with what might be our lowest common denominator.

But what comes next? That poo, after all, goes somewhere. And in America, it basically means that local municipalities make a choice: human waste can go to a landfill, a field, or an incinerator.

The landfill and the incinerator seem clear enough, but can we really put human waste on the fields where we grow our food, or play with our kids?

You bet!

Human waste becomes “sludge” when it passes through municipal water treatment facilities, and it can be treated and turned into Class A or Class B biosolids (as defined by the EPA). 

In terms of land application (spreading the biosolids on the ground as fertilizer), exceptional quality—Class A—biosolids can be used in small amounts with no buffer requirements or crop type/harvesting restrictions. These “exceptional quality” biosolids meet EPA requirements for low levels of metals and bugs.

If used in large quantities, Class A biosolids do have buffer requirements, but no crop harvesting restrictions. 

Class B biosolids, on the other hand, basically always have buffer requirements, public access, and crop harvesting restrictions (Class B biosolids are treated, but still contain detectible levels of metals/pathogens).

Contact Us, We Can Help Any City Produce Class A

Tags: Class 'A' Biosolids, Bisolids Handling, Wastewater Treatment

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