4.4 Compost and Pathogen Reduction

Ensuring that the composting process meets the minimum temperature and time criteria is vital to guarantee that the biosolids are pathogen-free. To simplify monitoring and reduce costs, monitoring the temperature during the process and adjusting the parameters accordingly is a common practice.

As an example, the US EPA (Part 503) categorizes biosolids depending on the level of treatment they received and the reduction of pathogens. If pathogens have been reduced below detectable levels, the biosolids meet Class A designation and can be sold to the public or land applied without further restrictions. Class B biosolids are not treated to the same extent and as such, may only be applied with site restrictions in place.

The temperature and time requirements for Class A and Class B biosolids are outlined in the following table.

Not only is the rate of reduction of pathogens dependent on time and temperature, but it is also highly dependent on the specific type of pathogen involved.

The following graph shows selected pathogens that are commonly found in sludge and their inactivation in relation to time and temperature.

In the graph presented above, the lines represent conservative upper boundaries for pathogen death or estimates of the time-temperature combinations required for pathogen inactivation. A treatment process with time-temperature effects falling within the “safety zone” should be lethal to all excreted pathogens (with the possible exception of hepatitis A virus at short retention times).

Indicated time temperature requirements are at least: 1 hour at >62°C, 1 day at >50’C, and 1 week at >46°C.

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