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More Fun Than Herding Cahts - Producing Class a Biosolids with an Innovative Heat-Treatment and Anaerobic Digestion Process

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A continuous-flow, heat-treatment system designed to meet Class A pathogen-reduction standards when combined with conventional, anaerobic digestion was developed and tested beginning in 1993 at the municipal wastewater treatment plant (WWTP) in Bremerton, Washington. The heat-treatment system, characterized by a high-temperature vessel located between first-stage and second-stage digesters, was called “Class A Heat Treatment System (CAHTS).”

The CAHTS was developed to be capable of meeting Class A pathogen reduction and vectorattraction reduction by achieving the following standards:

Time-temperature criteria of Class A, Alternative 1, to be achieved primarily in the high-temperature solids retention tank located between the two digesters


Salmonella density standard of 3 most-probable number (MPN) Salmonella per 4 grams of dry solids in digested solids sample


Achieving the vector-attraction reduction standard of at least 38% volatile-solids reduction during the whole process, consisting of the two-stage digestion system with high-temperature tank in series between the primary and secondary digester.


The vector-attraction reduction standard of >38% volatile-solids reduction would be met at the end of the second-stage digester, and the high-temperature vessel in between the two digestion stages would meet the Class A pathogen-reduction standard, prior to the vector-attraction reduction standard being met.

After some modifications, the CAHTS was tested via slug-injection, lithium-tracer technique to ensure that the time-temperature requirement of Class A, Alternative 1 was being met. The lithium tracer test was conducted several times at the maximum CAHTS design flow of 10 gallons per minute (gpm), with results showing the first trace of lithium above background levels at 35 minutes detention time in the CAHTS reactor. The minimum temperature maintained in the reactor is 67 degC (153 degF), for which a minimum of 30 minutes detention time is required to meet time-temperature Regime D of Class A, Alternative 1. The CAHTS reactor has been fitted with a number of temperature probes that demonstrate achieving this minimum temperature standard.

The other portion of the Class A pathogen-reduction standard that must be met is direct monitoring for either fecal coliform or Salmonella at a minimum of four times annually (quarterly basis) for the WWTP operated by West Sound Utility District (WSUD). The WSUD chose to monitor for Salmonella in the dewatered biosolids cake to demonstrate meeting the Class A pathogen standard. Results confirmed that Class A pathogen reduction was being consistently met.

This paper describes the CAHTS process, presents the procedures and results for demonstrating Class A pathogen reduction as well as vector attraction reduction, and summarizes the benefits of CAHTS to the biosolids management system.
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Keywords: Biosolids heat treatment; Class A pathogen reduction; anaerobic digestion; vector attraction reduction

Document Type: Research Article

Publication date: 2009-01-01

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