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Amendment Free, Mechanically Enhanced Biodrying & Composting of Dewatered Undigested Sewage Sludge

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Biodrying of manure is not an uncommon practice, but few active sewage sludge projects at full scale have been reported in the literature. During biodrying, heat from the composting microbiology is used to drive off moisture. Pathogen kill can also be accomplished provided the compost energetics support the required temperatures. In some facilities, the operator can choose between retaining the compost heat to achieve pathogen killing temperatures, or increasing aeration and turnings to maximize moisture evaporation, release to the atmosphere, and enhancing drying mechanically. In this project, two such mechanically enhanced drying trials using municipal waste water treatment plant (MWWTP) sewage sludge (biosolids) were conducted to investigate both options. These trials took place in a positive aeration, agitated bay type composting facility located in, and owned and operated by, the Town of Bristol, Rhode Island.

In the initial Phase I trial, heat-dried MWWTP sludge pellets were blended with dewatered undigested primary- plus waste activated- MWWTP sludge at 22% solids to increase the solids content from a target infeed mixture of 40% solids. The initial trial optimized drying by maintaining aeration rates to hold temperatures in the range of 25°C (i.e. greater than ambient) to 45°C. During the 24 day composting period, dry solids increased from approximately 43% to 88%. Odors were minimal and not considered to be objectionable. Fecal coliform counts in the dried compost ranged from 19 to 1,100 Most Probable Number (MPN) per g dry solids and averaged 500 MPN per g dry solids.

In Phase II, the 85% solids pellet and sludge compost discharged from the Phase I trial was blended with the dewatered cake to produce a target infeed mixture of about 40% solids. Aeration was managed in Phase II to achieve pathogen killing temperatures as the major objective while also achieving adequate drying. The material was retained in the bays for an average of 18 days. During this retention time, the solids content rose from about 41% to about 68%. Temperatures were maintained in the 40°C to at least 55°C range for 13 days, with temperatures exceeding 55°C for 8 consecutive days – more than sufficient time/temperature days to meet the United States Environmental Protection Agency Code of Federal Regulations Section 503 (USEPA 40 CFR 503) requirement for pathogen kill of 3 days at above 55°C (PFRP). Consistent with this, the fecal coliform counts in the dried compost product were consistently well below the 1,000 MPN per g dry solids USEPA requirement. We were unable to achieve the USEPA requirement for Vector Attraction Reduction (VAR) of 14 days at >40°C during this trial although the 14 day average temperature exceeded 55°C. Ironically, we feel this was due to the compost drying out too quickly and becoming dormant. In future trials, moisture in the compost will be managed to assure continued biological activity.

Anticipated problems that were not manifested in either trial included:

severe odors of the C:N < 10 composting mixture,

stickiness of the sludge-only mix (no woody amendment),

inability to drive off sufficient moisture,

inability to generate adequate heat for pathogen kill,

lack of porosity, and

overloading of the compost agitator
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Document Type: Research Article

Publication date: 2009-01-01

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