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Pilot Testing Used to Optimize Design of Class A Biosolids Processing System in Concord, NH

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The City of Concord, New Hampshire owns and operates a 10.1 million gallon per day (mgd) Wastewater Treatment Facility (WWTF) located on Hall Street. The WWTF provides secondary treatment using primary clarification followed by the fixed film “activated biofilter” secondary treatment process prior to discharge into the adjacent Merrimack River.

The WWTF currently dewaters primary and secondary sludge using two plate and frame style filter presses. The sludges are conditioned with lime and ferric chloride prior to dewatering, producing a United States Environmental Protection Agency (EPA) Class B Biosolids product. This material is beneficially reused on restricted agricultural sites.

In 1999, the City performed a Sludge Dewatering and Stabilization Alternatives Evaluation. The evaluation had the following objectives:

Evaluate options to produce a Class A Biosolids Product

Select an option which produces a final product with characteristics that are attractive to a diverse market

Reduce odors

Reduce chemical costs, increase solids throughput, and reduce operational manpower required for sludge dewatering

A total of 16 different alternatives were developed and evaluated in terms of overall life cycle costs, product utilization, odor potential, process reliability, and flexibility. The Evaluation recommended a high-alkaline stabilization system utilizing supplemental heat (pasteurization) to produce an EPA Class A Biosolids product. The recommended process included provisions to add an amendment to the process (such as wood ash, saw dust, or ground yard waste) to improve product appearance and reduce odors.

The recommended plan also evaluated various options to provide sludge dewatering. These alternatives included:

Expand and refurbish existing plate and frame presses

New Belt Filter Presses (BFPs)

New Centrifuges

New Rotary Presses

Based on the life cycle cost analysis, two new 2.0 meter heavy duty, high-solids, self-enclosed style belt filter presses were selected. In order to optimize the design a pilot scale test was developed to evaluate the proposed sludge dewatering and stabilization systems. A 0.6 meter, trailer mounted belt filter press was used to dewater a blend of raw primary and secondary sludge from the WWTF. The dewatered sludge from the trailer-mounted belt filter press was transported to another WWTF in the region which utilizes the recommended high-alkaline stabilization system with supplemental heat and amendment. The pilot testing allowed the City to confirm the following parameters which are instrumental in providing an efficient and economical design:

Belt filter press throughput

Belt filter press polymer dosage

Belt filter press cake solids concentration

Stabilization system calcium oxide lime dosage

Stabilization system amendment dosage

Final product characteristics

This paper presents the results of the pilot testing, conclusions, and design recommendations.
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Document Type: Research Article

Publication date: 2001-01-01

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