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Woodash Additives used to Produce Enhanced Alkaline-Stabilized EPA Class A Biosolids

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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 off of 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 City upgraded the solids processing system at the Hall Street WWTF in 2002 to produce a United States Environmental Protection Agency (EPA) Class A Biosolids product. This material can be beneficially reused on agricultural sites, or publicly distributed.

A high-alkaline stabilization system with supplemental heat (pasteurization) was installed to produce an EPA Class A Biosolids product. The new system includes equipment to automatically add an amendment to the process (such as wood ash or saw dust, ground yard waste or other additive) to:

Reduce odors


Improve product dry solids content


Improve aesthetic appearance.


The innovative system produces a dry, crumbly material, providing a solution to the pasty, odorous end product, which sometimes occurs with the alkaline stabilization process.

Two new 2.0 meter heavy duty, high-solids, self-enclosed style belt filter presses were installed to replace the existing plate and frame filter presses. A new lime day hopper with volumetric feed system was added to replace the lime and ferric sludge blending tanks. A heated sludge/lime mixer was added, and pasteurization equipment was installed to provide the required holding time for the pasteurization of the Class A Biosolids (30 minutes holding time at a minimum temperature of 70°C).

The amendment handling and feed system includes the following components:

One (1) new 2,400 square foot amendment storage building with three separate bays.


One (1) new amendment day bin with a holding volume of 20 cubic yards, including live bottom screw conveyors with variable speed controls.


Three (3) new amendment transfer conveyors. The conveyors are shaftless screw type. Two are horizontal, and one is vertical.


The amendment handling and feed system is capable of handling various materials including woodash, sawdust, woodchips, or ground yard waste. The amendment is added to the sludge processing system after the belt filter presses, prior to the sludge/lime mixer. The amendment handling and feed system has also been designed to handle dewatered sludge cake to allow the City to receive these materials from other WWTF's.

This paper presents performance characteristics, and provide full scale operating data from the new facility during its first two years of operation producing EPA Class A Biosolids. This data will be compared with the historical operating data from the full-scale pilot testing conducted in 2000, and the City's previous EPA Class B Lime Stabilization system.

The paper provides a summary of advantages achieved by converting to an EPA Class A process with provisions to add amendment to enhance finished biosolids product quality.
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Document Type: Research Article

Publication date: 2005-01-01

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