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Akron, Ohio- A Review of the First Year for Biosolids Management Using a High Solids Anaerobic Digestion System

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The City of Akron has been managing their wastewater solids over the last 20 years by composting using the in-vessel Paygro method. While the operations of the facility and the marketing of the end product have been a success, it has not come without its trials. Odors still prevail and the cost of operations is increasing at an unprecedented rate as a result of the changes occurring in the global economy. In a continual effort to minimize odors and find a way to stabilize the costs associated with handling the wastewater solids, the City turned to high solids anaerobic digestion process whereby fewer amenities were necessary for it to operate and labor was minimal.

The anaerobic digestion process being used in Akron comes from Schmack Biogas AG Germany, where they have been maximizing the generation of biogas while anaerobically digesting high feed stocks (whether they are an organic waste or specifically grown for use in the digester) over the last ten years. Phase I of the anaerobic digestion process in Akron was designed to handle up to 5,000 dry tons of municipal solids. This is approximately one third of the solids presently being composted. Construction of the anaerobic digestion system began in August of 2006 with major completion occurring by October of 2007 when 150,000 gallons of 4.35% seed sludge was provided by the City of Kent WWTP.

To process the solids anaerobically, one 600 cubic meter Euco (primary digestion tank) and one 2000 cubic meter Coccus (secondary digestion tank) were constructed. The solids feeding the Euco consist of a combination of 30 % cake solids and 5% liquid solids. Temperature in the tanks is maintained at 93°F to 98°F. The digested solids leaving the Coccus for pathogen reduction are approximately 11%. The biogas generated from the anaerobic process is used to run a 335 kW Jenbacher engine that is equipped with heat recovery equipment. The electricity generated is fed back into the grid to supplement the remaining composting operations while a portion of the heat off the combined heat power unit is recovered and is used to maintain the necessary temperatures in the two digestion tanks.

During the start-up phase, in addition to the seed sludge from the City of Kent, an additional 275,000 gallons of raw sludge at 5.50% from Akron was introduced into the Coccus. A temporary boiler was used to provide the heat necessary to get the material up to 90–95°F during the initial months of start up beginning in November. Late November, a transfer of 100,000 gallons to the Euco occurred and then the 30% cake solids were introduced as well into the digestion process. By March 11, 2008 the generator was running at full capacity and on April 23, 2008 there was excess biogas being produced such that the flare was running periodically.

During the preliminary design of the system, Biomethane Potential analyses were performed on the wastewater solids by Schmack's laboratory as well as an independent study by Applied Technologies (design engineer) and Marquette University, both out of Milwaukee, Wisconsin. Preliminary results indicated that Akron's solids were very suitable for the anaerobic digestion process with the biogas production value being 19.4 ft3/lb of volatile solids destroyed and approximately 50% of the volatile solids destroyed. This paper will take a closer look at how close the operations have come to meeting this criteria as well as meeting the necessary pathogen and vector attraction reduction requirements for Class A solids. Finally, with a full year of operational data available discussions on how the compost facility will phase the remaining anaerobic digestion process into it to accommodate for 100% of the solids to be digested will be discussed.
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Keywords: Anaerobic digestion; Biogas; Biosolids; Schmack technology

Document Type: Research Article

Publication date: 2009-01-01

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