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The biosolids pelletizing facility processes digested sludge from the water reclamation facility. Digested sludge is dewatered using belt filter presses and pelletized using one of the two solids pelletizing trains. The pelletizing train consists of a furnace, dryer, and settling chamber. The off-gas emissions from each pelletizing train are treated through dual cyclones, a venturi scrubber-cyclonic separator, and a common regenerative thermal oxidizer (RTO) prior to discharge to the stack. The pelletized product is stored on-site in two silos and sold to a private firm for use as a fertilizer.

The existing biosolids pelletizing trains have process limitations. The existing venturi scrubbers have been in operation for more than 20 years and are in need of replacement. Both pelletizing trains cannot be operated simultaneously due to the flow limitations of the existing RTO and limits the production capacity. The current system utilizes a non-recirculating exhaust gas configuration, which results in higher operation and maintenance (O&M) costs.

Three system configurations of the exhaust recirculation project were evaluated to select the best configuration. The first configuration (configuration 1) evaluated the recirculation after the existing venturi scrubber/cyclonic separator and immediately ahead of RTO. The second configuration (configuration 2) evaluated the recirculation after the existing dual cyclones and immediately ahead of new venturi scrubber cyclonic separator. The third configuration (configuration 3) evaluated recirculation after a new impingement tray scrubber cooler.

The configurations were evaluated and ranked based on distinct criteria to select the best configuration. These criteria consisted of marginal capital costs, O&M costs, water usage, and natural gas usage. Based on the evaluation, the City selected configuration 1 as the best configuration to design and construct the exhaust gas recirculation project.

The exhaust gas recirculation modification will substantially benefit the facility by dramatically increasing production and reducing associated O&M costs. Based on the current annual production of 2,800 tons, the proposed system will provide an annual cost savings of approximately 77,000 per year with the operation of one pelletizing train and up to 98,000 per year with the operation of two pelletizing trains. Additional O&M savings can be achieved by increasing the facility thruput by processing additional solids from other municipalities and from escalating utility and chemical costs.
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Document Type: Research Article

Publication date: 01 January 2007

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