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Chicago's Start-up of Renewed Large Diameter Anaerobic Digesters with World's Largest Membrane Covers

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This case study documents the results of the start-up of the recently upgraded anaerobic digestion facilities for the Metropolitan Water Reclamation District of Greater Chicago's (MWRDGC) Egan Water Reclamation Plant. This 30 mgd plant has 4 - 110-foot diameter digesters with floating covers. The design included the following objectives:

Replace the floating covers, improve the flexibility of operation, while increasing the effective liquid sludge storage.

Provide increased sludge mixing and heating to provide for more efficient and enhanced stabilization of biosolids.

Provide increased gas storage to improve gas utilization while safely handling digester gas.

To ensure a positive method of maintaining Class A or B biosolids, the detailed design included the following concepts with their related benefits:

Two fixed cover primary digesters operating in parallel, with confined gas mixing and external sludge heat exchangers to improve mixing and heating – provides enhanced pathogen reduction and increased volatile solids destruction.

Mesophilic digestion process in series configuration – ensures pathogen reduction and minimizes odor potential.

Two gas/sludge storage tanks operating in parallel with intermittent pumped mixing and membrane covers (the World's largest diameter membrane cover installation to date) – improved gas utilization and increased sludge dewatering operation flexibility.

The Egan WRP processes sludge from both the Egan (30 mgd) and Kirie (40 mgd) WRP's and is a critical component of the District's biosolids management program. During construction, digester volume has been reduced by 50%. Start-up and operational considerations include:

Maintain existing operations, including digestion, to the maximum extent possible

Construction of membrane cover digester – minimize digester down time

Start-up of new process train with one fixed cover digester followed by one membrane cover gas/sludge storage tank

Increased mixing in the primary digester – provides enhanced digestion

This project is currently under construction, with start-up of the first process train to begin in late summer of 2001. Steady-state conditions should be achieved in several months providing operational data to compare with the original construction. The figures and tables illustrate the process and document the results.
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Document Type: Research Article

Publication date: 2001-01-01

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