Enhanced Anaerobic Digester Performance and Efficiency with Improved Heating and Mixing

Authors:

  • Hunter Long, Todd Johnson, Ron Taylor, Brian Porter, Joe Rohrbacher – Hazen and Sawyer
  • Michelle Simpson, Jennifer Whitaker – Rivanna Water and Sewer Authority

The Rivanna Water and Sewer Authority owns and operates a 15 mgd capacity advanced water resource recovery facility (WRRF) in central Virginia. Nutrient removal improvements were completed in 2012. The nutrient removal project included installing a 335 kW internal combustion engine cogeneration facility, fire tube boiler, and hot water heat exchanger, enabling the facility to use digester gas for electricity generation and heat recovery. However the digester “in-tank” heat exchangers and coarse bubble mixing system were not providing adequate heating and mixing and the anaerobic digestion facilities were upgraded to gain full beneficial use of the cogeneration facility through enhanced digester gas production. This presentation will provide a summary of the project completed in 2014. The project has enabled the facility to consistently maintain effective digester mixing and temperatures above 95°F and increased volatile solids reduction and digester gas production.

Pumped mixing, draft tube mixing, and linear motion (LMTM) mixing were evaluated. LMTM mixers were selected due to their low energy requirement, low maintenance and ability to provide similar mixing effectiveness as conventional technologies. Tube-in-tube sludge to hot water heat exchangers with external sludge recirculation replaced the “in-tank” heat exchangers allowing for improved maintenance access, process and control monitoring. Piston type digester gas compressors were replaced with oil flooded screw compressors and an iron sponge hydrogen sulfide (H2S) removal system was added for gas purification. Numerous instruments, transmitters and control valves were added to the digestion system to enhance process control and monitoring. The entire digester process monitoring and control was made available on the facility-wide Supervisory Control and Data Acquisition (SCADA) system.

This presentation will provide a summary of design considerations, lessons learned, and summarize the benefits of improved anaerobic digester heating, mixing, gas handling, process control, and process monitoring including a comparison of digester performance before and after the improvements project.

For more information, contact the author at hlong@hazenandsawyer.com.

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