Evaluating Primary Sludge and Grease Trap Waste Fermentation for a Nutrient Removal Carbon Source

Authors:

  • Hunter Long, Ronald Latimer, Wendell Khunjar, Katya Bilyk - Hazen and Sawyer
  • Charles Bott, Bill Balzer - Hampton Roads Sanitation District
  • Steven Chiesa - Santa Clara University
  • Jeff Nicholson - Hampton Roads Sanitation District, Virginia Tech
  • Jon DeArmond - Hampton Roads Sanitation District, Old Dominion University

Grease trap waste (GTW) is the fat, oil, grease, and food particle containing wastewater that is removed from food service establishment grease abatement devices. GTW is ubiquitous, has high volatile solids (VS) content, high chemical oxygen demand (COD) content, and relatively low nutrient content. Consequently GTW has received a lot of recent publicity for its ability to increase anaerobic digester gas production when co-digested with municipal sewage sludge. However, these same characteristics (high COD and low nutrient content) also make GTW a candidate for fermentation to produce readily-biodegradable COD (rbCOD) like volatile fatty acids (VFAs) for use as a carbon source for denitrification.

Fermentation of GTW and municipal sewage solids for producing a carbon source for denitrification and/or biological phosphorus removal is particularly relevant for resource recovery facilities (RRFs) that do not have anaerobic digesters or may not have a use for digester gas and are required to meet stringent nutrient discharge limits. In some locations a RRF’s annual cost for supplemental carbon may even exceed the facility’s cost for electricity. Depending on local conditions, GTW may be able to provide a greater economic benefit as a supplemental carbon source or as a co-fermentation feed stock rather than a co-digestion feed stock. For example, if it assumed that between 3 to 7 lbs of COD are typically required to denitrify 1 lb of NO3-N and that supplemental carbon products such as methanol, ethanol or glycerol cost between $0.15 and $0.50 per lb of COD, RRFs can be required to spend between $0.45 and $3.50 to denitrify 1 lb of NO3-N. If diverted to the anaerobic digestion process, 1 lb of COD can produce approximately 6 scf of methane gas. Based on a natural gas price of $10 per thousand cubic feet, 6 scf of methane gas is worth about $0.06. Clearly, producing supplemental carbon onsite is worth considering, depending on the cost of electricity and natural gas in a particular region.

This technical paper and presentation will share an economic evaluation of fermentation of GTW and municipal sewage primary sludge (PS) based on results from a yearlong pilot scale study at the Nansemond Treatment Plant (NTP), located in Suffolk, VA. As part of this work, efforts were provided to maximize GTW and PS co-fermentation kinetics and yield.

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

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