Co-Fermentation of Primary Sludge and Grease Trap Waste for VFA Production
- Ronald Latimer and Hunter Long - Hazen and Sawyer
- Jeff Nicholson, Holy Anne Hillard, Bill Balzer, and Charles Bott - Hampton Roads Sanitation District
- Steven Chiesa - Santa Clara University
The Nansemond Wastewater Treatment Plant, located in Suffolk, VA and operated by the Hampton Roads Sanitation District (HRSD), discharges treated effluent into a tributary of Chesapeake Bay. This 1.31 m3/s (30 mgd) facility currently satisfies average annual phosphorus and total nitrogen limits of 2.0 mg/L and 8.0 mg/l, respectively, as well as being part of a multiple plant bubble permit limit that requires an annual effluent TN mass discharge of less than 2,730 tonnes (6M lb) of nitrogen. More stringent effluent nutrient concentration limits are anticipated within the next five years. The Nansemond Plant’s primary effluent contains insufficient levels of readily biodegradable organic carbon to meet its biological nutrient removal targets without supplemental carbon addition. The plant currently purchases concentrated waste methanol to assist with denitrification in its modified five-stage BNR process system at approximately $700K per year.
To potentially provide additional nutrient removal capacity and reduce the net cost of supplemental organic carbon addition, a pilot-scale investigation was conducted to characterize the local grease trap waste stream and examine the ability of blended settled primary solids and locally available grease trap waste to be co-fermented to produce a volatile fatty acid (VFA) – enriched product stream to enhance phosphorus and nitrogen removal. The plant currently receives, on average, 19.7 m3/d (5,200 gpd) of grease trap waste from local haulers and can receive additional grease trap waste by diverting such flows from other HRSD treatment facilities. This material has the high C:N and C:P ratios desired in a supplemental carbon source and a significant daily organic (COD) loading that makes it an attractive option for replacing a large fraction of the purchased chemical now used to augment nutrient removal.
Two parallel trains of completely mixed anaerobic fermenters, 300 gal, coupled with gravity thickeners, 90 gal, were operated to ferment primary solids (PS) and grease trap waste (GTW) from the plant. A control treatment train was fed only primary solids at a targeted organic loading rate of 10kg COD/m3/d while the experimental train was fed an elevated lipid load, an additional 20% COD load as grease trap waste (GTW), collected from the grease trap waste stream being disposed of at the treatment facility. The desired effluent product was a low solids, high readily biodegradable COD (rbCOD) and volatile fatty acids (VFA) stream that could be used for biological nutrient removal. At present the primary sludge has been found to have a particulate COD to soluble COD (sCOD) conversion rate of 6 to 15%. However, the amount of effluent sCOD contributed by VFAs (C2:C6) ranges from 25 to 80% depending on reactor operational parameters, while the contribution of acetic acid ranges from 20% to 40%. The incremental degradation of the added lipid load was also far below desired expectations at only 6 to 8%. Modifications are required to effectively transform the grease trap waste in a low pH, 5.2-5.5, anaerobic fermenter.
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