Sustainable Supplemental Carbon Sources
- Hunter Long - Hazen and Sawyer
In order to meet strict nutrient limits, wastewater treatment facilities with low influent COD:N may require supplemental carbon for denitrification, resulting in chemical costs of approximately $1.50 per pound nitrate removed. In efforts to reduce chemical costs, two wastewater treatment facilities in Virginia are investigating alternate methods for on-site production of supplemental carbon. The Nansemond Treatment Plant (NTP) is evaluating co-fermentation of primary sludge (PS) with truck hauled grease trap waste (GTW) to produce VFAs and soluble COD as a supplemental carbon source. The Henrico County Water Reclamation facility is evaluating the use of focused pulse treated (OpenCelTM) thickened waste activated sludge (TWAS) as a carbon source for denitrification. OpenCelTM treated TWAS purportedly achieves soluble COD production comparable to fermentation but without the associated odor potential and operational challenges of running a biological process.
The NTP co-fermentation pilot design consists of two 300-gpd reactor trains; one train ferments PS only and the other train ferments a combination of GTW and PS. GTW has the potential to significantly increase the COD loading rate to the fermenter with a minimal increase in phosphorus and nitrogen, making GTW an ideal candidate for fermentation. The pilot began operation in January and will run through July 2013. During Phase I (PS only, January-April) the pilot fermenter has been converting total COD to soluble COD at an average rate of 8.1%. Phase II (PS + GTW, April-July) will evaluate fermenter performance at multiple GTW loading rates wherein GTW represents 20%, 40%, 75% and 100% of the fermenter feed (by mass as COD). At the current NTP PS and GTW production rates, on-site co-fermentation is estimated to produce 5,000 lbs COD per day of supplemental carbon which would offset approximately half of the facility’s annual average daily supplemental carbon requirement.
Results from parallel investigations at the Henrico facility suggest that OpenCelTM treatment can yield up to 0.09 g soluble COD/g total COD, which is comparable to successful fermentation. Short-term denitrification tests also indicate that OpenCelTM treated TWAS has comparable nitrate reduction kinetics to glycerol. Full-scale addition of OpenCelTM treated TWAS to the ENR basins is scheduled for April through May of 2013.
The fermentate and OpenCelTM effluent produced from both pilots are also being evaluated in lab-scale sequencing batch reactors to determine their suitability to offset purchased carbon for denitrification in a 5-stage BNR process.
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