Evaluation of Alternative Supplemental Carbon Sources at Five BNR Facilities
- Joe Rohrbacher, P.E., Theresa Bruton, P.E., Paul Pitt, PhD, P.E., Ron Latimer, P.E., and Katya Bilyk, P.E. - Hazen and Sawyer
Supplemental carbon donors are generally added to the secondary anoxic zone of biological nutrient removal (BNR) processes or denitrification filters, where little or no wastewater carbon is available for denitrification. Methanol is the most prevalent carbon donor for denitrification at wastewater treatment plants; however, safety concerns, supply/cost volatility and the long-term sustainability of methanol usage have led many utilities to consider alternative sources for carbon augmentation. Several promising alternative carbon sources viewed as more sustainable replacements for methanol are waste byproducts from industrial applications, including corn syrup waste and glycerin/glycerol-based products derived from the production of biodiesel. Other supplemental carbon products include ethanol and proprietary compounds such as MicroC™. Process optimization including primary sludge fermentation is another sustainable solution to reduce supplemental carbon requirements.
The objective of this investigation is to compare the available supplemental carbon donors from both a process design and operations and cost aspect.
Full-scale evaluations of supplemental carbon alternatives have been conducted or are planned in the next couple of months at the facilities shown in Table 1. Bench-scale experiments were performed during the full-scale evaluation period to quantify the required chemical oxygen demand to nitrate-nitrogen ratio (COD:NO3-N) and the specific denitrification rate (SDNR) in a controlled environment. COD in the form of the carbon source and nitrate were added to the reactor at the start of each test, and ammonia, COD, nitrate and nitrite were monitored over time.
The supplemental carbon source was typically fed to one or all of the secondary anoxic zones in the BNR basins during the full-scale trial at each facility. In addition, glycerin was fed to the denitrification filters at the Neuse River WWTP (NRWWTP). Nutrient profiling was performed through the BNR basins to determine the efficacy of the supplemental carbon sources in denitrification. Nutrient profiling typically consisted of quantifying nitrate, nitrite and dissolved oxygen into and out of the secondary anoxic zones.
Process performance from all of the facilities listed in Table 1 will be presented in the paper. Full-scale and pilot testing at the Henrico WRF confirmed that the corn syrup solution could be successfully used as a carbon donor for post-anoxic denitrification. Full-scale pilot testing at the NRWWTP and the Parkway WWTP confirmed glycerin products could be used successfully for post-anoxic zone denitrification, although cold weather storage and pumping issues related to product viscosity need to be addressed. The full-scale glycerin pilots indicate that no acclimation period is required, as robust denitrification was observed within one day of adding these products. Results from the bench-scale experiment conducted at the NRWWTP with Brenntag’s glycerin product are shown in Figure 1. Limited nitrite accumulation (? 2 mg/L NO2-N) was observed during the bench-scale studies, which contrasts with some previous studies that indicated more significant nitrite accumulation. Nitrite accumulation was not observed during the full-scale nutrient profiles. COD and nitrate concentrations observed during bench scale testing at the NRWWTP are presented in Figure 2 and were used to calculate the COD:NO3-N removal ratio.
A cost comparison of the supplemental carbon sources was performed based on the cost to remove one pound of nitrate ($/NO3-N removed) and is summarized in Table 2. Costs were developed based on the following factors: COD:NO3-N removal ratio from the bench-scale testing, the mass of COD per mass of substrate, measured product COD concentration, specific weight and bulk delivery quotes.
Full-scale and bench-scale evaluations of alternative carbon products demonstrated that corn syrup, MicroC, glycerin-derived products, and methanol can all be used effectively in secondary anoxic zone denitrification. The results of this evaluation indicate that glycerin-based carbon donors are cost competitive with methanol; however, there was a significant variability in the cost and COD content of the different glycerin products, which should be taken into account when applying these results to other facilities. Bench-scale results indicate that corn syrup and Micro C are expected to be more expensive than methanol at current prices. Primary sludge fermentate was shown to improve biological phosphorus removal at the High Point WRF, and supplemental carbon requirements could be reduced if a portion of the primary effluent were redirected to the anoxic zone.
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