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TERTIARY TREATMENT TECHNOLOGIES & PRACTICES Optimization of Biological Nutrient Removal (BNR) Facilities to Achieve Consistent and Compliant Effluent Quality BY SARA ARABI, PH.D., P.ENG. AND MEHRAN ANDALIB, PH.D., P.ENG., BCEE, ENVIRONMENTAL OPERATING SOLUTIONS, INC. BOURNE, MA In addition to controlling eutrophication in receiving water body and environmental benefits, biological nutrient removal (BNR) facilities have demonstrated economical and operational benefits. The utilization of BNR processes is potentially more economical than conventional activated sludge treatment or physical/ chemical processes. Incorporation of an un-aerated zone ahead of the aerobic zone in a BNR process can result in a substantial reduction of the aeration energy costs. The aeration energy needs would be further reduced because most of the substrate removal and some stabilization of organics would occur in the un-aerated zone. The two most widely used BNR processes are enhanced biological phosphorus removal (EBPR) which ties up soluble phosphorus within the waste activated sludge without the use of metal salts, and biological nitrogen removal which removes oxidized nitrogen. EPBR provides an economic benefit through reduction or elimination of chemical addition for phosphorus removal. This BNR process produces less waste activated sludge due to a lower sludge yield. Biological nitrogen removal recovers approximately one-half of the alkalinity consumed during autotrophic nitrification. It has been shown that anaerobic and/or anoxic zones placed ahead of aerobic zones in BNR processes act as biological selectors that discourage the growth of filamentous organisms, generally improve the sludge settling properties, and enhance process stability. Process optimization: Carbon augmentation for nutrient removal The performance of a BNR system is strongly affected by the characteristics of the wastewater influent to each zone of the process. Neither biological nitrogen removal nor EBPR can be accomplished without sufficient biodegradable organic substrate, i.e., as measured by chemical oxygen demand (COD) or biochemical oxygen demand (BOD). Carbon augmentation is used when there is insufficient carbon available to achieve complete denitrification, which is normally the case when low levels of total nitrogen (TN) (e.g. < 5 mg/L TN) are required in the treated effluent. A shortfall of readily – biodegradable COD (rbCOD) is known to negatively impact the EBPR process and addition of an external carbon source is required. The choice of a carbon source can have profound implications not just on the efficacy of nutrient removal, but also on plant and personnel safety, sludge yields, aeration adequacy, environmental sustainability, overall effluent quality, cost, and other factors. Recent studies also indicate the type of carbon source could have differing effects on nitrogen and phosphorus removal, even in the same treatment process. Table 1 provides a summary of stoichiometric and kinetic coefficients of denitrification values reported in the literature from several sources. Soluble and readily degradable substrates support the highest rate of denitrification. Although methanol has been the most widely used external carbon source, it often requires an adaption period of up to seven months before denitrification rates significantly increase due to low methylotrophs growth rates (US EPA, 2013). The flammability, safety concerns and price fluctuations for methanol have limited its use for wastewater treatment (US EPA, 2013). Agriculturally-derived carbon sources such as molasses, glycerol, corn syrup, sucrose and MicroC (glycerin based product), tend to have more predictable and less volatile price profiles (US EPA, 2013). Recently, glycerin has drawn significant attention as an alternative to alcohols (methanol and ethanol) for denitrification application and acetate for enhanced biological phosphorus removal, as it is safer, noncorrosive and nonflammable. Its price, biodegradability, high COD value and ability to promote nutrient removal behavior are all advantages that make this supplemental carbon source a TABLE 1 Stoichiometric and kinetic parameters at 20°C for denitrifying biomass grown on different carbon sources. Carbon Source MicroC 2000 Methanol Ethanol Acetate Max Specific Growth Rate, µmax (d -1 ) 2.08 (a) 1.28 (b) 1.3 (d) 3.5 (g) COD:N 5.12 (a) 4.8 (c) 6.1 (e) 5.7 (c) Observed Yield (gVSS/g COD) 0.31(a) 0.29 (c) 0.38 (e) 0.35 (c) Specific Denitrification Rate, SDNR max (mgN/gVSS/h) 13.3 (a) 6.1(c) 10 (f) 13.6 (c) (a)Onnis-Hayden et al. (2011); (b) Dold et al. (2005); (c) Cherchi et al. (2009); (d) Dold et al. (2008); (e) deBarbadillo et al. (2008); (f) Nyberg et al. (1996); (g) Mokhayeri et al. (2006) 30 INFLUENTS Fall 2015 Click HERE to return to Table of contents
TERTIARY TREATMENT TECHNOLOGIES & PRACTICES viable alternative. In addition, glycerin’s abundance in nature has led to microbial adaptations for its uptake and use as a source of carbon and energy. Two case studies for application of MicroC products for biological nutrient removal are presented in this article. CASE STUDY 1: Industrial wastewater – Beef processing facility – Biological nitrogen removal Case Study 1 focuses on a beef slaughter and processing plant in Pennsylvania that recently completed an upgrade to the wastewater treatment facility (WWTF) (1.89 MLD) to comply with the total maximum daily load (TMDL) limits. The WWTF upgrades include a dual train, four-stage Bardenpho process (Photograph 1). Prior to the upgrade, the WWTF historically discharged >200 mg/L TN. Effective the start of October 2013, the WWTF’s new National Pollutant Discharge Elimination System (NPDES) permit required an annual TN average concentration of
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