Primary Sludge Fermentation: Sustainable and Economical Process of Supplementing Carbon for Short-Cut Nitrogen Removal Open Access
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Mainstream shortcut nitrogen removal offers vast opportunities for energy and treatment cost savings, making these processes sustainable and cost-effective for biological nutrient removal (BNR) systems within modern wastewater treatment plants. DC Water operates a 384 mgd (1400 MLD) advanced wastewater treatment plant to meet stringent nutrient limits (Total phosphorous (TP) < 0.17 mg P/L and Total nitrogen (TN) < 3.7 mg TN/L). The operating costs of the nitrogen removal facility are significant and include $8M per year for methanol, $0.6M per year for alkalinity supplementation, and $1.5M in energy demand for aeration. This notable cost for methanol addition as an external carbon source for BNR process is a clear driver for evaluation of short-cut nitrogen removal technologies. Nitrite oxidizing bacteria (NOB) out-selection, using aeration control strategies, proved to be tricky considering the stringent nutrient permit. Therefore, an alternative route selecting for partial denitrification (PdN) of NO3-N to NO2-N using acetate coupled with anammox (AnAOB) was explored to achieve higher removal of nitrogen. In contrast to the NOB out-selection based approach, the PdN coupled with annamox approach requires an external carbon source either in the form of acetate or glycerol. Because the cost of commercially available acetate and glycerol in the USA are respectively, more expensive than (factor >= 3) or comparable to the currently used methanol, alternative sources should be explored to increase economic viability. The overall objective of this research was to explore the viability of primary sludge fermentate as the recycled carbon source for selecting PdN in order to reduce chemical costs. In addition, the solids retention time needed to reach required soluble COD yields as well as the concurrent nutrient release during fermentation were evaluated to estimate the potential of PdN-AnAOB under mainstream conditions. Laboratory scale continuously stirred tank reactors (CSTR) with working volume of 4L were operated with equal solids retention time and hydraulic retention time (SRT=HRT) at room temperature (20-23C). The reactors were seeded and wasted with primary sludge influent (PS) to maintain 0.5 day, 1 day, 2 day and 4 day SRT. This study showed that incorporating primary sludge fermentation within the short-cut nitrogen removal scheme has a huge potential for AnAOB contribution in the mainstream system. Overall, this approach should allow to save 25-67% of operational cost for BNR at Blue Plains, almost $6.2M out of $10M currently spent.