he Effects of Carbon Choice & Nitrate Loading on IFAS Partial Denitrification/Anammox Processes

The Hampton Roads Sanitation District's James River Treatment Plant traditionally operated with an A2O configuration and aerobic IFAS. To improve nitrogen removal, eight of the nine treatment trains were reconfigured to a 5-stage process by converting their second anoxic zones to moving media IFAS with WWW2 media (World Water Works). This enabled the incorporation of the partial denitrification/anammox (PdNA) process. For effective partial denitrification in the second anoxic zone, a carbon source is required to convert NO3-N (nitrate-nitrogen) to NO2-N (nitrite-nitrogen), which anammox bacteria use. Ethanol was explored as a potential cost-effective alternative carbon source, hypothesized to exhibit a partial denitrification rate between that of methanol and glycerol. A pilot-scale experiment at the James River Treatment Plant was conducted to compare the efficacy of ethanol and methanol as external carbon sources for the PdNA process. Two identical PdNA IFAS reactors were operated in parallel, one fed with ethanol and the other with methanol, under varying COD loading conditions. The methanol-fed reactor consistently showed higher PdN efficiencies, better NH4 removal, and lower C/TIN values, indicating more efficient carbon utilization, thus establishing methanol as the preferred choice for PdNA. Subsequently, the study examined the resilience of PdNA systems to nitrate loading. The pilot setup was modified to include two reactors, both fed with methanol; one served as a baseline, and the other received supplemental nitrate. Thesereactors were operated in parallel and evaluated for removal rates and PdN efficiency. Over time, the nitrate-supplemented reactor developed a thicker biofilm and exhibited increased NO3 removal, but this came at the cost of reduced ammonia removal. The reactor favored full denitrification over partial denitrification, resulting in lower NO2 production and thus limited substrate availability for anammox bacteria, leading to lower in-situ NH4 removal. The results of this study are crucial for designing full-scale PdNA IFAS systems, particularly concerning the choice of external carbon sources and the impact of NO3 loading on system performance.