The Investigation of Nitrite Accumulation and Biological Phosphorus Removal in an Intermittently Aerated Process Combining Shortcut Nitrogen Removal and Sidestream Biological Phosphorus Removal

Abstract

The research in this thesis was conducted at the Hampton Road Sanitation District's biological nutrient removal pilot, located at the Chesapeake-Elizabeth WWTP in Virginia Beach, VA. The pilot is operated in an A/B process with a high-rate, carbon-diverting A-stage, followed by a biological nitrogen removal B-stage containing four intermittently aerated CSTRs, followed by an anammox polishing MBBR. The goal of this research was to successfully combine short-cut nitrogen removal with sidestream enhanced biological nutrient removal (EBPR) in the most efficient way possible, specifically aiming to decrease cost and energy requirements, divert the most amount of carbon possible before B-stage, and to achieve low effluent nitrogen and phosphorus concentrations. A RAS fermenter (SBPR) and an A-stage WAS fermenter that feeds VFA into the SBPR (the supernatant of the fermenter is called fermentate) were implemented in order to enhance biological phosphorus removal. About 8 months after the RAS and WAS fermenter implementation, there was a 28 day consecutive period of low B-stage effluent OP <1 mg/L, with an average of 0.5 ± 0.1 mg/L OP. Following this low effluent OP period, bio-P became more unstable and there was high nitrite accumulation in the B-stage effluent for 106 days with concentrations ranging from 1.1-5.9 mg/L NO2. The nitrite accumulation was not due to NOB out-selection, confirmed by AOB and NOB maximum activity tests. It was determined that the nitrite accumulation was due to partial denitrification of nitrate to nitrite by bacteria using internally stored carbon, because profiles and activity tests showed anoxic nitrite accumulation at the end of the aerobic process. Post-anoxic denitrification using internally stored carbon compounds has been observed in other EBPR systems (Vocks, Adam, Lesjean, Gnirss, and Kraume, 2005). Fermentate addition was then halted, and nitrite accumulation and bio-P activity ceased all together, linking the fermentate addition to both bio-P activity and nitrite accumulation. Fermentate was then controlled to dose at 60% of the sCOD/OP (fermentate sCOD g/day / total OP- fermentate + influent - g/day) of the first low effluent OP period. During this fermentate dosing period where the average sCOD/OP was 15.6 ± 3.0 g/g, no nitrite accumulation was observed, but another consecutive low effluent OP period was observed with an average of 0.6 ± 0.2 mg/L OP. Linear correlation analysis shows that the highest r2 values relating the low effluent OP periods and the COD loads to the SBPR for both periods were between VFA g/day vs OP effluent mg/L, at r2=0.18 for the first low effluent OP period and r2=0.65 for the second. There were also high tCOD r2 values for the second low effluent OP period showing that COD hydrolysis in the SBPR could have impacted bio-P activity. However, the VFA r2 value was higher than any tCOD r2 value, concluding that the fermentate dosing mainly worked to enhance biological phosphorus removal by increasing the VFA load in g VFA as acetate/day. Since no nitrite was observed in a period with a lower VFA/OP dose, then the probable VFA load needed to provide enough internal storage to produce nitrite accumulation by partial denitrification is between 5-9 (g VFA as acetate/ g total OP). If sidestream EBPR systems could be studied further to promote nitrite accumulation and bio-P activity to produce low effluent OP, then short-cut nitrogen removal and EBPR could be successfully combined in an efficient way.