Stepwise Increment of Salinity for Evaluating the Ammonium Removal Efficiency in Synthetic Wastewater Using an Aerated Bioreactor

Abstract

Using seawater for flushing toilets is a potential solution to reduce freshwater consumption in Guam, an island in the Western Pacific Region. However, elevated salinity levels from seawater can affect microbial treatment efficiency in wastewater plants. To study this effect, synthetic wastewater containing 30 mg/L of ammonium-nitrogen was used to evaluate the impact of various salinity levels on ammonium removal efficiency in a continuously operated polyvinyl-alcohol gel bead bioreactor. The bioreactor was incrementally exposed to salinity levels of 0%, 20%, 30%, 50%, 70%, 80%, and 100%, where 0% represented tap water salinity and 100% was equivalent to Guam’s seawater salinity. A recovery phase followed the 100% salinity exposure to assess any changes in treatment processes. The total operational period spanned 212 days. Between 0 and 30% salinity, the ammonium-nitrogen removal efficiency averaged 95.0%, indicating no inhibition of nitrification. However, at 50% salinity, the ammonium-nitrogen concentration began to decrease, and a trend that continued up to 80% salinity, resulting in over 60.0% removal efficiency. At 100% salinity, the removal efficiency rapidly dropped from 59.2% to 15.4% within ten days. When the bioreactor returned to 0% salinity, the removal efficiency quickly recovered to 93.7% within five days. This experiment identified 50% salinity as a critical threshold for nitrification efficiency, while 100% salinity was found to be unfavorable for nitrification.