Total Nitrogen Removal by Reverse Osmosis: Role of Biodegradable Dissolved Organic Nitrogen

Stringent water quality standards in recent times are requiring wastewater treatment plants to reduce total nitrogen to below 3 mg-N/L. Dissolved organic nitrogen (DON) is a major fraction of total dissolved nitrogen (TDN) in treated effluents ranging between 25 to 80%. Several advanced treatment technologies, including membranes have shown varying successes in removing DON and TDN. Studies have shown that DON in reverse osmosis (RO) treated water can be as high as 40% of TDN. Biodegradable portion of this DON (BDON) can lead to algal growth in the receiving waters. However it is unknown what fraction of this DON is BDON. Fate of DON and BDON was investigated in a combined ultrafiltration (UF) and a two-stage RO membrane treatment plant. UF/RO membrane process removed more than 95% of TDN, with majority of the removal occurring in RO membranes. The UF/RO facility is discharging its effluent with TDN and DON at 1.9 and 0.12 mg-N/L, respectively. BDON was completely removed by the RO membranes. RO concentrate contained high concentrations of TDN and DON, but relatively low concentration of BDON. RO was found to be an effective process to remove TDN to below 2 mg-N/L from a secondary treated effluent.