Pilot-scale ozone/biological activated carbon treatment of reverse osmosis concentrate: Potential for synergism between nitrate and contaminant removal and potable reuse

With limited budgets, utilities may forego potable reuse of municipal wastewater to meet regulations requiring nitrate and contaminant removal from discharges. At 85% recovery, reverse osmosis (RO)-based potable reuse rejects contaminants into a concentrate flowrate that is 6.7-fold lower than wastewater discharges. This study evaluated the footprint and cost savings associated with treating nitrate and contaminants in RO concentrate vs. wastewater effluent. Pilot-scale ozone and biological activated carbon (BAC) treatment of RO concentrate provided design parameters needed for this comparison. Addition of 60 mg C per L methanol as a carbon source was needed to achieve complete denitrification of 70 mg N per L nitrate within a 30 min BAC empty bed contact time. Combined with pre-ozonation at 0.5 mg O₃/mg DOC, this treatment removed >74% of fipronil, imidacloprid, atenolol, DEET and sulfamethoxazole. Estimates indicated that applying ozone/BAC treatment to RO concentrate rather than conventional wastewater effluent for contaminant removal prior to discharge would offset 60% of the footprint and 25% of the cost required for the RO-based potable reuse train. Considering these savings, the reuse train cost could be recouped if the product water were sold at rates below those current in southern California. These results suggest synergy rather than competition between potable reuse and contaminant removal.