The high-recovery RO treatment produced permeate of excellent quality in both the constant and increasing recovery experiments. The EC was reduced by ~97%, and the removal rate of all individual ions was >95%. The permeate pH (~3.4) was was higher than that of the feed (pH~2.5, likely due to the high rejection of acidic species (mainly H3PO4 and HSO4 -). An industrial RO spiral-wound element may provide even higher salt rejection at lower energy with higher recovery than the spacer-less flow cell used here. Because of its high quality, the permeate produced by RO could replace freshwater in many plant processes. Although the RO concentrate stream comprises only ~7% of the PAIWW volume, it remains an environmental risk because of its composition (Table ) and requires further treatment. Although the concentrate (pH ~2.4) may be slightly less acidic than the PIAWW (pH ~2.1), it is more hazardous because of the stronger buffering capacity induced by the higher phosphate concentration (~43 g L -1 ).