A pure oxygen activated sludge system was converted to a VIP configuration BNR (biological nutrient removal) system wherein three of the five pure oxygen sections were retained, and performance was compared to that of a side-by-side air aeration MUCT (modified UCT) system. Because the pure oxygen BNR system could not obtain good nitrification, its treatment capacity had to be downgraded from 113,550 m(3)/d to a flow of only 60,000 m(3)/d. At the lesser flow, it was determined that adequate nitrification and improved denitrification could be accomplished in the pure oxygen system by continuously seeding it with 100% of the WAS from the MUCT system. Fortunately, while the capacity of the pure oxygen system had to be downgraded, it was determined that the capacity of the MUCT system was substantially greater than its design flow, and the combined system is capable of treating the entire design flow. However, this requires increasing the operating sludge age of the MUCT system. The pure oxygen BNR system performed better phosphorus removal than the MUCT system, both before and after seeding with the MUCT WAS. Apparently this was because the MUCT system was operated at a substantially higher sludge age than the pure oxygen system. However, both systems have consistently discharged effluent phosphorus concentrations of less than 2.0 mg/L TP, which is the Chesapeake Bay standard. Even with improved nitrification and denitrification in the pure oxygen BNR system, neither it nor the MUCT system have proven to be capable of meeting the Virginia Chesapeake Bay goal of 10 mg/L total nitrogen in the effluent.