One of the two trains of the 37,000 m3/d Annapolis, Maryland step aeration activated sludge treatment plant was modified for single-sludge anoxic-aerobic operation, and then fixed-film media were integrated into the aerobic zone to enhance nitrification. Rope-like Ringlace media was selected for integration, and 30,000 meters were installed in a volume of 475 m3 for a pilot demonstration. The purpose of the integrated fixed-film media was to upgrade the short hydraulic retention time (HRT) basin (6 hrs nominal) for efficient, year-round nitrogen removal without construction to increase basin volume. An engineering study had concluded that upgrading the facility for year round complete nitrification, without nitrogen removal, would cost US$24 million. The modified train was operated for 12 months, six in the plug-flow MLE configuration, and six in a step-feed configuration. The integrated Ringlace media increased the nitrification rate per unit volume to 225% of that observed in the control section, attaining a value of 1.75 kg/d NH3-N per linear meter at 15°C. The media also increased denitrification in the aerobic media section to the extent that between 30 and 88% of the nitrates formed in the section were denitrified within it, permitting a potential 25% or more reduction in the volume of the anoxic zone. An IFAS retrofit design was developed which incorporated step-feed operation, and reduced the projected construction cost to US$9.2 million.
