Research was conducted in IFAS systems to identify problems that arose with the biofilm when the intensity of mixing and aeration fall below or rise above certain thresholds. The types of IFAS media evaluated include fixed bed cord media (Ringlace and Bioweb), moving bed sponge (sponge mediaLinpor and Captor; and plastic media -Kaldnes) and RBC media. An aeration process design model and operating strategies have been developed to address these problems. This paper presents the IFAS and MBBR design methodology and aeration process design component of the Aquifas Unified Model for Activated Sludge, IFAS and MBBR systems. The paper references specific instances in full-scale IFAS plants where problems occurred because of insufficient or improper mixing patterns and documents how they were resolved. The problems observed included nuisance predator development and plugging of media and screens. The IFAS model was upgraded to incorporate computation of biofilm thickness and the impact of the increase in biofilm thickness on the reduction in effective surface area available on a various types of media. Effective surface area is the m 2 of biofilm surface per m 3 of tank volume. The model uses the effective surface area and the amount of media in the tank to compute the substrate (COD, NH4N and Oxidized-N) profiles. Additional research is being conducted to establish certain thresholds for mixing necessary above and beyond the aeration requirements to maintain a thin biofilm.The paper shows how the process design model is applied to improve the design of IFAS and MBBR systems. In instances where the aeration requirement to satisfy the DO set-point is satisfied but the mixing is below the threshold to maintain a thin biofilm, the media fill volume fraction in an aerobic cell can be increased and the fraction of tank volume occupied by the aerobic cell can be increased. By increasing the media fill volume fraction in a situation where the biofilm in an aerobic cell is too thick, the total amount of biofilm surface area present in the aerobic cell is increased and the soluble COD concentration is decreased. This decreases the biofilm thickness observed for the same intensity of mixing and increases the effective surface area of biofilm. This improves the performance of the system. When the media fill volume fraction is increased, the air flow per unit of reactor volume has to be increased to satisfy the additional oxygen demand. This increase in air flow helps satisfy the mixing requirements to mix the media and induce the requisite amount of biofilm shear. In some instances with fixed bed media, supplemental aeration system is installed below the frames to increase the air flow during diurnal peak load hours and thin out the biofilm.

This project involves the upgrade of the 58,670 m 3 /d (15.5 mgd) Danbury, CT Water Pollution Control Plant (WPCP) to provide nitrogen removal using a creative approach that employed a modification of the Wuhrmann process, a nitrogen removal process dating back to the 1960's, that uses a post anoxic zone as the primary means to achieve nitrogen removal. Relatively low BOD and high dissolved oxygen in the influent to the WPCP's existing nitrification tanks posed a challenge to using a pre anoxic zone in a conventional MLE process modification for nitrogen removal. Moving the anoxic zone to the effluent end of the nitrification tanks (as a post anoxic zone) allowed the influent dissolved oxygen to be used for nitrification and BOD removal, eliminated the need for nitrate recycle, and the relatively low cost modification achieves lower effluent nitrogen (between 4 to 6 mg/l TN) than achievable by the MLE process.