FULL-SCALE COMPARISON OF DECLINING RATE VS. CONSTANT RATE FILTRATION   Under the AWWARF competitive technical proposal process EE&T was selected to perform a full scale test evaluation to compare the performance of filters hydraulically controlled by rate-of-flow controllers to filters operated by declining rate hydraulic conditions. The AWWARF rationale for solicitation on this project was that it would be the first time that such a direct full scale comparison of the two control methods has been researched. EE&T conducted the studies at the City of Durham, N.C. This plant was divided into two identical halves by the installation of baffles and flow meters. In one half of the plant, four new rate-of-flow controllers were installed. The flow controllers are operated to maintain constant flow through the filter based on a venturi feedback loop. They have an override set condition based on filter head changes which compensates for changes in flow rates to the filter such as occurs during plant flow changes or backwashing of one filter. The motorized butterfly valves close or open at a rate proportional to the distance away from the set point. The second side of the plant was converted to declining rate filters by installing flow measurement venturi meters and locking the valves in place at the calculated desired headloss. The filters were operated at average rates between 4 and 8 gpm/ft2. Data were collected comparing effluent turbidity, particle size distribution of the floc penetration through the media depth, headloss development, and bacteriologic quality comparisons. Final results showed that both operating methods provided excellent finished water quality. Declining rate filters offer the possibility of longer filter runs for the same system headloss, depending upon the magnitude of turbulent headlosses.