High Load Field Test of a Secondary Clarifier

Secondary clarifiers were field tested for hydraulic characteristics and performance and checked with the Clarifier Research Technical Committee's draft protocol at a Wisconsin, Ohio and North Carolina WWTP. The test clarifiers were two (2) 42.68m (140 ft.) diameter; 35.1m (115 ft.) and 48.78m (160 ft.) x peripheral-feed, inlet, peripheral-overflow clarifiers. Settled solids are collected by a rotating hydraulic header for all four (4) units.The WWTPs wanted to determine the capacity of the existing secondary clarifier system for use in planning of plant upgrades and permitting. Also, to determine if any upgrades or changes should be considered for the units.Multiple tests were conducted at average and max flow conditions approximating 800 to 1,700 GPD/sq. ft. (1.36 and 2.9 m/h). Influent mixed liquor concentrations RAS concentrations, effluent TSS, sludge blanket profiles, and flow rates were recorded to document the performance of the clarifier. Sludge volume index (SSVI) and SVI and in-situ dispersed solids (IDS) tests were conducted to quantify the settling and flocculation characteristics of the influent mixed liquor. Hydraulic characteristics of the clarifier were assessed by conducting slug dye and flow pattern/solids distribution and drogue tests.To run the tests, it was necessary to take basins off-line. It was possible to maintain an overflow rate of up to 1,700 GPD/sq. ft. (2.9 m/h) with a solids loading of up to 50 lbs./sq. ft./day (243 kgs/sq. m/day).Results of the high load test show a hydraulic and solids loading efficiency well above conventional design parameters. Solids studies showed effluent suspended solids (ESS) levels could be maintained below 20 ppm at hydraulic loading up to 1,700 GPD/sq. ft/day (2.9 m/h) and 50 lbs/sf/d.The flow pattern was away from the effluent weirs.The sludge blanket is controlled.The three (3) studies reported in this paper verify the original work done by Crosby in the late 1970's “that the PF/PO clarifier acts as an upflow clarifier and needs little or no modifications to its design.”It was possible to maintain, over an eight (8) hour and a 7 day period, an overflow rate of up to 1,700 gpd/sq ft. with a solids loading of up to 50 lbs./sq. ft/day.