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FIG. 7.35.9 Schematic diagram of two fluidized-bed reactors in series for biological wastewater treatment. (Reprinted, with permission, from E.T. Oppelt and J.M. Smith, 1981, U.S. Environmental Protection Agency research and current thinking on fluidised-bed biological treatment, in Biological fluidised bed treatment of water and waste water, edited by P.F. Cooper and B. Atkinson, Chichester: Ellis Horwood Ltd., Publishers.)

to 4.8 kg/m3/day for a standard oxygen-activated sludge process. The low food-to-biomass mass ratio of U = 0.3 kg COD/kg VSS/day achieved the low effluent BOD of 13.

The effluent BOD was a function of U. At U = 1.1, the effluent BOD was 30. Ecolotrol Inc. of Westbury, New

York used this value to design a large 10-mgd plant (1577 m3/hr) with five, parallel, fluidized-bed reactors at the Bay Park Wastewater Treatment Plant in Nassau County, New York (Jeris, Owens, and Flood 1981). These five reactors with a combined volume of 1000 m3 operated at an up-flow rate of 37 m/hr and a recycling ratio of 4.6. A PSA oxygen generator provided 6 tn/day of oxygen (5443 kg/day). The plant had various operating difficulties; controlling microbial growth was the most problematic. The plant was shut down in 1991.

Dorr-Oliver, a Connecticut engineering company, developed the Oxitron system for fluidized-bed wastewater treatment on the basis of the Ecolotrol design (Sutton, Shieh, and Kos 1981). Dorr-Oliver added an influent distributor and a proprietary influent oxygenator with a 20-sec hydraulic retention time to allow the use of at least 50 mg/l of oxygen per pass. They controlled the biofilm thickness on the fluidized sand by keeping the bed expansion at a specific level. The particles with more biofilm are lighter and raise the bed level. When that situation happens, this system pumps them with a rubber-lined pump to a hydroclone and sand washer and returns sand to the reactor.

In the pilot plant shown in Figure 7.35.10, tests were conducted with wastewater having a median BOD of 67 ppm and a median COD of 175 ppm. The best results, 78% BOD removal to 15 ppm, were obtained at an up-flow rate of 20.3 m/hr, a hydraulic retention time of 37 min, and a food-to-biomass ratio of U = 0.16 kg BOD/kg VSS/day.

Dorr-Oliver designed full-scale plants for municipal wastewater treatment at Haywood, California (70,000 m3/day) and Sherville, Indiana (35,000 m3/day). Based on operating costs, the OXITRON system is favored for

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