Oxygen Transfer in a Biological Aerated Filter

The City of San Diego (City) operates the Point Loma Wastewater Treatment Plant (PLWTP) which can normally treat up to 240 million gallons per day (MGD) of chemically enhanced primary treated (CEPT) effluent for ocean disposal. The City wanted to understand the performance capabilities of Biological Aerated Filters (BAFs) in treating CEPT effluent to secondary standards. To this end, the City conducted a one year pilot plant evaluation of BAF technology supplied by the two leading BAF manufacturers with experience in providing and operating BAFs greater than 30 MGD capacity. The pilot test results, which were reported at the 2005 WEFTEC conference, confirmed that the BAF technology is capable of producing secondary treated effluent that meets anticipated discharge requirements during simulated wet weather and dry weather conditions. This paper reports on oxygen transfer efficiencies, which were measured twice during the pilot study using the off-gas method. The first set of tests was largely unsuccessful due to mechanical problems that were unrelated to aeration equipment. The second set of tests was successful and showed process water oxygen transfer efficiencies (OTE) of 1.3 to 1.8 %/ft (4.1 to 5.7 %/m) and 1.8 to 2.4 %/ft (6 to 7.9 %/m) for the two different pilot plants at their nominal design conditions. A mass balance using chemical oxygen demand and dissolved organic showed similar transfer rates. These rates are higher than can be expected from fine pore diffusers for similar process conditions and depths.