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A new process, the BASIN process (Biofilm-Activated Sludge Innovative Nitrification), consisting of a MBBR (Moving Bed Biofilm Reactor) with separate heterotrophic wasting, followed by activated sludge (AS), has been proposed to reduce the volume requirements of the AS process for nitrification.
The basic principle is to complete COD removal on the biofilm carriers by heterotrophic organisms and then to waste a portion of the heterotrophic biomass before it has the chance to pass onto the aeration basin. By this means, the amount of heterotrophic organisms grown in the aeration tank
is reduced, thereby reducing the volume of that tank needed for nitrification. For nitrification applications, the simplest method for stripping biomass was to use an in-tank technique using high shearing rates with aeration. Bench scale testing was used to prove the concept. During Phase
1, intermittent biomass removal was practiced in one MBBR while another served as control. Compressed air was used to simulate the airlift pumping that was anticipated for full-scale use. It was demonstrated that about one-half of the heterotrophic biomass was stripped by the once per day
cleaning method established. MBBR functioning was not impaired by intermittent stripping, as overall sludge yields and COD removals were the same for both MBBRs tested. During Phase 2, three lines were tested: 1) the BASIN process, 2) a MBBR (without intermittent biomass removal) followed
by intermediate settling (IS) and then an AS process and 3) a control AS process. AS sludge yields in the BASIN process were one-half of that in the control and twice that of the MBBR/IS/AS line. Critical washout solids residence times for nitrifiers were the same for all three lines,
so AS volumes for the BASIN process could be reduced by 50 percent compared to the control. For comparison, the process with intermediate settling (MBBR/IS/AS line) could reduce its aeration volume for nitrification by 75 percent, although at the expense of the intermediate sedimentation
tank. Overall, originally conceived process concepts for the BASIN process have proven out. The next step is their confirmation at large pilot or demonstration-scale.
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