Incorporating MBR Features Into a IWA ASM2d and Biofilm Model Constructed on .NET Platform for Design, Control and Optimization
Abstract:A MBR model was developed to include biofilm and activated sludge components modeled along the guidelines established by the IWA Task Groups. The biofilm components allow the user to specify the biofilm specific surface area for media (m2 of media surface/m3 of tank volume with media) in the anoxic and aerobic cells upstream of the MBR cell. Within the MBR cell, the user can apply the specific surface area of the membranes for biofilm applications. The transport of substrates and electron acceptors to the biomass layer on the membrane can be modeled by two methods: (a) a diffusion transport as in traditional biofilm modeling; or (b) a permeate transport mechanism. For the biofilm on fixed bed or moving bed media in aerobic or anoxic cells upstream of the MBR cell, one uses the diffusion transport mechanism.
The study evaluated the effect of integration of biofilm in the aerobic and anoxic cells as an IFAS-MBR. When the IFAS-MBR was optimized for energy and nutrient removal, its energy consumption was similar to that for a hollow fiber conventiontal MBR optimized for energy. The conventional MBR was operated at low DO levels in the aerobic zone (0.75 to 1.25 mg/L), an MLSS of 5000 to 7500 mg/L, and with a 0.25 on-time fraction for air scour in the MBR cell. The IFAS was operated at a lower MLSS (2500 mg/L) which helped increase the alpha for aeration. However, the mixing requirements for the moving bed media required a higher operating DO in the aerobic cells compared to the optimized conventional MBR (2.5 to 3.5 mg/L at 12 C). The lower MLSS allowed the IFAS MBR to operate with a 0.5Q MBR recycle, as opposed to 3 to 4Q for conventional MBR. An active method of control of MBR recycle (ACTMBR ©) was developed and tested, where the MBR recycle was flow paced, and the flow pacing ratio was >0, and typically >1. A value of 0 would imply no flow pacing; a value of 1 would imply an increase in recycle flow rate that matches the increase in influent flow rate. For a ratio >1, an increase in influent flow rate to 2 times the average flow would increase the MBR recycle more than two times to >1Q. A ratio > 1 would help dilute out the MLSS in the MBR cell during high flows, as opposed to conventional MBRs where the MLSS increases substantially during the high flows. This helps reduce the solids flux on the membranes in the IFAS MBR. This can reduce the energy and membrane surface area required during high flows.
Document Type: Research Article
Publication date: January 1, 2010
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