Performance Evaluation and Model-Based Optimization of Membrane Bioreactors: The Case of Addis Ababa Package Treatment Plant
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Date
2017-11
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Addis Ababa University
Abstract
Addis Ababa Water and Sewerage Authority invested 750 million ETB for installing membrane
bioreactor units in the city. This new technology is expected to double the wastewater treatment
capacity of the city. Evaluating the performance of the units is necessary in estimating the
likelihood of meeting this expectation and predicting the sustainability of the treatment plants.
This research explores performance of the installed plants and suggests optimal operational
strategies to enhance the denitrification capacity of the plant while minimizing energy cost.
Measures of performance were effluent quality and membrane filtration processes. In order to
evaluate the effluent quality, samples were collected both in dry and wet seasons. The MBR module
was evaluated using recorded data of Transmembrane Pressure (TMP), flux & permeability of
five-month operation. Activated sludge model no1 based simulation was used to determine the
optimal operation strategy. To ensure energy reduction while improving the treated water quality;
the suitability of intermittent aeration was explored using the developed model. The average COD
removal efficiency was 97%. The ammonium effluent concentration was always lower than 0.5
mg/L. However, absence of anoxic tank resulted in poor denitrification with consequently increase
nitrate effluent concentration over 100 mg/l. The treated wastewater was found to be completely
clear of fecal coliform; making the treated water suitable for reuse. The MBR filtration showed
stable but low permeability condition. The average operational TMP was less than 20mbar.
However, flux and permeability plot showed the plant was operating with decreasing trend over
time and maximum of 80LMH/bar. This suggests the need for frequent cleaning or construction of
a primary tank. Results from the ASM1 simulation indicated that intermittent aeration with
aerobic-anoxic cycle of 43/54 min was the optimum duration. The proposed operational strategy
increased the total nitrogen removal efficiency from 42% to 71% without affecting COD removal.
These results indicate that creating optimal aerobic/anoxic conditions within the existing reactor
is the most competitive solution to upgrade the MBR treatment plant.
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Keywords
Membrane BioReactor, Activated sludge model, AQUASIM, Cyclic aeration, Response surface optimization