Development of a model of the aerobic membrane bioreactor treating Illovo wastewater.

Abstract

The Membrane Bioreactor (MBR) at Sezela, KwaZulu-Natal treats a process effluent emanating from a sugar industry by-products plant. Depending primarily on the effluent feed rate to the MBR as well as other less significant factors, the MBR tends to operate at a temperature that fluctuates between 40 and 50 °C. As a result of the temperature fluctuations the MBR may operate at either mesophilic or thermophilic temperatures. In an attempt to avoid the operational instability that accompanies the transition between temperature regimes, it would be conceivable to maintain mesophilic operation through either the removal of heat during feed increases or by continuously maintaining a low feed rate; alternatively to maintain thermophilic operation by providing auxiliary heat to the MBR when low feed rates are experienced, or by maintaining a high feed rate, possibly in conjunction with a buffer tank. A solution to the problem was sought through the formulation of a coupled dynamic mass and energy balance model, with an attached speciation routine. Development of a simulation model allowed the prediction of key operating parameters, namely the temperature, pH, substrate concentration, and volatile suspended solids (VSS) concentration. The sources of data used for modelling were laboratory experiments, historical MBR data, and literature data. Kinetic and stoichiometric coefficients of the model were determined from batch respirometric tests on the MBR furfural plant effluent feed and the activated sludge. The final model yielded a dynamic temperature (Root Mean Square Deviation (RMSD) of 1.61 and 1.34 °C) and pH (RMSD of 0.36 and 0.47) prediction over a continuous 69 day interval, where only the furfural plant effluent feed and sludge wasting rates were required as model inputs. The prediction of the substrate concentration and VSS concentration were found to be unreliable. The results of the comparison of mesophilic to thermophilic operation, through the final calibrated model, indicated that thermophilic operation was advantageous, however a rigorous economic analysis is required to substantiate this outcome. Thermophilic operation at 50 °C can handle feed rates 2.2 times higher than mesophilic operation at 40 °C, but may be more susceptible to process upsets.