Optimization of Reactor Depth in Membrane Bioreactors for Municipal Wastewater Treatment

Abstract

Aeration energy demand represents 45 to 80% of the total energy consumption of a wastewater treatment plant (WWTP). In membrane bioreactor (MBR) technology, biological aeration requirements are higher than in conventional activated sludge (CAS) because the transfer efficiency is lower. The evaluation of aeration transfer efficiency is imperative in order to find energy-saving opportunities and to make MBR cost effective in comparison to CAS. The effect of increasing the reactor depth was revised through the study of different scenarios from 3 to 9 m. Afterwards, the overall costs considering both capital and operating costs were studied. The results of this work indicate that air consumption is reduced on average by 9.1% per meter increased. This decrease affects the aeration system and represents capital savings in aeration equipment of 52%. Moreover, blower energy consumption shows a reduction of approximately 5% per meter increased. However, regarding civil work, an increase in reactor depth implies an increase of 21% in the constructions costs, showing an optimum depth of 5 m. When considering global costs, this optimum moves to 6.5 m, at which the total reduction is about 42%. From here the tendency is to stay similar, so a further increase in reactor depth does not compensate for the increase in civil work costs, even if it leads to lower operational costs.