Abstract
Membrane bioreactor is no longer just a promising technology. Full scale applications in urban wastewater treatment plants are rapidly growing in number and in terms of treatment capacity. This modification of the conventional activated sludge process may enhance the removal of xenobiotics for two main reasons: (1) the effluent (permeate) is virtually free from suspended solids and associated pollutants; (2) there is a major flexibility for the operation of the biological process, which is distinguished from the sedimentation properties of the activated sludge. This chapter deals with the removal of xenobiotics from real urban wastewater showing and discussing a 10-year activity carried out in Italy on pilot, demonstration and full scale membrane bioreactors. Target xenobiotics were metals (As, Cd, Cr, Hg, Ni, Pb), industrial organic chemicals and products, such as PAHs, BTEXs, PCBs, PCDDs/PCDFs, etc. As far as metals are concerned, besides Cd and Hg, which were almost completely removed both in conventional and membrane systems, generally the enhanced biosorption and/or retention capability allowed the membrane bioreactor to be more effective than the conventional activated sludge systems in removal of Cr, Cu and Ni. High sludge age seemed to enhance the bioconversion of hydrophobic and partially recalcitrant substances such as dioxins, hexachlorobenzene and poly-chlorinated bi-phenyls. As for the power requirements of the membrane bioreactors, which still represent a bottleneck for the widespread urban application of the technology, full scale data demonstrated were close to sustainable values, especially when membrane filtration is coupled to energy-saving biological processes such as the intermittent and automatically controlled aeration. Moreover, although a significant decrease is being observed for investment costs, land cost still represents a real major driver for membrane bioreactors urban implementation.
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