Selection and Design of Membrane Bioreactors in Environmental Bioengineering

Abstract

The membrane bioreactor (M BR) technology is nowadays widely consideredas one of the most important innovations in the field of wastewatertreatment in the last decades. MBRs couple suspended growth wastewatertreatment with membrane filtration, and early applications werepresented in late 1960s. However, the actual popularity occurred duringthe 1990s, with a higher and higher interest in the relevant strengthaspects of the process compared with conventional activated sludge (CAS)systems: High process compactness, excellent effluent quality (oftensuitable for water reuse) and lower sludge production. In urban sewagetreatment, the most important advantage derived from using membranefiltration is the elimination of the secondary settling tank for thetreated wastewater clarification. This can lead to some positiveconsequences summarized as follows: the obvious footprint reduction dueto the lack of the secondary settling tank; the indirect footprintreduction due to the possibility to operate at higher mixed liquorsuspended solids concentrations; the biomass selection is influenced bytheir degradation efficiency for pollutants rather than their ability toform well-settling flocs, as commonly happens in CAS plants. Two partscan be distinguished within the chapter. At first, a general descriptionof membrane processes is provided through a structure- andgeometry-based classification of membranes, a description of someconstituent materials, a brief introduction of the most importantmembrane processes and of the most relevant factors affecting membraneperformance. Then, the chapter focuses on membrane bioreactors forsolid/liquid separation. The possible MBR configurations are described(side-stream, membrane immersed in the biological tank and membraneimmersed in an external tank). Besides, the fouling phenomenon isdiscussed with special care for those MBR operational aspects, whichplay a relevant role in fouling mechanisms, mainly being thecharacteristics of the mixed liquor suspension, the membrane geometry,the hydrodynamic conditions and the hydraulic regime. Major strategiesfor fouling control are presented: wastewater pre-treatment facilities,air scouring, intermittent permeation and cyclic backwashes with eitherpermeate or chemical solutions. Furthermore, the ``critical flux{''}concept is introduced as a tool for the periodical assessment ofmembrane performances under various operating conditions; the mostsuitable version of the critical flux for MBRs (the sustainable flux) isproposed as possible fouling control strategy aimed to minimizeaggressive chemical cleanings, thus extending the membrane expectedlifetime. A specific section of the chapter is dedicated to some of themost diffused commercial applications of the :MBR technology, rangingfrom the flat sheet geometry (Kubota, Huber) to hollow fibre (Zenon,Memcor-US Filter, Mitsubishi) and tubular ones (X-Flow), from submergedto side-stream schemes. A COD-based approach for the design of suspendedgrowth wastewater treatment processes for total nitrogen removal understeady-state conditions is presented. The method is essentially based onthe well-consolidated approach proposed by the University of Cape Townin the early 1980s and formalised by International Water Associationwith the well-known activated sludge models (ASM1, ASM2 and AMS3). Themethod is based on the COD fractionation according to thebiodegradability of both its particulate and soluble aliquots.The design value of SRT (solids retention time) is determined as afunction of the requird ammonia nitrogen quality in the effluent, thenitrifiers biokinetics and the anoxic fraction of the overall biologicalprocess volume. An iterative determination of the anoxic fraction and ofthe recycle ratio is then suggested, in order to achieve the needednitrogen concentration in the effluent. The method presented and thedesign example are mainly aimed to make explicit the conventionalequations in terms of the required effluent standards for nitrogen formsas well as to show the possible differences between CAS systems and MBRsdue to the different biological kinetics.