Selection and Design of Membrane Bioreactors in Environmental Bioengineering

Abstract

The membrane bioreactor (MBR) technology is nowadays widely considered as one of the most important innovations in the field of wastewater treatment in the last decades. MBRs couple suspended growth wastewater treatment with membrane filtration, and early applications were presented in late 1960s. However, the actual popularity occurred during the 1990s, with a higher and higher interest in the relevant strength aspects of the process compared with conventional activated sludge (CAS) systems: High process compactness, excellent effluent quality (often suitable for water reuse) and lower sludge production. In urban sewage treatment, the most important advantage derived from using membrane filtration is the elimination of the secondary settling tank for the treated wastewater clarification. This can lead to some positive consequences summarized as follows: the obvious footprint reduction due to the lack of the secondary settling tank; the indirect footprint reduction due to the possibility to operate at higher mixed liquor suspended solids concentrations; the biomass selection is influenced by their degradation efficiency for pollutants rather than their ability to form well-settling flocs, as commonly happens in CAS plants. Two parts can be distinguished within the chapter. At first, a general description of membrane processes is provided through a structure- and geometry-based classification of membranes, a description of some constituent materials, a brief introduction of the most important membrane processes and of the most relevant factors affecting membrane performance. Then, the chapter focuses on membrane bioreactors for solid/liquid separation. The possible MBR configurations are described (side-stream, membrane immersed in the biological tank and membrane immersed in an external tank). Besides, the fouling phenomenon is discussed with special care for those MBR operational aspects, which play a relevant role in fouling mechanisms, mainly being the characteristics of the mixed liquor suspension, the membrane geometry, the hydrodynamic conditions and the hydraulic regime. Major strategies for fouling control are presented: wastewater pre-treatment facilities, air scouring, intermittent permeation and cyclic backwashes with either permeate or chemical solutions. Furthermore, the “critical flux” concept is introduced as a tool for the periodical assessment of membrane performances under various operating conditions; the most suitable version of the critical flux for MBRs (the sustainable flux) is proposed as possible fouling control strategy aimed to minimize aggressive chemical cleanings, thus extending the membrane expected lifetime. A specific section of the chapter is dedicated to some of the most diffused commercial applications of the MBR technology, ranging from the flat sheet geometry (Kubota, Huber) to hollow fibre (Zenon, Memcor-US Filter, Mitsubishi) and tubular ones (X-Flow), from submerged to side-stream schemes. A COD-based approach for the design of suspended growth wastewater treatment processes for total nitrogen removal under steady-state conditions is presented. The method is essentially based on the well-consolidated approach proposed by the University of Cape Town in the early 1980s and formalised by International Water Association with the well-known activated sludge models (ASM1, ASM2 and AMS3). The method is based on the COD fractionation according to the biodegradability of both its particulate and soluble aliquots. The design value of SRT (solids retention time) is determined as a function of the required ammonia nitrogen quality in the effluent, the nitrifiers biokinetics and the anoxic fraction of the overall biological process volume. An iterative determination of the anoxic fraction and of the recycle ratio is then suggested, in order to achieve the needed nitrogen concentration in the effluent. The method presented and the design example are mainly aimed to make explicit the conventional equations in terms of the required effluent standards for nitrogen forms as well as to show the possible differences between CAS systems and MBRs due to the different biological kinetics.