When used with domestic wastewater, MBR processes could produce effluent of high quality enough to be discharged to coastal, surface or brackish waterways or to be reclaimed for urban irrigation. Other advantages of MBRs over conventional processes include small footprint, easy retrofit and upgrade of old wastewater treatment plants. Two MBR configurations exist: internal, where the membranes are immersed in and integral to the biological reactor; and external/sidestream, where membranes are a separate unit process requiring an intermediate pumping step.
Recent technical innovation and significant membrane cost reduction have pushed MBRs to become an established process option to treat wastewaters . As a result, the MBR process has now become an attractive option for the treatment and reuse of industrial and municipal wastewaters, as evidenced by their constantly rising numbers and capacity. The current MBR market has been estimated to value around US$216 million in 2006 and to rise to US$363 million by 2010 .
The breakthrough for the MBR came in 1989 with the idea of Yamamoto and co-workers to submerge the membranes in the bioreactor. Until then, MBRs were designed with the separation device located external to the reactor (sidestream MBR) and relied on high transmembrane pressure (TMP) to maintain filtration. With the membrane directly immersed into the bioreactor, submerged MBR systems are usually preferred to sidestream configuration, especially for domestic wastewater treatment. The submerged configuration relies on coarse bubble aeration to produce mixing and limit fouling. The energy demand of the submerged system can be up to 2 orders of magnitude lower than that of the sidestream systems and submerged systems operate at a lower flux, demanding more membrane area. In submerged configurations, aeration is considered as one of the major parameter on process performances both hydraulic and biological. Aeration maintains solids in suspension, scours the membrane surface and provides oxygen to the biomass, leading to a better biodegradability and cell synthesis.
The other key steps in the recent MBR development were the acceptance of modest fluxes (25% or less of those in the first generation), and the idea to use two-phase bubbly flow to control fouling. The lower operating cost obtained with the submerged configuration along with the steady decrease in the membrane cost encouraged an exponential increase in MBR plant installations from the mid 90s. Since then, further improvements in the MBR design and operation have been introduced and incorporated into larger plants. While early MBRs were operated at solid retention times (SRT) as high as 100 days with mixed liquor suspended solids up to 30 g/L, the recent trend is to apply lower solid retention times (around 10-20 days), resulting in more manageable mixed liquor suspended solids (MLSS) levels (10-15 g/L). Thanks to these new operating conditions, the oxygen transfer and the pumping cost in the MBR have tended to decrease and overall maintenance has been simplified. There is now a range of MBR systems commercially available, most of which use submerged membranes although some external modules are available; these external systems also use two-phase flow for fouling control. Typical hydraulic retention times (HRT) range between 3 and 10 hours. In terms of membrane configurations, mainly hollow fibre and flat sheet membranes are applied for MBR applications .
In recent reviews covering membrane applications to bioreactors, it has been shown that, as with other membrane separation processes, membrane fouling is the most serious problem affecting system performance. Fouling leads to a significant increase in hydraulic resistance, manifested as permeate flux decline or transmembrane pressure (TMP) increase when the process is operated under constant-TMP or constant-flux conditions respectively. Frequent membrane cleaning and replacement is therefore required, increasing significantly the operating costs.
Membrane fouling results from interaction between the membrane material and the components of the activated sludge liquor, which include biological flocs formed by a large range of living or dead microorganisms along with soluble and colloidal compounds. The suspended biomass has no fixed composition and varies both with feed water composition and MBR operating conditions employed. Thus though many investigations of membrane fouling have been published, the diverse range of operating conditions and feedwater matrices employed, the different analytical methods used and the limited information reported in most studies on the suspended biomass composition, has made it difficult to establish any generic behaviour pertaining to membrane fouling in MBRs specifically.
The air-induced cross flow obtained in submerged MBR can efficiently remove or at least reduce the fouling layer on the membrane surface. A recent review reports the latest findings on applications of aeration in submerged membrane configuration and describes the enhancement of performances offered by gas bubbling . As an optimal air flow-rate has been identified behind which further increases in aeration have no effect on fouling removal, the choice of aeration rate is a key parameter in MBR design.
Many other anti-fouling strategies can be applied to MBR applications. They comprise, for example:
Strategy to optimize filtration and enhancing the flow by using the MPE-technology. This technology reduces significant fouling. In several case studies the effect is shown. More informations are provided by Nalco Membrane Performance Enhancer Technology
In addition, different types/intensities of chemical cleaning may also be recommended:
Intensive cleaning is also carried out when further filtration cannot be sustained because of an elevated transmembrane pressure (TMP). Each of the four main MBR suppliers (Kubota, Memcor, Mitsubishi and Zenon) have their own chemical cleaning recipes, which differ mainly in terms of concentration and methods (see Table 1). Under normal conditions, the prevalent cleaning agents remain NaOCl (Sodium Hypochlorite) and citric acid. It is common for MBR suppliers to adapt specific protocols for chemical cleanings (i.e. chemical concentrations and cleaning frequencies) for individual facilities .
Like in any other reactors, the hydrodynamics (or mixing) within an MBR plays an important role in determining the pollutant removal and fouling control within an MBR. It has a substantial effect on the energy usage and size requirements of an MBR, therefore the whole life cost of an MBR.
The removal of pollutants is greatly influenced by the length of time fluid elements spend in the MBR (i.e. the residence time distribution or RTD). The residence time distribution is a description of the hydrodynamics/mixing in the system and is determined by the design of the MBR (e.g. MBR size, inlet/recycle flowrates, wall/baffle/mixer/aerator positioning, mixing energy input). An example of the effect of mixing is that a continuous stirred-tank reactor will not have as high pollutant conversion per unit volume of reactor as a plug flow reactor.
The control of fouling, as previously mentioned, is primarily undertaken using coarse bubble aeration. The distribution of bubbles around the membranes, the shear at the membrane surface for cake removal and the size of the bubble are greatly influenced by the mixing/hydrodynamics of the system. The mixing within the system can also influence the production of possible foulants. For example, vessels not completely mixed (i.e. plug flow reactors) are more susceptible to the effects of shock loads which may cause cell lysis and release of soluble microbial products.
Many factors affect the hydrodynamics of wastewater processes and hence MBRs. These range from physical properties (e.g. mixture rheology and gas/liquid/solid density etc) to the fluid boundary conditions (e.g. inlet/outlet/recycle flowrates, baffle/mixer position etc). However, many factors are peculiar to MBRs, these cover the filtration tank design (e.g membrane type, multiple outlets attributed to membranes, membrane packing density, membrane orientation etc) and it’s operation (e.g. membrane relaxation, membrane back flush etc).
The mixing modelling and design techniques applied to MBRs are very similar to those used for conventional activated sludge systems. They include the relatively quick and easy compartmental modelling technique which will only derive the RTD of a process (e.g. the MBR) or the process unit (e.g. membrane filtration vessel) and relies on broad assumptions of the mixing properties of each sub unit. Computational fluid dynamics modelling (CFD) on the other hand does not rely on broad assumptions of the mixing characteristics and attempts to predict the hydrodynamics from a fundamental level. It is applicable to all scales of fluid flow and can reveal much information about the mixing in a process, ranging from the RTD to the shear profile on a membrane surface. Visualisation of MBR CFD modelling results is shown below.
Investigations of MBR hydrodynamics have occurred at many different scales, ranging from examination of shear stress at the membrane surface to RTD analysis of the whole MBR. Cui et al (2003) investigated the movement of Taylor bubbles through tubular membranes, Prieske et al (2007) examined the entire membrane filtration vessel using CFD and velocity measurements, while Brannock et al (2007) examined the entire MBR using tracer study experiments and RTD analysis.
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