AnMBRs are somehow more prone to inorganic fouling by the precipitates of calcium, phosphorus and sulphur than their aerobic counterparts. This is because of the high concentration of these elements in the industrial wastewater of interest for AnMBR, the applied high loading rates, and the chemistry of carbon dioxide equilibrium (Stuckey, 2010). Struvite was reported as one of the most important precipitates affecting the filtration performance of inorganic membranes (Choo and Lee, 1998; Choo et al., 2000; Kang et al., 2002). Inorganic fouling should not be underestimated when treating complex wastewaters such as industrial effluents with high concentrations of nitrogen and phosphorus. Inorganic species can interact with soluble microbial products in the reactor and enhance the mechanical stability of the fouling layer (Lin et al., 2009). Furthermore, the pH increase in permeate pipes due to release of carbon dioxide under atmospheric pressure can also cause scaling problems in the permeate lines.
Since particles with the smallest size determine filterability in membrane processes, continuous particle size reduction due to applied shear rate decreases the attainable flux in AnMBRs in long term operation. Torres et al. (2011) suggested that modification of sludge properties should be considered to obtain high fluxes in AnMBRs. For that purpose, additives such as PAC, coagulants/flocculants can be used. Choo et al. (2000) assessed that PAC addition increases the mean particle size resulting in a lower specific cake resistance. Moreover, PAC could also reduce fouling by sorbing and/or coagulating colloidal matter in the reactor. Park et al. (1999) obtained an increased flux at high cross-flow velocities by addition of PAC and attributed this result both to the scouring effect of PAC and removal of colloidal matter by adsorption. Further research is needed to determine the effects of additives on improvement of filterability characteristics for AnMBRs.
The complete retention of slow growing methanogenic biomass in the reactor by membrane filtration may enable a faster start up of AnMBRs compared to conventional high rate anaerobic reactors. However, AnMBRs still need an appropriate start-up strategy in order to achieve a good filtration performance for long term. Moreover, selection of a suitable inoculum free of fibrous material and inert solids may be required to obtain good filtration performance. The fibrous material in the inoculum
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may also clog the pumps and pipes before the membrane module entrance and inert material like silt can be abrasive for the membranes during long term operation. Trzcinski and Stuckey (2009) reported that in order to achieve a successful start-up period for AnMBRs, a low initial OLR, low shear rate, and long acclimation periods were needed, which are anyway shorter compared to other anaerobic reactor types. The sludge development in agreement with the influent composition would automatically occur in due time. The substrate composition determines the microbial diversity and it can play a role on both biological and long term filtration performance of AnMBRs.
Full scale implementation of AnMBRs will be largely dependent on the flux levels that can be achieved, on a long term basis. Membrane filtration was originally combined with anaerobic treatment by using side-stream configurations, where high levels of shear rate are provided at the expense of high energy requirements. Main disadvantages of high cross-flow side-stream configuration are the high energy demands, combined with the concern about a potential negative effect of a high shear rate over biomass activity (Liao et al., 2006). The down side of submerged full-scale configurations is, similar to aerobic MBRs, higher costs for maintenance procedures. Nonetheless, although lab-scale applications of AnMBR technology are generally carried out in side-stream configuration, interestingly almost all of the full scale installations are operated in submerged configuration. Apparently, full-scale experiences with submerged AnMBRs are thus far sufficiently satisfactory. However, according to Martin et al. (2011) energy efficiency of submerged AnMBRs are not much different than side-stream cross-flow AnMBRs as it is not the case for aerobic MBRs. This is attributed to the required high biogas flows for scouring the membrane in submerged AnMBRs. Apparently, biogas sparging for fouling control is an important cost factor and should be optimized to improve the feasibility of submerged AnMBRs.
Even though membrane separation represents a highly effective way for biomass retention, it inevitably involves higher operational and investment costs, compared to granular sludge or biofilm based technologies. However, a recent feasibility study revealed that AnMBRs are more suitable for wastewaters with COD concentrations exceeding 4-5 g·L-1 (Martin et al., 2011). Then, MBR feasibility under anaerobic conditions will be determined by the balance between the techno-economic benefits
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that the membrane enhanced retention can provide and the increase in treatment costs that comes from the application of membrane filtration processes. Capital and operational costs of MBR systems are directly related with the applied surface membrane area, which is proportional to operational flux. It is clear then that a high operational flux is a required condition for the economic feasibility of AnMBR full scale application for moderate to low strength wastewaters. However, for highly concentrated industrial wastewaters the operational flux and thus membrane costs may not be a limiting factor due to the low volumes of wastewater to be treated by the system. Therefore, the share of membrane costs in the whole investment may not be considerably high and the system may still be feasible considering the superior effluent quality and the reuse options.