Various regions of the world are facing unprecedented burdens in terms of water availability due to rapid population growth and urbanization. Approximately 700 million people are under water stress conditions. To deal with water scarcity issues, water reuse practices are required to be adopted. For advanced wastewater treatment and reuse, the membrane technology, especially membrane bioreactor (MBR), is the most evolving technique over the past few decades. Operations of MBR are similar to conventional wastewater treatment process but separation by low pressure driven membrane differentiates it from the activated sludge treatment.
The membrane bioreactor is now well-established treatment technology for both domestic and industrial wastewater because of high effluent quality, extended sludge retention time (SRT), low sludge production, high mixed liquor suspended solid (MLSS) concentration, high nutrient removal, and low area constraints. However, fouling of membrane is the major limitation in the widespread application of MBR. Sludge mixed liquor is the main cause of membrane fouling as the foulants instigate from MLSS. Therefore, several factors can affect membrane fouling propensity.
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The mixed liquor can disturb the filterability of membrane by influencing the viscosity, extracellular polymeric substances (EPS) and cell debris. Other factors affecting membrane fouling are operational conditions (such as aeration intensity, HRT, SRT, flux and frequency of backwashing) and physicochemical characteristics of membranes. Fouling, which leads to membrane cleaning and finally replacement of membranes, results in high trans-membrane pressure (TMP) and low flux and disrupts the MBR operations frequently. Membrane fouling takes place due to the formation of cake layer and blockage of pores. Pore blockage is dominant at initial stages of the operation, when the membrane is new but the cake layer formation takes over in the later stages of operation.
Despite of the high quality output of MBR, further improvements are still required to develop operational strategies to achieve best possible performance. Many researchers have tried to correlate membrane fouling with various membrane materials and operational conditions. Dvořák et al. (2011) examined the influence of different operating conditions on the performance of pilot scale submerged MBR and found that EPS formation results in rapid membrane fouling. Fu et al. (2012) reported that aeration rate and time has a positive influence over membrane fouling and effluent quality. Similarly, Lim et al. (2007) evaluated the impacts of operational parameters on aeration on/off time in MBR and observed that MLSS concentration and EPS components are influenced by aeration time. EPS components increase with increase in relaxation time of aeration. In a recent study by Campo et al. (2017), effect of EPS on membrane fouling and foaming was thoroughly investigated in MBR at three different aerated regimes, expressed as taeration/tcycle, i.e., (i) 60 min/180 min; (ii) 80 min/180 min and (iii) 30 min/90 min. They observed rapid membrane fouling and relatively higher EPS production in the first two aerated regimes having less number of cycles per day. Since EPS plays an important role in membrane fouling, a number of strategies to control EPS generation have been reviewed extensively by Lin et al. (2014). Maqbool et al. (2014) investigated the performance of submerged MBR for the treatment of synthetic wastewater under different filtration modes and reported that the short/frequent filtration time reduced the membrane fouling. Jiang et al. (2005) examined different scenarios of backwashing and reported that less frequent but longer backwashing provide better fouling control. Most of the research has been carried out on synthetic wastewater, which is the main hurdle in the applicability of these studies as the synthetic wastewater composed of readily degradable soluble organics i.e., glucose is mainly used to serve as soluble COD. The composition of real wastewater is much more diverse due to the presence of readily degradable organics, slowly degradable organics and non-degradable organics both in soluble and insoluble form. Furthermore, the colloidal particles, both organic and inorganic, in real wastewater are responsible for pore blockage at the initial stage of the MBR operations. This phenomenon of pore blockage due to colloidal particles is absent in the MBR operations with synthetic wastewater. Therefore, effectiveness of different operating conditions, especially relaxation time, is yet to be optimized for real wastewater. Relaxation, also known as temporary cessation or pausing, allow the foulants to move away from the membrane surface to the mixed liquor via concentration gradient. Intermittent relaxation allows for stable operation even above critical flux and high flux can be maintained for longer period of time. Relaxation is preferred over backwashing as backwashing with permeate causes pore blockage in membrane. A wealth of studies are available on fouling control by relaxation and backwashing. A few recent research articles are outlined below: