Ventilation plays a dual role in aerobic MBRs. It provides oxygen for biological processes and serves to dissolve the layer of cake formed on the surface of the membrane (air washing). The oxygen provided by aeration facilitates the biodegradability and cellular synthesis of biomass [12]. Research has shown that increasing the rate of ventilation in an MBR leads to a reduction in membrane contamination [59,60]. In a study examining the influence of ventilation speed on membrane fouling in a pilot-scale submerged MBR, Yigit et al. [61] reported that increasing ventilation speed had a positive effect on pollution control. However, the degree of such a positive effect has been significantly reduced with the increase in MLSS. This is due to increased viscosity resulting from increased MLSS levels. Air irritation rates in the literature range from 3 l air/min m2 to 12 l air/min m2 [17]. Biofoulants refer to bacteria or flakes whose deposition, growth and metabolism on the membrane lead to fouling [2].
First of all, a bacterial cell can attach to the surface of the membrane or in its pores, and after a while, the cell multiplies into a group of cells, which leads to the formation of biocakes and therefore reduced permeability. Bacteria (biofoulants) and their metabolic products contribute to fouling [28]. Essentially, biological fouling of the membrane is a two-step process that begins with early bacterial fixation followed by the proliferation of bacteria on the surface of the membrane [29]. Some publications expand the definition of biofoulants to include the metabolic products of these clusters of bacterial cells [28]. In this work, however, organic substances produced by microorganisms are considered to be foul-smelling organic substances (section 2.1.2.) in order to study their mitigation strategies. Membrane biological fouling is one of the most important operational problems of membrane systems [30]. Operationally, membrane fouling reduces permeate flow when the MBR is operating at constant transmembrane pressure (TMP) and results in an increase in TMP when the MBR is operating at a constant permeate flow. With constant flow operation, a sharp increase in TMP indicates high contamination of the membrane. This sudden increase in TMP is called a “TMP jump.” TMP skipping has been described as a three-step process [1,23]: Step 1 – an initial “conditioning” fouling caused by initial blockage and adsorption of the pores; Stage 2 – linear or low exponential gradual increase in PMO due to biofilm formation and additional blockage of membrane pores; and Level 3 – a sudden and rapid increase in the rate of increase in TMP (dTMP/dt) [3]. Stage 3 is thought to be the result of severe membrane pollution and is due to successive pore closures and changes in local flow resulting from pollution, resulting in local fluxes exceeding the critical value, resulting in an acceleration of particle deposition [24,25] and sudden changes in the structure of the cake layer [23].
Bacteria in internal biofilms tend to die due to oxygen restrictions, releasing more EPS [26]. As soon as step 3 occurs, a cleaning of the membrane is necessary. The practical consequence of this is that a delay in step 3 allows a reduction in the frequency of cleaning the membrane, which will ultimately lead to savings on the operating costs of the MBR. A key objective of pollution control is therefore to delay the TMP jump by changing the properties of the sludge (MLSS, flake size, EPS content and apparent viscosity) or by reducing the operating flow rate [2]. Membrane bioreactors (MBRs) are generally understood as the combination of membrane filtration and biological treatment with activated sludge. The development of a biofilm MBR combining a moving bed biofilm reactor with an underwater membrane biomass separation reactor was studied. Treatment efficiency in the consistent production of high-quality wastewater has been shown to be high, regardless of the load rates on the operating modes of the bioreactor or membrane reactor. The performance of the membrane (fouling) is a function of the wastewater quality of the biofilm reactor and varies according to the load rates (HRT).
It was found that long-lasting functioning throughout the treatment process correlated with the fate of the particle size fraction below the micron. MBBR is the short form of the moving bed bioreactor, which uses freely floating plastic filling media for the growth of connected biofilms. To keep plastic filling media in tension, their density is close to the density of water. Continuous aeration or mixing ensures good contact between the organic matter and the attached biofilm for effective boD removal. This article reviews the basics of membrane fouling and advances in fouling mitigation strategies in RMBs. Membrane fouling in RMBs can be divided into biofoulants, organic fouls, and inorganic fouls due to their biological and chemical properties. Of these, biofuses and organic fouling contribute the most to membrane fouling in RMBs. Most research on membrane fouling targets this fouling. Several factors affect membrane fouling in RMBs. These factors include: membrane properties (material type, water affinity, surface roughness, surface load and pore size), operating conditions (operating mode, aeration rate, SRT, HRT, F/M ratio, OLR, COD/N ratio and temperature), and feed and biomass properties (MLSS, apparent viscosity of sludge, EPS, flake size, alkalinity, pH and salinity). PES, in particular, contributes significantly to membrane contamination.
The MBR process was introduced in the late 1960s, when commercial ultrafiltration (UF) and microfiltration (MF) membranes became available. The original process was introduced by Dorr-Oliver Inc. and combined the use of an activated sludge bioreactor with a cross-flow membrane filtration loop. The flat membranes used in this process were polymeric and had pore sizes ranging from 0.003 to 0.01 μm. Although the idea of replacing the decanter of the conventional activated sludge process is attractive, it was difficult to justify the use of such a process due to the high cost of membranes, the low economic value of the product (tertiary wastewater) and the potential rapid loss of performance due to membrane contamination. As a result, the focus was on achieving high flows, and therefore it was necessary to pump the MLSS with a high cross-flow speed with a significant energy load (of the order of 10 kWh / m3 of product) to reduce pollution. Due to the poor profitability of first-generation RMBs, they have only found applications in niche areas with special needs, such as isolated trailer parks. B or ski resorts. However, the biggest technical problem with AGMBR is the long-term instability of the operation of the aerobic granulation system and the problems with granule decay [151,158]. Aerobic granules have been observed to disintegrate after prolonged surgery [149,159,160,161]. The deterioration of granular stability over time affects the efficiency of wastewater treatment and is a significant problem that affects the efficiency of aerobic granulation in full operation.
Applied to AGMBR, granular decay increases the concentration of soluble EPS, thereby increasing the tendency to membrane contamination [154]. Therefore, the production of pellets with long-term sustainable structural integrity is a key area that requires further research. The water affinity property (hydrophilicity or hydrophobia) of the membrane material affects fouling in RMBs. The water affinity behavior of a membrane material is determined by measuring the contact angle of a water droplet on its surface [47]. Smaller angles indicate hydrophilicity, while larger angles indicate hydrophobia. Due to the hydrophobic interactions between membrane material, microbial cell and dissolved substances, membrane fouling in hydrophobic membranes is more severe than in hydrophilic membranes [1]. The more hydrophilic a membrane material is, the more macrofreeze substances are adsorption in wastewater, such as proteins. B, is weak.
Hydrophobic materials, on the other hand, tend to adsorb hydrophobic substances into wastewater, resulting in pollution. To find a balance, composite membranes are produced by covering the hydrophobic membranes with a thin layer of hydrophilic material to combine the robustness of the former and the low tendency to contamination of the latter [47]. The pore size of membranes relative to the size of particles in the wastewater supply stream in RMBs can affect membrane fouling. The mechanism of pore blocking tends to increase with increasing pore size of the membrane [47]. This is because it is easier for fine particles (smaller than the size of the membrane pores) to penetrate and be trapped in the membrane pores, resulting in a blockage of the pores [53]. With smaller pores, large particles quickly form a top layer on the membrane and collect the smaller particles. The resulting layer that forms on the surface of the membrane can be easily removed by air friction or turbulence resulting from cross-flow filtration. This is illustrated schematically in Figure 3 below.
In general, the effects of membrane pore size on pollution depend heavily on the composition of the feed stream, especially the particle size distribution. When treating MBR wastewater, solid-liquid separation is achieved by microfiltration (MF) or ultrafiltration (UF). .
