Literature Review on Effect of Sludge Age on Microbial Community Structure in HMBR

The composite membrane biological treatment process has the characteristics of high quality effluent and low membrane pollution, which is widely used in domestic and international water treatment industry. Based on the review of the basic research on membrane fouling mechanism and the comparison of the research results of MBR at home and abroad, from the perspective of microbial ecology, based on the new generation of environmental microbial analysis technology of high-throughput sequencing, this paper expounds the microbial ecological mechanism that sludge age changes the content of microbial metabolites and membrane fouling process by affecting the microbial community structure and physiological characteristics.


Introduction
Faced with increasingly serious water environmental problems, Membrane bioreactor (MBR), as an emerging sewage treatment reactor, has the advantages of high volumetric load, low sludge yield, a small amount of land, and stable and safe effluent[1],which has received great attention and application (Figure 1.). With the technological progress and market development, the membrane price is gradually reduced, and membrane pollution has gradually become the most prominent problem. The formation mechanism of membrane pollution, the main influencing factors and how to effectively reduce membrane pollution are studied, in order to make due contribution to the promotion and application of MBR.

Definitions of MBR
Membrane bioreactor (MBR) is a sewage treatment system that utilizes hollow microfiltration membrane to replace secondary sedimentation tank to continuously discharge water under negative pressure [2].According to the structural mode, there are two main types of applications in the market at present, one is submerged as shown in Figure 2, and the other is circular as shown in Figure 3.
MBR can be divided into anaerobic MBR and aerobic MBR. Anaerobic MBR is mainly used to treat high concentration wastewater, make full use of the anaerobic microbial acid production fermentation process to reduce the refractory substances in the system, and improve the content of biochemical substances in wastewater, so as to treat wastewater. However, the chemical Oxygen Demand (COD) and biochemical oxygen demand(BOD)in the effluent are still very high, which cannot meet the discharge standards. Aerobic MBR can, on the basis of aeration, utilize the biochemical substances in the aerobic microbial treatment system to thoroughly oxidize the reducing substances in the system and make the concentration of pollutants in effluent reach a very low level. however, the concentration of pollutants in influent for general treatment cannot be too high and the biological load is low. A/O-MBR combine that above two reactor forms together, a pre-anoxic reaction tank, a subsequent aerobic aeration tank and a reflux device communicate with each other. The reactor not only has the characteristics of high pollutant load of an anoxic reactor, capable of treating refractory substances and improving the biodegradability of wastewater, but also has the characteristics of complete oxidation from an aerobic reactor and low concentration of effluent pollutants[3-7].

Application of MBR
With the increasing aggravation of the situation of water resources shortage, the continuous increase of effluent quality indexes of sewage treatment plants, and the renewal and development of meMBRane production technology, mbr is widely applied in practice in the field of water treatment, and its number is constantly increasing, and the scale is gradually expanding. The development of membrane industry drives and stimulates the development of MBR and will play a strategic role in the MBR industry [8].According to the estimation of membrane trading market, the sales volume in the world membrane market in 2012 is expected to reach US$ 15 billion [9].

Foreign applications. Brightwater Wastewater
Treatment Plant located in Washington, USA is currently the largest MBR treatment system in the world. It was completed and put into operation in 2010. Its designed average flow rate is 117,000 t/d, and the peak flow rate reaches 144,000 t/d. This project breaks the traditional concept that MBR is only applicable to small and medium-scale water treatment and also illustrates that both the technology and economy of MBR can reach the level of large-scale application. The technology of MBR has become increasingly mature [10],but it is still far from the traditional sewage treatment system with the treatment capacity of 5 million tons per day. At present, in developed countries in Europe and America, the MBR process is widely used for the treatment of domestic wastewater, industrial wastewater (pharmaceutical wastewater, petrochemical wastewater, port wastewater, printing and dyeing wastewater), and high-concentration organic wastewater [11].At present, the MBR technology is in a trend of comprehensive development, and the business scope has been expanded to such advanced treatment fields as nitrogen and phosphorus removal, seawater desalination, reclaimed water reuse, and drinking water purification [12].

Domestic applications.
The research and application of MBR in china are relatively backward. The earliest reports on the MBR were in the early 1990s. However, due to the concern and attention of the country in recent years, although it started late, it has a good prospect for related research and application and has developed rapidly. Into the 21st century, major universities, such as Tsinghua University, Harbin Institute of Technology, Tongji University, Nanjing University and other relevant laboratories to conduct research on the MBR, the content and depth of research has also made great progress. A large number of papers related to MBR are published in various journals. In 2012, Hou Jinchai et al. [13]conducted a study on the operation of MBR with sludge bulking. The results showed that under the sludge bulking condition, there was a critical membrane flux related to membrane pollution, and the critical value was related to sludge bulking. In 2013, Yang Wei et al. [14]studied the characteristics of MBR during the startup phase of drinking water treatment process, and the results showed that the removal of ammonia nitrogen was relatively difficult in the first 20 days, and the operation of MBR was stable after that, with the removal efficiency of ammonia nitrogen maintaining a relatively high stable state (about 90%). In 2010, Chen Qiang et al. [15]conducted a comparative experiment on the treatment of printing and dyeing wastewater with two different MBRs. The results showed that the seaweedtype MBR was significantly superior to the traditional curtain-type MBR in terms of various indicators, and had the advantages of convenient cleaning, low membrane pollution rate, and the like. It has high efficiency in treating printing and dyeing wastewater, and the removal rates of COD, BOD, chroma, ammonia nitrogen and total nitrogen are as high as 90%, 94%, 91.4%, 87.8% and 86.4%, respectively, which are higher than those of the traditional roller shutter type MBR. The results showed that an appropriate increase in temperature was conducive to the biological removal of CODcr by MBR, but it would affect the biological removal of BODs by MBR, but the temperature had no significant effect on the overall removal rate of CODcr-BODs system by MBR.

Definition of membrane fouling
Membrane fouling refers to the fact that during the operation of membrane bioreactor, sludge flocs, macromolecular organic matters and colloidal particles existing in the sludge mixed liquor in which the membrane is located are deposited and adsorbed on the surface of the membrane through physical and chemical action, resulting in the pore size of the membrane becoming smaller or even blocking. The main hazards are as follows: it causes the decrease of membrane flux and makes the MBR unable to operate normally for effluent. The membrane cleaning process needs a large amount of energy-consuming equipment investment, which greatly increases the operation cost of the mbr. In the backwashing process, secondary pollution is easily caused, and the service life of the membrane is reduced, thereby further increasing the cost investment of the mbr. Therefore, it can be seen that solving the membrane pollution problem is the key to reducing the operation cost of MBR [9].

Control studies for HRT and SRT.
HRT and SRT play a significant role in optimizing reactor operation. A shorter HRT means a smaller reactor size and lower operation cost. Under the longer SRT condition, the sludge yield is less [16].With the deepening of research, these two parameters also changed greatly. With the continuous development of anaerobic reactors, the separation of HRT and SRT makes the HRT gradually reduce from dozens of hours to more than ten hours or even a few hours, greatly increasing the treatment efficiency of anaerobic reactor.The submerged anaerobic membrane bioreactor was operated at the HRT of 4h and the temperature of 13C, and the COD removal rate reached the ninety-seven percent, and methane gas was collected in the gas phase [17].The results in the SRT study indicate that the SRT ranged from a minimum of 25 days to a maximum of 335d days. Obviously, the length of SRT can be controlled by regular sludge discharge. The longer the SRT is, the less excess sludge needs to be treated. However, if the SRT is too long, a large number of dead cells will remain in the reaction system, and the refractory substances released by the dead cells will become SMP, which in turn will contaminate the membrane modules and increase COD in the effluent water.

Microbial community control.
Although the SRT with infinite length greatly affected the microbial community structure in the reactor, the research in this field was scarce. OC Kolawole [18]applied a 10L anaerobic membrane bioreactor with artificial water distribution at 30°C to study the relationship between the microbial community structure and membrane pollution, and T-RFLP and clone library technologies were used to compare the differences in membrane pollution between PVDF and PEI. The results showed that the abundance of OP11 expressed in the gel layer polluted by membrane was very high, which was the main cause of membrane pollution. The abundance of Bacteroides and Sclerotinia was very low, while the abundance of Bacteroides and Sclerotinia in the reactor was very high. These results indicated that some microorganisms with very low index in the reactor, such as OP1I, had a direct effect on membrane fouling, while the species with very high abundance in the reactor had little contribution to membrane fouling. However, it was not certain that the microbial community structure and metabolites would affect the membrane pollution.

Conclusion
The membrane fouling mechanism of membrane bioreactor is generally studied from the structure and properties of the membrane, the operation conditions of the reactor and the microbial properties of the treatment liquid. In the future, the study on the formation process and influencing factors of various pollutants such as extracellular polymer(EPS) and soluble microbial product(SMP) will become an important topic in the study of membrane pollution.
At present, the understanding of membrane fouling mechanism is still relatively shallow, and the areas that need more attention are the biotoxicity of pollutants and the dynamic changes of bio-cakes on mbr. The formation and composition of membrane pollutants and the interaction between soluble microbial products and extracellular polymers need further study [20]. With the invention of cost-effective membrane materials and the gradual solution of membrane pollution in membrane bioreactor, membrane bioreactor will be more widely used.
At present, some researches have used denaturing gradient gel electrophoresis (DGGE) and temperature gradient gel electrophoresis (TGGE), microbial (quinone), molecular hybridization and other molecular biological methods to study the influence of microbial community structure on membrane fouling. Electrically enhanced membrane bioreactor (EMBR), as a new membrane pollution control technology, has attracted more and more attention in recent years. It uses the driving force of electric field to make pollutants flocculate or stay away from the membrane surface, so as to achieve the purpose of removing pollutants [19]. The new technology can make it easier for researchers to study the membrane fouling process microscopically.