Anaerobic membrane bioreactors for antibiotic wastewater treatment: Performance and membrane fouling issues.

Antibiotic wastewater has become a major concern due to the toxicity and recalcitrance of antibiotics. Anaerobic membrane bioreactors (AnMBRs) are considered alternative technology for treating antibiotic wastewater because of their advantages over the conventional anaerobic processes and aerobic MBRs. However, membrane fouling remains the most challenging issue in the AnMBRs' operation and this limits their application. This review critically discusses: (i) antibiotics removal and antibiotic resistance genes (ARGs) in different types of AnMBRs and the impact of antibiotics on membrane fouling and (ii) the integrated AnMBRs systems for fouling control and removal of antibiotics. The presence of antibiotics in AnMBRs could aggravate membrane fouling by influencing fouling-related factors (i.e., sludge particle size, extracellular polymeric substances (EPS), soluble microbial products (SMP), and fouling-related microbial communities). Conclusively, integrated AnMBR systems can be a practical technology for antibiotic wastewater treatment.


Introduction
Antibiotics are widely used to treat or prevent human and animal diseases, and promote livestock growth. Such behaviour results in high levels of antibiotic residues in municipal wastewater, livestock wastewater and other industrial effluents (Li, 2014, Sabri et al., 2018). It is widely known that the occurrence of antibiotics in the environment could cause serious risks to environmental security and public health due to the emergence and transfer of antibiotic resistance genes (ARGs) and bacteria (ARB)  Table 1, the integrated AnMBR processes performed better than AnMBR alone for removing antibiotics from wastewater. Specifically, the total removal of micropollutants in the combined AnMBR with nanofiltration membrane (AnMBR-NF) system was better than their removal in the individual AnMBR system, with the removal of SMX being above 98%. In this system, NF played an important role for the removal of micropollutants from wastewater with the average being 87% for all micropollutants (Wei et al., 2016). For example, the removal efficiency of SMX and trimethoprim in the AnMBR with PAC was more than 99% in comparison with 67.8 ± 13.9% and 94.2 ± 5.5% in the AnMBR without PAC under the same operating conditions. The enhancement of their removal in Previous studies have concluded that flocs with smaller sized pores contribute more to fouling than larger ones (Lin et al., 2011a, Lin et al., 2009). The formation rate of cake layer correlated with the fraction of smaller sized particles (Lin et al., 2010). One possible reason is that small flocs had a strong tendency to deposit on the membrane surface due to their low back transport force and the compaction of the cake layer.
Another reason is that the smaller flocs have a higher density than the larger flocs with It is reported that the instabilities such as exposure to toxic conditions, sudden organic load, temperature and pH changes, may cause floc breakage and result in the decrease of particle size in the AnMBRs (Shen et al., 2015). In response to cytostatic drugs presence in an anaerobic osmotic membrane bioreactor, the mean floc size of sludge decreased from 92 to 80 μm leading to higher layer formation rate and membrane This phenomenon might also decrease the particle size and contribute to the problem of membrane fouling in AnMBRs.
The production of EPS by microorganisms is their natural response to the toxic environment, as this plays an important role in protecting microorganisms to cope with the stress, that is, in the presence of heavy metals and or antibiotics (Avella et al., 2010, Sheng et al., 2010). The EPS are believed to be major contributors to membrane fouling, since they possess complex properties including surface charge, hydrophobicity/hydrophilicity, and adhesive characteristics, etc., which play roles in flocculation, stability and adhesion behaviors of sludge flocs (Lin et al., 2014). Thus, an increase in EPS will trigger a decline in sludge filterability, and the decrease in flux accompanied by an increase in specific cake resistance in MBRs (Wang et al., 2009). ). The reason is that SMP not only increase the sludge viscosity and leads to pore blockage, but also serve as the binding sites for cake layer formation, and thus facilitate cake formation on the membrane surface (Lin et al., 2010). Like EPS, the production of indicated that the protein was a negatively charged and sticky substance, and so it would reduce the surface potential of sludge particles and increase the viscosity of the sludge.
These sludge particles quickly gathered on the membrane surface and promoted the formation of the gel layer, ultimately leading to serious membrane fouling. Therefore, the aggravation of membrane fouling with the presence of antibiotics in the AnMBRs may result from the positive correlation between the antibiotics and protein concentration (Fang et al., 2002). As reported by Zheng et al. (2016), microorganisms, which were exposed to higher levels of antibiotics, would secrete more protein, probably due to the protein secretion metabolism of microorganisms being more sensitive to antibiotics than that of the polysaccharide secretion metabolism. Xu et al. (2013) stated that although the EPS production was not significantly influenced by sulfamethazine at 500 μg/L in an aerobic activated sludge system, the secondary structure of proteins in EPS altered.
Therefore, the presence of antibiotics in the AnMBRs can increase membrane fouling through their effects on anaerobic sludge properties.

Influences of antibiotics on fouling-related microbial communities
Microbial communities have been regarded as the ultimate factor responsible for the

Integrated AnMBRs for fouling mitigation and antibiotics removal
Membrane fouling is a major issue, in that it can seriously affect the membrane's

Integration of AnMBRs with biofilm carriers
The introduction of biofilm carriers (e.g., GAC, PAC, and Sponge) into the membrane bioreactor has been considered an effective method for controlling membrane

Economic evaluation of AnMBR technologies
Membrane fouling continues to be an important barrier for the application of the AnMBR system due to the costs of fouling control. Based on above discussion, the system. Therefore, the integrated AnMBR system is a promising technology for the treatment of antibiotic wastewater.

Future perspectives
As discussed in this paper, the selected antibiotics can be removed in large quantities from wastewater by AnMBRs, especially the integrated AnMBR systems. In summary, more studies on the behavior of antibiotics in AnMBRs systems are necessary for their elimination and influences on membrane fouling. The hybrid AnMBR systems have been considered as promising alternatives for removal of toxins and controlling membrane fouling with low energy costs, but further tests are required.
Essentially, both technically and economically feasible AnMBRs processes should be developed for treating antibiotic wastewaters.

Conclusion
The AnMBRs are effective technologies for removing antibiotics and ARGs from wastewater. Yet, antibiotics would aggravate membrane fouling by influencing the floc size, the production of EPS and SMP, and the microbial communities in the AnMBRs.
Integrating AnMBRs with carriers demonstrate a higher removal efficiency of antibiotics and a slower membrane fouling rate, but their long-term effects on membrane properties and microbial activities need further investigation. The integrated BES-AnMBRs not only can control fouling, degrade antibiotics and eliminate their antibacterial activity, but also enhance the energy recovery from wastewater. It is therefore a promising technology for antibiotic wastewater treatment.