Effect of ultraviolet disinfection on the fouling of reverse osmosis membranes for municipal wastewater reclamation
Graphical abstract
Introduction
Reclaimed water is becoming one of the most important water resources for the production of high-quality pure water for industrial use, such as low pressure boiler make-up water, recirculating cooling water, industrial water for semiconductor factories and so on (Navarro, 2018; Zhu et al., 2017). Reverse osmosis (RO) process is an inevitable unit in the production of pure water. This process is relatively stable and mature for seawater desalination and surface water or groundwater purification. However, the concentration of organic pollutants in reclaimed water is much higher than that in seawater and conventional water sources. Therefore, the RO system for wastewater reclamation is confronted with more serious membrane fouling compared with seawater desalination and conventional water source purification.
Membrane fouling could increase the operational energy and chemical consumption, and deteriorate the permeate quality of the RO system (Jiang et al., 2017; Madaeni and Samieirad, 2010; Wen et al., 2018). Among the complex and various kinds of RO membrane fouling mechanism, biofouling is the most complicated, changeable and difficult to control (Matin et al., 2011; Nagaraj et al., 2018). RO membrane biofouling was mainly induced by microorganisms and the surrounding extracellular polymeric substances (EPS) (Yu et al., 2017, 2018). EPS could increase the hydraulic resistance and thus cause the decline of the permeate flux (Herzberg et al., 2009).
To prevent and alleviate biofouling, disinfection was used as the pretreatment prior to the RO unit. It is considered that the reduction of microbial amount in the feed water meant the decrease of biofouling potential. In the industrial-scale RO systems, free chlorine was the most commonly-used reagent for disinfection pretreatment (Bai et al., 2020; Kim et al., 2009; Wang et al., 2019b). However, disinfection using free chlorine was reported to potentially aggravate RO membrane fouling as it could promote EPS production via changing the microbial community structure (Khan et al., 2015; Wang et al., 2019b, 2019c).
Chlorine, as a typical oxidizing disinfectant, could select microorganisms with higher EPS production from microbial community, and thus promoted the EPS secretion of the residual bacteria. Therefore, in order to prevent biofouling, it is necessary to investigate the effect of non-oxidizing disinfection process on RO membrane biofouling. Ultraviolet (UV) disinfection is also a commonly-used process to inactivate microorganisms in wastewater by damaging the DNA of microorganisms (Wolfe, 1990). Because of the non-oxidizing disinfection mechanism, UV might not increase the EPS production of the residual bacteria after disinfection, and thus show positive effect on the control of biofouling. However, the effect of UV disinfection on the biofouling of RO membranes has rarely been studied.
Therefore, the objective of this study was to investigate the effects of UV disinfection on the fouling of RO membranes for municipal wastewater reclamation and reveal the mechanism. The changes of permeate flux with and without UV disinfection as pretreatment during ~20-day operation were recorded and compared. After the operation, the fouled membranes and the foulants were analyzed and compared systematically, in terms of morphology, microbial and organic matter amount, organic matter composition and microbial community structure. Furthermore, the amount and properties of the EPS secreted by the residual bacteria after UV disinfection were also investigated.
Section snippets
Feed water quality and ultraviolet disinfection
Membrane bioreactor (MBR) effluent was taken every two d from a municipal wastewater treatment plant (WWTP) in Beijing, China and kept at 4 °C. The water quality parameters of the MBR effluent are shown in Table 1. The MBR effluent was used as feed water because MBR effluent had good water quantity with much less suspended solids, compared with traditional secondary effluent. Meanwhile, the applicant of MBR effluent as feed water could pose remarkable economic viability. Moreover, the MBR
Change of the RO membrane flux with different dosages of UV as disinfection pretreatment
The feed water of the RO system was disinfected with different dosages of UV and the disinfection performance was shown in Fig. 2a. The initial bacterial concentration in the feed water was 2.43 ± 1.24 × 105 CFU/mL, and the residual bacteria concentration was 1.86 ± 0.13 × 104, 6.9 ± 1.2 × 102 and 6 ± 1 × 101 CFU/mL after disinfection with the UV dosage of 20, 40 and 80 mJ/cm2, respectively. UV disinfection significantly inactivated the bacteria in the feed water with a log reduction of 1.11,
Conclusions
This study showed that disinfection pretreatment with high-dosage UV could aggravate membrane fouling, even though UV disinfection significantly inactivated the bacteria in the feed water of the RO system. At the end of operation, the HPC and ATP content in the foulants on RO membranes showed no significant difference in control group and UV-disinfection groups. However, the TOC in membrane foulants was significantly enhanced after UV disinfection, especial with the UV dosage of 80 mJ/cm2. This
Notes
The authors declare no competing financial interest.
Declaration of Competing Interest
There is no financial and personal relationship with other people or organizations that could inappropriately influence the current work.
Acknowledgement
This study was supported by the National Natural Science Foundation of China (No. 52000114) and Key Program of the National Natural Science Foundation of China (No. 51738005).
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