Operation of passive membrane systems for drinking water treatment
Introduction
Submerged hollow fibre ultrafiltration (UF) is an established technology viable for community-scale water treatment. It is particularly appealing for drinking water treatment because it can achieve up to 4-log removal of colloids, pathogenic bacteria, and viruses in a single process stage. However, the adoption of this technology in small communities or rural areas is currently hindered by the complexity and cost of UF systems (Peter-Varbanets et al., 2010). Most of the complexity is associated with the auxiliary processes used for fouling mitigation, notably backwashing, air sparging and chemical cleaning (Pearce, 2012). The reversal of the permeate flow during backwash promotes the expansion and back transport of accumulated foulants at the membrane surface (Ye et al., 2011) and dislodges material from the membrane pores (Akhondi et al., 2014). The scouring induced onto the membranes by air sparging minimizes the accumulation of foulants at the membrane surface during permeation (Cui et al., 2003) and enhances the back transport of foulants during backwash (Serra et al., 1999). Chemical cleaning using sodium hypochlorite and/or citric acid is used to oxidize and/or dissolve residual foulants that cannot be removed hydraulically by backwashing or air sparging (Porcelli and Judd, 2010). The membrane itself is a small portion of the entire system and accounts for only 10–40% of the capital cost of membrane systems (Pearce, 2012).
Passive filtration has been defined as a self-sustained physical transport process of biochemical and other atomic substances across the wall (i.e. membrane) of a living cell (Pont and de Bonting, 1981). UF systems are not normally self-sustained as steady operation is dependent on auxiliary processes such as backwashing, gas sparging and chemical cleaning that act as fouling control measures. However, recent studies have demonstrated the potential of membrane systems to function passively (i.e. without any fouling control measures) over extended periods when operated at an ultra-low permeate flux (Derlon et al., 2014, Kohler et al., 2014, Peter-Varbanets et al., 2010, Peter-Varbanets et al., 2011). The ability to sustain a consistent permeate flux was attributed to the microbial community that establishes itself on the membrane when operated under such conditions. Peter-Varbanets et al., 2010, Peter-Varbanets et al., 2011 determined that the microbial community promoted the development of cavities, channel networks, and heterogeneous structures within the foulant layer. In addition to reducing the resistance associated with the foulant layer, the microbial community could also degrade some of the organic contaminants present in raw waters (Derlon et al., 2014, Kimura et al., 2016, Kohler et al., 2014), further reducing fouling and improving the treatment efficacy of the system.
Although sustained operation is possible without any auxiliary fouling control measures, operation at an ultra low permeate flux is required (Derlon et al., 2014, Kohler et al., 2014, Peter-Varbanets et al., 2010, Peter-Varbanets et al., 2011). To overcome this limitation, it may be beneficial to retain some fouling control measures. However, limited knowledge exists regarding the optimal conditions under which extended operation can be sustained with minimal to no fouling control measures. The present study investigated the contribution of different auxiliary fouling control measures to the permeability that can be sustained. More specifically, the contributions of backwashing, air sparging, and chemical cleaning were considered because these fouling control measures are generally considered to be mechanically and operationally complex. The introduction of permeate flux interruptions (i.e. relaxation periods), as a mechanically and operationally simple fouling control measure, was also considered. The overall goal was to minimize the mechanical and operational complexity of a submerged hollow fiber UF membrane system, hereafter simply referred to as a membrane system, while maximizing its throughput capacity.
Section snippets
Bench-scale submerged hollow fibre membrane systems
Three different configurations of bench-scale submerged hollow-fibre membrane systems were used and presented in Fig. 1.
The conventional bench-scale systems (Fig. 1a) were used to assess the contribution of backwashing to the permeability that can be sustained. Each system consisted of a cylindrical system tank (diameter: 155 mm; height: 1270 mm; working volume: 19 L), three custom membrane modules (three 430 mm strands/module, 0.00717 m2 filtration area/module) made from ZeeWeed 500 type
Backwashing
To assess the impact of reducing or eliminating backwashing, filtration tests were performed with two conventional systems (Fig. 1a) operated in parallel, one with and one without periodic backwashing. The permeate flux of the three modules in each system was set to 10, 20 or 30 Lm−2h−1. The filtration tests were performed in triplicate for a period of 2 months (unless the trans-membrane pressure exceeded a maximum allowable value of 55 kPa, in which case, the system operation was terminated).
Conclusions
Sustained conditions could be achieved without backwashing, air sparging or chemical cleaning, indicating that these fouling control measures can be eliminated, simplifying the mechanical and operational complexity of submerged hollow fiber ultrafiltration systems. The ability to achieve sustained conditions was attributed to the microbial community that establishes itself on the membrane surface. The adoption of hydrostatic pressure to provide the driving force for permeation further reduced
Acknowledgement
The authors wish to thank IC IMPACTS (www.ic-impacts.com) for funding the research.
References (26)
- et al.
Evaluation of fouling deposition, fouling reversibility and energy consumption of submerged hollow fiber membrane systems with periodic backwash
J. Membr. Sci.
(2014) - et al.
Removal of disinfection by-product precursors with ozone-UV advanced oxidation process
Water Res.
(2005) - et al.
The use of gas bubbling to enhance membrane processes
J. Membr. Sci.
(2003) - et al.
Presence of biofilms on ultrafiltration membrane surfaces increases the quality of permeate produced during ultra-low pressure gravity-driven membrane filtration
Water Res.
(2014) - et al.
Removal of natural organic matter from drinking water supplies by ozone-biofiltration
Water Sci. Technol.
(1999) - et al.
Characterisation of aquatic humic and non-humic matter with size-exclusion chromatography – organic carbon detection – organic nitrogen detection (LC-OCD-OND)
Water Res.
(2011) - et al.
Comparison of membrane fouling at constant flux and constant transmembrane pressure conditions
J. Membr.
(2014) - et al.
Stabilization of flux during dead-end ultra-low pressure ultrafiltration
Water Res.
(2010) - et al.
Mechanisms of membrane fouling during ultra-low pressure ultrafiltration
J. Membr. Sci.
(2011) - et al.
Intermittent operation of ultra-low pressure ultrafiltration for decentralized drinking water treatment
Water Res.
(2012)