Elsevier

Water Research

Volume 115, 15 May 2017, Pages 287-296
Water Research

Operation of passive membrane systems for drinking water treatment

https://doi.org/10.1016/j.watres.2017.02.065Get rights and content

Highlights

  • 20% of the initial permeability could be sustained without any fouling control.

  • Sustained permeability can be doubled by retaining minimal air sparging and relaxation.

  • An acclimatization period was not required to achieve a sustained permeability.

  • Approximately 50% of influent organic matter removed during treatment.

Abstract

The widespread adoption of submerged hollow fibre ultrafiltration (UF) for drinking water treatment is currently hindered by the complexity and cost of these membrane systems, especially in small/remote communities. Most of the complexity is associated with auxiliary fouling control measures, which include backwashing, air sparging and chemical cleaning. Recent studies have demonstrated that sustained operation without fouling control measures is possible, but little is known regarding the 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, with the intent of minimizing the mechanical and operational complexity of submerged hollow fiber UF membrane systems while maximizing their throughput capacity.

Sustained conditions could be achieved without backwashing, air sparging or chemical cleaning (i.e. passive operation), indicating that these fouling control measures can be eliminated, substantially simplifying the mechanical and operational complexity of submerged hollow fiber UF systems. The adoption of hydrostatic pressure (i.e. gravity) to provide the driving force for permeation further reduced the system complexity. Approximately 50% of the organic material in the raw water was removed during treatment. The sustained passive operation and effective removal of organic material was likely due to the microbial community that established itself on the membrane surface. The permeability that could be sustained was however only approximately 20% of that which can be maintained with fouling control measures. Retaining a small amount of air sparging (i.e. a few minutes daily) and incorporating a daily 1-h relaxation (i.e. permeate flux interruption) period prior to sparging more than doubled the permeability that could be sustained. Neither the approach used to interrupt the permeate flux nor that developed to draw air into the system for sparging using gravity add substantial mechanical or operational complexity to the system. The high throughput capacity that can be sustained by eliminating all but a couple of simple fouling control measures make passive membrane systems ideally suited to provide high quality water especially where access to financial resources, technical expertise and/or electrical power is limited.

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.

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