A review of polymeric membranes and processes for potable water reuse

Abstract

Conventional water resources in many regions are insufficient to meet the water needs of growing populations, thus reuse is gaining acceptance as a method of water supply augmentation. Recent advancements in membrane technology have allowed for the reclamation of municipal wastewater for the production of drinking water, i.e., potable reuse. Although public perception can be a challenge, potable reuse is often the least energy-intensive method of providing additional drinking water to water stressed regions. A variety of membranes have been developed that can remove water contaminants ranging from particles and pathogens to dissolved organic compounds and salts. Typically, potable reuse treatment plants use polymeric membranes for microfiltration or ultrafiltration in conjunction with reverse osmosis and, in some cases, nanofiltration. Membrane properties, including pore size, wettability, surface charge, roughness, thermal resistance, chemical stability, permeability, thickness and mechanical strength, vary between membranes and applications. Advancements in membrane technology including new membrane materials, coatings, and manufacturing methods, as well as emerging membrane processes such as membrane bioreactors, electrodialysis, and forward osmosis have been developed to improve selectivity, energy consumption, fouling resistance, and/or capital cost. The purpose of this review is to provide a comprehensive summary of the role of polymeric membranes and process components in the treatment of wastewater to potable water quality and to highlight recent advancements and needs in separation processes. Beyond membranes themselves, this review covers the background and history of potable reuse, and commonly used potable reuse process chains, pretreatment steps, and advanced oxidation processes. Key trends in membrane technology include novel configurations, materials, and fouling prevention techniques. Challenges still facing membrane-based potable reuse applications, including chemical and biological contaminant removal, membrane fouling, and public perception, are highlighted as areas in need of further research and development.

Introduction

Dwindling water supplies and growing populations have made planned potable reuse an increasingly important component of water resource management for many urban areas around the world [[1], [2], [3]]. Although reuse can only be a portion of a water supply portfolio due to intrinsic water losses, reuse of wastewater can augment the supply of water for agriculture, industry, and potable use, even in regions where climate change and cyclical droughts make traditional supplies unreliable. Membranes, particularly polymeric membranes, play a crucial role in the purification of municipal wastewater to potable quality, and are the core part of many of these systems [[1], [2]]. However, despite the technologies available, several challenges including membrane fouling, contaminant permeation, energy consumption, high pretreatment costs, managing treatment residuals, membrane integrity, and public perception limit widespread implementation of potable reuse [[1], [3], [4], [5]]. For example, as of 2010, only about 0.1% of treated municipal wastewater was directed to planned potable reuse in the U.S. [6]. The purpose of this review is to summarize recent developments for polymeric membranes that relate to potable water reuse, and to identify areas in which future research and innovation are needed.

Planned potable reuse projects can be categorized as direct or indirect [3]. Direct potable reuse (DPR) is the direct addition of treated, reclaimed water to a drinking water treatment plant’s influent (referred to as “raw water augmentation”) or a drinking water distribution system (“treated drinking water augmentation”, Fig. 1a). Indirect potable reuse (IPR) is the planned addition of treated, reclaimed water) to an environmental buffer, namely a surface water reservoir (“reservoir water augmentation”) or groundwater aquifer (“groundwater augmentation”) [7], that is subsequently used for a drinking water supply (Fig. 1b). In addition, de facto potable reuse (i.e., unplanned potable reuse) refers to the production of drinking water from wastewater-impacted water resources (Fig. 1c). De facto potable reuse is common [8] and often unavoidable in major river systems such as adjacent to the Mississippi and Nile River (in the U.S. and Egypt, respectively).

As one implementation of IPR, water agencies may also inject or infiltrate reclaimed water at locations in between the ocean and drinking water production wells to slow or reverse seawater intrusion into coastal aquifers. While de facto potable reuse and IPR have been practiced for some time, DPR has more recently become a technically and (to a lesser extent) socially viable reuse option in many geographies world-wide [4].

The purification of wastewater has been examined as a means of augmenting conventional drinking water supplies for over 100 years [1]. In the 1920s, the Los Angeles Department of Water and Power constructed a wastewater purification plant to accommodate increased water demand due to rapid development and the lack of additional water supplies prior to access to Colorado River water [6]. By the 1930s, spreading basins were being used to augment groundwater with the effluent of a wastewater treatment in Southern California [1]. In 1968, the first DPR scheme was constructed in Windhoek, Namibia; in the years following, numerous IPR projects were established globally [7]. It was not until 1977 that membranes became an integral component of potable water reuse applications, when RO membranes were first used to purify wastewater at Orange County Water District’s Water Factory 21 [7].

As treatment technologies for potable reuse have evolved over 50 years, there has been a gradual shift away from traditional processes, such as lime softening, toward membrane filtration [6]. As the cost and performance of membrane products have improved, polymeric membranes have played an increasingly important role in potable reuse [9]. Polymeric membranes are now used to remove colloidal and dissolved materials in most potable reuse facilities. In the last 20 years, the number of IPR and DPR projects employing membrane technologies has increased significantly in the U.S., Australia, Singapore, and South Africa (Table 1). Globally, water reuse for potable and non-potable applications continues to be a critical water resource (Fig. 2), and has reached approximately 32 million m³/day [10].

Years after the first application of reverse osmosis (RO) for wastewater purification at Orange County Water District, California, a treatment train consisting of microfiltration (MF) or ultrafiltration (UF) followed by RO and an advanced oxidation process (AOP) has emerged as an industry standard for many potable reuse applications (Fig. 3) [8]. The RO and AOP components are referred to as “full advanced treatment” (FAT), which is defined as the treatment of an oxidized wastewater using RO and an oxidation treatment process (e.g., UV/AOP). The use of MF/UF in front of RO is referred to as an integrated membrane system (IMS) where MF/UF acts as pretreatment to RO [7]. In Fig. 3, the steps shown prior to membrane treatment help reduce membrane fouling, and the steps after (e.g., AOP) break down small neutral organic compounds that pass through the RO process. FAT and IMS are favored for many potable reuse projects due to high removal efficiencies of microbial pathogens, organic and inorganic contaminants, and other constituents relevant to potable water production (e.g., particles, inorganic nitrogen, and dissolved solids; see Table 1) [11]. Post-treatment disinfection at current IPR and DPR facilities is typically accomplished through the UV or UV-AOP processes. For groundwater and reservoir augmentation systems, post-treatment chlorine disinfection is generally not implemented as the water is sent to an environmental buffer. Primary disinfection for current DPR systems may be performed downstream at the water treatment plant treating potable reuse treatment system finished water [11]. Additionally, chloramine residual is often applied before the RO membranes in order to minimize membrane biofouling, and is used instead of the stronger biocide chlorine due to polyamide RO membrane sensitivity [3]. Water stabilization (adding chemicals like calcium hydroxide) as a final step is often done to minimize distribution pipe corrosion.

Compared to alternative unconventional water resources such as seawater desalination and water importation, potable water reuse generally requires less energy and is less costly [[12], [13]]. Although the FAT has become the backbone of most planned potable reuse projects and many consider it the standard for potable reuse worldwide, a number of drawbacks are associated with its implementation. Thus, alternative treatment trains may be used [14]. One such variation is to omit biologically activated carbon (BAC) from tertiary treatment plant, instead using other means for biological nutrient removal, which is often combined with ultrafiltration (UF) instead of microfiltration (MF). As another variation, biologically activated filtration may be used after RO and UV to remove byproducts of disinfection. Some trains even lack salinity control, omitting RO membranes. However, RO membranes are rarely omitted for direct potable reuse. Overall, several aims are always accomplished regardless of whether FAT or an alternative treatment train is used for potable reuse: physical removal, oxidation, and chemical inactivation [15].

Overall, the widespread use of RO membranes in potable reuse applications is due to: 1) demonstrated success in multiple installations worldwide enabling technology familiarity in a risk-averse industry responsible for protecting public health; 2) ability to handle variable input (i.e., operational reliability); 3) modularity; and 4) the very high quality of the product water, particularly with respect to pathogens, dissolved salts characterized as total dissolved solids (TDS; typically >99% removal), and wastewater-derived organic contaminants such as pharmaceuticals (removal varies, but typically >90%) [[16], [17], [18]]. While high pathogen removal is expected, current regulatory structures often give little or no disinfection credit for RO due to limitations in monitoring membrane integrity. Major advantages of membrane-based treatment systems for potable reuse include small footprints, modular designs, synergistic combinations with other treatment processes [19], fewer treatment stages and the ability to reject compounds that other processes cannot eliminate [20].