Potable reuse of water

Stuart J. Khan

Dr Stuart Khan is an Associate Professor in Civil & Environmental Engineering at the University of New South Wales and Hans Fischer Fellow at the Technical University of Munich. He has contributed to over 100 articles on water quality issues relating to drinking water, wastewater and recycled water. He is a member of the World Health Organization (WHO) Water Quality and Health Technical Advisory Group and the Water Quality Advisory Committee to the Australian National Health and Medical Research Council. Dr Khan has made major contributions to numerous water quality guidelines including the Australian Drinking Water Guidelines and Australian Guidelines for Water Recycling. He is currently contributing to the development of WHO guidelines for potable water reuse.

Following rapid population growth in the first half of the 20th century, Los Angeles County, California began to suffer a major overdraft on the Central Groundwater Basin, an essential component of clean water supplies for the region. The groundwater table dropped, shallow wells dried up and seawater began to intrude into the aquifer. In response, the Water Replenishment District of Southern California was formed to manage the basin by regulating extractions and obtaining additional water supplies to replenish and re-pressurise the aquifer. The Montebello Forebay Groundwater Recharge Project was born with the objective to recharge the aquifer by surface infiltration of water directed to the Montebello Forebay Spreading Grounds. New water for infiltration was initially obtained from local stormwater and imported river water. However, in 1962 an unusual source was added to the mix. The Whittier Narrows Water Reclamation Plant was constructed, treating local municipal wastewater by primary sedimentation and activated sludge treatment. The effluent from the plant was directed to the Montebello Forebay Spreading Grounds, from where it contributed to the recharge of the Central Groundwater Basin. While the reclamation plant has been upgraded numerous times since, The Montebello Forebay Groundwater Recharge Project has operated continuously for over half a century.

The residents of Los Angeles County were by no means the first community to have a major drinking water supply partially replenished by municipal wastewater. Indeed, thousands of cities throughout the world, taking water from riverine sources, have upstream neighbours discharging treated or untreated wastewater into those rivers. However, Los Angeles County pioneered the practice that we now refer to as ‘planned potable reuse’. That is, the Montebello Forebay Project was among the first in the world to acknowledge the value of municipal wastewater as a resource that can be appropriately treated and managed to safely and intentionally supplement local drinking water supplies. While the distinction between ‘unplanned’ and ‘planned’ potable reuse is often opaque, the number of widely acknowledged planned potable reuse projects remains low, with less than 40 projects throughout the world. Most of these acknowledged projects are in the USA with a few other scattered examples in Singapore, Namibia, South Africa, Belgium and Australia.

Scientific research has been an essential factor for developing the necessary treatment technologies, water quality monitoring and regulatory processes required to underpin the safe practice of potable reuse. Furthermore, research has led to improved appreciation for the important role of public perception and the application of optimised governance and economic frameworks to facilitate potable reuse. As population growth and climate change threaten to drastically increase pressure on water supplies for many of the world's cities, research to further increase the widespread viability of planned potable reuse has never been more important. It is thus appropriate that the first themed issue of Environmental Science: Water Research & Technology be devoted to potable reuse of water.

It is now well acknowledged that the technical ability to treat municipal wastewater to a quality suitable for potable reuse has been achieved. A range of water treatment processes including carbon adsorption, high pressure membranes, chemical oxidation and ultraviolet radiation may be variously employed to achieve almost any desired water quality. However, excellent water quality comes with various costs, which commonly include high energy consumption, associated high greenhouse footprint as well as high capital, operation and maintenance costs. Some processes produce additional waste streams presenting disposal challenges, or require large physical footprints presenting challenges around space availability. Consequently, there is a need for further research to facilitate the development of new or improved water treatment technologies which overcome these limitations. In this issue, we are pleased to present a critical review of such an emerging technology, the osmotic membrane bioreactor (OMBR). As Holloway et al. (DOI: 10.1039/C5EW00103J) reveal, the OMBR is a promising treatment technology for potable reuse applications due to the multiple treatment barriers inherent to its design. Furthermore, the OMBR has significant potential advantages for nutrient recovery. However, the OMBR also faces challenges that must be overcome before successful commercialisation may be achieved. These include the control of membrane fouling, bioreactor salinity and gradual degradation of the necessary draw solution. The authors have provided a clear outline of where future research investments should be applied in order to progress the future of OMBRs for potable reuse and other applications.

Two important contributions to progress the role of ozone in potable reuse treatment trains are made in this issue. Pisarenko et al. (DOI: 10.1039/C5EW00046G) have investigated the formation of toxic by-products which may be produced when ozone is used to treat municipal wastewaters. This work focuses on the formation of N-nitrosamines and perfluoroalkyl acids, both of which are currently included in focused drinking water contaminant assessments for future regulation in the USA. The authors present work in which they have discerned the potential roles of molecular ozone (O₃) and hydroxyl radical (˙OH) in the formation of these substances. In doing so, they show that the presence of molecular ozone is significant in promoting the production of N-nitrosodimethylamine (NDMA) as well as a number of perfluoroalkyl acids. NDMA is a byproduct which commonly has major implications for potable reuse projects since it presents cancer risks at very low concentrations (the 10⁻⁶ cancer risk is estimated to be at 0.7 ng L⁻¹) and it is poorly rejected by reverse osmosis membranes. Improved understanding of the factors that lead to the formation of this by-product will facilitate future treatment process design, potentially relieving the need to specifically target this chemical for subsequent removal.

The other side of the ozonation coin is presented by Park et al. (DOI: 10.1039/C5EW00120J) as they demonstrate the effectiveness of this oxidative treatment process for the removal of a range of trace chemical contaminants. It is known that ozonation can be highly effective for the attenuation of some amenable trace organic chemicals, but the ability to consistently and reliably achieve a prescribed level of treatment performance has been more elusive. This is due in part to an incomplete understanding of the variations in water quality that can impact performance through fluctuations in ozone decay. Through the implementation of cause–effect modelling techniques, these authors have shown that the expected treatment performance can be continuously monitored via the key controlling factor of ozone dose whereas variations in total organic carbon (TOC) concentration have negligible influence for a given water quality. The ability to relate contaminant removal to continuously monitored and real-time controllable parameters will be increasingly important as developments such as ‘direct’ potable reuse require increasingly rapid water quality monitoring and performance correction.

Responding to the need for continuous performance monitoring of reverse osmosis processes, Singh et al. (DOI: 10.1039/C5EW00090D) demonstrate the application of online fluorescence monitoring to observe process fluctuations associated with membrane fouling and integrity losses. In this work, a portable fluorescence sensor was tested at two Australian advanced water treatment plants. The sensor was able to discern differences between membrane permeates produced from sequential stages of a reverse osmosis system, thus demonstrating the ability to distinguish very subtle differences in water quality. In one case, parallel monitoring of electrical conductivity demonstrated improving permeate quality, while fluorescence measurements revealed decreasing quality. This observation highlights a limitation of relying on conductivity measurements, which are predominantly associated with inorganic ions, to imply performance in the removal of some groups of fluorescing organic substances. The imperfect relationship between the removal performance of inorganic ions and organic substances is likely explained by the differing mechanisms responsible for the rejection of each group of contaminants.

A variety of approaches for monitoring and demonstrating the removal of trace organic chemical substances from water, prior to potable reuse, are investigated. Busetti et al. (DOI: 10.1039/C4EW00104D) present the results of a comprehensive monitoring study undertaken to assist the treatment performance characterisation of a potable reuse project in Perth, Western Australia. From the target screening of 291 chemicals, 13 were detected following reverse osmosis membrane filtration and ultraviolet disinfection. These consisted of two corrosion inhibitors, three pesticides, three pharmaceuticals, one personal care product, three artificial sweeteners and one flame retardant, mostly at low ng L⁻¹ concentrations. Calculated risk quotients for each of these chemicals were 2 to 6 orders of magnitude below 1, implying a high degree of safety associated with human consumption of this water.

Since it is not possible to monitor all contaminants which may potentially be present in waters intended for reuse, Anumol et al. (DOI: 10.1039/C5EW00080G) have presented a prioritised list of twenty ‘indicator’ trace organic compounds in wastewater. This prioritised list was developed through a detailed literature review and scoring system. The twenty compounds include a range of pharmaceuticals, artificial sweeteners, personal care products, biomedical contrast media and flame retardants. They were selected based on their likely detectability in waters contaminated, in part, by sub-optimally treated municipal wastewaters. The authors then present a rapid and sensitive analytical method for measurement of these compounds in wastewaters and water treated for reuse. It is proposed that this method be applied to rapidly assess water quality based on chemicals which can be most reliably expected to be present in circumstances of wastewater contamination or sub-optimal treatment.

For many, the ultimate approach to assessing the presence and treatment performance for ‘unknown’ trace organic chemicals lies with the use of ‘bioanalytical tools’. In vitro bioassays can be used to detect the presence of chemicals imparting a biological impact, even when the identities of the responsible chemicals are unknown. Such bioassays are often thought of as novel tools by water stakeholders. However, Leusch and Snyder (DOI: 10.1039/C5EW00115C) reveal that such tools have been incorporated in assessment of water recycling schemes since the 1960s. Up until around 1998, the focus was on detecting reactive toxicity, predominantly mutagenicity and genotoxicity. This was followed by a decade with increasing interest in testing endocrine activity. But interest has grown and broadened since about 2007 largely in response to accelerating interest in potable water reuse. These authors provide a compelling case for bioanalytical tools as they offer a path towards more comprehensive chemical evaluations of water, which can provide greater public confidence in the ability of potable reuse schemes to produce clean and safe drinking water.

When comparing public health risks between two alternative water supply options, a lack of information required for ‘absolute’ assessment of risks is commonly a major limitation. Soller et al. (DOI: 10.1039/C5EW00038F) provide a powerful way around this problem by presenting quantitative relative risk assessments of direct potable reuse scenarios compared to a ‘no project’ or traditional water supply alternative. The study is focused on risks associated with trace chemical contaminants, but the authors argue that a similar approach could be applied for pathogen risks. In the two case studies presented, potable reuse scenarios are projected to provide protection from regulated chemical contaminants, and a range of ‘contaminants of emerging concern’, at levels that are comparable to or better than the ‘no project’ alternatives. These examples clearly demonstrate how quantitative relative risk assessment could inform future water supply options assessment.

In 2004, the World Health Organization published the third edition of the WHO Guidelines for Drinking-water Quality. These Guidelines presented a major departure from the traditional ‘end-of-pipe’ approach to assessing and managing drinking water safety. In doing so, they provided a framework for water quality risk management, based on the development of what they termed a ‘Water Safety Plan’ (WSP). Following the broad acceptance and success of WSPs in many parts of the world, Goodwin et al. (DOI: 10.1039/C5EW00070J) have investigated the extension of WSPs to water reuse applications. They conclude that Water Reuse Safety Plans (WRSPs) should be based on the existing WSP approach, but with a number of modifications. These include increased emphasis on supporting communication and engagement, and improvements in decision support mechanisms to better account for uncertainty, risk interactions and risk prioritisation.

The management of water quality and public health risks are commonly at the forefront of discussions of potable water reuse. However, when assessing and comparing alternative options for water supply augmentation, there are numerous other factors which must also be carefully considered. These include environmental impacts (including greenhouse gas emissions), economic impacts and issues relating to community acceptance and preference. Schimmoller et al. (DOI: 10.1039/C5EW00044K) present a framework for assessing the triple-bottom-line (TBL) costs of potable reuse projects. The focus of this work is to guide appropriate selection of water treatment processes, applicable for specific potable reuse circumstances. The objective is to prevent ‘overtreatment’ of water, defined as spending more than is necessary or causing adverse environmental impacts or social effects without providing counterbalancing benefits. The authors demonstrate that an ozone/activated carbon-based treatment train may have significantly reduced TBL costs compared to widely accepted ‘best available technology’ treatment trains based on membrane filtration and advanced oxidation.

Two communities with well-established successful histories of potable water reuse are Windhoek (Namibia) and Singapore. Both have been facing water scarcity for more than half a century and both have invested in a range of innovative solutions. Lafforgue and Lenouvel (DOI: 10.1039/C5EW00056D) have examined and contrasted these two case studies to reveal a range of insights demonstrating the need to adapt solutions to local contexts and constraints. The lessons from Windhoek are particularly insightful since they demonstrate that even in a harsh environment with a very low-income population, potable reuse can provide a feasible solution to water shortages. In contrast, Singapore has been a global leader in technological research, while providing an example of how water management can be addressed as a fundamental component of sustainable urban planning.

This themed issue includes two ‘perspective’ papers intended to provide insights on key issues relating to potable reuse practice and research. As a previous Commissioner of the Australian National Water Commission, Radcliffe (DOI: 10.1039/C5EW00048C) brings a unique perspective on water reform and water recycling in Australia. He describes how Australian governments made very large investments in potable reuse and seawater desalination at the height of a severe drought during around 2005–2008. However, extensive rainfall following that period has rendered many of these plants unnecessary in the short-to-medium term and many have ceased operation or been decommissioned. As Radcliffe describes, the change in the weather revealed numerous shortcomings in the way that much of this water supply augmentation was planned and implemented. He argues that several projects were built with over-capacity in responses to offers of additional funding without regard to clarifying how the additional production capacity would be integrated into the long-term likely base supply requirements. Furthermore, there were conflicting opinions on how and when to achieve the commitment to full cost recovery. Other issues which were not always well managed included transparency of decision-making processes and how to communicate to consumers the value of security of supply and the cost of achieving it.

Industry and academic readers alike will find great value in the perspective offered by Burgess et al. (DOI: 10.1039/C5EW00165J). This paper was composed by authors representing three research agencies, who together are leading the major component of the global research effort on potable reuse. These are the WateReuse Research Foundation (USA), the Water Research Commission (South Africa) and the Australian Water Recycling Centre of Excellence. It is clear that there is much current activity relating to potable reuse in each of these countries, which is not fully appreciated in a global context. For example, most Australian and US readers will have little idea of the range of indirect and direct potable reuse projects currently underway in South Africa, or the comprehensive research portfolio being undertaken in their support. International research trends are identified, along with key regional differences in research objectives and application. The perspective concludes with an assessment of future opportunities for mutual benefit by enhanced international cooperation.

Soliciting and processing the papers presented in this themed issue has been a pleasurable and rewarding task. We at Environmental Science: Water Research & Technology are proud to have been able to feature many leading international researchers contributing to progressing the viability and success of potable reuse globally. While the number of acknowledged planned potable reuse projects is growing internationally, it is arguable that they are still few and far between. This perspective is relative to the potentially enormous role that they may play in providing safe drinking water to large urban populations in the coming decades.

Water treatment technology, monitoring capability and risk management practices have developed significantly since commissioning of the Montebello Forebay Groundwater Recharge Project over half a century ago. During this time, regulatory requirements and public expectations around water quality and safety have also evolved appreciably. Technological challenges such as improved energy efficiency, reduced operational costs and solutions for the management of treatment byproducts continue to drive ongoing research and development. These challenges and others – perhaps as yet unthought of – will need to be addressed before the potentially great advantages of widespread, well-managed potable water reuse will be standard global practice. We hope that this themed issue will be seen as one important milestone along that road.

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