Managing emerging pathogen risks in recycled water

Keywords: COVID‐19, emerging pathogens, microbial risk assessment, mpox, quantitative risk-based management, recycled water supply, water recycling.

Access to clean drinking water and sanitation services is critical for maintaining public health. The absence of these services places a large burden on developing countries, causing an estimated 829 000 deaths annually, of which 432 000 are linked to poor sanitation.¹ Sanitation services, such as sewer networks and wastewater treatment facilities, break faecal–oral disease transmission by reducing levels of faecal contamination in the environment and thereby reducing subsequent exposure to pathogens via contaminated food and water. Reduced access to fresh water, due to population growth, increased industrial and agricultural use, and climate change, has driven increased wastewater recycling as jurisdictions seek out secure ‘climate-independent’ supplies. Direct use of untreated wastewater for irrigation is practiced in some countries;² however in Australia, the treatment and use of recycled water is highly regulated.

In Australia, major wastewater recycling initiatives commenced during the 1990s, with Class A recycled water supplied for horticultural irrigation (e.g. the Virginia Pipeline Scheme in South Australia) and residential use via dual reticulation systems (e.g. toilet flushing and outdoor use at Mawson Lakes in Adelaide and Rouse Hill in Sydney).³ Water recycling initiatives were significantly expanded in Australia during the so-called ‘millennium drought’ of 2000–2010⁴ and recycled water is now a key component of diverse water supply portfolios for many Australian water authorities nationwide⁵ (Fig. 1).

Fig. 1.  Filtration units as part of the large 12 gigalitre per year Northern Adelaide Irrigation Scheme water recycling plant in South Australia (image taken by Ben van den Akker).

Compared with treated wastewater, which is diluted when released into environmental waters, recycled wastewater poses increased risks to human health due to the types of end uses and exposure pathways. Consequently, recycled wastewater is more highly treated than discharged effluent and Australian Guidelines adopt a quantitative risk-based management approach to protect public health.⁶ A key element of the Guidelines is the characterisation of microbial hazards in sewage to ensure that sufficient treatment is used to ensure the safety of recycled water; this includes consideration of emerging pathogens. This paper introduces some emerging pathogens and describes how these emerging microbial human health risks are effectively managed in recycled water supplies for public health protection.

The COVID-19 pandemic has heightened the focus on emerging pathogens internationally. During the early days of the pandemic, it was unclear to what extent the SARS-CoV-2 virus posed human health risks from environmental exposure pathways, including exposure to potentially contaminated water and wastewater.⁷ As more information emerged, it was clear that human health risks from these exposure pathways were low and effectively managed by existing water industry protocols and public health protection measures.⁸ Since the genesis of COVID, other novel pathogens continue to emerge and wastewater monitoring has confirmed their presence in sewage (e.g. mpox⁹), which raises questions about potential human health risks from wastewater and recycled water. While much of the focus is on viral and bacterial pathogens, fungal pathogens including those resistant to antimicrobials also impose a substantial disease burden internationally, with cumulative global mortality rates for serious fungal disease now surpassing those of malaria, tuberculosis, breast or prostate cancer.¹⁰ Table 1 provides a synopsis of current high-profile emerging bacterial, fungal and viral diseases, and the likely level of associated risk posed by recycled water use.

Table 1.  Examples of high-profile (re)emerging viral, bacterial and fungal pathogens, and the likely human health risk associated with recycled water use.

Across Australia, health risks associated with the use of recycled wastewater are managed using the national Australian Guidelines for Water Recycling.⁶ A key feature of the Guidelines is a framework that promotes the application of quantitative microbial risk assessment principles using disability adjusted life years (DALYs). This marks a change from past management frameworks that were based on ‘too little, too late’ end-point monitoring of faecal indicators (e.g. E. coli). First introduced in the 2004 World Health Organization Guidelines for Drinking Water Quality,¹⁹ DALYs convert the likelihood of infection or illness into burdens of disease and set a tolerable risk benchmark of one micro-DALY (10⁻⁶ DALYs) per person per year. When applied to water recycling schemes, this benchmark is used to set microbial health-based targets (HBTs) as water treatment system performance targets, expressed as log reductions, for reference organisms that represent the major sewage pathogen groups (bacteria, viruses and protozoa). These HBTs can be met using multiple treatment barriers and by the adoption of on-site preventive measures that lower human exposure to recycled water at point of use.

The adoption of a risk-based approach inherently safeguards against emerging pathogen risks in the following ways:

1. Flexible framework capable of accommodating change: Risk management plans are an ongoing and iterative process, whereby risk is continually evaluated and updated based on new information including that pertaining to emerging pathogens (e.g. densities in source waters, exposure pathways, pathogen fate/response to treatment processes, dose–response and disease burden).

2. Use of conservative assumptions: In identifying hazards, it is impractical to set HBTs for all pathogens in sewage, therefore, the guidelines specify the use of reference pathogens which include Campylobacter for bacteria, rotavirus and adenovirus for viruses, and Cryptosporidium parvum for protozoa. These have been selected based on their high incidence within the community, high abundances in sewage, low infectious dose, high disease severity and resistance/resilience to treatment barriers. Hence, if these indicators are adequately controlled, the scheme should ensure protection against all pathogens of concern within the same class. Furthermore, conservative assumptions are also applied to estimate human exposure for many end-uses and both intended and unintended uses are considered.

3. Redundancy in water treatment: Reuse schemes that supply water for high exposure uses (e.g. dual reticulation for unrestricted municipal use) require the use of multiple barriers to meet HBTs such as secondary treatment, filtration, UV and chlorine disinfection (Fig. 2). In the Fig. 2 example, the multiple barrier system-wide organism LRVs for virus, bacteria and protozoa (8.0, 10.5 and 7.5-log₁₀ reductions respectively) exceed the stipulated guideline requirements for the highest risk recycled water end uses like dual reticulation (i.e. 6.5, 5.0 and 5.0-log₁₀ reductions respectively). Validation of individual treatment barriers is also assessed, often through challenge testing, whereby verified pathogen log reduction credits are assigned using worse case 5th percentile performance values. Additionally, some treatment barriers have considerable flexibility in terms of being able to respond quickly to emerging risks or hazardous events. For example, disinfection processes such as chlorination and to some extent UV can be rapidly adjusted to increase disinfection performance (UV and chlorine dose) to deliver enhanced microbial inactivation as required.

Fig. 2.  Example of a multiple barrier treatment approach often employed by water recycling schemes to manage pathogen risk. Barrier log₁₀ reductions for viruses (V), bacteria (B) and protozoa (P) shown in black, blue and red respectively.

Our experience with COVID has demonstrated that even with the emergence of new pathogens, the risks posed from recycled water are similar to those for known pathogens in terms of how these organisms respond to treatment barriers. Effectively, these emerging risks are managed by existing processes and management frameworks. Increased emphasis on wastewater monitoring²⁰ combined with detection methods that are increasingly rapid, informative, sensitive and low cost²¹ will continue to shine the spotlight on microbial contaminants in wastewater. However, an increased capacity to detect known and emerging pathogens in wastewater does not necessarily translate to elevated risk because presence measured by molecular methods does not equate to infectivity and risk.²² Additionally, recycled water exposure pathways for emerging viruses in particular are not the highest risk pathways (Table 1) and so risks are likely to be substantially reduced prima facie. Of course the water industry must remain alert to novel and emerging microbial hazards and the continued adoption of risk-based management of recycled water is the best way of ensuring ongoing public safety and acceptance of water recycling operations.

Although current approaches are effective in managing risk, knowledge gaps do exist in some aspects of recycled water supply and these should be the focus of future research. For example, better understanding water quality in recycled water distribution networks, especially under a future warming climate and how this might impact potential regrowth of opportunistic pathogens.¹⁷ Another area of uncertainty is the emerging risk posed by antimicrobial resistance (AMR), where there is potential for complex but as-yet uncharacterised risks (acute and diffuse) arising from both AMR genes and organisms at various exposure points across wastewater treatment and water recycling systems.^23–26 Some of these knowledge gaps will be the subject of future research to be undertaken by the newly established Cooperative Research Centre for Solving Antimicrobial Resistance in Agribusiness, Food, and Environments (CRC SAAFE; https://www.crcsaafe.com.au) (Fig. 3). There is also a level of uncertainty regarding non-enteric pathogens (bacterial and fungal) and the risks posed by opportunistic pathogens (e.g. Pseudomonas or Candida) for which dose-response models and exposure pathways are not well defined.^17,27,28

Fig. 3.  The horticulture sector is a large user of recycled water in Australia and is a key area of focus for CRC SAAFE research to understand and manage AMR risks in food and agribusiness (iStock image 1083286598).

There will continue to be new risks and uncertainties from emerging pathogens, but existing water industry processes and water recycling guidelines are able to manage these risks effectively as a result of in-built flexibility and conservatism in how water recycling systems are operated and how health risks are managed. As we move towards a future with increased climate variability and water supply insecurity, water recycling will play an increasingly important role in ensuring water security for our liveable urban environments and for future food production. Existing approaches to managing human health risks in recycled water supplies will be revised and updated in response to new risks to ensure that future risks from new and emerging pathogens continue to be managed appropriately.

Data sharing is not applicable as no new data were generated or analysed during this study.

Paul Monis and Ben van den Akker are employees of the South Australian Water Corporation, which is a supplier of recycled water, and both hold adjunct positions at the University of South Australia. The other authors declare no conflicts of interest.

This research did not receive any specific funding.