The absence of indicator organisms in drinking water does not provide sufficient guarantee for microbial safety. Therefore the water utilities are implementing water safety plans (WSP) to safeguard drinking water quality. Quantitative microbial risk assessment (QMRA) can be used to provide objective quantitative input for the WSP. This study presents several applications of treatment modelling in QMRA to answer the risk managers questions raised in the WSP. QMRA can estimate how safe the water is, how much the safety varies and how certain the estimate of safety is. This can be used in the WSP system assessment to determine whether treatment is meeting health-based targets with the required level of certainty. Quantitative data analysis showed that short events of only 8 hours per year can dominate the yearly average health risk for the consumer. QMRA also helps the design of physical and microbial monitoring. The study showed that the required monitoring frequency increases with increasing treatment efficacy. Daily monitoring can be sufficient to verify a treatment process achieving 2 log reduction of pathogens, but a process achieving 4 log reduction needs to be monitored every 15 minutes. Similarly, QMRA helps to prepare adequate corrective actions by determining the acceptable ‘down time’ of a process. For example, for a process achieving 2.5 log reduction a down time of maximum 6 hours per year is acceptable. These applications illustrate how QMRA can contribute to efficient and effective management of microbial drinking water safety.

Freshwater is a precious resource, and shortages can lead to water stress, impacting agriculture, industry, and other sectors. Wastewater reuse is increasingly considered as an opportunity to meet the freshwater demand. Legislative frameworks are under development to support the responsible reuse of wastewater, i.e., to balance benefits and risks. In an evaluation of the proposed European regulation for water reuse, we concluded that the proposed regulation is not practically feasible, as the water provider alone is responsible for the risk assessment and management, even beyond their span of control. The required knowledge and resources are extensive. Therefore, without clear guidance for implementation, the regulation would hinder implementation of reuse programs. As a consequence, the current practice of uncontrolled, unintentional, and indirect reuse continues, including related risks and inefficiency. Therefore, we provide an outline of the interdisciplinary approach required to design and achieve safe, responsible water reuse. Responsible water reuse requires knowledge of water demand and availability, quality and health, technology, and governance for the various types of application. Through this paper we want to provide a starting point for an interdisciplinary agenda to compile and generate knowledge (databases), approaches, guidelines, case examples, codes of practice, and legislation to help bring responsible water reuse into practice.

In the Netherlands, safe and sufficient drinking water is provided to the general population by ten drinking water companies. To guarantee safe drinking water the World Health Organization (WHO) developed a Water Safety Plan (WSP), a Risk Assessment and a Risk Management (RA/RM) framework. The objective of the study was to identify legally required RA approaches, to document application of RA/RM activities at Dutch drinking water companies and to determine to what extent these RA/RM activities as a whole cover all the elements of the WHO WSP approach. This study could be of interest to both managers of large water utilities and decision makers. The assessment was performed by means of a policy review and interviews with two to four staff members involved in RA/RM from all ten Dutch drinking water companies combined with a joint workshop. The drinking water companies are well aware of the potential hazards and risks that can influence the drinking water quality. To guarantee the supply of safe and sufficient drinking water, the Dutch drinking water sector uses six different legally required RA/RM approaches. This study shows that by using the six legally required RA/RM approaches, all WSP steps are covered. WSP entails a generic risk assessment for identifying all hazards and hazardous events from source to tap, whereas the six legally required RA/RM each focus on specific risks at an advanced level. Each risk assessment provides information on specific hazards and hazardous events covering a part of the water supply chain. These legal requirements are complemented with additional RA/RM activities at sector and water company level such as codes of practices and standard operating procedures. The outcomes of all RA/RM approaches combined provide information from source to tap. When using multiple RA/RM approaches, it is crucial to share and combine information derived from the different activities.

Access to safe drinking water is crucial for public health necessitating the use of effective water treatment processes. We conducted a systematic literature review on microorganism removal by physical treatment processes used in drinking water treatment systems with the aim of providing current summary data to update the World Health Organization’s Guidelines for Drinking Water Quality (GDWQ) and to reflect on the data available for comparison of treatment technologies. We reviewed peer-reviewed articles reporting original data that were published between 1997 and March 2022 on the following physical treatment technologies: roughing filters, storage reservoirs, bank filtration, conventional and high-rate clarification, dissolved air flotation, lime softening, granular media filtration, slow sand filtration, precoat filtration, membrane filtration, granular activated carbon, ceramic membrane filtration, and soil aquifer treatment. The literature search was conducted in several databases including Web of Science and PubMed. Data from 165 articles were included in the analysis and used to calculate Log Reduction Values (LRVs) for each technology by microbial contaminant type (bacteria, virus, or protozoa). The quantity and quality of data ranged widely for each technology. We found granular media, membranes (microfiltration (MF), ultrafiltration (UF), and reverse osmosis (RO)), and precoat filtration to remove the most protozoa with average LRVs of 3.0 (95% CI 2.8–3.3), 5.7 (95% CI 5.4–6.0), and 4.4 (95% CI 4.1–4.7), respectively. Bacteria was removed most effectively by membrane filtration (MF, UF, RO) with average LRVs of 4.5 (95% CI 3.9–5.1) and moderately by dissolved air flotation, lime softening, and soil aquifer treatment with average LRVs of 2.7, 2.6, and 2.4 respectively. Viruses were removed most effectively by reverse osmosis membrane filtration with an average LRV of 4.9 (95% CI 4.0–5.7). This data provides valuable information on pathogen reduction and areas of needed research. The variation in results underscores the importance of further consideration when selecting technologies to use and the need for standardized reporting in both lab and field studies. It is important to consider variables in water quality and technology operation that may impact treatment effectiveness when selecting treatment options for use. The findings contribute to ongoing efforts to revise the WHO’s GDWQ, offering updated insights into LRVs for different water treatment technologies.

Water safety plans (WSPs) are intended to assure safe drinking water (DW). WSPs involve assessing and managing risks associated with microbial, chemical, physical and radiological hazards from the catchment to the consumer. Currently, chemical hazards in WSPs are assessed by targeted chemical analysis, but this approach fails to account for the mixture effects of the many chemicals potentially present in water supplies and omits the possible effects of non-targeted chemicals. Consequently, effect-based monitoring (EBM) using in vitro bioassays and well plate-based in vivo assays are proposed as a complementary tool to targeted chemical analysis to support risk analysis, risk management and water quality verification within the WSP framework. EBM is frequently applied to DW and surface water and can be utilised in all defined monitoring categories within the WSP framework (including ‘system assessment’, ‘validation’, ‘operational’ and ‘verification’). Examples of how EBM can be applied within the different WSP modules are provided, along with guidance on where to apply EBM and how frequently. Since this is a new area, guidance documents, standard operating procedures (SOPs) and decision-making frameworks are required for both bioassay operators and WSP teams to facilitate the integration of EBM into WSPs, with these resources being developed currently.