Pathogens in the picture: safe drinking water with QMRA

No chlorine is used in the distribution network in the Netherlands. Drinking water utilities are therefore extra vigilant when it comes to microbiological safety. As part of the Water Quality and Health theme, KWR focuses on the question of which risks microbiological pathogens pose for people and the environment. Before we go into the practical details, KWR researcher Patrick Smeets first explains what QMRA involves.

Predicting water safety

You can translate the permitted health standard to about one pathogen per million litres of drinking water. “That’s a single micro-organism in an entire swimming pool,” compares Smeets. “It’s impossible to measure such small quantities and so we use computing models with QMRA. Those models predict water safety and they are based on measurements that are feasible: the number of pathogens in the source, in other words surface water or groundwater. Then we turn to the treatment stages, determining the removal efficiency for each stage. That allows us to calculate how many pathogens the drinking water will ultimately contain in theory.”

Development of QMRA

KWR has been working on QMRA since the beginning of this century. “Interest in model-based risk assessment developed in 1995,” says Smeets, who has now been working on the subject himself for 25 years. “There were major challenges in the United States relating to the increasing concentrations of pathogens in drinking water. KWR launched the European project MicroRisk in 2002. The aim was to develop the QMRA method for drinking water utilities. In addition to drinking water itself, we later developed a QMRA methodology for the distribution network. KWR is also working with partners to update and revise the tables used by the World Health Organization to show the microbiological impact of treatment. And in the recently completed European project PathoCERT, KWR contributed, among other things, knowledge about using QMRA to manage microbiological risks in emergency situations.”

Horticulture

What Smeets finds interesting is that end users outside the drinking water sector can also benefit from QMRA. “As in, for example, a TKI project that developed a virus sensor for horticulture,” he explains. “QMRA can also be of interest for organisations such as water authorities, for example when they want to use wastewater for purposes such as irrigation rather than discharging it.” In addition to its usefulness in decision-making about risk management, Smeets also points out another effect of QMRA. “When you try to quantify the treatment process, you start looking at it in a different way, and that resulted in improvements.”

Making drinking water from effluent

From a practical perspective, Han Vervaeren endorses the benefits of QMRA. In addition to the traditional sources of surface water and groundwater, drinking water utilities are also looking at alternative sources to make drinking water, such as effluent from wastewater treatment plants. The high level of microbiological contamination in this water means we need an understanding of how to arrive at technologically effective and robust treatment. A practical example is the European project B-Watersmart, in which De Watergroep participated. Vervaeren: “It used QMRA to look at how our current treatment approach would have to be modified to produce drinking water with good microbiological quality from effluent. The results of this study were decisive when it came to including this scenario in a broader evaluation that will hopefully help to improve the safety of water supplies in our region.”

Chemical safety: QCRA

In addition to microbiological risk analysis, the quantitative approach is also useful for estimating the chemical safety of water. In this context, it is known as QCRA (quantitative chemical risk assessment). Even so, the two methods are different, says KWR researcher Thomas ter Laak. “By comparison with micro-organisms, there are many more different chemicals that can cause problems. And the properties of those chemical substances also vary enormously. In addition, chemical substances have all sorts of metabolic products and mixtures of substances have different effects. In short, you often need to use a combination of techniques in your treatment processes to address the entire spectrum.”

Removal index

Whereas QMRA is a sort of methodology, Ter Laak describes QCRA as an ‘umbrella’ for studying the robustness of treatment plants in terms of removing chemicals. “With the drinking water sector, KWR has developed things like a removal index, which shows the treatment needed to ensure that drinking water meets the requirements of the Dutch Drinking Water Decree. In fact – like QMRA – it is a yardstick to determine the treatment required for a given source of drinking water.” This assessment is only possible if a standard exists for the chemicals in question, but those standards have not always been established. “For all the substances that are not on the list, we also want to know how they behave in the environment and during treatment,” is how Ter Laak describes the ambitions for QCRA. “The drinking water utilities measure many more chemicals than are required by law. This gives us the opportunity to test the extent to which standardized substances are representative for non-standardized substances.”

QMRA as a tool

Although QMRA is quite complicated as a theoretical model, the results it provides are not difficult to interpret, thinks Vervaeren. “For us, it is important for KWR to look for the right applications where QMRA is helpful. And in all honesty, drinking water treatment has been around longer than QMRA. We can already work on the basis of past experience, but I see QMRA as a tool to build up a deeper insight into the microbiological removal efficiency of different technologies. That allows you to design new concepts better and to optimise existing systems.”