Assessment of chemical water quality for safe recycling
Climate change is causing longer dry periods and a growing demand for water. This makes the future availability of sufficient freshwater of good quality a huge challenge. A circular approach could provide a solution: the recycling of purified sewage water or industrial wastewater can (temporarily) compensate for shortages. How can we improve guarantees that water is safe for recycling?
At present, the ‘water balance’ in the Netherlands is unhealthy. 977 Mm3/year more groundwater is drained than is supplemented naturally. At the same time, wastewater treatment plants (WWTPs) and industrial wastewater treatment plants in the Netherlands discharge 2160 Mm3/year into the surface water . Recycling this effluent could restrict desiccation and help meet the demand for water.
In principle, water can be recycled for many different uses. Examples are drinking water, agricultural irrigation, cooling, (industrial) process water, nature, recreation and supplementing groundwater.
Water recycling policy
However, recycling water also raises questions. Water may be polluted by biological and chemical components from previous use. This may make it unsuitable for recycling. However, it is striking to realise that we generally discharge purified wastewater into surface water. Downstream, this surface water is then used for various purposes, such as irrigation . Actually, this means that purified waste water is already being ‘reused’. However, any risks that this reuse entails are currently not structurally identified, nor are they minimised. Irrigation by means of WWTP-effluent is subject to a European directive . The directive provides for permit requirements, whereby risks must be identified and monitored. For other forms of water reuse, national and/or international policies or regulations are still lacking. For example infiltration in the soil, where monitoring the water quality could be important to preventing negative effects on the ecosystem. Quality requirements and the required legal framework for water reuse in the drinking water sector  have been considered in the research programme Water in the Circular Economy (WiCE).
Assessment of water quality for responsible reuse
Chemical safety plays an increasingly important role in reusing water from alternative sources, besides microbiological safety. A clear picture of the water quality is required to assess what purification efforts need to be made to make water suitable for responsible recycling.
However, assessing the chemical water quality is a special challenge due to the extremely high number of (often unknown) micropollutants that may be present in used water. Information about the origin, possible (mixture) toxicity and the fate (‘where do the substances end up in the environment’) of micropollutants is often lacking. The usual method for assessing chemical water quality often focuses on a limited number of substances (target substances), whether the monitoring of drinking water, surface water, purified WWTP-effluent or industrial wastewater is concerned. Given the large number of potential micropollutants, an incomplete picture may therefore be given. Presumably, this is too limited for a sound assessment of safety in the event of recycling. But there are more analysis methods available, namely target analysis, non-target screening and bioassays. A combination of these methods could increase confidence in the outcome of a water quality assessment (Figure 1).
Figure 1. Methods for water quality analysis may produce more results if they are applied in combination
Using innovative tools to discover relationships between substances and trends over time, and using new methods for toxicological determinations (for instance QSARs and machine learning), it is possible to prioritise smartly while still getting a grip on which micropollutants are the most important. Although the specific substance selected for targeted analysis are often carefully chosen, the drawback is that this approach fails to identify all the substances potentially present. In fact, the selected substances might not the most representative in every case for potential risks to people and the environment.
Combining analysis methods
In the chemical-analytic area, suspect screening and non-target screening (NTS) have really taken off. These technologies enable us to detect an extremely broad spectrum of substances. In just one analysis, suspect screening can identify many expected pollutants in water (due to be reused). Using non-target screening it is even possible to detect as yet unknown chemical substances in reusable water, and to provisionally identify these without reference material. Using data analysis, additional information can be obtained from the results, for instance about sub-structures, estimated concentrations or the formation of transformation products. For new unknown substances, knowledge about (sub)structures can be used to estimate possible toxicity by quantitative structure-activity relationships (QSARs) or recently developed machine learning models. Estimated concentrations can be tested against substance-specific safe concentrations or generic risk thresholds (threshold of toxicological concern; TTC). However, for more reliable testing against limit values, more accuracy is required regarding the concentration. This is not always easy to provide by means of suspect and NTS. A follow-up step could also be to confirm what substance is involved by means of target substance analyses, and to determine the precise concentrations.
Figure 2. Placing samples into the high resolution liquid chromatograph, for chemical analysis of a spectrum of known and unknown organic compounds as wide as possible
Biological test systems (bioassays, Figure 3) give insight in the combined effect of all substances present – whether or not detected chemically – on biological systems. A choice must be made as to which possible effects are looked into, as active substances may have different effects. Frequently, the bioassays chosen are those that are the most relevant for human health. There are also bioassays that are focussed on ecotoxicological parameters; these identify environmental risks. An increasing number of bioassays have so-called trigger values or signal values, which indicate when a response is so high that there might be a risk for people or the environment. Because harmful substances can nevertheless be present in the water in the case of a negative response in a bioassay, combining them with chemical analyses is important.
Figure 3. Bioassay in process
If chemical analyses or bioassays show non-acceptable exposure or risks for people and the environment, it is necessary to take measures. For some forms of water recycling, estimating risks for people and the environment is not simple. This is especially the case if the characteristics of substances indicate that they may also end up in food consumed by animals (cattle, wild animals, fish) or in crops. This broadens the issue from water quality to food safety. To ensure effective measures and set up routine monitoring, it is therefore necessary to understand the origin and the fate of the substances detected, including trends and conversions in the environment. Forensic methods are available for this purpose.
Water recycling: pilots and full scale applications
In recent years, multiple recycling pilots have been carried out. Below, three of these are described where WWTP-effluent and rainwater are reused as industrial water and for irrigation. In these pilots, diverse analysis methods were used to obtain insight into water quality, and we describe opportunities to obtain a yet more complete image.
1. Sub-irrigation with WWTP-effluent
Studies have been performed in Haaksbergen on irrigation with purified WWTP-effluent from an underground infiltration system. This pilot study examined the extent to which micropollutants in the soil are removed and can be washed into the groundwater. To this end, for several years, the groundwater was analysed for organic micropollutants at different depths and locations . More than 100 substances were measured, including transformation products. Also, their combined (mixture) ecotoxicity was calculated. A more complete picture thus arose of the distribution of the substances when purified effluent is used for irrigation. The results show that the irrigation of the soil does not just increase water availability during drought, but also that a considerable part of the organic pollution remains in the soil. Of the 133 substances, 89 were found in the field. Bioassays could provide more information about the possible effects of these remaining substances.
2. WWTP-effluent and rainwater as process water
Dow (previously: Dow Chemicals) in Terneuzen uses considerable amounts of process water. For this purpose, among other things, further purified effluent from WWTP Terneuzen is used. From 2019 to 2021, a pilot was carried out to examine whether this can be supplemented by rainwater . An important question was which substances are present in the various sources, how these substances behave in the process steps and whether they can form an obstacle for reuse as process water. Part of the study was also on the usability of ‘constructed wetlands’ for purification prior to reuse. Suspect analyses and NTS-analyses were especially used to screen more than 2,000 relevant compounds (including pharmaceutical products and pesticides). Some of these could not be identified in the samples measured with complete certainty, but nevertheless with sufficient certainty to give a general picture of the total burden. The suspect screening, for instance, showed the presence of the pharmaceutical metformin, as well as substances normally found in the effluent of WWTPs and not in the effluent of industrial wastewater treatment plants. Through NTS-analyses, the differences between influent and effluent samples were also identified. The location of specific substances in the Dow-process and of substances that are still unknown are could thus be seen. Insight into the suitability of the influent water as process water has been improved. It could also be important to examine the impact on the environment (in the ‘constructed wetlands’). Bioassays could provide more information about the potential (eco)toxicity of these compounds. This would enable an even more extensive assessment of the water quality.
3. WWTP-effluent or pre-purified surface water for high-quality recycling.
One example where, in addition to measurements of substances, bioassays have been used is the pilot study ‘Closing the Water Cycle in North-Holland’. This pilot aimed to examine whether WWTP-effluent or pre-purified surface water are suitable for high-quality recycling, for instance as irrigation water or even drinking water. Here, the effectiveness of various supplementary purification technologies for effluent of a WWTP plant were studied  by combining two techniques. The removal of eleven selected pharmaceuticals (also referred to as guide substances) was monitored by targeted analysis. The results were compared to outcomes of bioassays. The bioassays showed that when using advanced purification technologies, in most cases the levels of substances did indeed decrease. However, in some cases, hormone-disrupting activity, genotoxicity and oxidative stress increased. Consequently, additional measures are required for high-quality reuse. Using this combination of technologies (targeted analysis and bioassays), it is possible to determine the extent to which the guide substances are representative for the removal of a relevant toxicological effect. In this way, the pilot compares the quality of purified effluent to that of pre-purified surface water. The outcomes of the combination of technologies suggest that ozone treatment should be adjusted, in combination with other purification techniques, to enable high-quality recycling. The pilot therefore gives a more comprehensive picture of the improvement of water quality after purification than targeted analyses can give on their own.
Figure 4. Sampling groundwater from several depths (between groundwater level and twelve metres below ground level), on a plot in Haaksbergen. WWTP effluent has been infiltrating from drains here for several years. Monitoring well contains microfilters.
The described studies on the recycling of water require a combination of technologies to provide a broader picture of the number and the identity of (harmful) substances in water to be recycled. The pilot in North-Holland shows by additional research (bioassays) that the toxicity of these substances can also be determined, but this approach is not yet frequently used. We are therefore calling for a new framework for the assessment of the (chemical) water quality in the context of water recycling. We have shown that chemical and biological technologies are easy to apply. The technologies described here are available in various laboratories, both within and outside the Netherlands. However, to carry out the analyses, interpret the results and assess water quality requirements, specific expertise is necessary. It is therefore important to involve various experts in water quality assessment for water reuse. For a complete framework in which microbiological water quality is also assessed, experts in this area must also be brought in.
Summarising, in combination with knowledge of the origin and properties of the pollution, advanced chemical-analytical, biological and data-analysis technologies offer possibilities to properly assess water quality. This is necessary to consider possibilities for reuse and if necessary, to select risk-limiting measures.
An understanding is thereby required of the origin and the fate of the detected substances, including trends and conversions in the environment. Here too, different technologies supplement each other. Together, they give a more complete picture of the chemical water quality. This increases the reliability of the assessment of the water quality and reduces the likelihood of unexpected water quality issues.
(KWR Water Research Institute)
(KWR Water Research Institute)
(KWR Water Research Institute)
(KWR Water Research Institute)
Thomas ter Laak
(KWR Water Research Institute)
Sampling at sub-irrigation location Haaksbergen
Climate change causes longer dry periods and a growing demand for water. Recycling water, for instance for agricultural irrigation, cooling, (industrial) process water or nature (groundwater recharge) can help to meet the growing demand. The water for recycling can be rainwater collected, but can also come from sources such as WWTP-effluent or industrial waste water.
The contamination of water that is to be reused can lead to risks, depending on the intended use. A thorough assessment of the risks and the substantiation of the selection of risk-restricting measures, if any, are necessary. To this end, the usual analysis technologies to determine water quality produce information that is too limited. This article proposes using a combination of analysis technologies which together, give a broader and more reliable picture.
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