2021

'Limburg Water Board screens effluent from sewage treatment plants, as well as drinking water-relevant substances'

Since 2020, Limburg Water Board has been using the same screening technique as the one used by the drinking water sector. This is to gain a better view of the composition of effluents coming from these plants. Gabriël Zwart from the Limburg water board reports on new possibilities for monitoring new substances that are arising through the use of screening methods.

Water quality and sewage treatment plants

Gabriël Zwart is a senior consultant at Limburg Water Board. "My remit is broad: it concerns monitoring water quality and the analysis of the data, its interpretation, and finally providing recommendations to the organisation and management."

The topics related to surface water quality and sewage treatment plants (STPs) are two separate tracks at the water board. "However, the continuous attention for new substances that is demanded by the drinking water sector served as a motivation for the water board to make extra efforts to link these two tracks together.

From a practical standpoint this is now possible too because the methods for detecting organic micro-contaminations have improved and can be applied regularly. In 2019, the project 'Small Screening of the Meuse Region' shed light on the potential of these methods and what they can provide."

Close screening for a wide view

Gabriël draws attention to a large monitoring programme in the Meuse region, in which the drinking water companies, provinces, water managers, Rijkswaterstaat and the Brabant and Limburg Water Boards acted together. "The intention was to develop a wider view of the water quality of the Meuse. To this end, the library screening method, which is also used by drinking water companies, was applied to the screening of the effluent from STPs for organic micro-contaminations."

The screening method works as follows. "The starting point is the substances library, a list of 1,800 to 2,000 known organic substances. Next, water samples are analysed using chromatography (to separate the substances) followed by high resolution mass spectrometry (to identify the substances based on their mass). The peaks detected in the water sample are compared with the peak patterns in the substances’ library. In 2019, this screening was carried out by Het Water Laboratorium (HWL)."

Gabriël was immediately captivated by the method: "Of the 1800 substances in the library, it emerged that 500 different substances were actually present in the effluent from the STPs. In other words: if we want to know more about new substances in the surface water, we can extract more information out of the STP effluent with this technique."

Seventeen sewage treatment plants under the microscope

In 2020, following this inspiring Meuse-wide collaborative project Limburg Water Board decided to implement the screening method itself. "This involved 17 STPs in the area. In this approach the screening is repeated from two to six times. This is to see which other substances (besides all the medication residues) could also be relevant to the ecosystem.

With the results from the screening, we can't yet say anything about how problematic the substances that were found are, nor about possible breaches of standards. For this, target substance analyses are needed to determine the concentrations of the substances. However, after the screening you already have a good indication of the wide spectrum of substances that are present."

In 2021, the Aa en Maas Water Board also decided to investigate several STPs in the area with screening. "We then combined their data with ours.  It was time for the following step: the interpretation of the results. The question is: which of the detected substances can potentially become a problem for the water quality? For this assessment, we hired the external consultancy company Ecovide."

Interpretation of the results

The point of the assessment for the water board is mainly related to the risks posed to the ecosystem. "Which of the 500 substances we detect are problematic from an eco-toxicological perspective? In our case, it proved to be biocides, medication residues (800 in the substances library) and a few industrial substances. Besides these, we also found consumer products and (illicit) drugs. Particularly this last category is new, and therefore interesting for further research."

He continues: "Until now, the monitoring of drugs has been problematic, because these substances are strictly regulated. The laboratory must have a permit to be allowed to work with such target substances (drugs). This makes the analysis very expensive. But with the screening technique, you can just include the drugs in the substances library. There are around 40 types of (illicit) drugs in this list. Of these, we have actually found around 30 in the effluent from our sewage plants."

Together with the eco-toxicological interpretation, the Limburg Water Board's list of 500 detected substances was also presented to the drinking water companies. "Evides and Aqualab Zuid particularly helped us in the interpretation of the substances (more on this matter in following sections of the report). Based on the two angled approach (eco-toxicology and drinking water relevance), follows the development of a list of substances is which will be used by the Limburg Water Board in coming years for further investigation. This will happen with target analyses."

Innovation at sewage treatment plants

For example, the list of targets substances to be monitored will be used for research into innovations and at STPs in the area. "You can't do everything all at once. To allow the sewage plants to be prioritised in the coming years, we will first determine which belong to the 'hot-spots'. In other words: where does the discharge of effluent have the greatest impact on the abstracted water? To give an example: the Venlo STP discharges into the Meuse, where the effluent is highly diluted. This is not a hot spot. But another STP that discharges directly into the Geleenbeek, is one."

Gabriël continues: "Once we identify the hot-spots, we investigate the purification efficiency of particular measures at the STP in question. We are doing this for example at the Simpelveld STP in Zuid-Limburg, which discharges into a very small rivulet. There, the Waterschapsbedrijf Limburg (Limburg Water Board Company) -commissioned by the water board - is looking at the effects of active carbon dosing on the substances discharged in the effluent. Because wastewater is treated on two lines in this plant, we can compare the effect of dosing with carbon powder well. Carbon is added to one stream, but not to the other."

For the analysis of the effluent, the water board not only uses the prescribed list of guide substances, but also a number of relevant substances that emerged from the library screening. “There are also a number of drinking water-relevant substances in there."

Drinking water-relevant substances in one go

This is also new. Normally, the water board tracks the effects of discharges on the ecosystem. The fact that the water board can now also look out for drinking water-relevant substances is the result of successful recent collaborative projects in which screening techniques are being used.

Gabriël: "Drinking water companies have been banging the drum for ages because particular substances are causing them trouble. In the past for example, RIWA-Meuse was busy trying to get glyphosate and AMPA on the map. These substances were at that time less relevant to the water board because they are removed in the STPs. But we did then participate actively in monitoring them.

Besides this, we realised that we ought to pay more attention to other emerging substances. With the coming of the new screening techniques, this is also a practical thing to do. In other words: we've experienced the value of the library screening ourselves."