Please note that this article was previously published in H2O’s Water matters
Cover image courtesy: wastemanagement.in
A measurement campaign at 18 wastewater treatment plants in the east of the Netherlands provides insight into the removal efficiencies of pharmaceutical residues. The 11 guide substances reveal an average removal of around 30%.
Virtually all surface waters in the Netherlands contain traces of micro-pollutants: pharmaceutical residues, plant protection agents, household chemicals and industrial contaminants. These substances are in low concentrations, varying from a few nanograms to micrograms per litre.
Although the concentrations are low, there is growing evidence that these substances negatively impact the aquatic environment. A large portion of the micro-pollutants in surface water is directly related to effluent discharges from wastewater treatment plants (Moermond et al, 2016).
Wastewater treatment plants remove these substances to varying degrees, ranging from 0% and 99%. This is in part dependent of the properties of the substance (Watson database, Maas et al., 2018, and in comparison with STOWA 2018-02, STOWA 2018-46). The removal efficiency for specific substances can vary enormously per wastewater treatment plant (Maas et al, 2017).
This article is the follow-up to a publication in the previous issue of Water Matters (September 2019). That article described a measurement campaign that was designed to determine the impact of pharmaceuticals discharged from seven wastewater treatment plants operated by the Aa en Maas Water Authority (Evenblij et al, 2019).
This article briefly describes the insights into the removal efficiencies obtained during a measurement campaign at 18 wastewater treatment plants in the east of the Netherlands. It looks specifically at the results of removing pharmaceutical residues, and not at the other organic micro-pollutants measured.
Two research questions were developed prior to the study:
- What is the removal efficiency of the micro-pollutants?
- Are there any simple process parameters that can influence the removal efficiency?
The measurement campaign was carried out at 18 wastewater treatment plants in the management areas of five water boards: Drents Overijsselse Delta, Zuiderzeeland, Vallei en Veluwe, Vechtstromen and Rijn en IJssel. In February and July 2018, three 48-hour samples of influent and effluent were taken from these wastewater treatment plants. The samples were taken in a period of dry weather, over a period of approximately 10 days.
An analysis package was established for the project, comprising organic micro-pollutants, macro-parameters and metals. When compiling the package of substances, account was taken of problem substances that occur in the surface water, the RWS draft list (2017) with 11 (recommended) ‘guide substances’ and ‘other substances relevant for monitoring effluents’, the substances measured within comparable projects such as the PACAS project at the Papendrecht wastewater treatment plant and substances that are prominent in the Watson database.
Next, the available budget was used to align as far as possible with standard laboratory analysis packages. A deliberate decision was made in favour of ‘limited’ analysis of the number of micro-pollutants to allow for more frequent measuring.
This study did not consider the effects on the water system.
Analysis of pharmaceuticals
The pharmaceuticals were analysed using positive ionisation liquid chromatography–mass spectrometry (pos-LC-MSMS). Of these, 57 pharmaceuticals and two metabolites were determined. The effluent samples were taken by elution with added labelled internal standards without additional dilution, and injected into the LC-MSMS by direct injection. The influent samples were first diluted five times to reduce matrix effects, and subsequently measured in the same way as the effluent samples. The analyses were carried out by Aqualysis.
The influent concentrations of pharmaceutical residues measured at the 18 waste water treatment plants examined varied from below 0.1 µg/l to hundreds of µg/l. The total amount of the 59 pharmaceutical residues analysed in the influent was on average 464 µg/l. More than 75% of the load comprised paracetamol (painkiller) and metformin (controls the blood sugar). In effluent from the wastewater treatment plants, the total concentration was considerably lower (21.1 µg/l), and there is also a different ‘top 11’ here than in the influent (figure 1).
Figure 1 (a and b) Pharmaceutical residues in wastewater treatment plant influent and effluent, in micrograms per litre. The substances mentioned by name are those with the highest concentrations, averaged across 18 wastewater treatment plants. The numbers are the average concentration per substance.
Although a waste water treatment plant effectively removed metformin, the effluent concentration measured was still greater than 1 µg/l. The total removal efficiency across the total load of pharmaceutical residues was over 90%. This removal is largely influenced by the high load of the easily removable substances metformin and paracetamol. Without these two substances, the total load removal of pharmaceutical residues would be between 60-85%, depending on the wastewater treatment plant.
Removal of guide substances
The Micro-Pollutants Innovation Programme was established by STOWA in collaboration with the Ministry of Infrastructure and Water Management. This programme uses 11 guide substances to evaluate the effectiveness of the removal techniques. The average removal efficiency of these 11 guide substances, as derived from the measurement campaign, are presented in figure 2.
Figure 2 Average removal efficiency of the 11 guide substances from the Micro-Pollutants Innovation Programme, per wastewater treatment plant.
The average removal efficiency per wastewater treatment plant varies from 9% to 53%. Overall, across all 18 wastewater treatment plants, the average efficiency for the 11 substances is around 30%. In both the summer and the winter period, several substances regularly returned a negative removal efficiency: 22 in the summer and 37 in the winter. This demonstrates that a number of substances in the influent were not measured or were insufficiently measured, while those in the effluent can (apparently) be measured more accurately. It is also possible that a substance in the influent is included as a non-measured metabolite and is thus ‘invisible’, to then appear at the wastewater treatment plant as the parent component, which was measured, after the conversion processes.
Loads and concentrations of the substances from the ZORG project
In 2011, an inventory was completed of the emission of pharmaceuticals from care institutions (STOWA 2011-2). The study looked at the supply and emission of 25 substances at eight wastewater treatment plants. In the study, the average removal percentage for the substances measured at the time (excluding metformin) (total load removal) was 65%.
By comparison, this percentage for the 18 wastewater treatment plants in this study was also calculated and presented in figure 3 (the blue bars). The average removal efficiency was 78%, which is significantly higher than the 65% as measured in the ZORG project.
Figure 3 presents the removal of pharmaceutical residues in a different way, i.e. as an average removal percentage per substance. For this, the percentage reduction in concentration was calculated for each substance individually. Next, an average of these values was calculated for all the substances considered (in the same way as the calculation for the removal of the 11 guide substances): the green bars in figure 3. This was done for the same list of 25 substances from the ZORG project. On average, the concentrations of these substances in the effluent is 46% lower than in the influent.
Figure 3 Comparison between total load removal and percentage reduction in concentration of the 25 substances from the ZORG project, for the 18 wastewater treatment plants considered in the Rhine-East area.
Further, it appears that this approach shows (small) changes compared with the total load approach. Both approaches are necessary, however, to determine to what extent the discharge of pharmaceutical residues forms a risk for the receiving surface water.
That is a study in itself which involves a number of other factors, such as the function and quality of the surface water discharged to. With reference to the previous article about the measurement campaign at Aa en Maas Water Authority, it can be said that this type of measurement data provides input to determine a ‘ranking’ of wastewater treatment plants as further detailing of, for example, the Hotspot Analysis Pharmaceuticals at wastewater treatment plants, carried out in 2017.
Correlation between removal and operational characteristics of the wastewater treatment plant
In this project, a number of characteristics of the wastewater treatment plant are noted each time a measurement is taken so that possible connections can be made. The search here was focused on the correlation between the wastewater treatment plants’ process circumstances and the removal of pharmaceutical residues.
The following characteristics were analysed: temperature, hydraulic retention time, sludge age, quantity of heavy metals (with copper used as an indicator), and the presence of internal load from dewatering of digested sludge. The technological set-up of the wastewater treatment plant was also considered. As figure 3 shows, there is a wider distribution for the average removal than in the total load removal, such that the expectation was that any links here could be demonstrated more strongly. Therefore, the technological parameters mentioned are correlated to the average removal of the substances per wastewater treatment plant.
However, no statistically significant links were found between the parameters considered and the removal of pharmaceutical residues. The only significant relevant parameter appeared to be the temperature. In the summer period, the average removal efficiency across all the substances measured (excluding paracetamol and metformin) is 55%, and 32% in the winter.
A large level of pharmaceutical residues was found across all influents, ranging in concentration from less than 0.1 to hundreds of micrograms per litre. The average overall removal efficiency for micro-pollutants is highly dependent on the micro-pollutants considered, and whether the removal is calculated on the basis of the total load or on the basis of the average of the removal efficiencies for individual substances. Based on the average of efficiencies for individual substances, bigger differences were found between wastewater treatment plants than on the basis of the removal of total loads.
The 11 guide substances from the micro-pollutants innovation programme, which are also included in this study, reveal an average removal of around 30%. The performance of individual wastewater treatment plants ranges from 9% to 53% for the 11 guide substances.
The differences between wastewater treatment plants cannot be linked to simple technological parameters or the purification concept. It is possible that a wastewater treatment plant’s system configuration may influence individual substances or groups of substance; this was not examined in the study. The removal of micro-pollutants proved to be mostly strongly linked to temperature. A significantly higher removal efficiency was found in the warm summer period than in the cold winter.
The data collated for this project can be requested from Drents Overijsselse Delta Water Authority.
- Evenblij, H., Schoffelen, N., Knoben, R., Hulst, W. v.d. (2019) Rangschikking RWZI’s op basis van metingen aan geneesmiddelen, Water Matters 1 (9), 36-39.
- Maas, P. van der; B. Bult; H. de Vries; O. Kluiving; 2017; Verwijdering van acesulfaam in rioolwaterzuiveringsinstallaties: wat bepaalt het verschil?, H2O, 17 July 2017
- Moermond, C. et al, Geneesmiddelen en waterkwaliteit, RIVM, 2016-0111
- Wubbels et al. Biologische fingerprinting biedt inzicht in verwijdering van medicijnen en zoetstoffen in RWZI’s see here
- STOWA 2017-42 Landelijke Hotspotanalyse geneesmiddelen RWZI’s
- STOWA 2018-46 Zoetewaterfabriek awzi de Groot Lucht: pilotonderzoek ozonisatie en zandfiltratie
- STOWA 2018-02 PACAS – Poederkooldosering in actiefslib voor verwijdering van microverontreinigingen
- Watson database in the emissions register; http://www.emissieregistratie.nl/erpubliek/erpub/wsn/default.aspx