Please note that this article was previously published in H2O’s Water matters
The Watson database collates monitoring data from water managers concerning micropollutants in influent and effluent from wastewater treatment plants. These data can serve as input for emission estimates for substances. These estimates can provide information regarding the origin of micropollutants that are hard to assess using other methods, as this article demonstrates for (the personal use of) over-the-counter pharmaceuticals for humans and pets. With the most recent update to the Watson database, it has also become possible to derive trends for some (groups of) substances, such as PFAS, from the time series of measurements: a useful tool for evaluating the effectiveness of policy measures.
Water managers are tasked with ensuring good water quality in their management area. Insight into the origin of pollutants – and thus into how emissions can be limited – is a key part of this. The Netherlands Pollutant Release and Transfer Register (Emissions Register) and the accompanying Watson database are important tools in this. Deltares and AD eco advies have updated the Watson database. This article highlights the key results from the analysis of new data, and some of the resulting exciting applications.
About Watson Database:
The Watson database is often used to estimate emissions from the wastewater chain to surface water. The database contains monitoring data for micropollutants in the influent and effluent of Dutch wastewater treatment plants (WWTP), and is freely accessible to anyone (website: Nederlandse EmissieRegistratie). Emission Factors (EF) and Purification Efficiencies (PE) for substances from wastewater treatment plants are derived from the data (see box) as recorded in the Netherlands Pollutant Release and Transfer Register (Emissions Register). The EF is the emission of a substance (the occurrence in the wastewater treatment plant influent) in grams per inhabitant; the PE is the percentage of this that is removed by a wastewater treatment plant. These parameters can be used to determine emissions from the wastewater chain to the surface water.
Deltares and AD eco advies have completed an update of the Watson database, compiling data for the period from 2014 to 2018. These data were further analysed, and yielded a wealth of interesting data that can be used for all sorts of applications. Thanks to the update, the database now includes data about 1,310 substances, of which 112 have been included in the Emissions Register following further analysis. This article highlights the most interesting results of the analysis and a number of applications.
Determining emission factors and purification efficiencies
The monitoring data in the Watson database of the Netherlands Pollutant Release and Transfer Register (Emissions Register) are obtained from regional water managers. If they are to be usable in terms of extrapolating reliable emission factors and purification efficiencies, these data must meet a number of conditions. For example, to determine the emission factor, at least seven measurements must be available, taken within one year and from a minimum of three different wastewater treatment plants. To determine the purification efficiency, the above data must furthermore be available for three different years. If there are sufficient measurements above the reporting limit (RL), calculation of the emission factor is based on the median value (>50% RL) in the effluent or the influent. Is there is insufficient information available, the average is taken (>25% RL). Values <RL are not included in the calculation of the purification efficiency.
The Watson database already held data on a number of pharmaceutical residues. The recent update has increased the volume of data, and so for a number of a substances, a new or improved emission factor or purification efficiency value has been derived. For example, the key figures for 20 pharmaceutical residues were improved, and 12 substances were added to the Emissions Register. By way of validation, we compared our results with emissions estimates based on sales figures combined with excretion factors. The comparison shows that the results from the two methodologies are generally in line with one another, see figure 1. There are complementary results for a number of pharmaceuticals. These are pharmaceuticals whose sales and/or usage are not well documented, such as over the counter pharmaceuticals (medicines that can be sold freely, for example paracetamol, ibuprofen and diclofenac). The measurements provide a better picture of these. For substances that are broken down quickly, or for which there is no effective analysis method available, emission estimates based on sales figures provide a better overview. Examples of this latter category are cytostatics and metformin.
Figure 1. Concentrations of pharmaceutical residues in wastewater treatment plant influent, measured (x-axis) and estimated on the basis of use (y-axis)
Perfluoroalkyl and polyfluoroalkyl substances (PFAS) have recently attracted a lot of attention because of the ubiquitous nature of these toxic substances in soils, surface water and sediments, and the associated risks. Much remains unknown about the sources (type, location, size) from which PFAS reach the environment. The contribution of wastewater treatment plant effluents to the levels in surface water was determined based on the available calculations through to 2018. For a number of compounds in the PFAS group, there are sufficient data to derive both a purification efficiency and an emission factor.
Because the production, along with a large number of applications, of PFOS have been prohibited since 2010, the study looked at whether the concentration of PFOS in effluent has also decreased since then. The available measurements show that this is indeed the case; since 2010, the average concentration in effluents has decreased by 75% (figure 2). This is in line with the patterns of concentrations in surface water over the past ten years. These results suggest that emissions from wastewater treatment plants and flows from abroad together substantially determine the concentrations in surface water (and ultimately in water beds). Further study will be needed to confirm this.
It is notable that some wastewater treatment plants in South Holland have an increased concentration of PFOA compared with the rest of the Netherlands. This is probably the result of atmospheric deposition in the past, caused by the former PFOA production site in Dordrecht, which still causes increased concentrations in the wastewater chain through runoff and leaching from urban run-off and from wastewater.
Figure 2: Progression over time of PFOS concentrations in wastewater treatment plant effluent, based on available measurements. The wastewater treatment plants samples are taken from may vary each year. The boxplots indicate the median (horizontal line), the 25 and 75 percentile (the blue box), and the range (black ‘whiskers’) in concentrations.
Biocides and crop protection agents:
A group of substances also found regularly in both influents and effluents are pesticides for domestic applications. These include insecticides (such as fipronil and imidacloprid) and herbicides (for weed control on paved surfaces, such as glyphosate). The emissions from a number of these agents has already been estimated in the Emissions Register, but via different methods and routes. Since these emission estimates are quite rough, the study looked if the data from the Watson database could improve these emission estimates.
Limited use of imidacloprid is still permitted in greenhouse horticulture, and is also found in pet medical treatments (to combat fleas and ticks). Whereas waste water from greenhouses used to be discharged directly into surface water – usually barely purified – today the emission route is increasingly through the sewage system. It is estimated that in 2018, 91 kg reached the surface water via wastewater treatment plants. It is difficult to estimate what proportion of this came from greenhouse horticulture, and what proportion from private households. It is notable that relatively high concentrations are found at wastewater treatment plants that are not located in greenhouse horticulture areas. This implies that imidacloprid emissions as a result of use for animals are a substantial source.
This is also the case with the insecticide fipronil, which is used only for pets. From monitoring data, it was calculated that in 2018, 22 kg of fipronil reached the surface water via wastewater treatment plants. Provisional rough emission estimates were recently made for both fipronil and imidacloprid, based on application scenarios (Lahr et al., 2019). According to these scenarios, the loads for both substances calculated via Watson would be more than sufficient to exceed the standard for both substances. The use of veterinary medicines in pets will be further elaborated in the coming years in the theme ‘Veterinary medicines: sources, routes and risks’ within the Kennisimpuls Waterkwaliteit (Water Quality Knowledge Impulse) (see the Kennisimpuls Waterkwaliteit website).
Herbicides on paving:
Herbicides such as glyphosate are also regularly found in domestic wastewater. Glyphosate is rapidly converted into aminomethyl phosphonic acid (AMPA) in the environment, which means that the glyphosate concentrations measured underestimates the actual emissions Additionally, these substances may be used by both private individuals and public authorities, which makes the use of monitoring data less appropriate. Public authority use of herbicides on paved surfaces and in public green spaces has become severely restricted in recent years; private individuals are still permitted to use these products without restriction. In 2019, the CBS (Statistics Netherlands) and the RIVM (National Institute for Public Health and the Environment) published new figures on use by public authorities and private individuals. Based on this, we made new estimates for these agents. These estimates reveal that the emission of a substance such as glyphosate by private individuals (3,172 kg) is many times higher than use by public authorities (2 kg) and the agriculture sector (37 kg).
The monitoring data obtained from regional water managers, collated in the Watson database, are a good basis for estimating emissions for these substances. This can provide additional information about emission routes that is difficult to identify in other ways. With the most recent update to the Watson database, it has also become possible to derive trends for some (groups of) substances from the series of measurements. As a result, it is possible to evaluate the effectiveness of policy measures.
- Lahr et al.(2019) Diergeneesmiddelen in het milieu, een synthese van de huidige kennis. STOWA report 2019-26