A method to estimate the emissions of OTC drugs to the sewer system

  • O Schut

Student thesis: Master's Thesis


An increasing number of chemicals enter the market each day and reach via different pathways eventually the environment. Especially cities are hotspots of pollutant emissions of Contaminants of Emerging Concern (CEC) and other chemicals. One of these emissions origin from household consumption of over-the-counter (OTC) drugs, i.e. drugs that can be obtained without a prescription. OTC drugs consumption leads to emissions to the sewer system. Waste water transfer plants (WWTPs) are not capable of fully removing pharmaceuticals from the influent. Consequently, WWTPs’ effluents discharged to surface waters may contain varying small amounts of pharmaceuticals which may lead to negative effects on the environment.
The SUSPECt project has been initiated to “develop a set of flexible decision-support tools that can be used for location-specific analysis, risk-based prioritization, and cost-effective control of CEC and other chemicals”. This study contributes to one of SUSPECt’s project goals of developing a decision-support tool to analyse CEC emissions from households. Therefore, in this study, a method has been developed to quantify consumption in Nijmegen of OTC acetaminophen, diclofenac, ibuprofen, and naproxen, and accurately estimate emissions to the sewer system.
Based on our literature search we have reviewed which emission estimation methods for OTC drugs, and which (international) input data were used for these methods. Most emissions estimation methods have four important steps. First, to obtain sales data of the drugs via private or public sources. Second, calculation of sales volume in kg Active Pharmaceutical Ingredient (API) by multiplying wholesale data of prescription or OTC drugs with the total Defined Daily Dose (DDD)per drug. However, DDD, although the recommended methodology tocalculate daily drug use, may not reflect real consumption since a part of thepurchased OTC drugs is not consumed. A part may be trashed, kept at home,flushed down the toilet, etc. Third, calculation of excretion to the sewer by theuse of human excretion factors, for the parent form, and optionally in somestudies, also for metabolites. Both forms may have detrimental effects on theenvironment. Finally, validation of the estimates by comparing estimated WWTPsinfluent of OTC drugs with influent concentrations measured at several WWTPs,i.e. comparing Predicted Environmental Concentration (PEC) with the MeasuredEnvironmental concentration (MEC).
Our emissions estimation method uses a mix of largely local and partially national private wholesale data as input data. There is no public data on the sale of OTC drugs available in the Netherlands. Due to limitations in the local data, we partially used national data to fill in the missing data. However, usage may vary between regions. If a regional estimation is needed, and local data is available, this data may yield more accuracy. If local data is missing, national data can be used. If national and local data is missing, data from other countries may be an alternative. When national data per capita are converted to regional data, possible local differences are ignored. This could potentially lead to an increase in the uncertainty of the estimates.
Based on input data of sales of OTC acetaminophen, ibuprofen, naproxen, and diclofenac in Nijmegen we calculated oral/dermal/rectal OTC drugs consumption
(66,7%), leftovers (13,3%), flushed drugs (1% of leftovers), wash off (ibuprofen 88,32% and diclofenac 94,1%) and urinal and faecal excretion (mean values) for both metabolites, and parent compounds. Total emissions to the sewer system were the sum of flushing, was off and urinal and faecal excretion. Total predicted emissions of parent compounds in 2018 for OTC acetaminophen, ibuprofen, naproxen, and diclofenac, were respectively; 366.27, 91.66, 6.09, and 2.90 kg/API. Total predicted emissions of conjugates were significantly higher for acetaminophen, ibuprofen, and naproxen. Respectively; 5,711.09, 415.93, and 27.88 kg/API. In contrast, emissions of diclofenac conjugates were 3.16 kg/API, very similar to the emitted load of parent compounds.
Our validation showed underestimations in predicted emissions in all locations between 1.08 and 2.31 for acetaminophen and between 1.09 and 2.23 for ibuprofen. This may be due to the restrictions in data and method. However, for diclofenac, we found large overestimations with factors between 9.36 and 48.14 for all locations. One explanation for the overestimation of diclofenac could be due to using a too-high parameter for the washed-off part. For naproxen, we found overestimation factors between 2.40 and 15.52. A second explanation may be due to degradation in the sewer system of diclofenac and naproxen. Conjugates were not measured in the Nijmegen sewer system and could not be validated.
We explored the possibility of estimating sales of OTC drugs in Nijmegen based on readily available OTC and prescription drugs sales data from the Nordic countries. By applying the ratio of OTC/total sales from Norway to Nijmegen, we calculated the OTC drugs sales of paracetamol, ibuprofen, naproxen, and diclofenac in Nijmegen. The input data obtained via this method yielded underestimations between 1.9 and 5.6 compared to the sales data obtained via private data. Because of these underestimations, and having an alternative, we decided not to use the Norway data for estimating emissions. However, if there are no possibilities to obtain input data, this approach can be followed. Possible underestimations should then be taken into account.
Date of Award15 Nov 2022
Original languageEnglish
SupervisorAd Ragas (Supervisor), Jikke van Wijnen (Examiner) & Lily Fredrix (Co-assessor)

Master's Degree

  • Master Environmental Sciences

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