Strategies for mitigating organic micropollutants in drinking water

Groundwater is an important freshwater resource, from which more than 60% of European drinking water is produced. However, increasing detections of organic micropollutants (OMPs) in both groundwater and drinking water through enhanced analytical methods, have caused an emerging concern for wholesome and healthy drinking water as we know it. The frequencies and concentrations of recently detected OMPs e.g. pesticide metabolites challenge the current water supply. Groundwater protection as a sole strategy has failed to mitigate OMPs from drinking water, and since conventional treatment is insufficient to remove OMPs, additional treatment is necessary. Meanwhile, organic materials e.g. pipes of polyethylene (PE) are increasingly implemented in the distribution systems causing migration of OMPs, which may affect the drinking water quality after treatment, during storage and distribution.

The aim of this Ph.D. thesis was to identify and investigate strategies to mitigate OMPs with the focus on treatment and materials, in order to maintain security of supply and a high drinking water quality now and in the future.

Since knowledge on most recently detected OMPs is limited, a simple method was suggested to theoretically assess whether they can be removed with either granular activated carbon (GAC) or membrane filtration. Predicted sorption capacities and estimations of spherical molecular sizes were validated against reported scientific literature through which cut-off criteria were identified for efficient removal. GAC filtration was not identified as an economically efficient treatment strategy for all OMPs e.g. very polar pesticides metabolites like N,N-dimethylsulfamide (DMS), since it requires a frequent exchange of GAC material. Membrane filtration could retain most OMPs, however, water loss and discharge permits are major drawbacks of this treatment strategy. The potential for biological degradation in sand filters was explored as a sustainable retrofit treatment through various batch experiments. Nevertheless, the identified removal rates were insufficient for its further application in full scale, highlighting the persistence of recently detected pesticide metabolites e.g. desphenyl-chloridazone. Pilot scale investigations of advanced oxidation process (AOP) using H₂O₂ and UV were optimized for the removal of pesticide metabolites (i.e. DMS, dimethachlor ESA and alachlor ESA) from drinking water. Due to the unspecific nature of the treatment strategy, formation of various transformation and by-products including nitrite, ammonium, and in some cases, nitrosamines, were observed. AOP(UV/H₂O₂) was inadequate as a single treatment step to produce drinking water and therefore relies on post treatment. Implementation of additional treatment comes with an increased use of resources with economic and environmental impacts for which identification and proper handling of side-effects and waste products are essential and will affect which treatment strategy deems most optimal.

Obtaining a high drinking water quality through treatment, further imposes downstream requirements on the storage and distribution not to introduce new OMPs through migration from the used materials. Migration testing coupled with non-target screening methods were used to identify currently unevaluated migration from certified materials. New suspected lists were developed, which should be considered for adaptation to the positive list with appropriate maximum tolerable concentrations at tap (MTC_tap). To some extent, migration tests could predict the migration in full scale using appropriate surface area to volume ratios and contact time. However, results highlighted the need for further conversion and interpretation for a more comprehensive evaluation of the accumulated migration from materials used in contact with drinking water, for which field investigations are essential.

Epoxy coating used on the interior of filter tanks caused a high initial migration of toluene and bisphenol A, which was potentially further avoided through appropriate commissioning procedures in full-scale. Antioxidants and their degradation products migrated from high density (HD) PE-pipes, and some (i.e. 3,5-diterbuthyl-4-hydroxy-styrene) exceeded MTC_tap in conflict with certification. Thus, monitoring once installed in the distribution system was recommended. Targeted OMPs migrating from HDPE-pipes accounted for a decreasing share of the total migration over the lifetime, hence, long-term migration is currently unevaluated by certification schemes. Field investigations showed that extensive use of HDPE-pipes and synthetic rubber materials in e.g. valves and gaskets cause migration of various OMPs into the drinking water. Commissioning procedure, avoiding long term stagnation, or reducing the accumulated use of organic materials were identified as additional mitigation measures.

Overall, this PhD thesis demonstrated that mitigation of OMPs in drinking wa-ter is not trivial. The inherent physio-chemical properties and continuous de-tection of new OMPs, as well as insufficient toxicological data, challenges the formulation of appropriate objectives for intervention with the ultimate goal to reduce adverse human health effects of OMPs present in drinking water. Fur-thermore, identifying optimal mitigation strategies requires a holistic evalua-tion including both environmental impacts and related costs.