Understanding the fate of active pharmaceutical compounds in waters receiving poorly or untreated wastewater for the development of a dedicated environmental risk assessment approach

Abstract

The global increasing consumption and production of active pharmaceutical ingredients (APIs) is of a growing environmental concern. The inherently biological activity of APIs creates a concern of potential ecotoxicological effects even at very low concentrations. Such risk is particularly exacerbated in low and low-middle income countries (LLMICs) where it is common the practice of direct discharge of untreated wastewater (DDUW) into surface waters, which creates a heavily polluted area downstream, described as the “impact zone”. Little is known about the environmental fate of APIs in this area. Nevertheless, measured environmental concentrations (MECs) of LLMICs show higher concentrations than for high-income countries. Globally, the MECs for APIs in the “impact zone” are often above 0.01 µg L-1, which would be sufficient to trigger the environmental risk assessment (ERA) refinement phase. Environmental concentrations are calculated with a conservative default dilution factor (DF) of 10, but in many countries observations show a lower value. Additionally, the traditional ERA considers sorption on the wastewater sorbents in the wastewater treatment plant as removal process. This does not happen in the case of the DDUW, where instead the sorbent are released and diluted into the receiving waters. This peculiar widely spread environmental scenario translates in the necessity of a dedicated environmental risk assessment impact zone inclusive approach, and supports the generation of scientific robust data about the environmental behaviour of APIs along and beyond its boundaries. Such a situation is therefore giving rise to scientific questions such as the effects of dilution of the untreated wastewater on the distribution of APIs between water and sorbents, i.e. sorption and desorption; the effects of the dilution on the biodegradation of such molecules in the receiving compartment, e.i. the compound half-life; and the development of a scientific approach to model the boundaries of the impact zone and to predict the environmental concentrations within and beyond. With the aim of creating a framework for the generation of sound scientific information regarding such area a methodological approach was designed. A first phase of such methodology included laboratory studies about the most prominent occurring environmental fate processes, such as distribution and biodegradation. This information was used to propose a model for the determination of concentrations along and beyond the impact zone. In a subsequent phase it was conducted a fieldwork to both validate the first phase and to provide a real case for the modelling of environmental concentrations, and the inclusion of such methodology parallel to the traditional ERA. A first study was conducted to investigate the effect of DFs <10 on the distribution of APIs between wastewater sorbents and receiving waters. The sorption was consistent with the APIs chemical properties. Dilution increased desorption of the basic and neutral APIs and correlated with their lipophilicity (R2 >0.98). The data showed a clear trend in the desorption process of APIs that may lead to higher exposure risk than anticipated. A second study aimed at investigating low levels of dilution on the biodegradation of APIs. Also, the extent of the impact zone was modelled using the biochemical oxygen demand (BOD) as proxy. The degradation half-lives of acebutolol and diclofenac increased with increasing dilution and resulted in higher environmental persistence. The other APIs investigated were not degraded, but acetaminophen which was quickly lost from the solution (<24 h). The temporal end boundary of the impact zone was predicted as 24 h. Therefore, it was concluded that most of the investigated compounds would persist beyond the impact zone, as defined by BOD concentrations owing to the fact that they exhibited greater persistence than the DOM comprising the BOD within the samples. Thirdly, a sampling campaign of the Nairobi/Athi river catchment was performed to characterize the impact zone and the occurrence of APIs. The study showed a clear relationship with the DOM quality variation and the diffuse source areas of untreated wastewater within the city, as well as the occurrence of API trends. The study showed a first attempt of understanding the role of such an impact zone with respect to API source, occurrence and fate and furnishes pivotal information to further investigate such areas. In conclusion, from the results obtained by the laboratory studies and fieldwork it clearly results on the impact zone as an area of severe pollution which needs a dedicated ERA approach. Beyond its boundary the classical protocol of ERA can be applied, but to estimate the predicted environmental concentrations of APIs it is necessary to create a model to be applied to such area. Such model would output environmental concentrations beyond the end boundary of the impact zone that may be used for further risk assessment. Further studies are necessary to provide scientific robust endpoints useful to the development of such model.