## 1 Introduction  
Due to their excellent versatility and durability, plastics have come into widespread use, and their global production has increased markedly since the beginning of mass production, from 1.5 million tonnes in 1950 to almost 360 million tonnes in 2018 [1]. Although plastics have benefited our lives in cost and convenience, the indiscriminate disposal of end-of-life plastics poses serious environmental problems, as plastics progressively fragment into smaller pieces once they reach and accumulate in the aquatic environment due to their low biodegradability. Global concerns have been raised in recent years over the rampant proliferation of plastic debris in the environment, with particular attention paid to microplastics, which are small polymer particles less than 5 mm in size [2]. Microplastic occurrence has been extensively documented in freshwater and marine environments, such as rivers [3, 4], estuaries [5, 6], shorelines [7, 8] and oceans [9, 10]. Given the small size and ubiquity of microplastics, the topic of their effects on global ecosystems has been brought to the forefront. Recently, the introduction of microplastics into the food web via nonselective uptake, unintentional ingestion and ventilation by aquatic organisms was proven by microplastics recovered from the gastrointestinal tracts and/or gills of plankton (e.g., copepods and euphausiids) [11], filter feeders (e.g., bivalves) [12] and fish (e.g., solenette, dragonet and poor cod) [13]. Multiple laboratory studies have further indicated the bottom-up propagation of microplastics across trophic levels and their accumulation in predators [14, 15, 16]. Ingestion of microplastics may not only exert various physical impacts, such as external/internal abrasion, intestinal obstruction, reduced energy metabolism and reproductive malfunctioning, on aquatic biota [17], but also facilitate the transfer and absorption of chemicals, including hydrophobic organic contaminants (HOCs), from the environment to biota due to the favourable surface-area-to-volume ratio of microplastics [18]. Knowledge of the potential human health impacts associated with the uptake of microplastics is limited at the present stage. Questions have been raised concerning the possible human exposure pathways of microplastics [19]. In addition to commercially available seafoods, microplastics have been observed in table salts [20], beer [21], honey and sugar [22], and even drinking water (i.e., treated tap water and bottled water) [23; 24]. In 2018, the first study looking into the occurrence of microplastics in tap water reported that more than one-fifth of the tap water samples from 14 countries tested positive for micro-scaled plastic particles, with a vast majority (98.3%) of microplastics were fibres [25]. Given the presence of microplastics in food, it is possible that humans are exposed to microplastics through ingesting microplastic-contaminated food. However, information related to the microplastic concentration in Hong Kong's tap water remains lacking, exposing the need to understand the presence of microplastics throughout the water supply distribution chain with respect to human exposure and, more broadly, to safeguard drinking water quality. In Hong Kong, the growth of the urban population and economy has resulted in an increased freshwater demand, with an annual consumption that reached 1010.75 million m\({}^{3}\) from 1 April 2018 to 31 March 2019 [26]. Approximately 99.9% of the Hong Kong population receives tap water from surface water sources, hereafter referred to as raw water. Owing to its freshwater self-insufficiency, Hong Kong derives approximately 70% of its raw water from the East River (Dongjiang) in Guangdong Province and the rest from impounding reservoirs. There are now 20 water treatment works (WTWs) in Hong Kong with a total capacity for treating 5.31 million m\({}^{3}\) of water each day.