## 4 Discussion
However, although the above two factors seem to explain the major pathways of microplastics to raw water sources, a large-scale investigation on the seasonal variations in microplastic abundance in the source water bodies with a specific focus on the transport behaviour of microplastics, including the residence time and runoff dilution of microplastics, is required to further understand the sources and fates of microplastics in tap water. Water treatment is critically important, as it poses a barrier to microplastic entry into drinking water. Unfortunately, although current water treatment practices primarily aim to remove impurities, the removal is selective and ineffective for filtering out all microplastics. As demonstrated by [PERSON] et al. [23], a two-stage sedimentation-sand filtration technique for water purification was only capable of removing an average of 81% of the microplastics from surface water sources. Since interstage monitoring was not conducted in the present study, it was unable to confirm whetherthe presence of microplastics in water is attributable to the incomplete removal of plastic particles or the release of smaller microplastics following mechanical abrasion during treatment and distribution; for example, rapid mixing in coagulation may degrade stressed polymer particles under high shearing forces [19], and the abrasion of plastic-coated or plastic-lined water pipes and tanks could also result in microplastic fragmentation [35]. Thus, only very limited conclusions can be drawn at the current stage, and further research is needed to address this question. The detected microplastics were mostly tiny in size and nearly invisible to the naked eye. According to the World Health Organization [19], microplastics of a size greater than 150 \(\upmu\)m tend not to be absorbed in vivo due to their insolubility and are believed to be eliminated through direct excretion. In addition, the oral intake of smaller microplastics appears to be limited at the exposure levels recorded in the literature. Based on the actual amounts and sizes of microplastics reported by [PERSON] et al. [23] and [PERSON], [PERSON] [25] (Table 1), the estimated intake of microplastics from tap water was 2 \(\upmu\)g day\({}^{-1}\) or 0.03 \(\upmu\)g kg\({}^{-1}\) body weight per day [19]. Considering that an average local microplastic abundance is at most only half of the values reported in the above two studies, the daily intake of microplastics via Hong Kong tap water is likely to be at a very low level. Microplastic research on drinking water is still in its infancy. Since no research has been carried out to investigate the human health impacts of microplastics with particle sizes below 150 \(\upmu\)m, no firm conclusion can be reached with limited data on whether smaller microplastics cause harm to human health. In addition, due to a lack of standardized methods for microplastic analysis, studies are often incomparable and unable to provide meaningful results, as the approaches adopted by studies for sample collection, microplastic quantification and identification have greatly varied [40]. Since the presence of microplastics smaller than 1 \(\upmu\)m has been reported in treated water samples from the Czech Republic [23], the use of 2.7 \(\upmu\)m pore filter papers in this study could hinder accurate detection and quantification of smaller microplastics, which means that the abundance and size distribution of microplastics in the tap water in Hong Kong might have been underestimated. Despite the urgency of establishing an accurate and standardized method for paving the way towards efficient detection and analysis of micro- and nano-plastics in drinking water, this remains a significant challenge, as the reliability across published studies was not sufficient [41]. At the current stage, much attention should be paid to determining how to maintain a low-loss and contamination-free sample processing procedure to obtain robust and reproducible results.