Performance of bioretention systems by umbrella plant (Cyperus alternifolius L.) and common reed (Phragmites australis) for removal of microplastics

Domestic wastewater is one of the main sources of microplastic (MP) contamination in aquatic environment; however, most wastewater treatment plants (WWTPs) have not been designed for the removal of MPs. In this study, the release rate of MPs in the treated water from a WWTP into the river was estimated to be approximately 105 × 103 pcs/capita/day. Thus, bioretention systems are designed as tertiary wastewater treatment systems to reduce the number of MPs in water resources. The objective of this study was to evaluate the performance of bioretention systems using the umbrella plant (Cyperus alternifolius L.) and common reed (Phragmites australis) for MP removal. After 17 weeks, MPs were found in the common reed bioretention system. The common reed root system and substrate influence the MP retention process to a greater extent through physical filtration, substrate ratio, plant roots, and biofilms on the roots. Additionally, the common reed root system played an important role in the removal of low-density MPs. The accumulation of MPs in roots and root hairs inevitably impacts plants. The results showed that the common reed at a 3:2:9 ratio (gravel: sand: soil) had the highest relative height growth rate (RHGR) (1.8 mm cm−1 week−1). These results show that the HSFs–3 at a ratio of 3:2:9 was able to reduce the number of MPs, with an average removal efficiency of 62.89 % and 95.45 % for sizes >1000 µm, 89.21 % for sizes 500–1000 µm, and 44.16 % for sizes <500 µm. The highest average microbeads removal efficiency was 95.24 %. Our results suggest that plant bioretention using Horizontal Subsurface Flow system is an effective treatment for removing MPs from wastewater.