Horizontal subsurface flow constructed wetlands as tertiary treatment: Can they be an efficient barrier for microplastics pollution?

The presence and fate of microplastics (MPs) in wastewater represent a subject of major concern, as wastewater is one of the main inputs of MPs to the environment. This study deals with the ability of horizontal subsurface-flow constructed wetlands (CWs), as tertiary treatment, to reduce the MPs concentration of secondary effluents. Different locations of a wastewater treatment plant (WWTP) including raw wastewater, CW influent and final effluent, were sampled. Macroinvertebrates were collected from the CW to evaluate their potential role in the MPs distribution along the wetland. The global WWTP efficiency for MPs removal was 98%. MPs removal efficiency by CW was on average 88%, causing a significant reduction of the MPs concentration from 6.45 to 0.77 MP/L (p < 0.05), thus preventing them from entering vulnerable aquatic systems. The areal removal rate and the first order areal rate coefficient (kA) were estimated to be 3120 MPs/m2/d and 1.70 m/d, respectively. The most abundant size fraction was the one comprising MPs between 75 and 425 μm (51%), while the other size ranges analysed (40-75 and 425-5600 μm) accounted for 25 and 24%, respectively. Fiber was the most abundant shape in the WWTP influent (75%), the CW influent (54%) and effluent (71%). Non-significant differences were found between sites regarding size and shape distributions (p > 0.05). Macroinvertebrates can ingest a non-negligible quantity of MPs, with an average content of 166.2 MPs/g or 0.13 MPs/individual. Therefore, they could play a certain role in the MPs distribution inside CWs. Fiber was the most abundant shape for macroinvertebrates as well (89%), so attention should be paid to reduce their contamination at source. This study provides the first results on MPs removal in CWs as tertiary treatment and assesses the potential role of macroinvertebrates in their distribution along the CW, thus filling this gap of knowledge.