Abstract
Wastewater treatment plants (WWTPs) serve as important routes for microplastics (MP) to the environment. However, more effective MP sampling and detection methodologies, as well as a better understanding of their influence on MP occurrence and distributions in WWTP effluents, are needed for better removal and control. In this study, the efficiency of a municipal WWTP to remove MP was assessed by collecting samples from raw to tertiary effluent during a 12-month sampling campaign (season-based) using different sampling methods (containers, 24-h composite and large grab samples). MP retrieved from different treatment units within the WWTP were identified and quantified using plastic/non-plastic staining followed by optical microscopy, SEM and μ-Raman microscopy. Overall, the mean removal efficiency of MP in the WWTP was 97%, with most MP removed by the secondary stage and a mean effluent concentration of 1.97 MP L−1 after sand filtration. The relative abundance of fibers increased 74% of the total MP to 91% between raw wastewater and treated effluent, with a corresponding decrease in particles. Taking seasonal variations into account is important as total MP concentration in the effluent was notably higher in winter compared with the other seasons. Increasing the sampled volume using large samples or 24-h composite samples significantly reduced the variability between replicates. However, MP concentration post the tertiary stage was significantly lower using morning sampling (9 am) by large grab sampling method (1.2 MP L−1) compared to 24-h composite sampling (3.2 MP L−1) possibly due to intra-daily changes. Using a finer mesh size (0.45 μm) to capture MP beyond the size range typically studied (≥20 μm) effectively doubled the number of MP detected in the tertiary effluent and highlights the importance of standardizing sampling procedures.