Microplastics and nanoplastics in water: Improving removal in wastewater treatment plants with alternative coagulants

Coagulation, flocculation, and settling have been shown to be important processes to remove micro- and nano-sized plastic contaminants. However, limited research has been conducted to assess how challenging water chemistry characteristics (including water pH and presence of wastewater colloids) and treatment conditions (such as settling time, coagulant type, flocculant type) impact the removal of these contaminants. Our results show that plastic removal declines at higher pH values using conventional alum as a coagulant. Removal of pristine and aged nanoplastics dropped from 64% and 76%, respectively, at pH 7 to below 20% at pH >7.8. Similarly, polyester microfibre removal decreased from 97% at pH 7 to 85% at pH 8.6. Replacing alum with alternative coagulants resulted in improved plastic contaminant removal at pH >7.8 with an average microfibre removal of 94% (aluminium chlorohydrate) and an average nanoplastic removal of 66% (aluminium chlorohydrate + pDADMAC). Quartz crystal microbalance with dissipation monitoring measurements revealed that aluminium chlorohydrate coagulant species yield a thicker deposition layer on negatively charged surfaces compared to alum. Experiments with alternative flocculants revealed that anionic flocculants result in rapid nanoplastic removal, while cationic flocculants can achieve similar removal given sufficient settling times. Results from our study reveal how coagulant and flocculant selection and environmental conditions, including pH and interactions with wastewater colloids, can affect plastic contaminant removal during primary treatment. Our study also revealed that municipalities can enhance nanoplastic and microfibre removal using alternative aluminium-based coagulants under challenging pH conditions.