Microplastic pollution and removal in wastewater treatment plants in Thailand

Microplastic (MP) is an emerging pollutant that requires urgent attention. Wastewater treatment plants (WWTPs) are considered an important pathway to transport terrestrial MPs to water bodies. This study aims to investigate the level of MP pollution and removal at each treatment step to find the most efficient MP removal stage in four different WWTPs in Thailand. Removal efficiencies and the characteristics of MPs in the inlet of four WWTPs were compared. MP samples in three WWTPs were collected by filtration of wastewater through a set of sieves: 5 mm, 1 mm, 500 µm, and 53 µm. Grab sampling was performed in a WWTP built as a closed underground system. Sludge samples were collected from each WWTP by grab sampling. MPs found in the influent of four different WWTPs ranged from 3.50 – 77 particles per L, and the effluent ranged from 2.33 – 30.33 particles per L. The highest MP concentration in the inlet was found in Bang Sue WWTP. Despite the high MP loads in the influent, Bang Sue WWTP achieved the highest removal efficiency of 86.14% and 96.97% from the conventional treatment system and UF as a final polishing step, respectively. One of the reasons is that it was constructed as an underground treatment system to prevent atmospheric contamination. On the other hand, Sing Buri WWTP, with a waste stabilization pond, attained very low MP removal efficiency from both grab sampling and on-site filtration. There was a high concentration of MPs from atmospheric deposition in Sing Buri WWTP. The number of MPs detected in the influent of the largest WWTP in Bangkok (population equivalent to 1 million) is lower than Nong Khaem WWTP, which has a population equivalent to 520,000. Most of the MPs found in four WWTPs were fibers identified as polyethylene terephthalate (PET) or polyester. Fibers were derived from clothes shedding. Fragments identified as PE and films were less abundant. The results of this study show a variety of MP abundances in different WWTPs with different treatment technologies in Thailand. However, all of the studied conventional WWTPs, except the pond system, lack primary sedimentation which is reported to improve the removal of MPs. The addition of a primary sedimentation tank and advanced filtration as a final step in WWTP should be considered as it could decrease the number of MP released to the freshwater environment.