Microplastics Behaviour in Treated Waters by Real Biological Nutrient Removal Systems

Microplastics (MPs) with diameters less than 5 mm in wastewaters are being increasingly discharged into aquatic environment from plastic industries, personal care products, and synthetic textiles. Futher, MPs have a negative impact on the environment due to their extremely low biodegradability. This study aims at investigating the behaviour of MPs in treated waters using real biological nutrient removal (BNR) systems. In terms of shape, the majority of MPs in real wastewaters are fibers and fragments. Most BNR systems achieved high efficiency (>95%) in reducing the MPs from wastewaters. In the BNR systems, the profiles sizes of MPs removal are in the following order: aeration tank (AT) > primary settling tank > total phosphorus removal tank > return sludge > final settling tank (FST). Higher efficiency of MPs removal during aeration in the AT as compared to the other processes resulted in the absorption and fixation of MPs in the activated sludge and biofilm. In contrast, lower efficiency of FST resulted in the MPs not being absorbed, which were floating in treated water without particulate; thus, the floating MPs were discharged without settling due to their lower specific gravity. Therefore, a filtration process should be introduced to effectively reduce the MPs in the effluent before it enters the FTS.