Potential improvement for microplastic removal in Hong Kong Stonecutters Island Sewage Treatment Works (SCI) – dissolved air flotation (DAF)

Abstract

Microplastics (MPs), contaminants with sizes smaller than 5mm, acquire the rising concerns about its ecotoxicity and ubiquity towards the environment. Wastewater treatment plants gather all sewage sources containing MPs produced due to anthropogenic activities and perform contaminant treatment. Yet, without proper MPs removal method, a large abundance of MPs is released to the aquatic environment and affects the ecosystem. In Hong Kong Stonecutters Island Sewage Treatment Works, only 60.4% of removal efficiency was achieved, there is the urgency to seek a better MPs removal method. Therefore, in this report, the MP removal efficiency of dissolved air flotation (DAF) with Al-based coagulant is studied and compared to propose a potential improvement of DAF. We found that DAF could achieve a 95% of MP removal under the help of Al-based coagulation, the effect of coagulant addition and types are also discussed. However, the disinfection process in SCI was indicated that it could further break larger plastic matters and produce more MPs even after the treatment process, further investigation should be carried on. Besides, we observed that the large intra-day variation may not to the inaccurate estimation on MP daily loads, a more representative approach is suggested to perform for the better estimation and hence the design of the treatment system.