Application of intermittent sand and coke filters for the removal of microplastics in wastewater

Abstract

In recent years, microplastics (MPs) have become one of the most severe threats to the natural environment and aquatic life. The identification of urban wastewater treatment plants as one of the main pathways for these pollutants to enter the environment have led them to be the focus of several studies. The main interests are focused on the amount of MPs that are emitted into the environment, the efficiency of removal in wastewater treatment plants in addition to how their operation and design can be improved to reduce the presence of these pollutants in the effluent. One possible solution is the application of advanced treatment technologies that allow a more thorough water treatment, such as intermittent sand filters. In this study, the efficiency in removing MPs from the effluent of an urban wastewater treatment plant of three filters with different substrates (silica sand, sand/coke and coke) was analysed by comparing the performance on the three different substrates and working at different feed water flow rates. This provided useful information of retention mechanisms that affect MPs according to their morphology and how the operating parameters can influence the filtration systems performance. Several samples were collected from wastewater, before and after the filtration treatment, to analyse the presence of MPs. The results exposed a MP removal efficiency of up to 98.3%, with values in the treated water from 0.3 to 2.4 MP L−1. Among the most abundant polymers identified in the samples were polyethylene (PE), polyester (PES), polyethylene terephthalate (PET) and polypropylene (PP). Statistical analysis of the results verified that there are no significant differences between the different substrates or water flow rates object of study. Results obtained, in addition to the cost-benefit ratio, showed the filter filled with a top layer of sand and a bottom layer of coke as the best option among those studied. Experiments with laboratory-scale filters carried out without possible interferences of external and uncontrolled factors showed similar performance and MP removal efficiency to that obtained working in a real plant conditions.