The removal behavior of polyethylene (PE), which is readily suspended in water and is the main component of microplastic pollution, was studied [224] in a combined process where coagulation with commonly used Fe salts was followed by an ultrafiltration step. Coagulation was performed in JAR tests, and poly(vinylidene fluoride) membranes (100 kDa) were used for UF experiments in a Millipore stirred UF cell. The removal process was monitored using an optical microscope equipped with a CCD camera, and the zeta potential and particle size distribution, as well as the mass of microplastic particles, were measured. The results showed that although the removal efficiency was higher for smaller PE particles, the removability of PE by the conventional coagulation method was low (below 15%) and hardly affected by the water properties. Relative to solution pH, the addition of a flocculant (PAM) was important in increasing removal efficiency, especially at high doses of anionic PAM (efficiency up to 90.9%). Although ultrafiltration completely retained the PE particles, they caused little membrane fouling due to their large particle size. Membrane flux decreased after coagulation; however, membrane plugging was lower due to the heterogeneous nature of the filter layer caused by PE, even in the case of a coagulant dosed in large amounts. Based on the behavior shown during the combined coagulation and ultrafiltration, it is recommended to use the process outlined below in the treatment of drinking water (Figure 10).

 

 

In recent years, it has been reported that the content of microplastic particles smaller than 100 µm is increasing in samples from drinking water treatment plant effluents. Therefore, in a study by D. W. Skaf et al. [43] the removability, shape and surface morphology of microplastics of different sizes (10–100 µm) were investigated by a combined method in which Al coagulant was used together with a cationic polyamine-coated sand (PC). For identification and quantification, MPs were stained with the dye Nile Red and observed with a laser scanning fluorescence microscope. The removal of MPs increased when the Al doses were increased to 30 mg/L (70.7%), while efficiency decreased with further increases. Analysis of the different sizes showed that smaller MPs (10–30 µm) were less easily removed. The highest MP removal efficiency (92.7%) was obtained with the combination of PC sand (500 mg/L) and 20 mg Al/L. Removal was improved by 26.8% compared to Al coagulation alone. MP removal was in the following order: elongated-rough > elongated-smooth > spherical-rough > spherical-smooth. The latter was confirmed by a kinetic study of flocculation. Size, shape and surface morphology have been shown to play an important role in the removal of MP during drinking water treatment.