# Notes on Common Misconceptions in Microplastics Removal from Water
## 3 Microplastics Removal from Water
### Filtration  
Various types of filtration are the most widespread methods used in water treatment. Some of them may comprise rapid sand filtration, membrane filtration, activated carbon filtration, disc filtration, etc. There were also several studies focusing on removal of microplastic from water using filtration technologies and some of them were tested in real conditions in WWTPs. Various tertiary treatment stage technologies in WWTPs were studied for the removal of microplastics from water. Two types of filtration, disc-filter and rapid sand filtration, were compared with membrane photoreactor on removal of particles of different material (PE, PES, etc., size > 20 \(\upmu\)m) with more than 95% removal efficiency. The major drawback of this study is the size of microplastic particles, because only particles larger than 20 \(\upmu\)m were determined [112]. Another study focused on smaller microplastic particles (size > 10 \(\upmu\)m) and tested removal efficiency via disc filtration. The results were evaluated in terms of number and mass of particles in WWTPs. It has been shown that disc filtration was able to remove almost 90% of microparticles [113]. Filtration is often part of coagulation/flocculation experiments (see Section 3.1) and [PERSON] et al. [114] compared removal of microplastic particles (180 nm-125 \(\upmu\)m, PE, PS) via filtration and combined coagulation/flocculation/filtration process. The largest particles, in this study, were removed with almost absolute efficiency (106-125 \(\upmu\)m with >99% removal efficiency) and it was shown that the size of microplastic particles does not affect removal efficiency linearly; 180 nm particles were removed with nearly 99% efficiency, but particles with sizes of 10-20 \(\upmu\)m were removed with less than 90% efficiency. This is explained by the possibility of smaller particles being more likely retained in filtration grains through particle attachment or diffusion. [PERSON], [PERSON] and [PERSON] [115] compared rapid sand filtration (silica sand) with biochar filtration for removal of PS spherical microparticles (10 \(\upmu\)m) in a model solution. Two biochars of different origins (corn straw and hardwood), produced via a slow pyrolysis process at three temperatures (300 \({}^{\circ}\)C, 400 \({}^{\circ}\)C and 500 \({}^{\circ}\)C), revealed higher removal efficiencies than sand filtration (more than 95% for biochars, compared to 60-80% for sand filter). It has also been reported that different mechanisms of immobilisation occur ( biochars re-release less than 1% of retained microplastics by leaching, compared to 7% for sand filter). [PERSON], [PERSON] and [PERSON] [116] investigated membrane bioreactor and rapid sand filtration for the removal of 14 different microplastics in the WWTP final effluent and studied removal efficiencies of different forms of microplastics (fibres, films, fragments and beads; 6.3-210 \(\upmu\)m). The removal efficiencies of 79% for membrane reactor and 75% for rapid sand filtration were achieved. Based on these observations, the authors concluded that membrane bioreactor and rapid sand filtration are not better than conventional ones. Again, the main disadvantage of this study is the relative high dimensions of microplastic particles (>6 \(\upmu\)m). The influence of shape was not discussed. On the other hand, membrane bioreactor (pore size < 0.1 \(\upmu\)m) was successfully used for the removal of PVC (<5 \(\upmu\)m) in a model solution, however, the presence of microplastic led to irreversible membrane fouling [117]. It has also been reported that PE microplastic particles can effectively be removed from water via membrane filtrations using PVDF with an average pore size of 30 nm [118]. Problems with membranes fouling were also described by [PERSON] et al. [119]. Due to high pressure drop and immense price, it is unlikely that membrane filtrations would be applied in practice. Separation via various filters may be driven by different mechanisms and not only the large surface area enhanced adsorption must occur.