The processes of water and wastewater treatment are similar, except for the biological processes required in wastewater treatment due to the high content of organic matter. The main stages of wastewater treatment are preliminary and primary treatment to remove coarse particles, secondary treatment to treat dissolved organic matter and suspended solids with activated sludge, and tertiary treatment for disinfection. During primary cleaning, 45% of microplastic particles adhere to and are absorbed by floating fats and oils, and 50% settle out during secondary cleaning by flocculation and attachment to bacteria [186]. To remove the remaining 5%, the fraction with the smallest particles (20-100 µm), a membrane separation process (ultrafiltration, reverse osmosis) is required. The microplastic particles break up during the purification process (Figure 9, in the areas marked with red *), their average size decreases, but they can also aggregate due to the change in attractive and repulsive forces between them.

 

Flotation: due to their surface properties, NP/MP can adsorb impurities and aggregate with other particles in the water, changing their size and density; therefore, the size and quantity of bubbles must be optimized for effective removal of impurities.

Rapid sand filtration: Silica particles adsorb NP/MPs by hydrophilic interaction (hydrogen bonding), clogging the sand bed and reducing the performance of sand filter faster than expected, posing a potential hazard.

Activated sludge processes and sludge treatment: The toxicity of nano- and microplastic particles to activated sludge bacteria is not yet known. However, their inhibitory effect on some strains has already been established, so they may be hazardous to microorganisms in the sludge.

Membrane filtration: The pore size of membranes allows effective removal of NP /MPs from water, but at the same time the interaction between the membrane surface and the NP /MP increases. Problems arise: wear of membrane surface due to particle movement, especially in cross-flow systems; and membrane clogging is likely (clogging of pores mainly by particles smaller than 10 µm or plaque formation, typically by larger particles), although no articles have yet been published on clog formation associated with filtration of NPs/MPs. Understanding the fouling mechanism of membrane systems by NP /MPs is critical for determining their impact on filtration performance.

Disinfection: Due to biofilm formation, MPs can serve as a protective vector for bacteria that resist the disinfection process. The presence of MPs worsens the effectiveness of ozonation and UV irradiation, as they also decompose plastic particles.

The presence of microplastic (MP) particles in oceans, seas and freshwater is now well known, but few studies deal with their occurrence in drinking water. Since the potential toxicological effects of MPs are already being outlined (Chapter 4.2), it is important to pay attention to their presence in water intended for human consumption. Fortunately, drinking water treatment plants (DWTP) prevent MPs from entering the drinking water network. A review [218] summarizes the available information on MPs in drinking water sources and in potable water, discusses the current knowledge on MP removal by different water treatment processes, and identifies the research needs regarding MP removal by DWTP technologies. They found that:

According to literature data [218], the particles of MP are larger and typically have a hydrophobic surface, but their charge is negative, similar to the other particles which, without exception, are hydrophilic particles (organic algae, humic substances, clay minerals, etc.). When coagulated with Fe and Al salts, MP can be removed with very low efficiency (7–17%) compared to the other particles (e.g. organic algae: 50%, cyanobacterial cells: 99%).

The use of surface water contaminated with microplastics (at a concentration of 10 PVC particles/L) in drinking water treatment with membrane bioreactors (MBR) was studied, the removal of microplastic and its effect on membrane plugging was investigated [219]. The results showed that MBR is an effective technology for treating surface water contaminated with microplastics.

Combined techniques for the removal of MP in drinking water treatment: the combined use of coagulation/flocculation already improves removal significantly (in the combination of 3–15 mg/L anionic polyacrylamide [PAM], the removal of PE is about 85–90% and increases to 50-60% in the case of coagulation with Fe and Al salts); their combination with membrane filtration (e.g. ultrafiltration on a 100 kDa poly(vinylidene fluoride) [PVDF] membrane) is a promising technology for MP removal that deserves further attention [185].

MPs in the size range observed in drinking water are undetectable by human senses, and their potential health effects are still largely unknown (Section 4.2), but they are nondegradable, persistent substances. Due to the use of plastics, their presence may become a targeted contaminant in removal technologies.

A review published in 2020 reached mostly similar conclusions to those above [220]. Although the potential impact of microplastics in drinking water on humans is not yet fully understood, microplastics are a public health concern due to their high uptake by humans through drinking water systems (Section 4.2). They explain that current drinking water treatment systems prevent microplastics from entering daily drinking water from raw water. Therefore, it is very important to understand the drinking water treatment processes of the plants and the fate of microplastics.

Strategies for microplastic removal in drinking water treatment: it is very important that microplastic particles do not enter drinking water. However, currently neither the direct removal of microplastics nor the legal limits for the presence of microplastics in drinking water are known.

Conventional drinking water treatment: coagulation and flocculation are promising approaches that are more effective for smaller particles; according to studies [186], PE particles can be completely removed with additional ultrafiltration using a poly(vinylidene fluoride) membrane (average pore size 30 nm); although the coagulation and ultrafiltration used in the studies have shortcomings, their potential application in drinking water treatment is possible.

Electrocoagulation: as an environmentally friendly water treatment process, it has a removal efficiency of over 99% for microplastics and therefore has a good chance of being used to remove microplastics from drinking water. Considering its microplastic removal efficiency and estimated operating cost, this technique can be transferred from laboratory to industry and replicated. In a study where the removability of polyethylene microspheres from artificial wastewater was tested [221], electrocoagulation had a removal efficiency of over 90% at pH values between 3 and 10, with an optimum of 99.24% at pH 7.5.

Magnetic extraction [222]: an interesting new approach, where hydrophobized Fe0 nanoparticles are used, and removal efficiency of microplastic particles is size dependent (92% for < 20 mm PE and PS particles, 84% for medium size [200 µm – 1 mm] from freshwater and from sediment 78%); it is likely that this method is more suitable for drinking water treatment.

Membrane separation technology: it provides a practical method for removing microplastic contaminants from drinking water sources, with the membrane filter acting as a physical barrier. Results indicate that membrane separation technology could be used for rapid removal of microplastic particles from drinking water. We face many challenges in terms of the performance of microplastic handling processes.

Coagulation: it is advantageous that the surface of microplastic particles from the environment is usually negatively charged, but they can increase the chemical demand.

Clogging of membranes: a real problem, clogging of membranes by micro/nanoplastics will be an important issue.

Disinfection: the effects of micro/nanoplastics on the disinfection process of drinking water are likely to be harmful and require further research.

Improving removal efficiency: electrocoagulation, the membrane bioreactor (MBR) is one of the most effective methods for removing microplastics, the combination of coagulation and membrane separation processes (with a negatively charged membrane!) has some economic potential for removing microplastics from drinking water.

The main problem related to the impact of micro/nanoplastics on drinking water treatment performance is the lack of knowledge, so increasing our knowledge of micro/nanoplastics is essential to control their impact on water treatment processes.

Coagulation induced by aluminum salt was used to study the removal of microplastic particles from simulated drinking water [42]. Coagulation was performed in JAR tests, zeta potential was measured, and floc structure was observed with an optical microscope. The results of this work confirm that the usual conditions suitable for the removal of oxide colloids and kaolin are also effective for the removal of plastic beads with a density of 1.3 g/cm3 and a diameter of 1–5 µm. The turbidity of water obtained from solutions containing 5 mg/L microspheres with an initial turbidity of 16 NTU by coagulation with an Al2(SO4)3 solution with 5–10 mg/L Al content is less than 1.0 NTU. Although the presence of 20 mg/L surfactant in the solution changed the zeta potential of the microsphere, it did not affect the coagulation of the microsphere at low Al doses, but at higher aluminum doses, all solutions showed higher residual NTU. From floc photos and zeta potential measurements, it appears that flocculation is the dominant mechanism for microspheres removal under these test conditions. Dispersion of polyethylene microfibers (density 0.96 g/cm3, 5 μm diameter, cut to 0.1 mm length) in water was strongly affected by surfactants, but coagulation always effectively removed the fibers.

The mechanisms of coagulation and flocculation of microplastics (MP) have been studied only superficially. The removal of clean and aged plastic debris and the effect of plastic particle size on removal are largely unexplored. The coagulation, flocculation, and sedimentation abilities of neat and aged (UV photooxidation) MP textile fibers were used in model systems (MP [fluorescent 15 and 140 µm PE and 140 µm PS microspheres and ground ~580 µm polyester {PEST}] [223]). The aging processes changed the surface chemistry of the MPs (new functional groups - hydroxyl, carboxyl, and vinyl - appeared on the aged PE spheres; the former mainly due to UV radiation, the latter due to photooxidation and NOM adsorption), with the combined UV exposure promoting plastic erosion, increasing surface roughness and porosity, and thus affecting their affinity for coagulants and flocculants. The mechanisms during MP coagulation and flocculation (where opposite charges were preferred) were observed and identified using a quartz crystal microbalance. The measured deposition rate confirmed that the affinity between plastic surfaces and aluminum-based coagulants is relatively low compared to cationic polyacrylamide (PAM), but the efficiency of Al coagulant increased on the aged plastic surface. Removals of 97% and 99% were measured for PEST and aged PE, respectively. The larger pure PE MP was the most resistant to coagulation and flocculation with a removal of only 82%. The study found that turbidity of the water after settling is a suitable indicator of MP removal.