**2.5 Retention of MPs in the tertiary treatment stage of a WWTP**

Tertiary treatment methods have been studied extensively in regard to MP retention. In this regard, sand and gravel filtration is a common tertiary treatment, and quite a few of the early papers looking at the retaining capability of tertiary WWTPs also investigated the performance of such filters. In 2015, New York State had authorized a study of its WWTPs and found that some WWTPs using filtration processes in their tertiary stage still released MPs. The study focused on plastic microbeads, and here it can be said that certain WWTPs using membrane microfiltration, continuous backwash up flow dual sand (CBUDS) microfiltration or rapid sand filtration indeed did not show any plastic microbeads in the effluent at the time. Data on the retention of synthetic fibers was not released, however [120]. Two other studies came from New York State at that time, both citing release of microplastics downstream from WWTPs [84, 145]. One of the studies looked at a WWTP in Western New York State (Lake Erie) (12.000 people served, flow rate 13.000 m3 /day) using granular filtration (sand/anthracite coal) as the tertiary stage, with 0.009 MP/L found in the effluent, leading to a release of 101.000 MP/day, 68% of which were fibers [84]. The effluents of three tertiary WWTPs in the San Francisco Bay area with sand filtration or sand/anthracite coal filtration were found to have higher loadings with 0.064, 0.092, and 0.127 MP/L [84], leading the largest of the WWTPs to release more than 9.6 million MP/day [84]. Also, a later study from a WWTP in Northern Italy [126] showed that sand filtration as the tertiary stage with an overall MP retention rate of 84% can still lead to significant releases in the order of 160 million MPs day−1. For a WWTP in the Murcia region, Spain (serving 29.800 people; flow rate: 12.000 m3 /day) J. Bayo et al. gave a 75.5% MP retention rate for the gravity rapid sand filtration, with 3 sand filters installed in parallel [127]. Here, it was noted that RSF could retain plastic microparticulates (95.5%) better than synthetic fibers

#### *The Effect of Wastewater Treatment Methods on the Retainment of Plastic Microparticles DOI: http://dx.doi.org/10.5772/intechopen.97083*

(53.8%) [126]. In contrast, in cases. Rapid sand filtration has also been shown to lead to very significant reduction of MP concentration in the effluent, e.g., from 0.7 (±0.1) to 0.02 MP (±0.007) MP L−1 (97% MP retention) at the Kakolanmäki WWTP (Turku Region Waste Water Treatment Plant) in Turku, Southern Finland [122]. On the downside, there has been a report of fragmentation of MP material during sand filtration [146].

Another filtering technique uses disc filters (DF) as a final polishing step, removing particles from biologically treated wastewater. Disc filters are made of a stack of round filter meshes in a closed tank, where the filter mesh is a woven material, made of polypropylene, polyester, or polyamide with a pore size of 10–40 μm. The sludge cake formed from the retained particles is periodically removed by high-pressure back-flushing. M. Simon et al. looked [147] at the efficiency of DF in a WWTP in Grindsted, Denmark (flow rate 10.040 m3 /day). The effluent from the secondary clarifier was noted to carry 20 mg L−1 suspended solid. This was reduced by DF to 3–8 mg L−1. When passed through DF of a pore size of 18 μm, the MP content could be reduced from 29 MP L−1 to 3 MP L−1 (89.7% removal efficiency). Talvitie et al. looked at the filtration of the secondary effluent through a pilot-scale disc filter (Hydrotech HSF 1702-1F) consisting of two discs each composed of 24 filter panels at the Viikinmäki WWTP, located in Helsinki, Finland. Here, DF-10 (10 μm pore size) decreased the MP concentration from 0.5 (± 0.2) to 0.3 (± 0.1) (40% removal efficiency) and DF-20 (20 μm pore size) from 2.0 (± 1.3) to 0.03 (± 0.01) (98% removal efficiency). The results were noted to fluctuate from trial to trial [125].

There are a number of membrane filtration techniques. However, MP removal through micro- and ultrafiltration (UF) has been studied less frequently. Often, UF is used in combination with coagulation and can be used as a secondary or tertiary treatment method. Polymeric or ceramic membranes with a pore size between 1 and 100 nm are used, laid out to retain large organic molecules such as proteins as well as bacteria, protozoa, and viruses. UF is not specifically designed to retain micro- or nano-plastics. UF membranes can be fouled easily. To that effect, a coagulation step as pretreatment with iron-based coagulants has been advocated, especially in combination with an addition of polyacrylamide (PAM), which has been reported to increase the removal efficiency of small-sized polyethene particles (d < 0.5 mm) significantly from 13 to 91% [148, 149]. UF can also be used as a pretreatment for a reverse osmosis (RO) separation (see below) to protect the RO membrane. Nevertheless, fouling of membranes due to meso-particles, where MP have the same size, continues to be a problem [150].

An alternative membrane separation technique is that using dynamic membranes (DMs). DMs operate with a layer formed on a supporting membrane by particles in the influent. So, these particles in the influent create a filtration layer that can be supported by a larger pore-sized mesh or by low-cost porous materials. DMs have been run successfully with particles that are of a similar size to microplastics [151].

Reverse osmosis (RO) is the process filtering water from a region of high solute concentration through a semipermeable membrane to a region of low-solute concentration by applying a pressure larger than the osmotic pressure. RO units are used in desalination plants but are also used in drinking water treatment plants and in some WWTPs. Ziajahromi et al. [107] have looked at a WWTP in the Sydney area operating with a reverse osmosis (RO) unit (13.000 m3 day−1) as a tertiary treatment. Here, the MP concentration decreased from 2.2 MP L−1 in the primary effluent to 0.21 MP L−1, after the reverse osmosis (RO) process. This still leads to a discharge of 10 million MP day−1 into the tributary of a major urban river in Australia. It is thought that the occurrence of larger sized pores on the membrane, the membrane material and other membrane imperfections may contribute to the passage of the MP through the membrane [107].

Finally, dissolved air flotation (DAF) as a flocculation process can be used as a tertiary treatment method. It was found to remove 95% of MP remaining from the secondary treatment [125]. In this case, dissolved air flotation (DAF) was studied as a full-scale tertiary treatment at Paroinen WWTP (Hameenlinna Region Water Supply and Sewerage Ltd) located in city of Hameenlinna, Southern Finland. In DAF, water is saturated with air at high pressure and then pumped to a flotation tank at 1 atm, forming dispersed water. The formed air bubbles (typically 20–70 μm in size) in the dispersed water adhere to the suspended solids causing them to float to the surface, from which they are removed by skimming. The process necessitates only a small retention time of the treated water. At the Paroinen WWTP, before the flotation, flocculation chemical polyaluminum chloride was added to the wastewater with a dosage of 40 mg L−1 to enhance flocculation [125]. Y. Wang et al. studied DAF with three common types of MP in freshwater and found the hydrophilic-hydrophobic interaction not to be ideal for an efficient separation of MPs without additives, citing a removal of 32–38% of MPs, only. The efficiency could be increased by 13.6–33.7%, however, with two additives that modified the surface of the air bubbles [152].
