## 2 Centralized Treatment Plants
### Urban Wastewater Treatments
#### 2.1.3 Membrane Technologies  
Membrane technologies will be the ultimate, full-scale tertiary treatment once the main drawbacks affecting their long-term use are removed. [PERSON] et al. tested the feasibility of ultrafiltration (UF) in a semi-industrial pilot plant in which nominal capacity was 20 m3 water per day. Two campaigns were carried out (October 2019-March 2020 and July 2020) analyzing the UF permeate quality with comparison to the WHO guidelines and French and EU regulations for reuse. Results showed that the UF system can greatly retain _E. coli_, _Enterococci_, spores of anaerobic sulfito-reducers and bacteriophages, providing high-quality water to be reused in non-potable applications, as it met reuse guidelines from the WHO, reuse standards from France and the most recent EU regulation for agricultural irrigation. In addition, the UF was effective for most microplastic particle removal, especially on particles >70 um. Nevertheless, the management of the membrane fouling remains the most challenging aspect of the filtration treatments [62]. A complete, autonomous, compact and low-pressure UF pilot unit, including a stainless-steel prefilter with a pore size of 100-150 um, a mild washing subsystem (air-water) and a chemical cleaning system (air-water and chemicals) controlled by automated electric valves, was installed at the Patras sewage treatment plant for the treatment of its secondary settling tank effluent. The system consisted of eight LITREE LH3-1060 V membrane modules, each supplying 2.5 m3/h of permeate, with a maximum capacity equal to 400 m3/d. The membrane permeate ended up in the chlorination tank. Monitoring of membrane performance was carried out during a two-month period. The turbidity and TSS were reduced by more than 99% and 94%, respectively, producing a statistically significant effect on the values of TS, TP and TN. COD showed a reduction of 35%. _E. coli_ and _Enterococcus_ were completely removed, and other coliforms were reduced by 95%,reduced to zero after chlorination with 0.14% sodium hypochlorite, allowing permeate unrestricted reuse according to Greek legislation [63]. A possible solution to the problem of UF membrane fouling was proposed by [PERSON] et al., who introduced a microcoagulation step as pre-treatment on the effluent from a coastal WWTP. Poly-aluminum chloride was added before the flow was introduced into a UF-RO system. The microcoagulation step improved contaminant removal and cleaning efficiencies, including water backwash, chemical-enhanced backwash and cleaning in place processes [64]. The application of coagulation pre-treatment was further improved by the work of [PERSON] et al. The authors introduced an online light-scattering technology to monitor the colloidal particle concentration in the feed flow to inject the required amount of coagulant and to remove particles. Two pilot-scale membrane filtration systems operated in parallel, and the one managed by the online technology showed only 4-20% of the transmembrane pressure increase measured in the system without monitoring. The filtrate water quality in terms of colloidal particle concentration, chemical oxygen demand (COD) and turbidity was, however, very similar [65]. UF, nanofiltration (NF) and RO were evaluated in parallel, at a pilot scale, for the removal of selected inorganic compounds and COD from municipal sewage wastewater treatment plant (MSWWTP)-MBR secondary effluent. Three commercial spiral wound membranes were tested: (1) XLE, a polyamide extra-low-energy RO membrane with a Molecular Weight Cut-Off (MWCO) of approximately 200 Da; (2) a polyamide loose NF membrane with an MWCO of approximately 400 Da; and (3) a piperazine loose UF membrane with an MWCO of approximately 1000 Da. When the effluent was treated with RO, an excellent removal rate (>90%) for all selected inorganics was obtained, and water met the standard requirements for reuse in cooling and irrigation.