## 2 Centralized Treatment Plants
### Urban Wastewater Treatments
#### 2.1.4 Multibarrier Systems  
The multibarrier approaches have been proposed to reduce the contaminant content in wastewater and are mainly implemented by combining coagulation, absorbent materials, AOPs and/or membrane filtration technologies. As an example, a powder-activated carbon (PAC) pre-coating membrane (PM) was developed as a WWTP effluent pre-treatment for the UF system. The PM is a filter cake layer formed by pre-coated agent on the surface of base membrane, which could reduce the fouling of the base membrane by removing different kinds of organic compounds, colloids and particles in the influent, protecting the UF membrane. In a pilot-scale study, the PACPM-UF effective filtration area was 0.5 m\({}^{2}\), and 150 g of PAC (300 g/m\({}^{2}\)) particles were suspended. The PACPM showed its better removal ability for organic matter with weight under 1 kDa and higher than 100 kDa, which could be explained by a combined effect of adsorption and filtration. Experiments of continuous operation durability were conducted (24 h), and the PACPM was almost unable to remove organic matter after 8 h of operation, but the improving effect on the UF operation was maintained up to more than 12 h. PACPM requires a regular replace-regeneration process [68]. Among absorbing materials, activated carbon from vegetal wastes is an economical and sustainable solution. In a pilot-scale study, [PERSON] et al. analyzed the performance of coconut-shell granular activated carbon in absorbing different CECs and investigated the effectiveness of a multibarrier system (RO, photoperiodation and filtration) in producing water for direct potable reuse. The absorption capacity was different for each contaminant, but the multibarrier system was able to reduce CECs to less than 1% of the initial content, also reducing the water color, turbidity, nitrogen and phosphorous concentration [69]. Another multibarrier train aimed at water potable reuse was adapted and tested on tertiary effluents by [PERSON] et al. through a pilot-scale water reuse train including flocculation/sedimentation, sedimentation, monochromamine supplemented ozonation, biologically active carbon (BAC) and granular activated carbon (GAC) filtrations and ultraviolet disinfection. The authors employed the biodegradable dissolved organic carbon (BDOC) analysis to evaluate the removal extent of various TOC fractions. GAC and ozone treatments were the critical steps for non-BDOC removal, and the results highlighted the need to optimize BAC performance to maximize GAC adsorption capacity [70]. The research group of [PERSON] et al. investigated a novel, advanced integrated pilot-scale treatment system combining pressure sand filter (PSF), UF, ozone and GAC to treat WWTP secondary effluent and evaluated the reclaimed water suitability for reuse. Various combinations of the multibarrier components were tested:  
PSF + O\({}_{3}\) + GAC;  
PSF + UF + GAC. The first configuration was found to be more effective, producing almost complete removal of contaminants before GAC filtration, which obtained complete removal. The GAC process removed ozone oxidation by-products, controlled the complex odor issues and attained a TOC value removal efficiency of about 99%. Also, this multibarrier pilot-scale process was effective in removing contaminants to the required level of discharge in the environment or reuse [71]. The presence of persistent organic contaminants can prevent the potable reuse of water exactly like a microbial contamination. At the City of Altamonte Springs's permanent potable reuse demonstration facility (pureALTA), the challenging removal of perfluoroxyll acids (PFAAs) from WWTP effluent was faced by employing a carbon-based treatment: a multibarrier able to provide potable water that meets regulatory requirements and health guidelines for unregulated contaminants. The multibarrier treatment included O3, BAC, UF, GAC and UV-AOP.