## 2 Centralized Treatment Plants
### Concentrated Stream Remediation for Water Reuse  
The concentrate resulting from the WW treatment can be also subjected to further treatments to recover water. A 'fourth-stage' concentrate treatment facility could generate \(\sim\)30,283 m\({}^{3}\)/d of additional permeate, reducing concentrate disposal. In the concentrated streams, the unusual high content of pathogens and contaminants represents a particular challenge. One of the most recent pilot-scale research studies on this topic employed membrane distillation (MD) technology. The rejection of MS2 and PhiX174 was quantified at low (40 \({}^{\circ}\)C) and high (70 \({}^{\circ}\)C) inlet temperatures. Results highlighted that in case of MD, the bench-scale data cannot be used to predict pilot-scale contaminant rejection and removal of viruses. During the rejection experiments, both viruses were detected in the distillate, and virus rejection was lower in the pilot-scale experiments because of increased pore flow. Indeed, the increase in membrane area increases the contribution of convective pore flow and thus reduces virus rejection. Operating parameters minimizing the hydraulic pressure difference are important to reduce the convective pore flow. Moreover, a minimum residence time of the brine in the MD system is required to ensure complete removal of viruses from the concentrate [74]. Closed-circuit reverse osmosis (CCRO) and FO-RO were tested and compared at a pilot scale for the treatment of RO concentrate from a full-scale advanced water purification facility. Both systems obtained inorganic, organic and microbiological constituent removal, making permeate suitable for blending with primary RO effluent, since it respected the microbiological limits for coliforms, _E. coli_, somatic and male-specific (MS) coliphages. The FO-RO barrier produced a higher MS coliphage log removal as well as greater removal for certain chemical constituents. The permeates were both suitable for subsequent UV-AOP process, and at a full-scale implementation, they can be treated once blended with primary RO permeate. The estimated full-scale total unit cost of water produced by the two technologies ranges from USD 0.91 to USD 1.12 per cubic meter [75]. \begin{table}
\begin{tabular}{c c c} \hline
**Process and Methods** & **Notes** & **Ref.** \\ \hline UWW effluent disinfection & Clarified effluent treated for 6 months in 5 m\({}^{3}\) tank. W.Q. \({}^{1}\) Class 15\% peracetic acid, 24\% H\({}_{2}\)O\({}_{2}\), 16\% acetic acid & B of EU 741/2020 for microbial contaminants. \\  & Tertiary effluent treated at 1000–6000 Lh\({}^{-1}\). W.Q. up to Class A of \\ UV + O\({}_{3}\) + H\({}_{2}\)O\({}_{2}\) & EU 741/2020 for microbial contaminants depending on water \\  & quality (particles and organic content). \\  & Secondary effluent treated in UV C reactor, 5 min to remove _E._ \\ UV C + H\({}_{2}\)O\({}_{2}\) 15 mg L\({}^{-1}\) & _coli_, 10 min for _Enterocccus_. W.Q. Class A of EU 741/2020 for microbial contaminants. High energy demanding process. \\  & Secondary effluent treated in raceway pond reactor 15 L. W.Q. \\ Solar photo-Fenton (Fe:IDS) & Class A of EU 741/2020 for microbial contaminants. No bacterial \\  & regrowth. Ideal for small WWTPs. \\ \hline \end{tabular}
\end{table} 
Table 1: Recently published (2020–2024) applications at pilot- or full-scale level of water remediation technologies on effluents from centralized urban wastewater treatment plants. \begin{table}
\begin{tabular}{c c c} \hline \multicolumn{1}{c}{**Process and Methods**} & **Notes** & **Ref. ** \\ \hline Fe3+-NTA 5 mgL-1 + H2O2 25 mg L-1 + NaOCl 10 mg L-1 & Secondary effluent treated in 80 L raceway pond reactor, 60 min & [50] \\  & & HRT. W.Q. Class B of EU 741/2020 for microbial contaminants. \\  & & Costs: reagents 0.29 EUR/m-3, energy 0.94 EUR/m-3. \\  & & Treatment in 2 parallel ring photoreceptors, max flow rate 650 L-1 \\  & & each one, 2.77 s HRT. W.Q. Class A of EU 741/2020 for microbial contaminants. Removal of CECs and ARGs: 94\%. Treated water is \\  & & phytotoxic. Costs: 0.70 EUR/m-3.