## 3 Decentralized Treatment Systems
### Graywater Treatment  
To use reclaimed graywater (GW) would also be a valuable practice in rural areas where GW is often used untreated in irrigation. This would prevent food contamination, salinization and clogging of soils and groundwater pollution. CWs have become the more widely studied NBS for small-scale decentralized waste- and graywater treatment and reuse, on household or small community levels, able to remove even recalcitrant compounds like antibiotic molecules [123]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Process and Methods** & **Notes** & **Ref.** \\ \hline MF; UF & MF; silicon carbide membrane, UF ceramic membrane. After 4 days, permeability was reduced 95\% and 37\%, respectively. TS, turbidity, COD and microplastics were reduced \(\sim\)100\%. UF was better in removing microplastics: 99.2 vs. 98.55\% & [115] \\  & Electroflotocoaulation was performed in a 500 L pilot plant by soluble Al electrode, polymer nanocomposite absorber, 2.5 m\({}^{3}\) h\({}^{-1}\). After filtration (30 min), result was surfactants and color \(-\)95\%, turbidity \(-\)97\%, and 50\% energy savings by implementing heat recovery. & [116] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._[PERSON] et al. reported the results of a GW treatment carried out for more than four months by a horizontal flow constructed wetland (HFCW). _Phragmites australis_, _Carex osmiensis_ and _Cyperus papyrus_ were tested separately in preliminary laboratory-scale experiments showing very similar results. Pilot-scale tests were conducted to confirm the performance at a larger scale and evaluate the influence of hydraulic retention time. Removal values >92%, >85%, >89% and >88% for turbidity, total suspended solids (TSSs), COD and five-day BOD\({}_{5}\), respectively, were obtained [124]. A different kind of CW, called a vertical ecosystem (vertECO), has been installed in a large hotel with separated collection of GW. The vertECO can be installed indoors and/or outdoors and has a vertical structure with four cascading stages combined with horizontal water flow. Each level is placed 0.5 m above the next level below. The plants are cultivated in a hydroponic system with expanded clay. The total high is 2.5 m and the total horizontal surface 7.2 m\({}^{2}\). The amount of GW generated in tourist facilities was reported to lie between 50 and 150 L/day/person, but it can be much higher. The vertical ecosystem was operated continuously at 2 m\({}^{3}\)/day for 12 months, and water exited the system after an HRT of 0.55 days. The limits required for the irrigation purpose (class B) in the European regulation were met, but the reclaimed water was reused only for toilet flushing after a gridding and disinfection with HCIO at around 1 mg/L free chlorine [125]. A similar setup, the Total Value Wall (TVW) with a surface of 14.4 m\({}^{2}\), has been tested in a full-scale system to produce water for toilet flushing, installed at a terraced house and loaded with graywater at 100 L per day. Eleven different species of plant were selected and planted in a mixture (lava 50%, organic soil 25%, biochar 25%), which proved to have optimal percolation rates and removal efficiencies. The TVW effluent had BOD\({}_{5}\) values always under the standard limit (20 mg O\({}_{2}\cdot\)L\({}^{-1}\)), while TSS fluctuated around the standard value of 10 mg\(\cdot\)L\({}^{-1}\). The total coliform count remained very low, and a removal of 99% (2 log units) was obtained. _Legionella_ sp. was under the admitted limit. Purified water was safe for reuse, but some problems were created by the compounds released by the organic soil (water was an intense yellow color probably because of humic acid release) and from the plant plugs (phosphate and nitrogen). Nevertheless, an additional benefit of the TVW was its acting as an insulating layer, keeping the heat outside on warmer days and keeping the heat inside on colder days [126]. The presence of CECs in GW could prevent the obtaining of high-quality reclaimed water.