## 3 Decentralized Treatment Systems
### Wastewater Treatment
This system operated by treating WWs from a public toilet facility at an informal settlement community for about one year and half. The AnMBR removed about 95% of the influent COD, and the nutrient capture system was efficient in removing total nitrogen, residual COD and color, but the regeneration process was lengthy. The EC provided effective disinfection, and no _E. coli_ or helminths ova were detected in the effluent. The system is promising, but it still requires additional improvement to consistently meet the entirety of the ISO 30500 standards [140; 141]. The synergistic effect between natural solar radiation and a heterojunction with perovskite-type strontium stannate and graphitic carbon nitride for actual wastewater purification has been the main component of the treatment proposed by [PERSON] et al. The raw sewage was exposed to sunlight for 240 min in the presence of 100 mg L\({}^{-1}\) SrSnO\({}_{3}\)/g-C\({}_{3}\)N\({}_{4}\), and a 56.1% organic matter mineralization was observed, with an efficiency 2.5 times higher than photolysis under sunlight. The total dissolved solids (TDSs) were reduced by 30%. Treated effluent showed no coliforms or heterotrophic bacteria growth and can be used for irrigation as a viable alternative to water supply and for the nutritional needs of plants [142]. Similarly, a SrSnO\({}_{3}\)/g-C\({}_{3}\)N\({}_{4}\)-solar radiation-based wastewater treatment was tested as secondary treatment of the effluent from a CW receiving wastewater from a hotel with a convention center. The pilot-scale plant included a solar Compound Parabolic Collector photoreceptor. The bacteria inactivation at the D.L. of 2 CFU/mL was reached after 15 min with 1 mM of PMS. The 80% degradation in three CECs (diclofenac, sulfamethoxazole and trimethoprim) was reached after 27 min. ARB were successfully inactivated to values below the DL, while the ARGs were not completely removed. The effluent did not show any eco- or phytotoxic effects [143]. A low-cost, pilot-scale decentralized WW treatment system integrating aerobic granular sludge (AGS) with a gravity-driven membrane (GDM), with a capacity of about 0.1 m\({}^{3}\) per day, was developed by [PERSON] et al. and operated continuously for 1 year in an arid climate, showing superior performance with respect to MBR. The AGS reactor was seeded with granular biomass, and the cycle time was set at 4 h under steady-state conditions. The height of the AGS tank would provide a sufficient water pressure head to drive the GDM filtration process without energy input. The system was able to simultaneously remove carbon and nutrients (P and N) in a single reactor tank by sequential batch process relying on microbes and to withstand toxic shock loading. Both TSS and _E. coli_ in the membrane effluent were below the detection limit [144]. The application of decentralized WW treatment systems can become very interesting in dense urban environments, where the reuse of treated WW would require the establishment of an adequate distribution net from the centralized WWTPs, usually far away from the city center, to the town buildings. A first example is represented by the already mentioned ecological treatment plant where the anodic oxidation process and planted filter were selected and combined because of their low cost and significant purification performance, an example of a solution with minimum expense and maximum efficiency [54]. Another urban decentralized wastewater treatment is represented by the \"NextGen\" sewer mining hybrid system. The mobile, modular treatment system consists of an MBR unit along with a UV disinfection unit. This technology extracts untreated wastewater from the local sewers, treats it at the point of demand and produces freshwater that can be used directly for irrigation or for aquifer recharge during the winter, while waste by-products are returned to the main sewerage plant. The system has been tested at the Athens plant nursery since 2021 and until now. The operational stability and performance were greatly satisfactory.