## 3 Decentralized Treatment Systems
### Wastewater Treatment
The flow of 60 L/min is enough to remove organic content and obtain high rates of nitrification. The microbial load is also completely removed after the UV disinfection, producing safe water for any kind of reuse, as it meets even the strictest criteria. The system seems ideal for implementing decentralized wastewater treatment to address water scarcity issues in urban environments or to produce freshwater in emergency situations [145]. Domestic WW originating from a settlement with a population of 17,500 inhabitants was treated by an EC process in a real-scale WWTP with a capacity of 2400 m\({}^{3}\)/day. The WW plant consisted of screens, equalization tank, EC reactors, filtration unit, treated WW tank and filter press. Iron was used as electrode, the current was between 90 and 120 A and the voltage was around 400 V. The HRT was 5-6 min. The iron hydroxides formed by anode dissolution caused flocculation, and the hydrogen bubbles from cathode floated the flocculated sludge. Moreover, 0.002 kg/m\({}^{3}\) cationic polyelectrolyte was also added. The control parameters, monitored for 12 months, were SS, BOD, COD, total nitrogen (TN), total phosphorus (TP), pH and conductivity. The removal efficiencies were in the ranges of 72-83% (SS), 67-80% (COD), 69-81% (BOD), 21-47% (TN) and 27-46% (TP). Considering the Turkish wastewater discharge regulation standards, only those for TN and TP were not achieved in certain periods. The energy consumption and the operating cost were determined to be 0.49-0.54 kWh/m\({}^{3}\) and 0.24-0.28 EUR/m\({}^{3}\), respectively. An EC process is economically feasible in non-sensitive areas (TN and TP removal not required), especially in settlements where mobile population is high or there is not enough area for the installation of a WWTP [146]. Unconventional sewage waste needing onsite treatment is represented by open drains in rural areas, villages and satellite towns. Sewage in open drains has a high content of microorganisms, organic load, suspended solids, sulfides, nitrates, phosphates, pharmaceuticals and pesticides, since the flow comprises raw sewage and domestic and agricultural WWs, representing a serious threat to the environment and human health. The treatment of an open drain was made at The Energy and Resources Institute (TERI) in Gwal Pahari, India, by using the TADOX(r) system, which includes a primary coagulation-flocculation treatment, a heterogeneous photocatalysis (UV/n-TiO\({}_{2}\)) and a nanomaterial recovery. TADOX(r) is designed to be modular, so it could be installed adjacent to the drain, and the upstream flow could be channelized to the system. Total nitrogen reduction of 61%, phosphate reduction of 96% and nitrate reduction of 24% were observed within 5 h. TADOX(r) obtained a reduction of 3 log unit for _E. coli_, 4 log unit for total coliforms and 4 log unit for total count (CFU/mL). The 5 h treatment removed both primary pollutants (63.5% COD, 99% BOD, 96% phosphate and 61% TKN) and micropollutants (93.5% acetaminophen, 96% sulfamethoxazole, 96% ibuprofen and 89% caffeine) [147]. A natural-based, in situ treatment of mixed agricultural-domestic-industrial drainage water has been carried out using a microalgae layer with the aim to obtain reclaimed water for agriculture irrigation. The assessment of the layer optimal thickness to obtain the maximum removal efficiency would help to restore the spontaneous self-purification activities in the drain. The pilot system was built including five parallel channels (40 cm width, 70 cm depth and 10 m length) treated with different thicknesses of immobilized microalgae layers. The water flow from the drain was 10 L/s, with 0.035 m/s velocity inside the channels and 14 s HRT inside the microalgae layer. This was carried out for six weeks, six working days per week. The 10 mm layer of microalgae was the most effective, with removal values of 29% for BOD, 46.9% for COD and 56.1% for TSS [148].