## 2 Centralized Treatment Plants
### Urban Wastewater Treatments
#### 2.1.2 Nature-Based Solutions (NBSs)  
Effective treatment trains can be obtained by using natural processes, as well as by the correct combination of natural and physico-chemical techniques, like the coupling of the anodic oxidation process to a planted filter system [54]. The electro-oxidation was combined both as pre-treatment and post-treatment, according to the characteristics of the influent to be treated. Urban sewage was treated first by filters planted with reeds, then by anodic oxidation on platinum using photovoltaic panels. The pilot plant was composed of three beds arranged in parallel, filled with gravel, and of two beds filled with sand and gravel. All filters were equipped with photovoltaic panels with a capacity of 8 KW for the operation of the immersed pumps supplying UWW. The system was evaluated for six months and obtained a complete elimination of organic matter, ammonium nitrogen and pathogenic bacteria, producing water that can be reused without restriction in different sectors. A pilot-scale algal bioreactor has been applied for UWW remediation and nutrient recovery, investigating the impact of seasonal variation and operating conditions on the water quality and results compared with those of a full-scale conventional WWTP. Its performance was comparable or better than conventional WWTP in nutrient removal and reduction in other contaminants, which resulted in below standard values for water discharge. The algal system achieved the secondary effluent discharge standards for biochemical oxygen demand and nutrients within 2-3 days during warm weather and within 1-2 days in winter by using different algal strains. The effluent met metal standards for domestic, irrigation and livestock uses [55]. Afterwards, the algal system has been coupled with the forward osmosis (FO)-reverse osmosis (RO) system in pilot-scale experiments aimed to obtain effluent for potable water reuse. An effective separation of algae for production of biofuels and fertilizers was obtained, recovering high-quality water for reuse. At 90% water recovery, the FO-RO achieved over 99% rejection of all contaminants. Water quality analysis indicated that the produced water met both the primary and secondary drinking water standards [56]. By using a photobioreactor (PBR) pilot installation in a WWTP, [PERSON] and his research group investigated the possibility of employing a mixed consortium of microalgae, bacteria, protozoa and other organisms as a more environmentally friendly tertiary treatment with respect to the nitrification-denitrification-UV system currently operating at the WWTP. In the consortium, bacteria supported algal growth, removing the organic carbon and releasing carbon dioxide and metabolites. The microalgae removed nitrogen and phosphorus nutrients as well as emerging pollutants. The self-sustainable ecosystem, in terms of O\({}_{2}\) and CO\({}_{2}\) production and exchange, was capable of performing the tertiary treatment of urban wastewater using an HRT of up to 48 h or 24 h (in winter season), removing up to 95% of NH\({}_{4}\)+. The PBR occupies around three times lower land space than a raceway pond, and it is simple to scale up. It cannot substitute for the great WWTPs due to the required retention time and land area, but when employed in combination with conventional treatments, it will reduce the environmental impact and treatment costs and produce a biomass with potential as biofertilizer or biofuel [57]. The recent advances in an emerging technology realized by integrating the microalgal-bacteria (MB) consortia with the membrane separation technology have been extensively reviewed by [PERSON] et al. This new application of biological consortia revealed many advantages not achievable with current technologies.