## 2 Centralized Treatment Plants
### Urban Wastewater Treatments
#### 2.1.2 Nature-Based Solutions (NBSs)
No MB biomass separation/washout issues are involved, ensuring a healthy and stable MB consortium; it realizes the decoupling of the HRT from the solid retention time; the concentrated MB biomass facilitates harvesting and biodegradation; and the simultaneous COD, N and P removal with superior quality of effluent is obtained with a significant energy reduction for aeration and/or carbonation, in this case not necessary. The evolution of these emerging systems has reached the stage of pilot-scale testing; they are now being investigated to obtain design parameters and economic analysis for full-scale application in the near future [58]. Conversely, the microbial community composition and the process performance of aerobic granular sludge (AGS) plants have been investigated for the first time, at the full-scale, in a Nordic country (Sweden). The AGS plant, made by two AGS reactors, treated 60% or 100% of the flow from a WWTP receiving 30,000 population equivalent (PE) urban WW. The influence of seasonal fluctuations in nutrient concentration, flow intensity and temperature was investigated. After 15 months, stable organic matter, nitrogen and phosphorus removal were attained with low effluent concentrations. During the start-up,the BOD7/ N ratio, influent flow and temperature were important factors influencing the performance of the AGS plant [59]. The long-term performance (6-7 years) of two classical NBSs, a constructed wetland (CW) and a lagoon system (LS) employed as tertiary treatment of UWWTP effluents, was assessed and compared to the new required standards for agricultural irrigation. The CW treated the effluent from a 5500 PE WWTP and the LS from a 75,000 PE one. In both cases, _E. coli_ (about 100 CFU 100 mL1) and BOD5 (lower than 25 mg L-1) mean effluent concentration need to be further reduced in reclaimed water to be suitable for unlimited reuse. Moreover, in both NBSs, an increase in TSS mean concentration in the effluent was observed, making reclaimed water unsuitable for agricultural reuse. Suggestions to improve the performance of the systems included the implementation of vegetation islands in both systems, the addition of horizontal flow (HF) wetlands after the LS and the addition of a disinfection system after both NBSs. To reach the requested water quality is definitely important, since this research showed that the reclaimed water would satisfy water needs of the crops cultivated in that area, as well as a certain reduction in chemical fertilizer application because of the nutrients (mainly nitrogen and phosphorous) still present in it [60]. A unique tertiary treatment system was applied to the WWTP effluent for a one-year period. The ecosystem included a treated wastewater pond (TWP) which received 100% effluent from the WWTP and an aquaculture pond (AP) for fish production receiving treated water from the TWP. The fate of a wide range of pharmaceutically active compounds (PhACs) such as telmisartan, carbamazepine, diclofenac and venlafaxine was monitored in this system and in a control pond fed by river water. The water quality hazard produced by PhACs and antibiotic resistance factors, as well as the ecotoxicological effects, were consistently reduced following treatment in the TWP prior to water reuse in the aquaculture industry [61].