## 3 Decentralized Treatment Systems
### Surface, Open Water Systems and Groundwater Treatment  
Water from open sources such as rivers, ponds and lakes can be contaminated by diffuse pollution and must be remediated before entering drinking water systems. Agricultural runoff or other wastewaters spread on the surface also require suitable treatments. The pilot- and full-scale applications of the most employed NBS in this field, the Floating Treating Wetland (FTW), have been reviewed by [PERSON] et al. In this paper, all key factors that have a considerable role in the design, operation, maintenance and pollutant treatment performance of large-scale FTWs are examined, and the various mechanisms contributing to the water cleanup are illustrated. Design factors including rack design, HRT and plant selection are discussed, along with the effects of seasonal variations, microbial supplementation and harvesting schemes. Valorization methods for the harvested plant biomass are overviewed, and suggestions for future performance improvements are provided [154]. A bioaugmented FTW (1000 L) has been employed in pilot scale to remediate natural water contaminated with crude oil. A four-member bacterial consortium (_Klebsiella_ sp. strain LCR187, _B. subtilis_ LOR166, _Acinetobacter_ sp. LCRH8 and _A. juini_ TYRH47), all carrying the alkane hydroxylase gene _alkB_ involved in aerobic hydrocarbon degradation, was added to FTWs planted with _Phragmites australis_ or _Typha domingensis_, and its influence was determined. During a treatment period of 36 days, bioaugmentation improved the treatment of oil-contaminated water by at least a factor of two, with removal efficiencies for hydrocarbons of 95% and 85% in _P. australis_ or _T. domingensis_, respectively. Copy numbers of the _alkB_ gene and its mRNA increased over time in plant-associated samples, suggesting that the performance of full-scale systems can be improved at only small costs. The toxicity of the contaminated water was reduced significantly, as shown by much better plant growth in the systems and increased survival of fish in the treated water [155]. Polluted water from Marriot Lake in Egypt was treated by combining a pre-treatment step using a series of connected settling tanks, limiting the clogging potential and the best-performing configuration of a horizontal subsurface flow constructed wetland (HFCW) unit. The HFCW unit was planted with _Typha angustifolia_ and contained a perforated pipe network for enhanced passive aeration. The system operated under a continuous flow mode and showed the highest performance at a hydraulic Load Rate of 1000 L m\({}^{-2}\)/d and 4-6 h HRT. The integrated system was proved to achieve high removal efficiency, with \(-95.3\%\) turbidity, \(-83\%\) BOD\({}_{5}\), \(-99.3\%\) NH\({}_{4}\)-N, \(-70.8\%\) TN and \(-66.7\%\) TP, while higher NO\({}_{3}\)-N and NO\({}_{2}\)-N effluent concentrations were observed. Metals were also reduced and accumulated in plant tissues. The quality of the effluent reached the national discharge limits [156]. The presence of microcontaminants in natural water can affect the quality of drinking water obtained from contaminated sources. The effectiveness of a solar-driven free chlorine advanced oxidation process in the simultaneous elimination of six organic microcontaminants (OMCs) and three bacteria from natural water was assessed at a pilot scale under natural solar irradiation. The combination of free chlorine with solar irradiation provided a simple and energy-efficient process for OMC and bacteria removal. At the optimal free chlorine concentration (2.5 mg L\({}^{-1}\)), determined at lab-scale experiments, 80% of total degradation was obtained in 5 min without \(\text{ClO}_{3}{}^{-}\) generation. Moreover, the process effectively avoided the possible bacterial regrowth in the post-treated sample after six days [157].