## 3 Decentralized Treatment Systems
### Aquaculture Effluent Treatment  
Aquaculture requires significant inflow of high-quality freshwater, but the effluents contain a heavy load of chemicals and nutrients. Recirculating aquaculture systems (RASs) reduce the aquaculture environmental impact, allowing water reuse. Biofilters, utilizing specific prokaryotic groups such as nitrifiers, are considered a key aspect of these systems. CWs have been successfully used for the treatment of intensive aquaculture basins in developing countries. An interesting prototype of a modular CW was designed and tested by [PERSON] et al. It consisted of a small and modular horizontal subsurface flow (HSSF) CW, in which successive compartments are dedicated to the sequential elimination of the contaminants. The first compartment, continuously aerated, was intended for organic matter oxidation and nitrification and was filled with polypropylene plastic pellets for microbial biomass fixation. The second compartment was designed for the reduction in dissolved oxygen and particle sedimentation. The third compartment was intended for denitrification, organic matter and phosphorus removal by anaerobic microbial activities, plant uptake and passive adsorption. The fourth compartment was set to stabilize water flow and retain possible particulates. The fifth compartment, free from any solid, was equipped with a 20-Watt 254 nm UV LED lamp for disinfection. Finally, in the sixth compartment, the effluent water was oxygenated. The CW was connected to a shrimp-breeding tank, and its performance was measured over a complete growth cycle of White-leg shrimps (_Litopeneus vannamei_). Complete removal of nitrite was attained, with a concomitant reduction in nitrate and COD of 78% and 76%, respectively. Bacteria enumeration tests showed that _Vibrio_ sp. cells were fully removed and that a 3-log reduction was reached in total aerobic bacteria [167]. A very simple, passive filtration system for nutrient removal and water reuse was employed in an RAS rearing rainbow trout. Denitrification in a woodchip bioreactor filled with birch wood (_Betula pendula_) followed by sand filtration was introduced into a side-loop of the RAS. Denitrification efficiency remained high (96%) throughout the experiment (7 months) and reached a nitrogen removal rate of 15 g NO\({}_{3}\)-N m\({}^{3}\) d\({}^{-1}\). Sand filtration removed dissolved and particulate matter and improved water quality before being returned to water circulation. The chemical analyses showed that harmful compounds or elements did not accumulate in the system [168]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Process and Methods** & **Notes** & **Ref.** \\ \hline Graywater & \begin{tabular}{c} Shower and sink, kitchen sink and washing machine water treated for \\ \textgreater{} 4 months in pilot CW 200 L, after settling tank. HRT 3 d. Removal \\ \end{tabular} \\ HFCW & \begin{tabular}{c} Effectiveness: turbidity \textgreater{} 92\%, TSS \textgreater{} 85\%, COD \textgreater{} 89\%BODS \textgreater{} 88\%, W.Q. Class \\ \textgreater{} C of EU 741/2020 for microbial contaminants and BOD5. \\ \end{tabular} \\ Vertical CW & \
\begin{tabular}{c} VertECO system, surface of 7.2 m\({}^{2}\), 4 stages with hydroponic cultivation \\ treated 2 m\({}^{3}\) d\({}^{-1}\) from a 441-room hotel. Total HRT 0.55 d. Followed by UV \\ disinfection. W.Q. Class B of EU 741/2020 (toilet flushing and irrigation). \\ \end{tabular} \\  &
\begin{tabular}{c} Full-scale plant, 14.4 m\({}^{2}\), 100 L d\({}^{-1}\) during 1 year. Plants in lava, organic soil, \\ biochar mixture: nutrients release produce yellowish color. Water reused for \\ toilet flushing. Energy cost: 1.79 kWh d\({}^{-1}\), but the GW \\ provides thermal insulation. \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Recently published (2020–2024) applications at pilot- or full-scale level of water remediation technologies as decentralized systems for wastewater treatments. The various applications are gathered according to the characteristics of the remediated water.