In the near future, the demand for drinking water could do nothing but increase, leading to the necessity of investigating new sources of freshwater, in addition to the frequently used groundwater and surface water. Brackishwater, occurring at the interface between fresh and seawater, represents a potential source of freshwater and is often treated with reverse osmosis due to its high salinity. The waste stream of this treatment, usually discharged into the sea after proper treatment, represents a potential additional source of freshwater that could be treated by sand filtration.

The aim of this study is to assess the viability of adopting sand filtration as a treatment method against ammonium and manganese for the reverse osmosis concentrate. Sand filtration has proven to be effective for waters characterized by a relatively low salt content; hence, the influence of salinity on the removal of the two aforementioned contaminants needs to be evaluated. This is done by comparing sand filtration in two different conditions: fresh and saline water, where the composition of the latter is taken from the concentrate of the reverse osmosis from Dunea's Freshman Project.

The results showed that manganese removal characterized the initial period in both systems; nevertheless, it became inhibited by the subsequent onset of nitrification. In the freshwater system, this occurred naturally, while in the saline one, nitrification needed to be enhanced by the addition of sludge coming from a SWWTP, as the natural onset turned out to be delayed. A strong correlation between the two processes relies on the effect that nitrite presence in the water has on the release of adsorbed manganese from the sand medium, a behaviour that was observed in both systems and therefore independent of water salinity. By the end of the experiments, ammonium was successfully removed in both systems. In the freshwater one, nitrification and manganese oxidation were complete, unlike the saline one, where nitrite was detected in the effluent and manganese removal didn't recover since the onset of nitrification.

Overall, the findings suggest that salinity has a delaying effect on the development of the biological processes involved in the removal of manganese and ammonium. Over time, the sand medium proved to increase its performance and potentially promote the growth of the necessary bacteria, indicating that manganese removal might also recover. By investigating how these processes respond to changing salinity conditions, this study aims to contribute to a better understanding of the potential application of sand filtration for the treatment of brackish groundwater reverse osmosis concentrate.