Brine that is produced during reverse osmosis (RO) could be further concentrated by another RO step. In the absence of bivalent cations, silica is a limiting factor because a high concentration of silica can cause irreversible fouling of the membranes. To prevent silica scale formation during the further concentration of brine with RO, silica should be removed in a separate process. Silica gel beads have a large specific surface area and can be used to precipitate silica. Previous research on the removal of silica using silica gel beads for solutions with a relatively high silica concentration showed that a removal of 30-60% is possible.
In this research the effect of the dosage of silica gel beads, pH, temperature and the presence of 1 or 10 mg/L of ferric and aluminum on the removal of silica using silica gel beads from a solution with a low silica concentration was investigated. Starting with a silica concentration of 180 mg SiO₂/L a removal up to 30% was obtained after 8 hours (pH 8.5, 21°C).
The removal of silica using silica gel beads was limited, because silica gel dissolved when the solution is unsaturated with regard to silica. The equilibrium concentration was approximately 102 mg SiO₂/L. The difference between the concentration of silica in solution and the equilibrium concentration appeared to be a driver of the removal. The increase in removal efficiency as a result of using a higher dose of silica gel seemed to be limited. Operating at pH 10 instead of 8.5 resulted in a lower removal efficiency. Lowering the pH to 7 or 5 did not result in a decrease of the removal efficiency. Changing the temperature to 10 or 35°C or adding 1 or 10mg/L of ferric or aluminum did not increase the removal efficiency either, but even decreased the efficiency.
The obtained removal efficiency might be too low for direct application of fixed bed column filtration with silica gel beads as interstage silica removal technique in a RO system that is used to concentrate brine. A combination of filtration with silica gel and a technology that can concentrate the brine without scaling to concentrations up to 500 mg SiO₂/L could be an interesting solution to make brine treatment more energy-efficient, because the need for an evaporation step is minimized.

Ammonia in aqueous environments leads to eutrophication and toxicity of the receiving water body (Metcalf & Eddy, 2003). In order to prevent environmental pollution, ammonia needs to be removed from residual water streams, before the water can be discharged to the aqueous environment (Song et al., 2012). Waste water treatment plants (WWTPs) reduce the ammonia-nitrogen concentration by applying biological treatment technologies. However, the oxidation of ammonia by bacteria requires high amounts of oxygen and consequently has a high energy consumption. The nitrification and denitrification of wastewater requires 15.83 kWh per kg-N (Magrí et al., 2013). Moreover, the reject water from the digested sludge contains high amounts of nitrogen, which is fed back to the biological treatment, and can contribute 15-20% of the nitrogen load (Fux et al., 2002). Concentrating ammonium fluxes lead to the production of a concentrated ammonium stream, used as potential energy source in the N2kWh project, and a sweet stream.