Remineralization of reverse osmosis produced drinking water: Impact of magnesium on biological stability and microbial community

• Magnesium up to 50 mg/L does not increase bacterial growth in RO water. • Magnesium addition enriches biofilm genera like Piscinibacter spp. and Afipia spp. • Legionella spp. was only detected in the control without magnesium addition. • Study supports safe magnesium dosing to improve RO tap water quality. Membrane desalination produces soft, corrosive water with low mineral content, requiring remineralization. In the Kingdom of Saudi Arabia, where reverse osmosis (RO) desalination is the main drinking water source, remineralization is commonly limited to calcium, although current regulations require a minimum of 5 ppm magnesium. This study evaluated whether magnesium remineralization influences microbial regrowth, biofilm development, and bacterial community composition in RO-produced tap water. Bacterial growth potential was evaluated under varying magnesium concentrations, salt types, magnesium-to-calcium ratios, and with the addition of nutrients (carbon, nitrogen, and phosphorus). Simulated distribution experiments were conducted using two miniature networks operated for six months: one supplemented with magnesium chloride (MgCl₂) (50 mg/L Mg²⁺) and one control without supplementation. Microbial abundance was quantified by flow cytometry and ATP analysis, while bacterial communities were profiled using 16S rRNA gene sequencing. The results showed that: (i) magnesium up to the WHO upper limit (50 mg/L) did not increase bacterial growth potential, which was instead limited by assimilable organic carbon (AOC); (ii) magnesium-to-calcium ratios had no measurable effect on bacterial yield or biofilm growth; and (iii) in the simulated network study, magnesium addition did not increase microbial abundance in outlet water or biofilm, but shifted biofilm communities toward higher richness, dominance of Piscinibacter spp., and absence of Legionella spp.. These findings demonstrate that magnesium remineralization does not compromise biological stability or promote excessive biofilm formation. Its safe inclusion in remineralization may improve taste, enhance corrosion control, and support the sustainability of RO-produced drinking water.