Promoting the growth of Rubrivivax gelatinosus in sewage purification by the addition of magnesium ions

•5–15 mg/L Mg2+ improved the feasibility of R. gelatinosus biomass recycling.•5–10 mg/L Mg2+ improved R. gelatinosus yield in wastewater to level in medium.•The optimal 10 mg/L Mg2+ reduced hydraulic retention time of SPW by 25%.•There were upper limit (15 mg/L) and optimal values (6 mg/L) for intracellular Mg2+.•Different Mg2+ concentrations had different impacting mechanisms on biomass.

This work investigated the improvement in biomass yield and production and organics reduction of Rubrivivax gelatinosus (R. gelatinosus) by adding magnesium (Mg2+) for sewage purification under natural light micro-oxygen conditions. The results showed that with the optimal Mg2+ dosage (10 mg/L), biomass production (4125.6 mg/L) was enhanced by 59%. The biomass yield was improved by 43.3%. COD and protein removal increased more than 90%. Hydraulic retention time was decreased by 25%. Moreover, 5–10 mg/L Mg2+ improved R. gelatinosus yield in wastewater to medium levels. Analyses revealed that different Mg2+ concentrations had different mechanisms for impacting biomass production and COD removal. This was because there was an upper limit (15 mg/L) and an optimal value (6 mg/L) of intracellular Mg2+ concentrations for R. gelatinosus. Appropriate Mg2+ concentrations enhanced ATP production, which improved the secretion and activity of protease, indicating that more protein and COD were removed and more biomass accumulated. However, excessive Mg2+ was harmful for biomass accumulation and COD removal because excess Mg2+ decreased ATP production.