GSE192713
Successful Sulphide-driven Partial Denitrification: efficiency, stability and resiliency in SRT-controlled conditions
The aim of the present work was to maximise  NO2- conversion over NO3- reduction coupled with complete sulphide (HS-) oxidation, by the selective control of influent S/N ratio and sludge retention time (SRT). A 2.5-L chemostat was operated for 115 days and three operational phases were tested at decreasing SRT of 40, 23 and 13 h, testing S/N ratios in the range of 0.5-1 gS/gN. Successful sulphide-driven PAD was achieved and lead to average NO2- conversion efficiencies as high as 77±17% at all the conditions tested, with the highest value of 99% at the lowest S/N of 0.58 gS/gN and SRT of 23 h. Respirometric tests showed that NO3- uptake rate was stable at 90±10 mgN/gVSS/h, either when NO3- was present as sole electron acceptor or at NO2- levels as high as 120 mgN/l; on the contrary, NO2- uptake rates were very sensitive to the applied conditions. Metabarcoding analyses revealed that the microbial community was highly enriched in Sulphur Oxidizing Bacteria (SOB>80%) and strict and stable S-limiting conditions appeared to favour Thiobacillus over Sulfurimonas genus. A preliminary assessment of N2O potential emission was also performed. To the best of our knowledge, this is the first work evaluating the synergic effect of SRT and influent S/N ratio on nitrite accumulation in highly SOB-enriched systems and the NO2- conversion efficiencies achieved are among the highest reported in literature. Overall design: Lab-scale chemostat seeded with enriched Sulphur Oxidizing Biomass and fed with sythetic mineral medium rich in sulphide and nitrate