Effect of iron on enhanced nitrogen removal from wastewater by sulfur autotrophic denitrification coupled to heterotrophic denitrification under different substrate ratios

Autotrophic denitrification has become the main contributor to the nitrogen removal from wastewater due to its high efficiency, safety and economy. Effect of Fe on sulfur autotrophic denitrification coupled to heterotrophic denitrification was investigated to enhance the nitrogen removal. Under the substrate ratios of S2O32-:Fe (II): CH3COO- of 1:1:3, 1:3:1 and 3:1:1, the microorganisms were wrapped by Fe minerals, and the normal metabolic pathways were blocked. By adjusting the substrate ratios of S2O32-:CH3COO- of 1:3, 2:2 and 3:1 without Fe (II), the denitrification recovered with the removal efficiency of NO3--N to 99%. Insufficient addition of S2O32- and CH3COO- promoted the production of NO2--N and N2O with high percentage of N2O of 5.70%. When S2O32- and CH3COO- were sufficient, the nitrogen removal was effective and N2O production was low (<1.35%). Modified Blotzman model and Logistic model were suitable to describe the removal of NO3--N, modified Logistic model was suitable to reveal the generation of SO42-. Dechloromonas acted as denitrifying bacteria in substrate ratios of S2O32-: CH3COO- of 1:3 and 3:1 up to 58% and 56% at the genus level. Autotrophic Thiobacillus bacteria decreased from 19% (inoculated sludge) to 1% (1:3), 16% (2:2) and 17% (3:1), respectively. Thauera was corresponded to the heterotrophic denitrification which was successfully enriched from 0 (inoculated sludge) to 13% (1:3), 49% (2:2) and 2% (3:1). The different substrate ratios promoted the coexistence of autotrophic and heterotrophic bacteria and cooperated with Fe minerals to improve nitrogen removal from wastewater.