Simultaneous heterotrophic and sulfur-oxidizing autotrophic denitrification process for drinking water treatment: Control of sulfate production

A long-term performance of a packed-bed bioreactor containing sulfur and limestone was evaluated for the denitrification of drinking water. Autotrophic denitrification rate was limited by the slow dissolution rate of sulfur and limestone. Dissolution of limestone for alkalinity supplementation increased hardness due to release of Ca2+. Sulfate production is the main disadvantage of the sulfur autotrophic denitrification process. The effluent sulfate concentration was reduced to values below drinking water guidelines by stimulating the simultaneous heterotrophic and autotrophic denitrification with methanol supplementation. Complete removal of 75 mg/L NO3–N with effluent sulfate concentration of around 225 mg/L was achieved when methanol was supplemented at methanol/NO3–N ratio of 1.67 (mg/mg), which was much lower than the theoretical value of 2.47 for heterotrophic denitrification. Batch studies showed that sulfur-based autotrophic NO2–N reduction rate was around three times lower than the reduction rate of NO3–N, which led to NO2–N accumulation at high loadings.

► Autotrophic denitrification rate was limited by the slow dissolution rate of sulfur and limestone.
► Autotrophic denitrification increased sulfate concentration and hardness of treated effluent.
► Simultaneous autotrophic and heterotrophic denitrification was achieved.
► Mixotrophic denitrification increased denitrification performance.
► Mixotrophic denitrification decreased sulfate and hardness production.