Effects of sulfide on mixotrophic denitrification by Thauera-dominated denitrifying sludge

Abstract

Sulfide can be used as a complementary electron donor to enhance the denitrification efficiency when organic carbon is insufficient to reduce nitrate. However, sulfide is toxic to denitrifiers and may significantly reduce the denitrification efficiency. In this study, Thauera, well-known facultative denitrifiers, were successfully enriched in denitrifying sludge that was adapted to sulfide. The effects of sulfide on heterotrophic and autotrophic denitrification (mixotrophic denitrification) by the Thauera-dominated denitrifying sludge were investigated. The sludge was tolerant to the sulfide effects, and performed stably in mixotrophic nitrate reduction at a sulfide concentration of up to 72 mg S per L. The nitrate reduction rate reached 62.3 mg N per g VSS h⁻¹. However, the nitrite reduction capacity of the sludge was low (up to 23.6 mg N per g VSS h⁻¹). As a comparison, nitrate and nitrite reduction by the heterotrophic denitrifying sludge was significantly inhibited by sulfide, with 70% reduction. The sulfide inhibition of the denitrifying sludge was likely due to the toxicity of molecular hydrogen sulfide (H₂S_aq), because the inhibitory effects decreased at higher pH. Using a modified two-step denitrification model, the inhibitory effects of H₂S_aq on mixotrophic nitrate and nitrite reduction were well simulated. Cultivation of Thauera-dominated denitrifying sludge can improve nitrate reduction with sulfide effects, but nitrite accumulation should be considered when using sulfide as a complementary electron donor to treat wastewater with a low C/N ratio.