-Sulfur-Based Heterotrophic and Autotrophic Denitrification Process for Nitrate-Rich Agricultural Runoff Treatment.

-sulfur-based heterotrophic and autotrophic denitrification (HSHAD) process was developed in free water surface constructed wetland mesocosms for the treatment of nitrate-rich agricultural runoff with low chemical oxygen demand/total nitrogen (C/N) ratio, whose feasibility and mechanism were extensively studied and compared with those of heterotrophic denitrification (HHD) mesocosms through a 273-day operation. The results showed that the heterotrophic and autotrophic denitrification can be combined successfully in HSHAD mesocosms, and achieve satisfactory nitrate removal performance. The average NO-N removal efficiency and denitrification rate of HSHAD were 94.4% and 1.3 g NO-N m·d in steady phase II (7-118 d). Most nitrate was reduced by heterotrophic denitrification with sufficient organic carbon in phase I (0-6 d) and II, i.e., the C/N ratio exceeded 4.0, and no significant difference of nitrate removal capacity was observed between HSHAD and HHD mesocosms. During phase III (119-273 d), sulfur autotrophic denitrification gradually dominated the HSHAD process with the C/N ratio less than 4.0, and HSHAD mesocosms obtained higher NO-N removal efficiency and denitrification rate (79.1% and 1.1 g NO-N m·d) than HHD mesocosms (65.3% and 1.0 g NO-N m·d). As a whole, HSHAD mesocosms removed 58.8 mg NO-N more than HHD mesocosms. pH fluctuated between 6.9-9.0 without any pH buffer. In general, HSHAD mesocosms were more stable and efficient than HHD mesocosms for NO-N removal from agricultural runoff during long-term operation. The containing (1.67 × 10 ± 1.28 × 10 copies g mixture-soil), (8.25 × 10 ± 8.95 × 10 copies g mixture-soil), and (1.56 × 10 ± 1.60 × 10 copies g mixture-soil) of litter bags and bottoms in HSHAD were higher than those in HHD, which indicated that the combined heterotrophic and autotrophic denitrification can increase the abundance of containing , and , thus leading to better denitrification performance.