Hydrilla verticillata–Sulfur-Based Heterotrophic and Autotrophic Denitrification Process for Nitrate-Rich Agricultural Runoff Treatment

Abstract

Hydrilla verticillata-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 H. verticillata 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 3 − -N removal efficiency and denitrification rate of HSHAD were 94.4% and 1.3 g NO 3 − -N m −3 ·d −1 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 3 − -N removal efficiency and denitrification rate (79.1% and 1.1 g NO 3 − -N m −3 ·d −1 ) than HHD mesocosms (65.3% and 1.0 g NO 3 − -N m −3 ·d −1 ). As a whole, HSHAD mesocosms removed 58.8 mg NO 3 − -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 3 − -N removal from agricultural runoff during long-term operation.The denitrificans containing narG (1.67 × 10 8 ± 1.28 × 10 7 copies g −1 mixture-soil −1 ), nirS (8.25 × 10 7 ± 8.95 × 10 6 copies g −1 mixture-soil −1 ), and nosZ (1.56 × 10 6 ± 1.60 × 10 5 copies g −1 mixture-soil −1 ) 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 denitrificans containing narG, nirS, and nosZ, thus leading to better denitrification performance.

Release Mechanism, Secondary Pollutants and Denitrification Performance Comparison of Six Kinds of Agricultural Wastes as Solid Carbon Sources for Nitrate Removal

“…It can be seen from Figure 7a that the denitrification process could be divided into two phases, i.e., the start-up phase (phase I, 1-11 d) and the stable phase (phase II 11-27 d). In phase I, the nitrate removal by denitrification fluctuated due to the acclimation of microorganisms, which was similar to the research results of Hang et al [33]. Initially, the agricultural wastes released abundant soluble and small molecular organics for the denitrification process, but different nitrate removal performance was achieved because of the different amounts and biodegradability of the carbon sources.…”

The role of hydrilla (Hydrilla verticillata (L.f.) Royle) as soil protectant in improving soil physical, chemical, and biological properties

“…The results of the research showed that the vegetative reproductive structures of hydrilla (tuber and rhizome parts) were able to store carbohydrates (Shi et al, 2021;Fuad et al, 2013;Safitri et al, 2019;Shrivastava dan Sudhakar, 2021). It was also stated that hydrilla contained 3.29% nitrogen (N₂), 0.52% phosphorus (P₂O₅), and 6.34% potassium oxide (K₂O), in which nitrogen, phosphorus, and potassium are macro elements needed by plants in large quantities to grow and develop (Jain dan Kalamdhad, 2019;Hang, et al, 2019;Hang, et al, 2020). It was stated by Jain and Kalamdhad (2018a) that in the hydrilla decomposition process, a denitrification process occurs, namely the reduction of nitrate to nitrogen gas.…”

Sulfur-based mixotrophic denitrification with the stoichiometric S0/N ratio and methanol supplementation: effect of the C/N ratio on the process

“…Other studies compared the autotrophic denitrification process performance with the combined heterotrophic-autotrophic process to remove nitrate from agriculture wastewater and showed that the combined process could be more efficient in nitrate removal, and was more stable than the single process of autotrophic denitrification [21] .…”