Nitrate Removal from Groundwater by Heterotrophic/Autotrophic Denitrification Using Easily DegradableOrganics and Nano-Zero Valent Iron as Co-Electron Donors

Heterotrophic/autotrophic denitrification (HAD) is an effective approach to remove nitrate from contaminated groundwater. To improve its performance, easily degradable organics (methanol, ethanol, oxalic acid, and sodium acetate) and nano-zero valent iron (nZVI) were selected as co-electron donors for HAD, and their effectiveness in enhancing HAD to remove nitrate from simulated groundwater was evaluated. It was found that the removal efficiency of HAD to nitrate was significantly affected by the species of easily degradable organics as their different biological availability. Among the tested organics, ethanol-supported HAD system exhibited a better removal efficiency, and after 10 days reaction, it could achieve a high nitrate removal rate to 85.6% with an initial concentration of 90.94 mg/l, and at the end of the test (27 days), nitrate was almost completely removed in the interaction of heterotrophic denitrification (HD) and autotrophic denitrification (AD), and there was no nitrite and ammonium accumulation (<0.1 and 1.0 mg/l). Moreover, the initial C/N ratios (0.2, 0.5, 1.0, 2.0, and 4.0) of simulated groundwater had a significant influence on nitrate removal by HAD. Increasing the C/N from 0.2 to 2.0 could markedly enhance nitrate removal efficiency, but continuously increased to 4.0 the removal rate just decreased; nevertheless, the accumulation of nitrite and ammonium were closely related to both the C/N ratios and species of organics. The synergistic effect between HD and AD process plays a vital role in the mixotrophic environment. Therefore, this research provides an effective method for nitrate removal from contaminated water with low organic carbon.