In Situ Remediation of Combined Ammonia and Nitrate Nitrogen Contamination Using Zero-Valent Iron-Enhanced Microorganisms in Acidic Groundwater: A Laboratory and Pilot-Scale Study

In acidic groundwater, effectively removing both ammonia nitrogen (NH4+-N) and nitrate nitrogen (NO3−-N) poses a challenge. This study focused on studying the removal of NH4+-N and NO3−-N combined contaminations by zero-valent iron (ZVI) combined with microbial agents in both laboratory and field pilot-scale studies. Laboratory experiments showed that ZVI could reduce the denitrification stage from 15 days to 10 days by increasing solution pH and improving NO3−-N reduction efficiency. In a field pilot test (at Qingyuan, Guangdong Province, China), high-pressure injection pumps were used to inject alkaline reagents to raise the pH to 7~8. Meanwhile, compressors were applied to aerate the groundwater to increase the dissolved oxygen (DO) concentration above 2 mg·L−1. Subsequently, microbial agents of nitrobacteria were injected to initiate aerobic nitrification. As the DO level dropped below 2 mg·L−1, agents of micro-ZVI and denitrifying bacteria were injected to stimulate autotrophic denitrification. Intermittent aeration was employed to modify the redox conditions in the groundwater to gradually eliminate NH4+-N and NO3−-N. However, due to the effect of the low-permeability layers, adjustments in the frequency of remediation agent injection and aeration were necessary to achieve removal efficiencies exceeding 80% for both NH4+-N and NO3−-N. This work aims to overcome the limitations of microbial remediation methods in the laboratory and the field and advance nitrogen pollution remediation technologies in groundwater.