Influence of Multiple Parameters on Groundwater Iron Removal in the Presence of Ammonium Nitrogen

This paper studies the effect of the initial iron (2.1–6.9 mg/L) and ammonium nitrogen (2.5–6.0 mg/L) concentrations and aeration parameters on the efficiency of two-stage purification of complex groundwater. The experimental setup utilized two stages of filters with air-to-water ratios of up to 5:1 and 1.5:1, respectively. It was found that secondary aeration reduced residual iron to the standard 0.3 mg/L, while the ammonium nitrogen concentration dropped below 1.5 mg/L in most modes. The maximum efficiency of ammonium removal was observed with optimal second-stage aeration: in the absence of aeration, the purification efficiency was only 39.5%, while with optimal parameters, the efficiency reached 75–80%. Correlation analysis revealed the key role of the initial concentrations and aeration parameters: for iron, the critical factor was Fe2+ and its interaction with N-NH4+, and for ammonium nitrogen, it was the intensity of second-stage aeration. Machine learning models (CatBoost and GPR) showed R2 determination coefficients of up to 1.000, confirming the high accuracy of forecasting. Practical implementation of the second stage of purification at the station made it possible to reduce residual iron concentrations by an additional 35–45% and reduce energy costs by 15–20% due to aeration optimization, ensuring stable compliance with Fe and N-NH4+ standards.