Effect of synthetic iron colloids on the microbiological NH4+ removal process during groundwater purification

Subsurface aeration is used to oxidise Fe in situ in groundwater that is used to make drinking water potable. In a groundwater system with pH>7 subsurface aeration results in non-mobile Fe precipitate and mobile Fe colloids. Since originally the goal of subsurface aeration is to remove iron in situ, the formation of non-mobile iron precipitate, which facilitates the metalʹs removal, is the desired result. In addition to this intended effect, subsurface aeration may also strongly enhance the microbiological removal of ammonium (NH4+) in the purification station. Mobile iron colloids could be the link between subsurface aeration and the positive effect on the NH4+ removal process. Therefore, the objective of this study was to assess whether synthetic iron colloids could improve the NH4+ removal process. The effect of synthetic iron colloids on the NH4+ removal process was studied using an artificial purification set-up on a laboratory scale. Columns that purified groundwater with or without added synthetic iron colloids were set up in duplicate. The results showed that the NH4+ removal was significantly (α=0.05) increased in columns treated with the synthetic iron colloids. Cumulative after 4 months about 10% more NH4+ was nitrified in the columns that was treated with the groundwater containing synthetic iron colloids. The results support the hypothesis that mobile iron colloids could be the link between subsurface aeration and the positive effect on the NH4+ removal process.