How salinity affects drinking water biofiltration of anaerobic brackish aquifer recharge water

Managed aquifer recharge has been the primary source for drinking water for decades in the most populated, western part of the Netherlands. In the coastal dune areas, pre-treated surface water is infiltrated and, after a residence time of months, abstracted through wells or open channels. Due to increasing drinking water demands and extended periods of drought, this sustainable water source is under pressure.

To increase water availability, research is ongoing to enlarge the subsurface freshwater lens in the dunes through extraction of anaerobic brackish groundwater at the fresh-saline interface. Consequently, the freshwater lens will be pulled downward, with the added benefit that the abstracted brackish groundwater itself can serve as a source for drinking. This new drinking water source, however, contains elevated levels of salinity, as well as ammonium (NH₄⁺) and manganese (Mn²⁺) above drinking water standards, which thus needs treatment.

A sustainable treatment method for NH₄⁺ and Mn²⁺ is aeration followed by biofiltration, to be combined with reverse osmosis for salts removal. There is, however, only limited knowledge on biofiltration of saline waters, hampering its uptake by water utilities. 

In this study we therefore investigated the growth of NH₄⁺ and nitrite (NO₂^-) oxidizing bacteria in aerated biofilters under fresh and saline conditions, to understand their conversion kinetics, interaction with Mn²⁺ oxidizing bacteria and metal reaction products.

Results demonstrated that under saline conditions, both nitritation and nitration did not develop spontaneously. After inoculation, growth of NO₂^- oxidizers was extremely slow and more sensitive to salinity than NH₄⁺ oxidation. Upon the onset of NO₂^- production, immediate Mn²⁺ release was observed, presumably caused by chemical reduction of Mn oxides present on the filter sand grains.

These results suggest that salinity strongly constrains nitrification and alters manganese cycling, which must be considered when implementing biofiltration for anaerobic brackish groundwater as a drinking water source. Furthermore, this insight helps to understand the fate of NH₄⁺ and Mn²⁺ in natural saline environments where they interact with NO₂^-oxidizing bacteria, including coastal marshes and ocean sediments.