HOW TO REMOVE METALS FROM GROUNDWATER FOR HIGH QUALITY DRINKING WATER PRODUCTION: PRINCIPLES, PRACTICES AND DRIVERS FOR RESEARCH (Invited Presentation)
Iron and manganese are the main metals to remove from the raw groundwater. Aeration followed by one- or two-step rapid sand filtration is applied at most of the treatment plants, during which iron may oxidize homogeneously, heterogeneously and biologically, or by a combined mechanism involving these processes, leaving hydrous ferric oxide precipitates in the supernatant and in the pores and on the surface of the filter media. Oxidation of Mn(II) by oxygen alone is negligible and bacteria and surface catalysts on the filter media grains are known to transform Mn(II) to hydrous manganese oxide.
Calcium is removed from water by the pellet softening process, in which lime or caustic soda is dosed in the process water to precipitate calcite. In order to make the separation of calcite efficient, sand or recycled calcite particles are introduced in the softening reactor to provide a surface for calcite precipitation. These calcite-coated grains (pellets) are then removed by gravity settling in the same reactor and the carry-over calcium is removed in the rapid sand filtration.
For the removal of trace metals such as As and Ni, the typical aeration-filtration process is adjusted by dosing iron or manganese based salts in the process water to coprecipitated these trace metals with iron or manganese oxides. The maximum allowable concentration of nickel according to the Dutch law was lowered from 50 to 20 µg/L in 2002. Through systematic research it was found that dosing MnCl2 between the two rapid sand filtration steps could effectively remove nickel from water. The health effects of low arsenic concentrations are uncertain, therefore Brabant Water targets an arsenic concentration of lower than 1 µg/L in drinking water since 2016. This is achieved by dosing FeCl3 in the rapid sand filter influent (after aeration), coupled with pre-oxidation of arsenic by permanganate.
In this talk, we will describe these processes in detail and share the experiences that we have gained over the years.