Knowledge to the surface: formation of iron precipitates in the upper water layer of rapid filters

Substances that are naturally present in the soil enter the groundwater and ultimately the surface water. This is a factor in drinking water production. High levels of iron are not harmful to health but, thanks to the formation of iron precipitates (in precipitation), they turn drinking water an undesirable yellowish colour. They also make microbial growth in the mains network more probable. High levels of manganese or arsenic can pose a health risk.

In this project, the focus is on iron and how it can be removed from the groundwater with sand filtration. In the case of rapid sand filtration, the iron particles that occur naturally are filtered out because they bind in certain conditions to other metals and to the sand grains. The processes in the water layer on top of the sand filter can affect how the sand bed works. Important factors here are the formation of iron precipitates and the oxidation of iron. This study focuses on improving our understanding of these processes and removal mechanisms and it will therefore help solve problems in, or improve, operations.

Poorly understood effects

An example from practice of an improvement in the operation of rapid filters was at BrabantWater. The intensity of the decalcification of the pumped groundwater was reduced at a groundwater treatment site owned by this drinking water company, resulting in the improved removal of iron by the rapid filters. It is still not known exactly how the underlying mechanism works. However, an industry-wide study conducted in the past (Ferramentum) has shown that high alkalinity slows down flocculation (and therefore precipitation).

Knowledge to the surface

This project is generating fundamental knowledge about iron removal mechanisms in water and groundwater with different compositions. It is looking at the starting point where these processes begin: the upper water layer of wet rapid filters. That is bringing knowledge to the surface, almost literally. The focus is on learning more about:

  • the relationship between iron oxidation and the composition of the water matrix, including alkalinity;
  • the relationship between precipitation/co-precipitation kinetics and oxidation kinetics.

A thorough understanding of iron-related processes can be used to optimise the working of rapid filters and determine more effectively why filters fail to work properly.