The role of sludge in infiltration ponds in OMP removal during dune infiltration

Drinking water utilities that use dune infiltration or riverbank filtration employ infiltration ponds with a sludge layer. This project investigates the role played by the thickness and conditions of this sludge layer in the removal of organic micropollutants (OMPs). The results may affect the maintenance regime for infiltration ponds, which is currently based primarily on sludge resistance.

Role of sludge in OMP removal

Whether and how the thickness and conditions of sludge layers affect OMP removal is not yet known. This study therefore aims to determine which conditions are optimal for OMP removal so that sludge layer management in infiltration ponds can be tailored accordingly. The accumulation of sludge and its properties are determined by factors including the quality of the inlet water, the transit rate, and the shape and depth of the infiltration pool. The vegetation and biological processes are also important. To prevent blockages, DPWE companies regularly decide to remove sludge when this layer becomes too thick and gives too much resistance. If the results of this study show that management based on layer thickness or other conditions such as redox potential make a functional contribution to OMP degradation, the water utilities could act on this in the future. For example, in the maintenance regime for infiltration ponds, adjustments in pre-treatment, pond level and/or residence time in the ponds may affect sludge conditions and the thickness of the sludge layer.

Soil studies in infiltration ponds

In this project, redox potential, sludge layer thickness, water quality and the removal of three model OMPs will be measured in several infiltration ponds. To begin with, changes in the redox potential in and below the sludge layer must be determined at several locations in a range of infiltration ponds, and the same applies to the sludge thickness. In the field, this is done with modified redox sticks and other equipment developed for the purpose such as gravity-corer samplers. The measurements provide a direct picture of the relationship between the sludge layer and the redox potential.

Three infiltration properties are then chosen with a range of baseline conditions:

  • recently dredged, almost no sludge layer present, similar redox potential in and below sludge layer;
  • selected for dredging, thick sludge layer, large difference in redox potential in and below sludge layer;
  • between these extreme conditions.

Water quality and OMP removal during infiltration were also measured for these three selected ponds. This is done by measuring the water quality and three model OMPs in the pond water and in a monitoring well. This monitoring well may still have to be installed for this study such that it is representative for a soil passage of 1 metre.


The project will produce the following results:

  • Methods to measure the thickness and conditions/redox conditions in the sludge layer in infiltration ponds.
  • Knowledge about the relationship between sludge conditions and OMP removal.
  • A clearer picture of the parameters that determine OMP removal in the sludge layer
  • Guidelines for managing infiltration ponds as a treatment step for OMP removal.
  • Possible direct effect on maintenance regime for infiltration ponds.