In this project, a design tool is developed based on simple input, enabling the optimization of water flow
and retention of environmental contaminants in filter media. The design tool is at pilot scale.
The tool is divided into two models: a 1D numerical water transport model based on the MMS model and
an analytical chemical transport model. These are developed based on a case study - St. Restrup Fælled for
the location and PFAS as the chemical. Samples have been taken from St. Restrup Fælled, and laboratory
experiments have been conducted to compare and evaluate various empirical models with the samples taken.
The empirical models are being investigated to minimize the amount of laboratory experiments and the time
it takes before a calculation can be performed.
The tool can be used to determine the depth of an infiltration basin and the distribution of grain size in filter
media. These are determined based on a desired criterion regarding how quickly a given amount of substance
should pass through the filter media (5%, 50%, or 90%). The grain size distribution is found through an
iterative process. In this iterative process, the soil fractions (clay, silt, fine sand, coarse sand, and organic
matter), bulk density, and surface area of the basin can be changed until the desired result is achieved.
In the designed tool, the content of organic matter is considered to have the greatest impact on chemical
transport, as it is assumed that the chemical adsorbs to the organic matter. However, adding a large amount
of organic matter is not realistic as it would make the soil unstable and prone to collapsing. A content of
organic matter up to 10% is recommended. It is also assessed that a clay content of over 35% has a significant
effect on the depth in the basin but not on retention of the chemical.
For the St. Restrup Fælled case, a filter media consisting of 20% clay, 2% silt, 35,5% fine sand, 35,5% coarse
sand, and 7% clay is designed. With this filter media, it will take 6,7 years, 17,2 years, 7,2 years, and 11,9 years
for 50% of PFHxS, PFOS, PFNA, and PFOA, respectively, to travel through 1 meter of the recommended
filter media when the basin is 20m x 20m in size. The basin will then have a maximum depth of 0,62m and
a volume of 250m2 over the calculated 31 years.