Aarhus University tests P removal from drainage water in Denmark

23 October 2019 - Published by Charlotte Boeckaert
An experimental P filter system was constructed and is currently being tested by the Department of Agroecology at Aarhus University (AU) in a small agricultural catchment in Denmark. The system receives tile drainage water from an 8,4 ha field with sandy loam soils under arable cropping. A part of the P in the drainage water is bound on particles, another part is solubilized in the water.

The main compartments of the system that is built at the Fensholt fieldsite are i) a patented sediment filter for particle-bound P removal, ii) a reactive filter for retention of soluble P and iii) a sediment sludge storage tank.

Sediment filter

The sediment filter consists of lamellae and a mesh filter that retains particles. Periodically it is automatically flushed with resident water to resuspend particles before the slurry is pumped into the storage tank. After allowing for sediment settling, water flows back from the storage tank to the filter system.


Reactive filter

The reactive filter consists of a cylindrical iron net cage with a volume of 2.53 m3 (figure below). The reactive filter is packed with 2-4 mm granules of Diapure™, a calcinated diatomaceous earth with an iron oxide coating. Drainage water flows from the outside to the inside of the filter due to the hydraulic gradient. This specific design combines a high cross-sectional area with a high hydraulic residence time.



The P filter system is additionally equipped with a flow meter and a number of pressure transducers, recording continuously the water levels in the different wells. Drainage water samples are collected automatically by ISCO samplers, located at the inlet (ISCO1), between the sediment and reactive filter (ISCO2) and at the outlet (ISCO3).



Monitoring has started in March 2019, at the end of the drainage season. Thus, the data set is currently too small for a system evaluation. However, initial results indicate that total phosphorus (TP) concentrations were reduced by about half in the P filter system (figure below).

The monitoring will be continued in the next drainage season.