About the Lake
Project Summary
Aim
To reduce the phosphorus loading within the lake.
Site Context
The Kralingse Plas was created in the early 1800s from peat mining and is situated northeast of the city of Rotterdam in the suburb of Kralingen. It is visited by more than three million people annually and is primarily used for fishing, swimming, sailing, and other water sports.
Why Treatment Was Needed
Monitoring of water quality over the past two decades suggested that poor water quality in the lake could be attributed to external influences from the surrounding catchment and internal phosphorus release from the sediments. Work had already been done by the City of Rotterdam for several years to improve lake water quality and to comply with Water Framework Directive targets for its lake type.
Phoslock Application
November 2021
A Phoslock application was carried out over 24 days.
Two pontoon systems
Phoslock granules were mixed with lake water in a custom mixing chamber and sprayed across the lake surface as a slurry.
Uniform placement
GPS technology was used on the boats to help assure even application across the treatment area.
Phoslock dosing was based on phosphorus concentrations in the water column and the amount of potentially releasable phosphorus from the sediments.
How Performance Was Evaluated
The Waterboard monitored a number of different water quality parameters in the Kralingse Plas on a monthly basis to assess changes in water quality. Both pre- and post-treatment data were available to assess the effectiveness of the Phoslock application.
Prior to treatment, the lake was divided into 42 zones and 10 sediment cores were taken in each zone. From each core, the top 10 cm layer of sediments was homogenized and sent to the laboratory for analysis. Sequential phosphorus extraction was used to determine relevant phosphorus fractions and compare concentrations of potentially releasable and stable phosphorus forms before and after treatment.
Sediment samples were also collected during the application at five different locations in the lake, and this monitoring continued through the post-application period.
Measured Water Quality Response
No significant changes were measured in pH, turbidity, conductivity, or alkalinity as a result of treatment.
Phoslock rapidly, in less than two weeks, and significantly reduced total phosphorus by more than 80% and free reactive phosphorus by more than 95% in the water column 16 months after treatment.
Phoslock also bound phosphorus in the sediment into more stable fractions that are not considered as easily releasable under natural lake conditions.
Outcome
- Phoslock significantly decreased phosphorus concentrations in the water column.
- Lower phosphorus concentrations were sustained more than one year after treatment.
- Sediment phosphorus fractions shifted to highly stable forms compared to pre-treatment conditions.
- The City of Rotterdam gained an ecologically sound solution to address phosphorus pollution, restore water quality, and support WFD targets.
