Opti-Float – Optimized Reduction of Fine Suspended Solids, Heavy Metals and Priority Substances from Highway Runoff Using Floating Treatment Wetlands


The reduction of priority substances from road runoff becomes increasingly important due to the progressive implementation of the Water Framework Directive. Numerous studies in recent years have shown how heavily road wastewater is contaminated with heavy metals such as cadmium, copper, lead and zinc as well as polycyclic aromatic hydrocarbons, PAHs for the short, and mineral oil hydrocarbons, MOHs for the short.

About 1,000 stormwater sedimentation tanks are an important part of runoff treatment in the state of Northrhine-Westphalia, of which approximately 600 are operated in a permanently impounded mode. Stormwater sedimentation tanks in permanent backwater are almost identical in construction to the RiStWag systems (guidelines for constructional measures on roads in water protection areas), which are used for the treatment of road runoff. The federal road authority currently operates approx. 1,000 RiStWag systems.

The discharges from these basins partly represent a considerable impact on water bodies and infiltration areas. The effectiveness of their retention of fine particles (SS) has not yet been clearly determined. For this reason, two large-scale basin facilities on the A3 federal motorway, AK Köln-West and AS Mülheim (eastern Cologne motorway ring) will be used to investigate how the retention of SS, selected heavy metals, PAHs and MOHs performs with the current design and if the retention can be improved using "Floating Treatment Wetlands". These are planted rafts whose vegetation forms particularly long roots in the water. The investigation covers also whether the combination of RiStWag plants with Floating Treatment Wetlands, FTW for the short,has a comparable or even improved cleaning performance compared to the combination of RiStWag plants with a subsequent treatment in a retention soil filter. The evaluation should also include the aspects of regenerative capacity of the absorption capacity of heavy metals, operational expenditure and cost comparison calculations. Comparative cost calculations for the combination of RiStWag systems and RBF with regard to SS and the combination of RiStWag systems with FTW are to be drawn up from the gained knowledge and their performance for the sustainable retention of the specified target parameters assessed. In addition, the possible service life of the plants is to be compared with each other. Recommendations for action and operation are to be developed from the experience gained with operation.


Information about Research Project Opti-Float

Funding Agency

The Ministry for Environment, Agriculture, Conservation and Consumer Protection of the State of North Rhine-Westphalia


May 2016 to May 2019

Project Manager

Jan Ruppelt M.Sc.

Project Partner

Institute for Molecular Ecology of the Rhizosphere,RWTH Aachen University
Federal road authority of the State of Northrhine-Westphalia