We often picture an oil slick in a river, caused by illegal dumping or some kind of spill. However, most pollutants in rainwater are in particulate form, i.e. “attached” to suspended matter, and their entry into water bodies is not at all spectacular. Ultimately, the spectacular accidental pollution that appears in the press represents a tiny fraction of the amount of hydrocarbons that are discharged into water bodies in particulate form. Similarly, other types of pollutants enter the water cycle in a discreet and chronic manner.
How can this pollution be limited?
Rainwater is not intended to be conveyed to a conventional treatment plant due to the volumes and variability of flows. Older sewerage networks mix wastewater and rainwater, leading to polluting overflows when it rains. Newer networks separate these two types of water, storing rainwater before discharge, but without systematic treatment.
However, there are simple and robust solutions for passively treating this rainwater before discharge, and Luxplan has been working and taking action in this area for over ten years.
What has Luxplan achieved in this field?
Luxplan is actively involved in making passive solutions for treating rainwater before discharge more widely available. Ten years ago, the first sand filter planted with reeds for a dedicated rainwater network was installed at the Grass business park (ZARO, Steinfort). This is a simple, robust, gravity-fed solution which, in addition to providing high-quality landscaping, significantly improves the quality of the rainwater discharged into the Eisch river.
We are also working for the Public Works Department on the motorway network with numerous projects involving retention basins coupled with sand filtration systems (new or renovated), one of which came into service at the end of 2023 in Raemerich.
In both cases, we have improved a simple rainwater retention system by integrating a treatment device. To do this, we create an approximately 70 cm deep excavation at the bottom of which we place the drains to collect the treated water. This extra depth is then filled with sand and gravel of different grain sizes in different layers. Reeds are then planted in the sand. It takes them about two years to grow and cover the entire surface of the filter. They serve multiple purposes: they help to remove clogging, provide shade over the water to prevent algae growth, develop a rhizosphere capable of breaking down certain organic pollutants, provide a refuge for wildlife, etc.
Some key figures for the Grass basin:
– Peak inlet flow 7 m³/s (for a ten-year HQ10 rainfall)
– Three inlet sludge traps of 54, 138 and 150 m³ with siphon partitions
– Four separate sand filter cells with a total capacity of 3,244 m³, enabling the complete treatment of a biennial rainfall (HQ2)
– 2,400 m³ of sand and gravel used in a 70 cm thick layer
– Maximum water level in the filters: 1 m
The Raemerich basin is more compact and more technical; it is a motorway basin with no public access.
How effective is the system?
The principle is based on filtration. The water passes vertically through the sand and gravel to reach the drains. During its vertical journey, the rainwater loses a large part of its particulate pollution load (around 90%). The recovered water is drained by gravity to the outlet. A research project with the University of Luxembourg, called STORMLAND and funded by the National Research Fund, will begin in June 2024 to verify the effectiveness of the system and possibly optimise it. The first phase will involve characterising the pollution, as it is so diverse and varied that it is impossible to carry out exhaustive monitoring. The NTS (non-target screening) technique will be used to maintain a broad spectrum of monitoring in order to characterise the water entering and leaving the site. In a second phase, optimisation solutions will be installed in in situ pilot projects. Luxplan’s objective is to confirm its solution and improve it for future projects and certain applications, particularly reuse.
The solution of reusing this runoff water after treatment has emerged recently, particularly with recurring episodes of drought. The aim is to reuse this treated water in more sensitive applications that could not necessarily use untreated runoff water (agriculture, market gardening, certain industrial applications, etc.).
The project will last three years, with interim results expected in 2025.
