Mobile constructed wetland at river bank to protect vulnerable nature

11 December 2020 - Published by Charlotte Boeckaert
Surface water quality in Lower Saxony (Germany) is often not fulfilling the requirements of the European Water Frame Directive. The nitrogen concentrations of the local river Lethe, a little stream in the county of Oldenburg, approximately 100 km south of the North Sea is affecting a Natura 2000 nature protected area downstream. As a project partner of NuReDrain, OOWV focuses on the potential of denitrification processes enforced by the installation of a constructed wetland. A mobile constructed treatment plant has been developed by a company in the study area and placed at the side of the river bank.

The denitrification potential of the mobile wetland is being tested since September 2020 as part of the OOWV case study. The plant was built next to the river Lethe. The river water is pumped into the mobile wetland using an electric cell and a solar module for power supply. 



Figure 1: Mobile wetland during construction

The mobile wetland consists of a sequence of six pots that are connected with pipes (Figure 1). The pots are placed on plastic tiles to prevent the underlying soil from consolidating. The pots are filled with plastic carriers and burnt clay as solid substratum (Figure 2) to provide micro-bacteria a surface to grow on. On the top, regional native plants are planted. This helps the aeration of the pots and also might increase denitrification due to plant uptake. The pots are filled via down facing pipes at the bottom to ensure that the water flows entirely through the carriers, roots and clay. Nitrates in the water will be removed by means of micro-biological denitrification converting nitrate into nitrogen gas.

Six pots of 250 liter
The mobile wetland is designed for the treatment of small streams or for connection to drainage outlets. This means that the installation can be dug into the ground. More pots can be attached to increase the treatable volume flow. The design at Lethe river uses six pots with an individual volume of 0,75 m3. Each pot holds around 250 l of water. The treated volume flow is around 60-70 ml/s. 


Figure 2: Plastic carriers and burnt clay inside the pots

In the inflow and outflow of the constructed wetland plant, two multi-parameter sensors (powered with a solar module) from TriOS are installed (Figure 3).


Figure 3: TriOS Opus multiparameter sensors for continuous monitoring of Nitrates, CODeq, water temperature and oxygen concentrations

Concentrations of COD eq, Nitrate, Oxygen and water temperature are measured in a 15 minutes interval and sent daily to a cloud for real-time analysis. Lab analysis will be done at regular time intervals to evaluate accuracy of the online measurements.

Preliminary results
Current denitrification potential is below 10%. This is due to insufficient carbon concentrations in the water for microbial growth and low water temperatures during autumn. Therefore, the next step includes adding a carbon source into the first pot (e.g. wood chips or straw) and to evaluate the subsequent changes in nitrate reduction. Measurements will also be carried out during winter, to get information about the variability of nitrate concentrations in winter time when the activity of the micro-bacteria is neglectable.