Fieldcase on surface water at Lethe River by OOWV

The river Lethe in Lower Saxony (Germany) suffers from water shortage in the summer time and discharge of diffuse pollution of phosphorus and nitrogen through drainage water and recharge of the connected ground water. The Lethe flows into fish ponds which also have to cope with water shortage and diffuse pollution. OOWV, the Water Board of Oldenburg and East Frisia is working on field demonstrations to protect surface and groundwater in the river catchment.

The wetland consists of 6 identical pots that are used to stimulate denitrification/nitrification processes and to trap ochre via aeration. The denitrification pots consist of two different sections. The upper section of the pot contains gravel and various types of plants. The plants are fixed to the upper section but their roots reach into the lower section to build a root system that is adapted to free-water areas. This enhances the active surface that is in contact with the water and therefore with nutrients as well. The plants take up nutrients continuously during their growth period. The gravel allows for the wetland to be walked upon without harming the plants or the installation as such.

The lower section consists of the root zone, water and plastic carriers for microbial growth. Due to the nutrients in the water, microbial growth will start as soon as construction and filling the plants with water is finished. With time, the nitrogen reduction efficiency will increase with an increasing mass of denitrifying bacteria attached to the carriers.

The water moves from chamber to chamber. The pots are connected in a way that only if water is flowing into a pot, water will flow out of a pot. This ensures that the plants/flowers will not dry out during dryer months in summer.

Each pot has a volume of 750 liters. The plastic carriers are filled with fired clay. The complete system of six denitrification pots holds a total of about 1.5 m³ of water. The green nets prevent the clay from moving freely. The inlet is located at the bottom of the pots so that there is no short circuit. The cost of the six pots is about 6.000 €. A 50W pond pump was installed to pump water (about 70 ml/s).

Power is supplied by solar panels and a fuel cell as a buffer to fill the battery at night. This allows power supply in remote locations. However, this leads to high fuel consumption in cloudy weather and at night.


The ochre trap benefits from a gradient from pipe to water surface to increase aeration. This will oxidize the dissolved iron. Stones in the trap will help the ochre flocks to settle.

More details can be found in the field case description document.