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.

Different filter materials and filter configurations will be tested on site for their nutrient removal capacity.The type of material used and the filter configuration tested will be based on the outcome of the bench scale evaluation.

Transportable constructed wetlands
Underground bioreactors using wood chips as C source for microbial growth were disregarded because of site limitations regarding limited space and high groundwater tables. The focus was then put on transportable constructed wetlands that are placed directly into drainage ditches. This measure was further discussed with the local water managers. This exchange was necessary to discuss possible obstacles, such as plants materials to be used for the wetland.

It was stressed that locally occurring plants are necessary for the water managers to support the wetlands. Other points of discussion were the dimension of the measure, probability of backwater and susceptibility to clogging. Following this meeting specifications for tenders were formulated. At the end of February 2019 the specifications were submitted to the responsible department at the OOWV in order to start the restricted tender.

In order to present the project and the transportable wetlands to a broader audience, governmental and non-governmental stakeholders from the water and agricultural sector were invited to an information event which took place in April after the end of the bidding process.

The wetland consists of different 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 various types of plants and gravel. 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.

The ochre trap uses a ledge from pipe to water surface to increase aeration. This will oxidize the dissolved Iron. Stones in the trap will help the forming ochre flocks to settle.