Treatment of landfill leachate using an innovative biological pilot installation

Since 2005, the Weber pit, an old landfill located at the Nedereindse plassen, has been the subject of remediation projects. Rainwater infiltrates the landfill body and flows out as a polluted stream. This stream contains a complex composition of different chemicals. The most relevant are calcium, magnesium, mineral oil, BTEX and naphthalene. Less relevant for this remediation, but still noteworthy, are ammonium and sulfate. Due to the high amount of salts, the water has a high conductivity, which poses a challenge for the remediation method. Due to the size of the landfill body, the flow rate is fairly stable, between 2–3 m³ hour⁻¹. Currently, the polluted water is being treated in a conventional treatment plant, which is nearing the end of its life. For this reason, a sustainable alternative has been chosen, requiring little maintenance, namely a constructed wetland (CW). To test whether the water is suitable for treatment via a CW, HMVT is conducting a pilot test. In this pilot test, the incoming water for the treatment is directed to a cascade step, where the water comes into contact with oxygen step by step. Through oxygen infiltration, the pH of the water increases, causing calcium to precipitate as sludge in a settling tank. After this, the water is directed to the biological step of the pilot: a container with 3 tanks, filled with different layers of soil containing plants. These plants root deeply and provide the microbiome with the extra boost it needs to break down the incoming pollution. The figure below provides an overview of all the processes taking place during the degradation:

This whole setup serves to simulate a helophyte filter, and as an intermediate step from the drawing board to a scale-up situation. In the 3 tanks, one can experiment with aerobic and anaerobic degradation conditions and with the materials. For the current situation, there is an alternation between aerobic and anaerobic conditions in tank 1, followed by anaerobic conditions in tank 2, and finally aerobic conditions again in tank 3, to create a varied palette of degradation conditions. In addition, these tanks can be used to test whether the plants can withstand the conditions of the water. So far, most measurements have been taken in autumn and winter, and during this period the pH values appeared to be fairly stable. The conductivity seems to decrease throughout the system, which indicates that there is adsorption of ions onto the soil matrix. The cascade step is doing its job successfully: the incoming water is rich in oxygen. In the system itself, the oxygen is low, but it rises again in tank 3. This is also reflected in the RedOx conditions, which indicates that the system functions as designed and intended. Finally, we tested whether the system is capable of breaking down mineral oil. So far, this seems to be going well, as hardly any mineral oil is measured in the effluent. DNA analysis has shown that the microbes responsible for breaking down mineral oil under anaerobic conditions are present. However, their number does not seem to increase in the constructed wetland, but in the future aerobic microbes will also be included in the analysis. Based on the results, the pilot can be translated into a full-scale constructed wetland. This will ultimately provide a sustainable, robust, cheap, and low-maintenance solution compared to the current treatment. This full-scale constructed wetland can then contribute to an increase in local biodiversity, and has a low CO₂ emission and energy consumption.