NBS for secondary wastewater effluents infiltration based on soil and woodchips as drainage material: laboratory study

Wastewater management and treatment are key points in maintaining the quality and the sustainability of water resources. To preserve receiving  water environments, efforts are being conducted to improve the  treatment efficiency . Soil infiltration can therefore be used as a  nature-based solution tertiary treatment, in some areas without surface  water available, or with supplementary water bodies’ protection  regulations. Secondary wastewater effluents (SWE) infiltration surfaces mainly consist of infiltration trenches or flood-meadows. Among the main issues encountered with soil infiltration, two can be highlighted:  the possible low hydraulic conductivity induced by soil clogging, on the  one hand, and the use of non-renewable draining materials such as  pebbles or gravel to ensure the distribution of water in trenches, on  the other hand. In France, in order to overcome those issues,  stakeholders are now considering the replacement of the gravel with  woodchips, a renewable biodegradable material, also prone to  biodiversity in soils. It has been demonstrated through a previous field study that the use of woodchips in infiltration trenches helps maintain infiltration over time, and even improves their performance. However, understanding the underlying mechanisms remains a significant scientific challenge. To better understand the soil and woodchip evolution processes, four columns were set up in a laboratory and fed with secondary treated effluents from a wastewater treatment plant.

 These four columns (with a diameter of 37 cm) are composed as follows:

• a) Column #1: 80 cm of soil,
• b) Column #2: 40 cm of wood chips and 40 cm of soil,
• c) Column #3: 80 cm of soil inoculated with a selection of earthworms ,
• d) Column #4: 40 cm of wood chips and 40 cm of soil, inoculated with a selection of earthworms .

During the presentation, hydraulic monitoring of the columns will be presented (inlet and outlet flow, column weight monitoring), showing the evolution of the infiltration rate. To analyze the evolution of physical properties within the columns, including parameters like saturated hydraulic conductivity, a modeling study was carried out using Comsol Multiphysics. Specifically, the Richards model (van Genuchten-Mualem) was employed to simulate and understand the changes occurring over time. The models fit the data well. They mainly show that the soil columns (1 and 3) tend to clog early if the hydraulic loads are too excessive. This is reflected by a reduction of hydraulic conductivity at saturation and porosity. In comparison, columns with wood chips seem to maintain their properties, with no major difference between columns with or without earthworms, after two years of monitoring. These results will be compared to the monitoring of physicochemical parameters of the inflow and outflow waters from the columns, allowing for a better understanding of the processes involving woodchips, soil, and macrofauna.