ReCon Soil: developing sustainable soils from construction waste

The soil's surface is an ally in human activities, transformed to support its biggest buildings and means of transportation. Its surface is reshaped and fertilized to produce food for people and green spaces for their well-being. Beyond those uses, soil is an important carbon sink on Earth and performs many functions that we are gradually becoming aware of. Another challenge is finding ways to better manage materials today seen as waste, which are often difficult and costly to manage and store and produce a significant environmental footprint. For example, large volumes of soil are excavated during development projects. This excavated earth needs to be managed. At the same time, the dredging of sediment for river maintenance raises the problem of how to manage it on land. Finally, certain industries produce mineral waste, such as washing aggregate sludge, which is also costly to manage. At present, these soil, sediment and mineral waste are either buried in landfills or managed on land, at significant cost. The environmental footprint of current management is high, notably in terms of greenhouse gas emissions produced by transporting waste to storage facilities. The ReCon Soil project offers an innovative recycling solution by returning them to the soil for reuse rather than storing them. Contributors to this project will now tell you more. This project is based on the simple observation that construction waste represents a major part of the waste generated in France and England every year. 150 million tons of waste are generated in France from excavated land alone. As long as these materials are not polluted, they are worth recovering and can be put to good use, in agricultural projects. For the ReCon Soil project, that was the starting principle, and we did a series of experiments to try and improve agricultural soils using construction waste, excavated earth and dredged sludge in particular. This is the Caté experimental station, part of the ReCon Soil project and the INTERREG France (Channel) England program, whose partners are the Caté station, BRGM and Le Havre University in France and the University of Plymouth and University of East Anglia in England. Caté, for "Technical and Economic Action Committee," is a regional experimentation station. Our job is to acquire technical references to help farmers in their daily activities. We work on open-field and under-cover vegetable crops, ornamental horticulture and cultivated mushrooms. Dredged sediment is rich in organic matter, which makes it good for growing vegetables. That is why we use sediment dredged from the port of Le Havre to enrich soils that are poor in minerals and organic matter. As a member of the GEMP laboratory, our mission is to work on polluted sediments using an electrokinetic process to reduce contaminant levels. We are back at Caté nine months after setting up lysimeter tanks. In these tanks, we mixed Caté soil with two types of materials: excavated soil from a building site and washing aggregate sludge from a quarry. These materials contain clay, and we want to use them to improve the structural stability of the Caté soil and in particular its porosity. The materials are also used to try to stabilize the carbon in soil so it releases fewer greenhouse gases into the atmosphere. That is the ultimate aim. We analyze the CO2 emissions from the soil and take samples to characterize them in the laboratory. We measure what we call the CO2 flux in these tanks. Measuring CO2 flux simply consists of determining how much CO2 escapes from the ground and is released into the atmosphere. To measure that, we place the chamber you see here onto the soil for three minutes and measure the increase in CO2 in the soil, before moving on to the next point. In these tanks, we take about 15 measurements. In soil, there are abiotic components, such as clay, organic matter and sand, for example, and biological components, like micro-organisms, fungi and bacteria. The interactions between the organic and inorganic components allow the soil to perform ecological functions: organic matter dynamics, recycling nutrients, making them available to plants, retaining water in the soil... In ReCon Soil, we wanted to look at how these ecological functions changed after different treatment tests. To examine the dynamics in the soil, we used the technique Hafida presented for soil respiration. The micro-organisms in the soil use available organic matter and transform it into CO2. We use a system called MicroResp. It uses a dye to measure the release of CO2 in the soil over time. We also use other approaches to examine activity and nutrient dynamics in the soil. We are particularly interested in enzymes related to the carbon, nitrogen and phosphorus cycles. We also measure enzyme activity reactions in the soil over time. In addition to enzyme activities, or measuring activity in the soil, we use more indirect approaches based on the bacterial DNA in the soil. We extract the DNA present in the soil and look for specific genes involved in the nitrogen cycle, for example. We search for and quantify the gene using PCR or qPCR methods. The last approach we use in ReCon Soil is sequencing. We amplify a gene, the universal gene for bacteria, the 16S gene in this DNA, and then we sequence it to try and identify the bacteria present in the soil. The ReCon Soil project shows different ways to manage materials such as excavated earth, sediment and mineral waste. Rather than always sending these materials to be stored, we can apply the circular economy principle and recycle them nearby. Using short-distance recycling sites reduces transportation and storage costs and the environmental footprint by reducing greenhouse gas emissions from transporting materials. ReCon Soil also shows that these filler materials enrich and improve soil structure and functions and preserve carbon stocks in the soil. The restored soil ecosystem better supports plant growth while helping combat climate change. s