Sustainable Restoration of Depleted Quarries by the Utilization of Biomass Energy By-Products: The Case of Olive Kernel Residuals

The combustion of biomass has a neutral atmospheric CO2 fingerprint, because the overall produced CO2 emissions are balanced by the CO2 uptake from the plants during their growth. The current study evaluates the environmental impact of the biomass ash wastes originating from the combustion of olive-kernel residuals for electricity production in accordance with Directive EE/2003. Additionally, the study investigates the potential use of such waste in the restoration of depleted calcareous aggregate quarries in the frame of the circular economy, as a substrate or as a soil amendment. Olive-kernel residual ash, obtained from a 5 MW operating electricity power plant, was mixed with soil and tested for its adequacy for use as a substrate or soil amendment in a depleted calcareous aggregate quarry. The positive effects of the olive-kernel residual bottom ashes in the availability and the mobility of major and trace elements were assessed in both batch and column experiments. The effect of biomass ash in soil amelioration was assessed via pot experiments, by examining the growth of two plant species Cupressus sempervirens (cypress) and Dichondra repens (alfalfa). The environmental characterization of the olive-kernel residual bottom ash indicates that the water-leachable concentrations of controlled elements are, generally, within the acceptable limits for disposal as inert waste in landfills. However, the bottom ash was found to contain significant amounts of K, Ca and Mg, which are macro-nutrients for the growth of plants, serving as a slow-release fertilizer by adding nutrients in the soil. The application of bottom ash in the alkaline soil had a minor positive effect in plant growth while the addition of the ash in the acidic soil exhibited considerable effect in the growth of Dichondra repens and Cupressus sempervirens due to the release of nutrients and to the pH conditioning. Olive-kernel residual bottom ash has been proved to be appropriate as a soil amendment, and as a soil substrate for the restoration of depleted quarries, decreasing the requirement for commercial inorganic fertilizers