Mining operations are a cause of soil degradation. They are associated with a legacy of abandoned metalliferous mine wastes and acid mine drainage, which contributes to around 2% of soil contamination in Europe. There is therefore an urgent need for sustainable site re-development strategies and remediation technologies that are effective, both in decontaminating and in preserving soil functions, at affordable costs. Low-cost technologies for the recovery of degraded mining areas increasingly use ash from combustion processes as a resource for the remediation of contaminated soils. A total of 175 degraded mining areas (including 114 metallic sulphide mines) were identified throughout Portugal. There is also between 150 000 and 200 000 tonnes of biomass ash generated annually in the country that is typically disposed of in landfills, which could be used to help recover degraded soils in former mining areas.
The LIFE No_Waste project aimed to evaluate, demonstrate and disseminate the sustainable use of ash (from forest biomass residues combustion) combined with organic waste materials (sludge from the pulp and paper industry or compost) to regenerate degraded soils from mining areas, in compliance with the EU Thematic Strategy for Soil Protection.
The project also aimed to reduce the impact of wastes from the pulp and paper industry on the environment, while making better use of valuable resources according to the end-of-waste criteria, while also contributing to the mitigation of greenhouse gas (GHG) emissions. A pilot-scale application of soil additives, produced by the mixture of ash with organic waste materials, would demonstrate soil recovery in three degraded mining areas (on a total of 12 test plots of 100 m2 each) located within the Iberian Pyrite Belt in Portugal.
The LIFE No_Waste project demonstrated the sustainable use of ash (from residual forest biomass combustion, or simply, biomass ash), combined with biological sludge or composted biological sludge originated from the pulp and paper industry (PPI), to regenerate degraded soils in mining areas. The project team conducted two pilot-scale field applications of the obtained soil improvers. The efficacy and safe use of these biomass ash-based materials was demonstrated in the recovery of soil functions at mining sites in the Iberian Pyrite Belt in Portugal.
The first pilot was implemented in 12 soil plots of 100 m2 each at the S. Domingos mine. New soil improvers were tested for efficacy in recovering degraded soils, and to monitor potential risks, over at least 18 months. Soils from three mines (Aljustrel, Lousal, and S. Domingos) were used with three formulations of biomass ash-based soil correctives: (i) un-stabilised mixture of ash and biological sludge; (ii) granulated ash; and (iii) granulated ash mixed with composted sludge. The project team monitored soil, water, plants, and physical-chemical, biological, and ecotoxicological parameters. All three biomass ash-based materials formulations restored the chemical and ecological quality of severely-degraded mining soils suffering from extreme pollution with heavy metals, salinity and low pH. They neutralised soil acidity, increased soil organic carbon content, increased water-holding capacity and nutrients, and enhanced the capacity of soils to grow natural vegetation. Though all three were effective, biomass ash granules mixed with composted sludge showed the best balance between benefits and potential risks.
In the second pilot, the soil improvers having the best scientific and technical results in the first pilot were applied at a larger scale in S. Domingos mine (1 250 m2), and monitored over five months (two sampling campaigns).
Approximately 50 tons of biomass ash and 70 tons of biological sludge from the PPI were recycled in the two pilot-scale field applications. The project team developed and optimised granulation and stabilisation methods for materials preparation, including optimising the binding agent and the time needed for ash granulation. The technical advances and the economic, social, and environmental viability study established that ash-based materials have good potential to be applied during the large-scale recovery of degraded soils.
The project team’s technical guide for future preparation and application of the tested soil improvers in other degraded soils; a multi-criteria decision making plan; and a technical risk assessment protocol for the identification, monitoring and mitigation of potential risks associated with the recycling of ash and organic waste materials in mining soils.
There is a huge replicability potential, as the use of the project’s solution is feasible on thousands of hectares of soils in Portugal alone. Nevertheless, the de-classification of biomass ash waste is key for the uptake of the approach. It is estimated that between 100 000 and 200 000 tons of both biomass ash and biological sludge could be recycled to improve soils over the next 10 years in Portugal using the project’s method, if biomass ash ceases to be categorised as a waste.
Further information on the project can be found in the project's layman report and After-LIFE Communication Plan (see "Read more" section).