Digested sludge used in the restoration of mining areas

The digested sludge used to restore the land at Boliden’s Swedish mines comes from, amongst other places, treatment plants in Stockholm. The measurements carried out by Boliden at the Aitik mine in Gällivare, where digested sludge has been used in conjunction with grass seeding for almost 10 years, show no signs of elevated levels of phosphorus in nearby watercourses. 

Why do you use digested sludge?
Mining operations have a relatively large impact on nature and the landscape around the mine, primarily by the formation of waste rock deposits and tailings ponds. Boliden has a responsibility to reclaim and restore the land areas it uses. Re-establishing plant growth on a waste rock deposit or tailings pond is important, both from a visual viewpoint and because a layer of vegetation reduces the risk of erosion. If erosion occurs, the underlying coverage of the waste rock is damaged. 

During the restoration process, nutrient-rich digested sludge is used to cover the surfaces, which are then sown with grass. Digested sludge has been shown to have numerous positive properties in this context, helping to bind in moisture and reduce the risk of erosion, elevating the pH level, binding metals, and, to some extent, consuming acids. These properties are very useful when handling waste that may discharge metals and acidify water. Sludge has been shown to be by far the most effective method of establishing plant growth and to be far more effective than, for example, artificial fertiliser. 

By using digested sludge in its reclamation work, Boliden helps restore the natural features affected by the mining operations. The high nutrient content creates excellent preconditions for long-term re-establishment of new plant growth and the use of digested sludge also minimises the need to bring in soil or artificial fertiliser during the reclamation process. 

How much digested sludge do you use?
Boliden has used a total of 600,000 tonnes of digested sludge since 2002, the majority of which comes from treatment plants in Stockholm, with additional supplies coming from treatment plants in Skellefteå, Luleå, Sundsvall, Ludvika and Arvidsjaur. Just over half of this digested sludge has been used at the Aitik mine, with the remainder primarily used in the reclamation of a number of Boliden’s decommissioned mines.

How is the digested sludge used?
We restore the countryside after mining operations are wound up by covering the waste rock deposit with, amongst other things, an approximately 30 cm deep layer of nutrient-rich digested sludge that improves plant growth. The sludge also helps protect against erosion and adds organic materials to the mix, in that the till that we also use is nutrient-poor and contains very little organic material.

What is digested sludge and what does it contain?
In simple terms, digested sludge is a treated form of the materials separated out in treatment plants, i.e. excrement and other waste products from the sewage systems. The actual digestion is a fermentation process that increases the nutrient content of the sludge. Digested sludge is used as a fertiliser in the agricultural sector or when establishing other forms of plant growth. Biogas, which is used, amongst other things, as fuel for vehicles and to heat buildings, is one of the by-products of the digestion process. 

Does Boliden have a permit to use digested sludge?
Boliden has the requisite permits from the respective supervisory authorities to transport, store and use the sludge. We also have internal regulations governing the way in which the sludge is handled, both when it arrives at our facilities and when it is spread, in order to minimise any negative effects on the environment. The digested sludge we use also complies with the quality standards that apply to sludge used for agricultural purposes (with regard to metals, organic substances, etc.), even if Boliden’s areas will not be used for these purposes.

What does the digested sludge cost?
Boliden receives negotiated payments from the treatment plants for taking charge of the digested sludge, but the actual amounts paid vary in line with e.g. demand. Boliden’s costs in connection with transportation and handling of the sludge do, however, exceed the income received.  

Is there any truth in the claims of an increase in phosphorus leakage?
Boliden has used digested sludge in its reclamation work at the Aitik mine for ten years now. Both Boliden’s environmental staff and the licensing authorities have carefully monitored any environmental impact during that time. Boliden has, amongst other things, measured the phosphorus levels in the nearby Leipojoki watercourse, but no signs of raised levels of phosphorus were detected. 

Agricultural applications use very small amounts of digested sludge and dilute it with soil. Why does Boliden use so much larger quantities and not dilute it?
The conditions when covering waste rock deposits or tailings ponds are completely different, in that there is no soil or other nutrient-bearing materials with which the digested sludge can be diluted. Were we to dilute it, we would have to add large quantities of soil (till), which would, in turn, have to be excavated and transported to the mining areas. In Boliden’s opinion, the use of digested sludge is the most eco-friendly method and enables the establishment of plant growth without the need to add new soil or artificial fertiliser.

Will Boliden continue using digested sludge?
The results of using digested sludge are so positive when it comes to the re-establishment of plant growth that we intend to continue using digested sludge. We will, of course, continue to monitor the usage carefully to ensure that there are no undesirable environmental effects.