11.4.1 Landfills and Mining Waste Deposits

Combustion residues are used to provide a barrier against the penetration of water or oxygen, or both, into the body of the landfill or of the heap or impoundment of mining waste. The properties that make fly ash from solid biofuels suitable are their high pH and their self-binding properties. The potential for use in several contexts is quite high and the demand could easily exceed the availability of suitable residues.

Fly ash mixed into sewage sludge in equal dry substance proportions raises the pH of the sludge, thus preventing its biological degradation. The percolation rates achieved in field experiments with fly ash mixed into sewage sludge are on the order of 12 l/m2/year, both initially and after a few years, which is sufficiently low for sealing layers on landfills for non-hazardous waste (Carling et al. 2006; Macsik et al. 2005). Its shear strength is acceptable and it withstands settling in the body of the landfill.

On the Tveta landfill, the functional requirements on a sealing layer are fulfilled using monolithic layers of ash through diffusional processes, i. e. particle size distribution, moisture content and reactivity of the combustion residues, all con­tributing to the minimisation of the pore volume (Tham and Andreas 2008).

A sealing layer of fly ash from solid biofuels, covered by a protective layer of vegetated sewage sludge, prevents oxygen from reaching sulphidic tailings from mining. The experimental object is the 80-ha large tailings impoundment of Gillervattnet, where an experimental field covering 3 ha was established in 2003-2005. The hardened layer of ash and the toxicity of the fresh ash prevent roots from breaking through the sealing layer and providing channels for air entry.

However, reed canary grass has some capacity to weaken even hardened fly ash sealing layers with a resistance of approximately 5 MPa. The study suggests that the secretion of saccharides by some plant roots may contribute to this effect (Greger et al. 2006, 2009). It is thus best to avoid these particular plants.