To protect the natural aggregates and promote the circular economy the suitable secondary aggregates have been studied intensively in last decades in Finland. One promising secondary aggregate is bottom ash from the municipal solid waste incineration process.
The use of municipal solid waste incineration bottom ash in earth structures is well-founded, since the environmental impacts of MSWI bottom ash are rather low and the technical properties of MSWI bottom ash nearly correspond to the natural aggregates. The ash is sieved into various fractions in a recovery process, where non-ferrous and ferrous metals are extracted from bottom ash, generating typically fractions of grain sizes 0-2 mm, 2-5 mm, (or 0-5mm), 5-12 mm and 12-50 mm. The amount of the fines depends on the recovery process. The grain size distribution in typically very homogenous.
The MSWI bottom slag contains heavy metals and other contaminants limiting its environmental acceptability. The fines contain typically the highest concentrations of contaminants. The portion of inert particles such as rock, glass and mineral particles is higher in coarser fractions.
The aim of the study was to assess the suitability of the fines to the mineral liner in landfill capping. Based on the environmental permit, the target value for capping liner is k ≤ 1 x 10-9 m/s. The permeability of the fines of MSWI bottom ash is typically around 1 x 10-6 m/s when well compacted. In order to achieve the required permeability, bentonite or other additives are needed. The grains are porous and the pH is high, typically 10-12, which effects on the amount of bentonite required. In addition, the MSWI bottom slag contains high concentrations of diluting chlorides, sulfides and calcium, which decrease the swelling properties of the bentonite. The swelling capacity of bentonite decreases when permeating aggressive leakages with high cation concentration. Therefore a special polymer modified bentonite was chosen for the tests. The addition of superabsorbent polymers, which have much higher resistance to aggressive leakages, greatly improve the performance and self-healing capacity of bentonite.
First, laboratory tests were performed to estimate the proper amount and quality of the bentonite needed to achieve the permeability required. Two bentonite types were tested, the common natural bentonite and a special polymer modified bentonite produced by Cetco. The swelling index of both types of bentonite was tested by an eluate of MSWI bottom ash. Several permeability tests were performed to evaluate the effect of dry density, bentonite quality and dose, and increase of coarser fraction (2-5 mm).
After laboratory testing, a test area was constructed on an old waste fill at Ämmässuo. During the construction, it was noticed that the water content effects significantly on the compaction result. The mineral liner was covered by a 1,0 mm thick LLDPE geomembrane and protective geotextile. The liner structure is covered only by a 0,5 m thick drainage layer from crushed rock. No surface layer was constructed. After one year, the liner was exposed and studied. The quality of the structures, especially the mineral liner was visually evaluated and gas emissions were measured from the surface. The density of the layer was measured by rubber-balloon method and troxler, and the water content and permeability were measured in laboratory. Based on the visual inspection the surface of the mineral liner was smooth, and the layer homogenious and hardened.
Lessons learnt from practical projects , Proof of compliance - requirements and quality assurance , Monitoring and long term performance, both technical and environmental
