Grain-size specific characterisation and resource potentials of municipal solid waste incineration (MSWI) bottom ash: A German case study

Municipal solid waste incineration (MSWI) is a major element of modern waste management and produces annually around 5.7 million tonnes of bottom ash (BA) in Germany. In order to save natural resources and protect the environment, utilisable materials need to be recovered from BA. It was the aim of the present study to determine metal and mineral resource potentials of MSWI BA based on a characterisation study of raw and aged BA of the MSWI plant in Kassel (Germany). The BA investigated consisted of 82.2% mineral materials, 16.3% metals, and 1.5% unburnt organic matter. Overall, 12.1% and 3.6% of the MSWI BA were theoretically recoverable as native ferrous (Fe) and non-ferrous (NFe) metals, respectively. Assuming state-of-the-art recovery technology, 10.7% and 2.0% of the BA were actually extractable as Fe and NFe metals. The processed BA, as a mixture, did not comply with current German limit values for use as a construction material mainly due to excessive soluble salt contents. Coarser grain size fractions were less contaminated, resulting in a utilisable potential of less than 30% of the BA as a construction material. Hence, grain-size specific processing routes need to be developed for MSWI BA to fully exploit its mineral resource potential.


S.1. German limit values for utilisation of MSWI bottom ash
In Germany there is a draft regulation on the use of secondary building materials (SBM).In this draft it is distinguished between three qualities of the MSWI BA, called HMVA-1, HMVA-2, and HMVA-3.HMVA-1 represent the best quality for MSWI BA utilisation with strictest limit values [1].
LAGA M 20 is a recommendation with requirements for the utilisation of mineral residues and eastes [2].According to LAGA M 20 MSWI BA may be utilised in technical constructions with defined safety measures, so called construction class Z 2. Examples of such technical constructions are: o base layer under a impermeable top layer (concrete) o bound base layer under a less permeable top layer (paving)  Anti-noise barrier.
TL Gestein-StB 04 presents requirements on aggregates in road construction.In this recommendation the utilisation of MSWI BA as aggregate in road construction is characterised by the leaching contents.
Table S1 summarises the limit values of the mentioned regulations and recommendations.

S.2. Samples overview
The required sample mass was determined by estimating the material heterogeneity and by a maximum of acceptable sampling error.The material heterogeneity was estimated by means of the Heterogeneity Invariant HIL (cf.Equation ( 1)) [3].All formulas and definitions can be seen in Gy (1992).
HIL Heterogeneity Invariant mL mass of the lot L mi mass of the particle i cL concentration of the analyte in the lot L ci concentration of the analyte in the particle i The sampling aim was to determine the content of non-ferrous metals (NFe) with a maximum relative standard error of 10%.The non-ferrous metals were chosen as an analyte due to their low mass in the MSWI BA.It was assumed that the concentration of the NFe metals in the lot is 1.7% and the mass of the lot is 1000 kg.
Following was assumed for the mass and concentration in different grain sizes: These assumptions are based on previous unpublished investigations of the MSWI BA from the WtE plant in Kassel.With this data the HIL was calcalated as 19.2 kg.Furthermore, the required sample mass was calculated according to Equation (2).
The calculated required sample mass is 1,917 kg.
In total 40 increments with a total mass of 1,724 kg were taken.There is a deviation from the calculated required sample mass due to the operational process of the plant.The mass variation of the increment is shown in Figure S1.

S.3 Determination of the metallic aluminium content in the fine fraction
The metallic aluminium reacts with sodium hydroxide solution according to Equation (3).The content of the metallic aluminium was calculated by means of the volume of hydrogen gas generated (Eq.( 4)).
∆V volume of the generated hydrogen gas [l] R gas constant (0.0416 bar .l .g - For the determination of the metallic aluminium appr.10 g of BA were put in contact with sodium hydroxide solution with a concentration of 30 wt.%.BA from the grain size fractions 2-4 mm, 1-2 mm, 0.063-1 mm, and < 0.063 mm was analysed.The analysis was carried out in triplicate.

S.4 Designing a mixture of the mineral fraction according to the Fuller curve
A mixture of the mineral fraction was designed after metal recovery.The assumed recovery rates for the metals to assess utilisable resource potentials of metals and the processed BA are shown in Table S3.The Fuller curve is a grading curve providing an ideal particle size distribution for aggregates resulting in best properties of concrete [6].The Fuller curve depends on the aggregate shape.
In this study, it was assumed that the aggregates have spherical shapes.An example for a particle size distribution after Fuller with spherical aggregate is shown in Table S4.The calculation can be seen in Wriggers and Moftah (2006) [6].The particle size distribution of the residual mineral fraction after metal recovery was compared with the particle size distribution of the Fuller curve (Table S4).For example, the coarse fraction above 31.5 mm was not usable as aggregate in concrete.In this way, a mixture of aggregate was designed that is comparable to the Fuller curve.The leaching concentrations, residual metal contents and glass content of this mixture were determined.

S.5.2 Composition of the non-ferrous metals
The composition of the non-ferrous metals over 4 mm determined by the manual sorting is shown in Figure S6.The most common metal in the coarse fraction over 31.5 mm is steel.In all other grain size fraction Al has the highest content which is between 36.9% and 66.8%.The aluminium content in the NFe-metal fraction is lower compared to previous studies [4,7,8].

Figure S1 :Figure S2 :
Figure S1: Distribution of the mass of the increments in the nine daily samples and in the whole sample

Figure S4 :
Figure S4: Total material composition of the raw and aged MSWI BA; error bars show the standard error of the nine daily samples

Figure S8 :
Figure S8: Leaching concentrations for evaluation of the utilisable resource potential as construction material in contained structures (such as road subbase layers) under the regulation of LAGA M 20

Figure S10 :
Figure S10: Leaching concentrations for evaluation of the utilisable resource potential as construction material in contained structures (such as road subbase layers) under the regulation of TL Gestein-StB 04

Figure S11 :
Figure S11: Content of metallic aluminium in the Fuller-curve mixture

Table S1 :
Limit values for recycling of mineral residues and wastes in Germany according to different regulations and recommendations

Table S2 :
Assumed values for calculating the HIL

Table S3 :
Assumed recovery rate of the metals for assessment of the utilisation potential of the mineral

Table S5 :
Statistical measures of the material contents of the raw and aged MSWI BA > 2 mm considering the nine daily samples.Abbreviations: SD: standard deviation; RSD: relative standard deviation; SE: standard error; RSE: relative standard error

Table S6 :
Mass fraction of all elements analysed in the non-ferrous metals fraction above 4 mm of the raw BA in [mg/kg dry matter]; LOD: limit of determination; ND: not determined

Table S7 :
Mass fraction of all elements analysed in the ferrous metals fraction above 4 mm of the raw BA in [mg/kg dry matter]; LOD: limit of determination; ND: not determined

Table S8 :
Mass fraction of all elements analysed in mineral fraction above 4 mm of the raw BA in [mg/kg dry matter]; LOD: limit of determination; ND: not determined

Table S9 :
Mass fraction of all elements analysed in the mineral fraction below 4 mm of the raw BA in [mg/kg dry matter]; LOD: limit of determination; ND: not determined

Table S10 :
Mass fraction of all elements analysed in the non-ferrous metals fraction of the aged BA in [mg/kg dry matter]; LOD: limit of determination; ND: not determined

Table S11 :
Mass fraction of all elements analysed in the ferrous metals fraction of the aged BA in [mg/kg dry matter]; LOD: limit of determination; ND: not determined

Table S12 :
Mass fraction of all elements analysed in the mineral fraction above 4 mm of the aged BA in [mg/kg dry matter]; LOD: limit of determination; ND: not determined

Table S13 :
Mass fraction of all elements analysed in the mineral fraction below 4 mm of the aged BA in [mg/kg dry matter]; LOD: limit of determination; ND: not determined

Table S14 :
Mass fraction of all elements analysed in the glass fraction of the raw and aged BA in [mg/kg dry matter]; LOD: limit of determination; ND: not determined

Table S15 :
Concentration of metallic aluminium in the grain sizes below 4 mm of the raw BA, determined by soda attack method

Table S16 :
Concentration of metallic aluminium in the grain sizes below 4 mm of the aged BA, determined by soda attack method

Table S17 :
Share of the mineral content of the metals in the raw and aged MSWI BA

Table S18 :
Leaching concentrations in the mineral fraction above 4 mm of the raw BA; LOD: limit of determination

Table S19 :
Leaching concentrations in the mineral fraction below 4 mm of the raw BA; LOD: limit of determination

Table S20 :
Leaching concentrations in the mineral fraction above 4 mm of the aged BA; LOD: limit of determination

Table S21 :
Leaching concentrations in the mineral fraction below 4 mm of the aged BA; LOD: limit of determination

Table S22 :
Leaching concentrations in the glass of the raw and aged BA; LOD: limit of determination