Ecotoxicity is a key property from a regulatory point of view, H14 in the European Waste Catalogue. Although the EU is expected to issue rules on criteria 2011, today there is no legally binding rule. There is no accepted test procedure for ecotoxicity yet, and an H14 value is computed using knowledge of the chemical composition of the waste and of the ecotoxicity of the pure chemical substances.

There is a difference of opinion in Sweden on the level at which a waste is to be considered ecotoxic. When a chemical product contains more than 2.5% of an ecotoxic substance, it is hazardous enough for organisms to warrant labelling as such. Analyses of ash usually yield the elemental composition, very seldom the concentration of chemical compounds. A study for the Ash Programme suggested a conservative selection of substances for the key trace elements and recommended that this value, 2.5%, be used to determine whether an ash is ecotoxic or not. The environmental authorities are of the opinion that the limit should be 0.25% because this is the limit for aquatic organisms.

The key element in this discussion is zinc, an essential nutrient. To be conserva­tive and not underestimate the ecotoxicity, the above-mentioned study assumed zinc was present as zinc oxide. At approximately the same time, zinc oxide was reclassified as ecotoxic, which has severe consequences for bioash. The zinc content in clean wood ash recommended by the Swedish Forest Agency is up to

7,0 mg/kg, i. e. 0.7%, which is larger than 0.25%. Concentrations of approxi­mately 0.4% are not uncommon for bark ash.

One way of tackling this difficulty is to examine whether zinc oxide is a reasonable choice as a reference substance after all, rather than just a conservative one. Three investigations should be mentioned here:

• Van der Sloot at ECN utilised the database LeachXS on leaching properties of MSWI residues and concluded that zinc is present mostly as willemite, a zinc silicate (H. van der Sloot, oral communication).

• Sjoblom (2007) reexamined the chemical thermodynamics of zinc in combus­tion residues, in particular the solubility equilibria, and concluded that zinc is present probably as franklinite, a mixed iron and zinc oxide, rather than willemite.

• Steenari and Noren (2008) studied the binding distances for zinc in different combustion residues using extended X-ray absorption fine structure spectros­copy, and found that the distances agree with zinc being bound in silicates or aluminates. They also found that in fresh ash, some of the more soluble zinc compounds are also present in minor quantities. When ash is wetted, zinc is rapidly redistributed to insoluble compounds.

Thus, zinc does not occur as oxide in combustion residues, but rather in much more stable compounds. Choosing oxide in the ecotoxicity assessment was overly conservative and significantly overestimates the environmental impact of zinc in ash. Settling for a realistic model compound is not straightforward: it seems more

probable that zinc is found as willemite or a silicate, but there is no official assessment of these compounds. On the other hand, less probable franklinite but with similar properties has been assessed as non-ecotoxic.

At the end of the day, when there are doubts, an ecotoxicity test will clinch the discussion. In a study co-financed by the Ash Programme, the Swedish Waste Management Association and the Swedish Environmental Protection Agency, a battery of tests were performed on different combustion residues (Stiernstrom et al. 2009). Care was taken to choose organisms that naturally occur in brackish waters rather than the freshwater organisms that are sensitive to the salt concentration.

At a liquid-to-solid leaching ratio of 10 (waste test condition to be on the conservative side, but for chemical products the liquid-to-solid leaching ratio should be 10,000 according to OECD good practice) almost all residues were ecotoxic. A more detailed examination revealed that the ecotoxicity is due to the high concentrations of potassium and calcium (nutrients) and of aluminium. This is scarcely a reason to consider the residues as ecotoxic.

Results from tests of bottom ash from MSWI are illustrative. Ash which had been stored and allowed to mature for 3 months was somewhat ecotoxic. Ash which had been aged and exposed to the weather (actual liquid-to-solid leaching ratio of 3.4, rainwater as leachant) for 12 years was not ecotoxic at all to the three organisms tested. And yet, its composition had not changed, except for a lower content of chlorides and probably of sulphates, too.

11.3 Conclusion

Projects within the Ash Programme have contributed increased knowledge of the properties of combustion residues as well as demonstrated large-scale uses for residues. The Ash Programme has been active since 2002, and it is quite probable that it will be extended for another 3 years at the end of 2011.

With ash from clean solid biofuels, priority should be given to recycling the ash to forest soils as a compensation for the extraction of mineral nutrients in an intensive harvesting, i. e. whole-tree harvesting. If that is not possible, the residues should be used in civil works, e. g. for building roads or hard surfaces. One should put all such ash to use, instead of rejecting particular streams.

The Ash Programme has striven for a common understanding of the level of risk for human health and the environment when using ash in civil works. Boundaries between low risk and not low risk have been suggested. The Swedish Environmen­tal Protection Agency has developed another boundary, below which one does not even need to pay notice to the environmental authorities.

All reports are public and are available from Varmeforsk’s Web site (http:// www. varmeforsk. se). The language is Swedish, but there is always a summary in English.