Sampling of heterogeneous bottom ash from municipal waste-incineration plants

Bottom ash from waste-incineration plants is a very heterogeneous material and sampling requires special attention. The objective of the sampling should be carefully defined before a sampling protocol can be made. In Denmark, 500,000 tonnes of bottom ash is produced annually from a number of municipal and privately owned incineration plants. For legal reasons, the bottom ash has to be sampled and evaluated since reuse options of the bottom ash depends on comparing fixed thresholds with these analytical results. Legally, a sample of 5 kg has to be taken from every batch of 5000 tonnes of bottom ash. The amount of trace elements leaking into the environment (leachates) play the key role in deciding if and where bottom ash can be reused. Test for systematic error (bias) and estimation of the sampling and preparation errors were the main objectives of this study. alternative methods of sampling, comprising mechanical sampling, “stopped belt sampling” and manual sampling from a stockpile have been evaluated and compared here. The primary and secondary sampling uncertainty is shown to be the dominant factor in the overall uncertainty of the analytical results. Stopped belt sampling of four different types of bottom ashes has been carried out in a baseline study to estimate the primary increment variance, enabling estimation of the different uncertainties associated with the final analytical result. on this study, the owners and operators of incinerating plants in Denmark now recommend to the Danish Environmental Agency to revise the current governmental protocol for sampling of bottom ash in order to minimize the risk for biased samples and to significantly reduce the uncertainty of the final result.