Bottom ash is fallout from the grate of mass-burn waste incinerators. Large quantities are produced and this residue has negative value.

The makeup of Irish hazardous waste generated changed in 2020. In 2019, incinerator bottom ash (from Dublin Waste to Energy facility) was the largest component, whereas in 2020 dredging spoil became the largest component, having increased from just over 10,500 tonnes in 2019 to over 90,000 tonnes in 2020.

Testing of incinerator bottom ash in 2020 led to its re-classification from hazardous to non-hazardous waste, this reduced Ireland’s overall hazardous waste generation.