Wood Ash

Compositional variation and hazards of wood ash in Ireland

PhD project of Lucas Jagodzinski

Supervisors: Prof John O’Halloran, Dr Frank van Pelt, and Prof Marcel Jansen


Wood as a source of energy is becoming increasingly popular, not only in forest related industry where the fuel accrues as a by-product (i.e. sawmills), but also amongst non-forestry businesses where the environmental image is important. A major selling-point is that energy from renewable fuels such as wood is considered sustainable. Yet, “true sustainability” requires the nutrient minerals in wood ash to be returned to the fuel producing soils, a fact that remains widely unacknowledged. The primary obstacle to closing the nutrient cycle and achieving sustainable status for wood energy is that ash materials are classified as waste, which can preclude ash spreading on forest soils. As per European regulation, a waste may cease to be such via end-of-waste and/or by-product status declarations from the producer. The relevant paragraphs of European regulations have been transposed into Irish law in 2011 but demonstration of compliance with the criteria therein is hampered by lack of local data on ash material composition and environmental safety of its potential after-use as a fertiliser.

This study compiled the first compositional account of wood ash as it accrues at ten wood-fired power plants in Ireland, which burn untreated wood. Two principal waste-constituting ash types, bottom and fly ash as well as their leachates, were distinguished. A more detailed case study on a combined heat and power (CHP) wood energy plant investigated bottom and fly ash composition over time. The ashes were also examined for their quality as plant nutrient sources and toxicological properties were investigated using a test battery of aquatic species. The MICROTOX test, Pseudokirchneriella subspicata growth inhibition test, Daphnia magna immobilization assay, Lemna minor growth inhibition and Oncorhynchus mykiss acute and prolonged survival tests comprised the ecotoxicological test battery. Further to this, the two ash types from the case study site were sieve fractionated and the particle size fractions tested using MICROTOX and L. minor.

The findings from this study show that, from a compositional perspective, leachates are more variable than their parent solid ashes. A large part of the variability and chemical hazards observed in the wood energy waste may stem from the fly ash portion therein. The segregation of the ash types (bottom and fly ash) allows for higher compositional consistency and thus better predictability of the materials’ hazardous as well as desired (fertilising) properties. Ecotoxicological assessment revealed fly ash from the case study site to be relatively more variable and more toxic than bottom ash. Single cell organisms within the test battery were most likely to be adversely affected by exposure to wood ash. Measurable effects of ash on aquatic species were affected by the test regime used, with both growth promoting and toxicological effects noted on Lemna minor. The data emphasise that a case-by-case evaluation is recommended when considering the fate of wood ash.