In vitro reporter gene activity of PCDD/Fs and dl-PCBs in wood ash from domestic heating systems (Belgium) using the H1L7.5c1 CALUX Bioassay

Wood ash can be used as a fertilizer because it contains some important nutrients (K, Mg, Ca, P) as well as certain microelements such as Fe, Mn, Cu and Zn, and this in either pure form, in a pelleted version, or in combination with organic waste to form compost. It has been demonstrated that compost amended with (wood) ash (8-16%) provides better fertilizing capacity/ies than without. As wood ash is a byproduct of the combustion of biomass, it would be profitable if these nutrient-containing ashes could return back to the ecosystem by using them as a fertilizer, and thereby making this a sustainable use of biomass.

There are, however, also disadvantages to the use of wood ash as fertilizer. Mainly the presence of pollutants, such as potentially toxic metals and dioxin-like compounds can be a concern. Also, the high alkalinity of these ashes, ranging in pH between 12 and 13 can be a problem since they can alter the pH of the soil and interfere with the natural biogeochemical cycle.

The chemistry of wood ash is highly dependent on a number of different factors, among them the type of wood that is being burned. For example, hardwood ash contains more macronutrients (such as K and P) than softwood ash, making it a more efficient fertilizer. There is also a difference when burning different parts of a tree, since the concentration of nutrients in the ash can differ greatly between ash generated from the bark of the tree or from the stem. Unfortunately, little is known about these differences in accumulations of POPs (such as dioxins and PCBs). Also the incineration temperature is an important variable whereby, for instance, the highest level of macronutrients in ash is retained between 500-800°C, whereas a temperature higher than 900°C can cause heavy metal volatization. There is also an important distinction between fly ash and bottom ash. Fly ash is the lightest component, which accumulates in the chimney and generally contains the highest concentration of possible harmful substances, such as heavy metals and dioxin-like compounds.

In this study, wood ash samples from domestic heating systems in Belgium were analysed for dioxin-like activity. Extra attention was given to samples originating from the combustion of wood pellets as these are marketed as eco-friendly, thereby driving sales and increasing their contribution to the total amount of wood used as combustion fuel. The crude wood ash samples (total extract) and the separated dioxin and PCB fractions were measured with the CALUX (Chemically Activated Luciferase gene eXpression) method. The final results were compared to legislation concerning the use of fertilizers and limit values for the use thereof to see if wood ash can be safely used for this purpose.