Biomass ash characterisation for reuse as additive in composting process

Ash generated by biomass combustion has been envisaged as a potential composting additive in order to address some of the most common critical issues concerning the composting process and final product quality. Nonetheless, a deeper awareness for the effective feasibility of such ash reuse option should be pursued. In this work, all the ash types produced at different sections of a typical biomass combustion plant (namely bottom, boiler and fly ash) were characterised with the scope of identifying the weaknesses and strengths of each residue in the light of their reuse as a composting additive. The results manifest that boiler and fly ash were very enriched in nutrients such as calcium, potassium, magnesium and phosphorus. They also had high porosity and water holding capacity. Because of these properties, they could act as a mineral additive and a physical amendment in composting. The pH values for boiler and fly ash were 12 and 13, respectively, so they could also act as a liming agent in composting of acidic substrates. The electrical conductivity was high (16 and 33 mS·cm−1, respectively) and could be harmful for plant growth. On the other hand, bottom ash had low salinity and alkalinity, but the physical properties and nutrient content were not of interest for composting. Heavy metals release was not a concern in all the ash types.