Reuse of biomass ash as an additive in the composting process

This work investigated the effects of using selected types of biomass combustion ash as an additive in the composting process. An experimental campaign was conducted where source separated organic fractions of municipal waste was mixed with 0%, 2%, 4% and 8% (w/w) of biomass ash. Both the composting process and the final compost quality were positively influenced by ash addition. In the ash-amended composting admixtures a higher volatile solids degradation and enhanced biological stability were found, because ash were found to be effective as physical conditioner. Improved humification of the organic matter, better germination performances and higher total Ca, Mg, K and P contents were observed in the ash-amended composts comparing to the unamended one. In addition, nutrients fraction readily availability for plants was favored by ash addition with regards to Ca, Mg and K. Nitrogen content in the final composts was reduced by ash addition, but the Ctotal/Ntotal ratio was not negatively affected. The content of heavy metals and their solubility, that is regarded as the main environmental disadvantage when using combustion ash as additive, did not affect the final compost quality negatively. On the other hand, some controversial effects were observed, such as higher pH and electrical conductivity levels comparing to the unamended one. Moreover, higher temperature values and lower moisture levels were observed at the beginning of the composting process in the ash-amended composting admixtures.