Co-composting of municipal biowaste and biomass power plant ash: effect on the biological process and the final compost quality

In this work the use of selected types of biomass combustion ash as an additive in the composting process was studied and discussed. A composting experimental campaign was conducted in four pilot-scale composters where 0%, 2%, 4% and 8% (w/w) of biomass ash were added to the organic fraction of the municipal solid waste mixed with wood prunings, that served as bulking agent. Biomass ash influenced both the composting process and the final product quality positively. The volatile solids reduction and the stability of the final composts, the latter being measured by means of the oxygen consumption, were greatly enhanced, and the temperature rise at the early stage of the process was improved. The ash-amended composts were enriched in nutrients (Ca, Mg, K and P) and expected to be beneficial for the plant growth, as the germination tests showed. Some adverse effects were also observed, such as the higher pH and electrical conductivity in the ash-amended composts comparing to the unamended one. Also, moisture was greatly affected by ash addition, to an extent which required external control. As for the heavy metal content, that was expected to be the main disadvantage related to the ash use, it does not seem to constitute an environmental concern for the ash content considered.