Fly ash addition into anaerobic digestion of biowaste: effects on biomethane production and fertilizing properties of the digestate

The effects of adding fly ash into anaerobic digestion process were investigated by evaluating the biomethane production of biowaste with and without ash addition and fertilizing properties of the digestate. The addition of fly ash at specific quantities (2.5–7.5 g of ash per kg) increased the probability that methane production with ash addition was higher than without, while at higher ash doses (10–23.3 g kg −1 ) the effect for methane production was negative. White mustard ( Sinapis alba L.) was used as an experimental crop, and the responses followed were seed germination, plant growth, and number of flowers. Potential accumulation of heavy metals and nutrient uptake of the plants were also analyzed. Fly ash addition also enhanced the growth of mustard, with the highest ash addition of 23.3 g kg −1 leading to approximately 30 % increase in biomass growth. Germination rate was increased by the fly ash addition from 74 % in the digestate fertilized soil to 82–84 % with ash. Flowering of the mustard plants was affected similarly, where ash addition increased the flowering rate to 59 % compared to 33 % with digestate addition. Analysis of the growing media and the mustard plants showed no significant increase in heavy metal concentrations due to addition of digestate or ash. Fly ash was hence demonstrated to be a viable additive to anaerobic digestion process by increasing biomethane production and benefitting fertilizer quality of the digestate, when used in specific quantities. • Ash addition of 2.5–7.5 g kg −1 had positive effect on biomethane production potential. • Higher ash dosage affected the biomethane production potential negatively. • Wood ash was found more beneficial for biomethane production compared to peat ash. • Ash addition increased biomass, flowering and germination compared to digestate alone. • Digestate/ash addition did not increase metal levels in growing media or plants.