Kinetic study of the stabilization of an agro-industrial digestate by adding wood bottom ash

The addition of wood bottom ash to an agrowaste anaerobic digestate has been proposed for improving the carbon to nutrient ratio of the soil organic amendment (C/N/P < 100/10/1). The ash-based treatment also aimed to improve properties of the anaerobic digestate as controlled-release fertilizer by decreasing the availability of nitrogen, carbon, and phosphorus. Treated (39.86 ± 0.94 g) and untreated (33.51 ± 0.84 g) digestates were incubated for 7 h at 20 °C and 100 rpm, in a 250-mL chamber with a 0.11 M sulfuric acid trap of 4.38 ± 0.02 mL. By the end of the incubation, the blend of 13.12 ± 2.87% ash and 86.88 ± 2.87% digestate released 8.95 ± 2.19 times more ammonia than the digestate alone. This was in agreement with the lower content of water-soluble ammoniacal nitrogen in the treated digestate (93.35 ± 21.00 mg/kg) than in the digestate incubated without the addition of the ash (357.64 ± 54.10 mg/kg). As per the very low levels of water-soluble nitric nitrogen found in the treated (0.16 ± 0.05 mg/kg) and the untreated (0.06 ± 0.02 mg/kg) digestate, the emissions of nitrous oxide were discarded. The availability of carbon increased 1.43 ± 0.82 times and the availability of phosphorus decreased approximately 100 times due to the addition of the ash to the digestate. The ash-based treatment was successful in improving the C/N/P of the digestate from 121.51 ± 57.18/9.94 ± 4.21/1 to 42.05 ± 9.88/2.48 ± 0.58/1