The effect of acidogenic and methanogenic conditions on the availability and stability of carbon, nitrogen and phosphorus in a digestate

Abstract

Acidification and drying of digestate are important post-treatment for, respectively, improving nutrient availability and hygiene. These approaches are expected to reduce digestate soil application mass and increase its value. This study compared eleven organic feedstocks under acidogenic and methanogenic conditions as a sustainable approach to improving phosphorus availablity, organic carbon and stabilising ammoniacal nitrogen of the resulting digestate under thermal drying. The result showed increases in phosphate concentration under acidogenic conditions and reduction in ammonium nitrogen after drying at 100 °C. The highest phosphate values of 3.2 ± 0.38 g/kg were achieved using whey permeate substrate while the effect of drying on ammonium nitrogen concentration was lowest for acidogenic bird seed fermentation with an ammonium loss of 59.7%. Both results were facilitated by high total volatile fatty acid concentration produced from available organic carbon which reached a maximum value of 5.71 ± 0.53 g/L, respectively. Increases in phosphate and ammonium nitrogen stability under acidogenic conditions was a consequence of lower pH, a condition synonymous with acidogenic only fermentation. The accumulated volatile fatty acid contributed to higher carbon to nitrogen ratio under acidogenic fermentation. Higher labile carbon to nitrogen ratio can trigger immobilization of ammonium nitrogen in the soil and this presents a case for subsequent experimentation into acidogenic digestate application in soil.