Acidification of livestock slurry and digestate to reduce NH3 emissions: Predicting needed H2SO4 dosage and pH trends over time based on their chemical-physical composition

Acidification is a well-known treatment to reduce NH3 emissions from livestock slurries by lowering their pH, but its application at the farm scale is still limited. The acid dosage depends on the acid strength and slurries composition. Acidification does not have a lasting effect and after the acid addition the slurry pH tends to rise due to its buffer system. By studying 54 samples of pig slurry, dairy cattle slurry and digestate, this study aimed to: (i) identify the chemical-physical parameters related to the sulphuric acid (98% w/w) dosage necessary to reduce pH at 5.5, and pH variation over time; (ii) develop predictive models for the acid dosage and the pH after one (pH1w) and two weeks (pH2w) of storage based on slurry composition by using regression trees (RT) and random forests (RF). Acid dosage ranged between 0.8 and 11.7 ml kg−1 increasing with slurry alkalinity, with digestate requiring significantly higher dosage than slurries. Pig slurry showed significantly higher pH increase than the other two slurries. Finally, the pH trend over time was negatively correlated with the solids content. The RF identified the alkalinity and the initial slurry pH as the most important variables in explaining the required acid dosage, while for pH1w and pH2w it identified the total organic carbon and volatile solids. Based on RF results, RT models accurately predicted required acid dosage (r2 = 0.881), the pH1w (r2 = 0.728) and pH2w (r2 = 0.667). Therefore, these simple models can have practical applications for reducing NH3 emissions.