A SIMPLE-MODEL OF PH IN SLURRY

Ammonia volatilization from stored slurry or surface-applied slurry in the field is strongly affected by pH. Thus a simple, iterative model was developed to predict pH changes in slurry. Concentrations of NH4+/ NH3, CO2/HCO3-/CO32- and volatile fatty acids were input data to the m odel. The model was validated by titrating 17 slurry samples collected from four cattle farms, seven pig farms and three biogas plants. Pred ictions of pH agreed well for 14 slurries with titration data in the p H interval from 4 to 10. Simulations indicated that microbial degradat ion of VFA to methane and carbon dioxide resulted in a pH increase if the carbon dioxide produced was lost to the atmosphere. There was litt le change in pH if the produced carbon dioxide dissolved in the slurry .