The influence of pH and ammonia concentration on the methane production in high-solids digestion processes

The influence of pH and ammonium-nitrogen on methane production in a high-solids sludge digestion process was investigated using a mesophilic batch digester fed with a sludge cake. A simple model developed from the Gompertz equation was applied to the quantitative measurement of the methane production rate and lag-phase time at pHs ranging from 6.5 to 9.0 and ammonium-nitrogen concentrations ranging from 100 to 6 000 mg/L. The results indicate that the ammonium-nitrogen concentration was a more significant factor than the free ammonia in affecting the methanogenic activity of a well-acclimatized system. The simulated results reveal that the methanogenic activity decreased with an increase in ammonium-nitrogen, dropped 10% at an ammonium-nitrogen concentration of 1 670 to 3 720 mg/L, dropped 50% at 4 090 to 5 550 mg/L, and dropped to zero at 5 880 to 6 000 mg/L. The lag-phase time in the batch experiment was dependent on the ammonia level, but not ammonium, and when the free ammonia concentration was higher than 500 mg/L, a notable shock load was observed. In addition, the maximum methane-converting capacity of sludge changed from 28 to 0.9 mL CH₄/g VS·d when the ammonium-nitrogen concentration increased from 100 to 6 000 mg/L.