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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.

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