Impact of ammonia and sulphate concentration on thermophilic anaerobic digestion

The effect of increasing concentrations of ammonia and sulphate on thermophilic anaerobic digestion (52 degrees C) was studied at laboratory-scale. The substrate consisted of a synthetic solution supplemented with ammonia and sodium sulphate. In terms of biogas production, the results showed that the C/N and C/SO(4)(2-) thresholds were 4.40 and 1.60, respectively, corresponding to 620 mg FA (free ammonia)/L and 1400 mg SO(4)(2-)/L. No reduction in biogas production was observed until reaching the above concentration of sulphate in the sulphate toxicity test. However, when the concentration of ammonia was increased to 620 mg FA/L in the ammonia toxicity test, a gradual decrease of 21% was observed for the biogas. In order to characterise each set of experiments kinetically, a biogas production first-order kinetic model was used to fit the experimental data. The proposed model accurately predicted the behaviour of the microorganisms affecting the thermophilic anaerobic digestion, allowing its evolution to be predicted.