Abstract
Anaerobic digestion is a renewable energy technology that converts organic waste to bioenergy.
Several nitrogen-rich waste, such as animal manure and food waste, have been considered as
substrate for biogas production. However, ammonia is a major toxicant and is produced during
anaerobic degradation of organic nitrogen. Ammonia inhibition or an overload on anaerobic
digestion is often evidenced through an increase in volatile fatty acids. When the ammonia or
volatile fatty acid concentration reaches a certain limit, its impact becomes inhibitory, which can
cause damage on anaerobic digestion. When the microorganisms acclimate to the inhibitory
concentration, reversibility of inhibition occurs. Inhibition is said to be irreversible when the
biogas production does not recover. Semi-continuously-fed anaerobic digesters with addition of
vacuum stripped digestate were operated for mesophilic digestion of food waste and dairy
manure and compared to digesters with regular feeding of food waste and dairy manure. The
digestate was vacuum stripped to recover ammonia and returned to anaerobic digesters so
ammonia inhibition to methanogenesis is avoided and volatile fatty acids are converted to biogas.
A batch digestion experiment with serial addition of propionate was performed to investigate
volatile fatty acid inhibition in anaerobic digestion. The acclimation of microbial populations and
their correlations to biogas production and concentrations of ammonia and volatile fatty acids
were examined. Provided sufficient exposure and duration of operation, reversibility of
inhibition occurs that extends the inhibition thresholds of total and free ammonia and volatile
fatty acids. This study found that inhibition in anaerobic digestion coupled with vacuum
stripping can be reversed through microbial acclimation and a high-rate degradation of
wastewater can be achieved. The methodology was optimized to find applicable inhibition
thresholds of total ammonia and volatile fatty acids.