Volatile fatty acid production during thermophilic aerobic digestion pre-treatment

Abstract

This study was initiated to demonstrate the application of thermophilic aerobic digestion pre-treatment as a method of producing volatile fatty acids (VFA) for potential use as a denitrification carbon source. In order to achieve this goal, volatile fatty acid production and nitrogen release were examined under two oxygenation states with detention times ranging from 6 to 24 hours at 55$\sp\circ$C. The semi-continuous laboratory scale reactors, with 3 L operating volumes, were fed primary solids obtained from the City of Winnipeg's North End Water Pollution Control Centre. VFAs accumulated under all oxygen-deprived conditions (O$\sb2$ flow rate: 0.025 $\rm m\sp3/m\sp3\cdot h;$ ORP: $-$330 mV to $-$390 mV) and the production rate increased with increasing retention time from 0.029 mg HAc/mg VS$\cdot$d at 6 hours to 0.057 mg HAc/mg VS$\cdot$d at 24 hours. VFAs were consumed under the oxygen-satisfied condition (O$\sb2$ flow rate: 0.14 $\rm m\sp3/m\sp3\cdot h;$ ORP: $-$10 mV to $-$225 mV). The VFA specific production rate did not appear to vary with the influent solids concentration. Acetic acid constituted the largest fraction of VFAs in both the oxygen-satisfied and oxygen-deprived experiments and ranged from 59.6% to 64.3%. The oxygen-satisfied condition exhibited much higher propionic acid concentrations (average of 30.3%) than did the oxygen-deprived conditions (average of 17.2%). Ammonia nitrogen accumulated under all conditions studied, but more so with a decreasing oxygen supply and increasing retention time. Under the oxygen-deprived condition, the 6 hour SRT/HRT exhibited the lowest ammonia nitrogen increase of 41.6% while the 24 hour SRT/HRT exhibited the highest at 142.8%. The soluble organic carbon to ammonia ratio increased with a decreasing oxygen supply and detention time. Returning the VFA-rich supernatant to the plant influent would increase the overall chemical oxygen demand to ammonia ratio and aid in the denitrification process.