Effect of Microaerophilic Conditions on Autothermal Thermophilic Aerobic Digestion Process

A pilot-scale, first-stage, autothermal thermophilic aerobic digestion reactor was used to study the effect of microaerophilic conditions on sludge solids destruction, volatile fatty acids (VFA) production, and phosphorus release. For the aeration rates of 0-100 mL/min and the reactor sludge volume of 72 L, with a primary to secondary sludge ratio of 35:65, the solids destruction efficiency ranged between 19.5 and 23.8%, as measured by total suspended solids (TSS). The maximum increase in VFA concentration (483 mg/L as acetic acid) occurred at the low airflow rate of 25 mL/min. The unit VFA production ranged from 0.009 to 0.183 mg of VFA generated/mg of TSS destroyed, with the dominance of acetic acid. The milligrams of phosphorus released per milligrams of TSS destroyed was from 0.018 to 0.0312, with the maximum measured when no air (nitrogen) was supplied; but the maximum ratio of VFA to PO4 (equal to 8.2) was measured when the air supply was at 25 mL/min.