Oxidation and ozonation of waste activated sludge

Abstract

In this bench-scale study, the treatment of waste activated sludge (WAS) was evaluated using aerobic digestion and ozonation. Two, 2-L batch digesters, one aerated and the other one ozonated, were operated for 30 days in each phase of the study. The aerated digester simulated the aerobic digestion process and served as control to the ozonated digester. In Phase I, the aerated digester was supplied 810 mg O₂ min^{− 1}, whereas, the ozonated digester was supplied 0.88 mg O₃ min^{− 1}. In Phase II, the oxygenation rate to the aerobic digester was increased to 1,200 mg O₂ min^{− 1} while the ozonation rate was reduced to 0.44 mg O₃ min^{− 1}. Ozone was more effective than air at oxidizing and reducing both total solids (TS) and volatile solids (VS) in the WAS. TS removals of 50% and 56% were observed for the ozonated digester versus TS removals of 23% and 35% for the aerated digester. VS removals of 40% and 42% were observed for the aerobic digester versus 57% and 74% for the ozonated digester. Aerobic digestion barely met the 38% reduction in VS required by the U.S. Environmental Protection Agency (EPA). The degradation rate constant (K _d ) based on degradable TS for the ozonated digester varied from 0.082 to 0.11 days^{− 1} and from 0.067 to 0.09 days^{− 1} for the aerobic digester. Total chemical oxygen demand (TCOD) removal in the aerobic digester increased from 30% to 40% from Phase I to Phase II. TCOD removal increased slightly from 57% to 58% in the ozonated digester from Phase I to Phase II. Soluble chemical oxygen demand (SCOD) concentrations in the sludge supernatant increased with digestion time, especially in the ozonated digester. Approximately 0.12 to 0.22 mg SCOD was produced per mg of TS destroyed during ozonation. The specific oxygen uptake rate (SOUR) was consistently below the EPA standard of 1.5 mg O₂ per hr per g TS, indicating that the sludge was well stabilized. The average quantity of oxygen required during aerobic digestion was 1.53 g O₂ per g of TS destroyed. Actual ozone consumption rates were 0.57 mg O₃ per mg TS destroyed and 1.09 mg O₃ per mg TS destroyed for Phase II and Phase I, respectively.

Keywords:

• Aerobic digestion
• ozonation
• waste activated sludge
• volatile solids reduction

Notes

* TCOD value from March 2, 2006.