Ultrasonic Pretreatment: Impact on Solubilization, Biogas Production and Kinetics of Anaerobic Digestion of Conventional and Biofilm Waste Sludges

Abstract

Anaerobic digestion is a useful method for stabilizing and reducing the waste activated sludges (WAS) produced from biological secondary treatment. Pretreatments can make anaerobic digestion more efficient. However, the study of anaerobic digestion and pretreatments is limited to a focus in treating conventional WAS. Therefore, WAS from three non-conventional municipal wastewater treatment systems, a rotating biological contactor (RBC), a lagoon, and a moving bed bioreactor (MBBR), were digested anaerobically to determine the sludges’ biogas potentials compared to a conventional WAS. All three WAS had lower biogas potential normalized per volatile solids than conventional sludge by 46% + 6 (MBBR), 63% + 6 (RBC), and 77% + 7 (lagoon). The four sludges were pretreated with ultrasonic energies of 800 - 6550 kJ/kg TS to illustrate impact of sludge type on biogas production, solubilization, and digestion kinetics. All four sludge types responded uniquely to the same levels of sonication energies. The greatest increase in biogas production over the control of pretreated sludge did not coincide consistently with greater sonication energy but occurred within a solubilization range of 2.9 – 7.4% degree of disintegration (DD) and are as follows: 5% + 3 biogas increase for conventional sludge, 12% + 9 for lagoon, 15% + 2 for MBBR and 20% + 2 for RBC. The yield of biogas production related to soluble COD decreases with increased sonication energy. Hence it is likely that sonication produces refractory COD or causes inhibition in biogas production. The effect of sonication on digestion kinetics was inconclusive with the application of Modified Gompertz, Reaction Curve, and First Order models to biogas production. Diauxic growth patterns of biogas production of sonicated conventional waste demonstrates that the active time of digestion can be decreased through the conversion of less preferential substrates into existing, preferential substrates.