Acoustic Stimulation of Anaerobic Digestion: Effects on Biogas Production and Wastewater Malodors

Abstract

Wastewater created from various solid wastes and agricultural residues was treated by anaerobic digestion, and the biogas and wastewater odors were quantified. One digester was exposed to low-frequency sound (<5 kHz) from underwater loudspeakers, while the other received no sonic treatment. It was hypothesized that low-frequency sound, by accelerating the breakdown of sludge via mechanisms such as cavitation induction and mechanical vibration, and enhancing biogas production, could also affect the concentrations of wastewater odors. During warm seasons, biogas production from the sound-treated digester was 29% higher than that from the control digester, and 184% higher during the cool season. Malodors—Mainly consisting of typical aromatic malodorants such as p-cresol and skatole, aliphatic secondary ketones, and dimethyl disulfide—were quantified. In contrast to the findings for biogas production, little difference was found in the concentrations of volatile compounds in the control and sound-treated digestates. Concentrations of dimethyl polysulfides increased over time in both the control and sound-treated digestates, likely due to the use of recycled system effluent that contained precipitated elemental sulfur. The digestate contained considerable concentrations of volatile fatty acids and ammonium, but due to the near neutral pH of the digestate it was surmised that neither made appreciable contributions to the wastewater’s malodor. However, the volatile fatty acid concentrations were reduced by sonic treatment, which was not unexpected, since volatile fatty acids are precursors to methane. Therefore, although sonic treatment of the anaerobic digestate boosted biogas production, it did not markedly affect the wastewater malodors. The biosynthetic origins of wastewater malodors are discussed in this paper.