Acoustic stimulation of anaerobic digestion: Effects on biogas production and wastewater malodors

Authors: Loughrin, John; Silva, Philip - Phil; Lovanh, Nanh; Sistani, Karamat

Submitted to: Environments
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/10/2022
Publication Date: 8/15/2022
Citation: Loughrin, J.H., Silva, P.J., Lovanh, N.C., Sistani, K.R. 2022. Acoustic stimulation of anaerobic digestion: Effects on biogas production and wastewater malodors. Environments. 9(8). Article 102. https://doi.org/10.3390/environments9080102.
DOI: https://doi.org/10.3390/environments9080102

Interpretive Summary: Gas production and offensive odors were measured from two anaerobic digesters used to treat wastewater and produce useful biogas for energy production. One digester was treated with low frequency sound (less than 5,000 cycles per second) to accelerate gas production and wastewater treatment and the other digester served as a control. The study was conducted in the spring to fall of 2018, and the fall of 2019 until the summer of 2020. During warm weather gas production from the sound treated digester was 29% higher than that of the control digester and 184% higher during the cool season. Malodors, typical of those contained in animal manures and sewage were measured in the wastewater. In contrast to the findings for gas production, little difference was found in the concentrations of malodors in control and sound treated digestate. Concentrations of offensive smelling long chain sulfur containing compounds increased over time in both control and sound treated digestate likely due to using recycled system discharge water that contained elemental sulfur. The digestate contained considerable malodorous organic acids and ammonia, but at the near neutral pH of the wastewater both the acids and ammonia were mostly in their ionized forms, and therefore nonvolatile. Thus, it was judged that neither short chain acids nor ammonia made much contribution to the wastewater odor. Although sound treatment of the anaerobic digestate boosted biogas production, it did not markedly affect wastewater malodors. Still, this study demonstrated an inexpensive yet effective means of enhancing energy production from animal waste and for treating wastewater.

Technical Abstract: Biogas production and wastewater malodors were quantified from anaerobic digesters. One di-gester was exposed to low frequency sound (< 5 kHz) to accelerate biogas production and the other digester served as a control. The study period encompassed the spring to early fall of 2018, and the fall of 2019 until the summer of 2020. During warm seasons biogas production from the sound treated digester was 29% higher than that of 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 control and sound treated digestate. Concentrations of dimethyl polysulfides increased over time in both control and sound treated digestate likely due to using recycled system effluent that contained precipitated elemental sulfur. The digestate contained considerable volatile fatty acids and ammonium, but due to the near neutral pH of the digestate it was surmised that neither made appreciable contributions to wastewater malodor. Therefore, although sound treatment of the anaerobic digestate boosted biogas production, it did not markedly affect wastewater malodors. The biosynthetic origins of wastewater malodors are discussed.