Sound enhances wastewater degradation and improves anaerobic digester performance

Abstract

Biogas production and wastewater quality from anaerobic digesters were studied to determine whether sound at sonic frequencies (< 20,000 Hz) could enhance their performance. In three trials with increasing waste loading rates, each of 100-day duration, the performance of control and sound-treated digesters was compared. Anaerobic digesters exposed to sound produced approximately 12% more biogas than did non-exposed digesters, and sound-treated digestate had significantly lower chemical oxygen demand. Sludge at the end of the 100-day digestion averaged 19% less carbon and 18% less nitrogen in sound-treated digesters as compared to sludge from untreated digesters. Although the mechanism(s) responsible for enhanced biogas production due to sound exposure are unknown, recordings of sound-treated digesters indicate that acoustically induced cavitation may play a role.

In Situ Acoustic Treatment of Anaerobic Digesters to Improve Biogas Yields

“…Furthermore, sludge from the digesters exposed to sound was more degraded than that of control digesters. Although we could not conclusively determine the mechanisms involved, we did observe bubble harmonics indicative of acoustic cavitation (as discussed below), and at infrequent intervals, broad-band noise indicative of shock waves propagated from cavitation cloud collapse [5].…”

“…The first series represents stable or acoustic cavitation with linear bubble oscillations, whereas the latter series is indicative of chaotic oscillations and incipient bubble collapse. In the earlier investigation [5], the appearance of ultraharmonic peaks was a prelude to broadband noise, indicating widespread cavitation cloud collapse [9]. The acoustic emissions due to bubble harmonics, ultraharmonics, and cavitational collapse may be quite intense.…”

“…Previously [5], wastewater was exposed to 1000-Hz sine waves as a means of enhancing biogas production, as well as sludge breakdown. One of the interesting aspects of this study was that the bubble harmonics of the excitation frequency were evident in acoustic spectra of the wastewater, often followed by subharmonics symptomatic of incipient inertial cavitation.…”

“…Recently, we investigated if sound could also be used to enhance anaerobic digestion by accelerating sludge breakdown and thereby improve biogas yields [5]. Phenomena that sound waves could induce in wastewater include, but are not limited to, mechanical vibration of the wastewater and sludge [6], acoustic streaming of the liquid to enhance nutrient mixing [7], and induction of acoustic cavitation in which bubbles oscillate in the presence of an applied acoustic field [8,9].…”

“…Phenomena that sound waves could induce in wastewater include, but are not limited to, mechanical vibration of the wastewater and sludge [6], acoustic streaming of the liquid to enhance nutrient mixing [7], and induction of acoustic cavitation in which bubbles oscillate in the presence of an applied acoustic field [8,9]. This experiment was conducted by contrasting the performances of control and sound-treated digesters in three 100-day trials [5]. The sound-treated digesters were exposed to sound at sonic frequencies (<20,000 Hz) by means of underwater speakers.…”