In Situ Sonification of Anaerobic Digestion: Extended Evaluation of Performance in a Temperate Climate

John Loughrin,Jason Simmons,Nanh Lovanh,Karamat Sistani,Stacy Antle

doi:10.3390/en13205349

John Loughrin, Jason Simmons + Show 3 more

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https://doi.org/10.3390/en13205349

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Abstract

Increasing the efficiency of anaerobic digesters and improving sludge breakdown is vital to reducing the cost of biogas production and reducing the environmental consequences of sludge disposal. The performance of two unheated anaerobic digestion systems, one exposed to sound at <20 kHz by waterproofed speakers and one acting as a control, were compared for over a year. The digester systems were both composed of primary (11.4 m3) and secondary (3.8 m3) anaerobic tanks, facultative tertiary (3.0 m3) tanks and an aerobic holding tank from which effluent was mixed with feed and recirculated back to the system. Exposure of the gas saturated digestate to a low frequency sine wave induced numerous bubble harmonics up to, and presumably beyond, ultrasonic range, showing that sonification of a highly gaseous liquid might be used to accomplish low power ultrasonication of digestate at greater distances than is possible with conventional ultrasonic technology. Through the summer of 2019, the sound-treated system produced 27% more biogas than the control system, and 74 times more during the winter when biogas production by the control systems essentially ceased. Afterwards, the control system produced more biogas due to depletion of volatile solids in the sound-treated digester. Results show that sound can be used for faster digester startup and substitute for a share of heating requirements during cool months.

Highlights

Waste generated by concentrated animal feeding operations (CAFO) is a challenging environmental problem
Ultrasonification of waste is generally limited to pretreatment of waste to reduce sludge particle size and partially solubilize the waste, as high-frequency sound is attenuated in proportion to distance much more rapidly than is low frequency sound
The possibility exists that ultrasonification of waste could be achieved in volumes otherwise impractical by conventional means by exploiting bubble harmonics

Summary

Introduction

Waste generated by concentrated animal feeding operations (CAFO) is a challenging environmental problem. In CAFO, sludge handling most often requires large volumes of water for flushing from feeding lots and housing, whereas in sewage treatment plants, excess generated sludge, composed of aerobic bacteria with complex cell walls and gelatinous extracellular matrices, often requires landfilling due to high concentrations of heavy metals [2]. In the former case, the flushed waste is typically stored/treated in facultative lagoons and subsequently applied to fields as fertilizer. This often leads to groundwater and surface water pollution due to leaching of organic matter and plant nutrients [3,4]

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