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The performance of a novel double-tube ultrasound (US) reactor for waste activated sludge (WAS) pretreatment was assessed at a full-scale wastewater treatment plant (WWTP). For high transferability of the results, a well-performing WWTP with rather typical operating conditions was selected. The effects of the treatment (conducted at a specific energy input of 200 kJ per kg of total solids) were monitored regarding improvements in sludge viscosity, methane production, biosolids removal, and digestate dewaterability. The pretreatment caused a significant reduction of WAS viscosity (−5.8% on average, at p < 0.01) and a maximum yet insignificant increase in methane yield (+6.2%, at p < 0.1). No effect was observed for solids content, viscosity, or dewaterability of the digestate. The economic benefit of the reduced WAS viscosity was negligible, as the reduced pumping costs were less than 1‰ of the US reactor’s electricity costs. Additional methane yields enabled for partial cost recovery (roughly equivalent to the energy costs), while investment costs of the US equipment could not be regained. Yet, the incorporation of improved methane production in the economic assessment remains somewhat speculative, given the only insignificant increase. Results suggest that an economical application of ultrasonic WAS pretreatment is challenging for typical, well-performing WWTPs.

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