Abstract
Effects of temperature (T) rise in conditions of isothermal, adiabatic sonication -US (the same power input -PUS), and sole thermal hydrolysis, then its effects associated with PUS for the same specific energy input (ES) and the same treatment duration were investigated. The main results were that the evolution of sludge T depended on PUS. In cases of the same ES (different PUS then different US duration), for the small probe, high PUS were still beneficial for sludge disintegration. However, for the big probe, a low disintegration efficiency was achieved at high PUS due to the high sludge T which leads to a significant damp of cavitation intensity. In cases of the same ES and treatment time, the sludge disintegration still benefited from high PUS if enough time was let for subsequent thermal hydrolysis. Therefore, the combined effect should be taken into account in optimization of US process: cavitation acts mainly during the early stage of the adiabatic US, then US being progressively damped by the increasing T, thermal hydrolysis takes over, being “boosted” by the initial work of US.
Highlights
The wastewater treatment via the activated sludge process produces a large amount of biomass, of which improper disposal generates a significant threat to ecosystems
It could be seen that (i) cavitation and thermal hydrolysis seem to show almost additional effects during the adiabatic Ultrasonic irradiation (US), (ii) thermal hydrolysis of early disrupted sludge is faster than that of raw sludge (Fig. 2B); the combined effect is more complex: cavitation acts mainly during the early stage of the adiabatic US, the US being progressively damped by the increasing T, thermal hydrolysis takes over, being “boosted” by the initial work of US
Effects of T were investigated in conditions of isothermal, adiabatic US, and sole thermal hydrolysis
Summary
The wastewater treatment via the activated sludge process produces a large amount of biomass, of which improper disposal generates a significant threat to ecosystems. Ultrasonic irradiation (US) is proved as a feasible and promising mechanical disruption technique for the sludge pretreatment according to the treatment time and power, equating to specific energy input (ES): efficient sludge disintegration, improvement in biodegradability and bio-solid quality, increase in biogas/methane production, no need for chemical additives, less sludge retention time, and the sludge reduction [1]. Theory-based, increasing temperature (T) will decrease the surface tension and raise the equilibrium vapour pressure of the medium, leading to easier bubble formation. These kinds of cavitation bubbles contain more vapors that reduce the US energy produced by the cavitation, reduce the amount of free radicals and mechanical effects. Great numbers of cavitation bubbles generating simultaneously will provoke the attenuation or dampening effect on the propagation of US energy from the emitter through the system [2]
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