Sludge particle size reduction (PSR) via pretreatment stage has been believed to accelerate the hydrolysis of sludge anaerobic digestion and enhance the degradation of organic matters. This work aimed at investigating the evolution of PSR under individual and combined pretreatment methods, including ultrasound (US), thermal, and alkaline pretreatments. In addition, the relationship between sludge PSR and COD release in the aqueous phase was taken into detailed consideration for the first time. US pretreatment showed the most predominant role in reducing sludge particle size compared to other methods. The reduction of the volume moment mean diameter of sludge (D[4,3]) were dependent on US specific energy input (ES) (68-77 %). Moreover, the PSR was slightly improved in adiabatic conditions and applied pressure (about 10 %). Alkaline addition prior to US also further accelerated the PSR. However, the strong D[4,3] reduction observed in the early stage of the process (low ES) was insufficient to affect COD solubilisation. It is therefore necessary combine with other factors (chemical, biological parameters), not only base on PSR for process optimization.
Related to ultrasonic (US) pretreatment of sludge, changing the hydrostatic pressure will change the resonance condition of cavitation bubbles and then may drive the system toward resonance conditions, consequently increase the rate and yield of reactions. Nevertheless, nearly all the US experiments had been carried out at atmospheric pressure. Only a few studies had been focusing on how increasing static pressure affects cavitation. The effect of hydrostatic pressure on sludge disintegration was studied for the first time in the last few years. This work aimed at reviewing the effect of pressure on sludge ultrasonic pretreatment efficiency in different conditions. The major result was that the optimum pressure depends on power input -P US , intensity -I US (or probe size), and temperature -T, but not on specific energy input -ES, frequency -F S , nor sludge type. In general, sludge disintegration efficacy was significantly improved by sonication at the optimum pressure, especially at low ES, leading to a potential of energy input savings in sludge sonication pretreatment, but also in most of ultrasound assisted processes.
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