Executive review of hydrostatic pressure effects on sludge pretreatment by sonication

Abstract

The effect of hydrostatic pressure on sludge disintegration was studied for the first time using pressurized nitrogen in different operating conditions: sludge types, specific energy input ES, power input P_US, probe size, temperature T, and frequency F_S.

Optimum pressures (> 1 bar) were found as a function of acoustic intensity due to an increase in both cavitation threshold and cavitation intensity when increasing hydrostatic pressure. Under convenient hydrostatic pressure, the effect of US intensity I_US on DD_COD was much higher than that at atmospheric pressure. The most effective isothermal sonication would be high P_US, low F_S, convenient excess pressure, and adequate total solid concentration, i.e. 360 W (35 mm diameter probe), 12 kHz, 3.25 bar, and 28 g_TS/L, respectively according to our equipment possibilities. Besides, positive effects of pressure associated with high power adiabatic US were also achieved. The major result was that the optimum pressure depends on P­_US, I_US (or probe size), and T, but not on ES, 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.