Improving sewage sludge ultrasonic pretreatment under pressure by changing initial pH

This work aimed at understanding the combined effect of sludge pH, temperature, and external pressure on the efficiency of sewage sludge ultrasound (US) pretreatment. Based on the evolution of both the degree of sludge disintegration (DDCOD) and pH, application of 40 mgNaOH/gTS during 30 min was selected for chemical pretreatment. Mechanical and thermal effects induced by cavitation contributed in similar proportion to sludge disruption, but the role of the latter effect tended to be weakened after mild alkalisation of sludge. When applying external pressure, DDCOD was always improved, by about 10% at the optimal value of 2 bar. The optimal combination was an addition of 40 mgNaOH/gTS prior to adiabatic sonication at 2 bar, resulting in a DDCOD value of about 46% at 75,000 kJ/kgTS (as compared to 35% for sole US) for the investigated mixed sludge. Very short time US application yielded a drastic reduction of the volume mean particle size, mainly due to the erosion and disruption of large flocs (>90 μm), yet this was not sufficient to initiate significant subsequent COD solubilisation under stirring.