SLUDGE PARTICLE SIZE REDUCTION UNDER ALKALINE, ULTRASONIC, THERMAL, AND COMBINED PRETREATMENTS

Abstract

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. It is therefore one of important factors used to describe the effect of pretreatment on sludge.

This work aimed at investigating the evolution of PSR under individual and combined pretreatment methods, including ultrasound (US), thermal, and alkaline pretreatments. Different conditions of temperature and pressure were applied to compare and optimize the PSR efficiency. In addition, the relationship between sludge PSR and COD release in the aqueous phase was taken into detailed consideration for the first time. Apart from analysis of Mastersizer 2000 particle size distribution, effects of US and temperature rise on particle size and morphological parameters were also examined by image processing of sludge photographs from Morphologi G3.

US pretreatment showed the most predominant role in reducing sludge particle size compared to other methods. The variations of the volume moment mean diameter of different sludge types were dependent on ultrasonic specific energy input (ES). Moreover, the PSR was slightly improved in adiabatic conditions and applied pressure as compared to standard conditions. Besides, alkaline addition prior to US further accelerated the PSR. However, the strong reduction of mean particle size 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.