Evaluating the effect of ultrasonication on sewage biodegradation by measuring particle size distribution

Abstract

Application of ultrasonication in optimum conditions as a pretreatment step before activated sludge process could increase the readily biodegradable fraction of wastewaters, but optimization of ultrasonication requires a reliable and simple control parameter. There is a direct relationship between biological degradation rate and particle size of organic substrates and distribution of chemical oxygen demand (COD) in terms of particle size in a wastewater can be determined by particle size distribution (PSD) analysis. The objective of this study is to investigate the applicability of PSD as a tool for monitoring the effect of ultrasonication on the biological treatability of municipal wastewaters. A municipal wastewater sample taken from an activated sludge process influent with high particulate COD (75.4 of 690 mg/L total COD) was subjected to ultrasonic pretreatment with 35 kHz frequency and 0.25 W/mL constant specific power for 2, 4, and 8 min reaction periods. After pretreatment, PSD of each ultrasonicated wastewater sample was determined by sequential filtration-ultrafiltration using eight filters with different pore sizes between 700 and 2 nm. The particulate COD sourced from the organic particles above 700 nm decreased gradually with increased ultrasonication periods from 520 to 326 mg/L (47.2 of total COD), while soluble COD (2 nm particle size) increased from 85 (12.3) to 142 mg/L (20.6). The experimental results demonstrated that PSD analysis can reflect the disintegration of organic particles by ultrasonication and there is a linear relationship between sonication period and COD concentration variations of particle size intervals. Therefore, PSD analysis can be used as a reliable tool for determination of optimal ultrasonication conditions.