Effects of chemically assisted ultrasonication treatments on anaerobic digestion of sewage sludge

• Main Author: Tian, Xinbo
• Other Authors: Ng Wun Jern
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Environmental engineering
• Online Access: https://hdl.handle.net/10356/144043
• Institution: Nanyang Technological University

Sewage sludge is a problematic by-product of wastewater treatment. Due to the high organic and pathogenic content, it needs to be stabilized before safe disposal. Anaerobic digestion is generally applied to stabilize sewage sludge, because it not only reduces the organic content in sludge but also generates biogas which can be used for energy supply. However, anaerobic sludge digestion is a time-consuming process because hydrolysis of particulate organics in sludge limits the overall anaerobic digestion rate. Sludge treatment is often integrated with an anaerobic digester to solubilize the solids as well as to enhance the overall anaerobic digestion performance. Sludge treatment can be integrated in two different configurations. It can be applied to feed sludge before anaerobic digestion as pre-treatment. Alternatively, it can be achieved by treating the digested sludge after anaerobic digestion and recycling the treated digested sludge to the original reactor, which is known as post-treatment. In this study, the effects of individual ultrasonication (ULS), sequentially combined ULS and ozonation (ULS-Ozone) and simultaneously combined ULS and alkaline (ULS+ALK) treatments on feed sewage sludge (i.e. pre-treatment) and digested sludge (i.e. post-treatment) were investigated. ULS pre-treatment showed limited performance in solubilizing feed sewage sludge and increasing the sludge biodegradability because it is essentially “single” effect - mechanical disintegration. Synergistic VSS solubilization was observed in feed sludge after the ULS-Ozone pre-treatment. In addition, some of the organics solubilized by ultrasound can be further broken down by ozone, resulting in solubilization of low molecular weight (MW) components (MW < 27 kDa). The ULS+ALK pre-treatment not only induced synergistic COD solubilization but also generated smaller organics with MW around 5.6 kDa in feed sludge. The biogas production increase following anaerobic digestion of ULS, ULS-Ozone, ULS+ALK treated feed sludge with semi-continuous anaerobic reactors were 20.7%, 35.5% and 24.6%, respectively at SRT of 10 days. ULS-Ozone and ULS+ALK post-treatments were found to result in synergistic COD solubilization in digested sludge. Despite the similar extent of COD solubilization (from 200 mg/L to around 2,600 mg/L), the solubilization products were different after the ULS-Ozone and ULS+ALK treatments of digested sludge. The ULS+ALK treatment of digested sludge primarily solubilized macromolecules with MW over 500 kDa. However, the ULS-Ozone treatment of digested sludge not only solubilized macromolecules with MW higher than 500 kDa but also released organics with MW around 103 kDa. The biogas production from the semi-continuous anaerobic digesters increased by 9.8%, 10.7%, 17.8% after the ULS, ULS-Ozone and ULS+ALK treated digested sludge were recycled to the anaerobic digester, respectively at SRT of 20 days and recycle ratio of 5%.