Transformation and utilization of solubilized substances from pretreated sludge

• Main Author: Lu, Dan
• Other Authors: Zhou Yan
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Environmental engineering
• Online Access: https://hdl.handle.net/10356/143889
• Institution: Nanyang Technological University

Waste activated sludge (WAS) is not only a potential environmental pollution but also an alternative resource. Technologies that could achieve both sludge reduction and resource recovery are studied in this Ph.D project. The primary aim of this research is to investigate and utilize the solubilized substances produced from sludge pretreatment. Pretreatment methods employed in this thesis are alkaline (ALK), ultrasonic (ULS) and thermal hydrolysis pretreatment (THP). Firstly, the organics released by ALK, ULS and combined ALK-ULS pretreatment as well as their transformation in anaerobic digestion (AD) are investigated. ALK-ULS pretreated sludge has the maximum methane production. However, such pretreatment releases not only easily biodegradable substances but also more recalcitrants. Thus, more residual dissolved organic matters (DOMs) are detected after digestion. The steroid-like matters, alkanes and aromatics are the main components of the residual low molecular weight (LMW) DOMs. With the investigation of higher molecular weight (MW) residuals, recalcitrant compounds like flavonoids and benzene derivatives are detected. The results suggest that further treatment should be considered to reduce these refractory residuals. Secondly, DOMs in THP sludge liquor and their transformation during AD are characterized. Sludge with 172 °C treatment shows the maximum methane production due to the significant release of LMW DOMs. The effluent from THP sludge digesters contains more DOMs residues. Over half of the residual LMW components are steroid-like compounds and aromatics. Further profiling of higher MW residuals also detects recalcitrant compounds. Therefore, polishing step should be considered to reduce the recalcitrant residuals in THP AD liquor. When the liquid and solids fractions of THP sludge (THP-L and THP-S) are separately digested, most of organics in THP-L can be removed within 15 days, while THP-S only contributes 31.0% to the total methane production. In order to improve the total resource recovery from THP sludge, an integrated process, namely, methane generation from THP-L coupled with biochar production from THP-S is proposed, which results in an energy surplus. Additionally, THP-L digested sludge has better dewaterability and reduced viscosity. The proposed new sludge treatment process therefore has lower operating cost and higher value returns. Finally, the feasibility of using THP-L as fertilizer is evaluated. With 165 ºC treatment, the contents of nitrogen, phosphorus and free amino acids (FAAs) in the THP-L are comparable to the commercial fertilizer. Especially, plant growth-promoting compounds (e.g. FAAs and indole-3-acetic acid) can be generated under this temperature. More importantly, THP-L products (< 180 ºC) do not have inhibition but have promotion effect on the plant and soil bacteria growth. By comparing nutrients content, organics content, the potential phytotoxicity and inhibition effect, it is concluded that THP-L from 165 ºC treatment is expected to be a valuable organic fertilizer. The findings lead to a new resource recovery approach and help to solve sludge treatment problem. Overall, this thesis provides new and detailed insights into the transformation of DOMs during sludge pretreatment and AD, proposes new approaches to utilize these DOMs, offers new direction for sludge treatment and resource recovery.