Understanding the stability of cyclic aerobic-anoxic conditioned sludge flocs under deflocculative stresses

• Main Author: Suresh, Akshaykumar
• Other Authors: Ng Wun Jern
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Environmental engineering::Water treatment
• Online Access: https://hdl.handle.net/10356/137573
• Institution: Nanyang Technological University

Biological wastewater treatment with activated sludge followed by a liquid-solids separation step has been an established process for over a century. Effective liquid-solids separation can be made through biological or chemical flocculation. Biological flocculation involves the aggregation of organic and inorganic components into flocs through the action of microbes and their by-products, i.e. extracellular polymeric substances (EPS). Adverse conditions can promote floc break-up, i.e. deflocculation, reducing effluent quality and increasing overall treatment costs. It is essential to have a better understanding of the biological flocculation process and factors influencing it, especially in cyclically conditioned (aerobic/anoxic/anaerobic) treatment processes. The exposure to low dissolved oxygen (DO) (<0.5 mg/L) conditions has been reported to promote deflocculation. However, in this dissertation studies, sludge from three different sources repeatedly showed a lack of deflocculation even after 24 hours subject to low DO and substrate depletion. Divalent cationic interactions, high bound EPS content and the activities of facultative bacterial communities were factors argued to have contributed to increased floc integrity, and which helped mitigate the deflocculation under low DO conditions. The importance of facultative bacterial communities on floc maintenance and mitigation of deflocculation under low DO conditions was confirmed through microbial inhibition experiments. Deflocculation, in the form of floc erosion, was observed under low DO conditions only when microbial inhibitory conditions were promoted using a mitochondrial uncoupler, sodium azide. Sodium azide also exhibited interactive tendencies with the EPS, which was confirmed through EPS fluorescence quenching studies and this was another mechanism through which floc integrity could be influenced. It was the microbial communities, their activities and metabolic by-products (EPS), and not necessarily the short-term DO transients, that influenced the flocculation state. The role of cations and their interactions in the stabilization of floc ecosystem was also confirmed when deflocculation occurred upon interaction of sludge samples with a chelating agent, ethylenediaminetetraacetic acid (EDTA). The chelation of calcium and magnesium ions from the floc body not only reduced the floc stabilizing ionic interactions, but also resulted in hampered microbial viability and activities. Consequently, irreversible deflocculation was observed above a threshold concentration of the chelating agent. Pre-complexation of EDTA with calcium ions counteracted its cation chelating, anti-bacterial and deflocculative tendencies. This highlighted the cations played a more important role in the overall floc ecosystem than previously considered, which helped maintain SBR sludge floc stability and integrity under different DO conditions. The results of this study confirmed the importance of overall microbial activities (including facultative bacteria) and cationic interactions in the maintenance of the floc ecosystem while disagreeing with the importance of short-term low DO transients. Unless a considerable adversarial stress was applied on the microbial communities, the EPS or cationic interactions, the sludge flocs would remain stable in a short-time disruption event. Thus, biological floc maintenance is a complex process that should be investigated relative to the conditions the flocs are most exposed to on a long-term basis.