Investigating the Impact of Salinity Variations on Aerobic Granules and Activated Sludge Flocs: Emphasis on Performance Monitoring and Identification of Influential Salinity Components
Creators: name: Sivasubramanian, Raghavendran
Publisher: name: Manara - Qatar Research Repository
Year: 2024
Subjects: classificationCode: 40
subject: Engineering

description: Biological wastewater treatment processes can effectively treat large volumes of wastewater at relative economic advantage. However, their effectiveness is sensitive to high salinity conditions that may occur in many industries, where disrupted morphology, bioactivity and reduced treatment performance have been reported. These observations are more prevalent with unacclimated seed sludge. Moreover, most studies have been limited in the different salts used, reducing the ability to understand the characteristics of salinity (molar concentration, ionic strength, density etc) that leads to system failure. Hence, this study performed analysis of sludge cultivated directly from seawater to investigate the nature of varying salinity in hypersaline industrial wastewaters to gain further understanding of impacts from underlying salinity characteristics and mechanisms. The first part of this study investigated salinity impacts on aerobic granular sludge (AGS), a compact densely aggregated type of sludge formed through specific selection pressures that has seen wide uptake for industrial wastewater treatment. Granules were cultivated from seawater and acclimated to a saline mixture comprising of NaCl, KCl, and Na<sub>2</sub>SO<sub>4</sub> at a total salt concentration of 0.8 mol.L-1, similar to the seawater salinity. Salinity impacts were explored by switching to single salt media for each of the three salts at the same total molar salt concentration as the mixture. Shift to single salt media resulted in granule disaggregation, poor settling, sludge washout and development of fluffy or slimy flocs. Immediate increase in polysaccharide concentrations was observed where Na<sub>2</sub>SO<sub>4</sub> &gt;&gt; KCl &gt; NaCl and was the predominant reason for the large changes in sludge morphology. Consequent to the changes in the extracellular polymeric substances (EPS), there was an immediate decrease in sludge hydrophobicity resulting in a deterioration of settling, indicated by an increase in the sludge volume index (SVI), and loss of biomass for all the reactors. Cell viability remained unaffected for the reactors operating under single salt solutions of NaCl, while there was a significant reduction in live viabilities following the change to KCl and Na<sub>2</sub>SO<sub>4</sub> solutions to ~ 50%. For both KCl and Na<sub>2</sub>SO<sub>4</sub> reactors the TOC removal also dropped to around 10% for at least 15 days after the change. The final TOC removal measured ~ 70% for both the reactors compared to ~ 95% attained for the NaCl reactors. A subsequent study was performed to identify the most influential salinity components across all the performance indicators of the reactors using the same seawater and mixed salt cultured seed sludge. Each reactor was subjected individually to one of NaCl, KCl or Na<sub>2</sub>SO<sub>4</sub> at either equal molar (0.8 mol.L<sup>-1</sup> each) or equal ionic strength (1.57 mol.L<sup>-1</sup>) concentrations to the culture condition. The resulting changes on performance monitoring variables were regressed as dependent variables against a multitude of salinity components as predictors. Ionic strength affected the morphology the most, where ionic strength disrupted the production of EPS components leading to increased negative sludge surface charge, poorer sludge adhesion and eventually significant reduction in sludge settling abilities. Ionic strength correlated strongly with polysaccharide to protein ratio (R<sup>2</sup>adj = 0.98), which in turn correlated with zeta potential, hydrophobicity and various shape descriptors (R<sup>2</sup>adj = 0.7-0.97). Tonicity, caused by changes in monovalent cation concentrations, were responsible for the decrease in cell viability and biomass respiration activities (R<sup>2</sup>adj = 0.72 and 0.96, respectively). A three-way mechanism is described for salinity impacts caused by tonicity changes on cellular viability and activity, ionic activity on EPS production and morphology and combined effect of morphology changes and substrate density increase on settling and biomass retention.The final component of the thesis was to determine whether the same benefits of divalent cation addition, reported at low salinity concentrations, extend to hypersaline conditions. The ratio of monovalent over divalent cation concentrations (M/D) was varied, and the corresponding effects were investigated on the performance evaluators. Nine sequencing batch reactors were operated at a constant monovalent cation concentration of 0.8 mol.L<sup>-1</sup>, five reactors containing Na+ and four containing K+ cations, to which divalent cations (Ca<sup>2</sup><sup>+</sup> and Mg<sup>2</sup><sup>+</sup>) were varied at increasing proportions. Increase in M/D ratio led to decreased polysaccharide production, higher hydrophobicity and lower SVI irrespective of the type of monovalent cation in the reactor. All ten performance indicators were affected by variations in the M/D ratio. Even small improvements in M/D ratio, equivalent to addition of a few hundred mg/L of the divalent ion, realized significant improvements in system performance and sludge structure. The specific monovalent cation present was influential on the changes observed in EPS and morphology. The dominant type of divalent cation only influenced sludge settling, where it was found Mg<sup>2</sup>+ was more beneficial than Ca<sup>2</sup>+. Overall, this study shows that sludge is highly sensitive to ionic strength and tonicity fluctuations, and that limited addition of divalent cations can greatly benefit high salinity activated sludge systems by improving the sludge structure, enhancing settling, and reducing oxygen requirements.