Unpacking sludge flotation problems in sulfate-reducing upflow sludge bed reactors

Abstract

Sludge flotation and the consequent biomass loss is a long-standing issue that hinders not only the widespread implementation of the upflow anaerobic sludge bed (UASB)-type bioreactors, but also the development of anaerobic treatment biotechnologies such as the traditional methanogenesis and the emerging anammox and sulfidogenesis in wastewater treatment. To date, a fundamental understanding of anaerobic sludge flotation and feasible control strategies remain elusive. In particular, little information about sulfidogenic sludge flotation can be found in the literature. This PhD study investigates the underlying causes of and devises effective control strategies for the sludge flotation problem in Sulfate-Reducing Upflow Sludge Bed (SRUSB) reactors. Three lab-scale SRUSB reactors were operated in parallel with different modes of mixing (hydraulic, mechanical, and pneumatic) at various mixing intensity shear rates (γ) ranging from 0.7 to 6.6 s^-1. The sludge flotation potential (SFP) and its relationships with reactor mixing, reactor performance, and sludge properties were also studied. The intrinsic causality of sulfidogenic sludge flotation was elucidated as follows: improper reactor mixing (i.e. ineffective mode and/or insufficient intensity) altered the sludge extracellular polymeric substances (EPS) to be bulky in structure (i.e. indicated by a high loosely bound (LB)-EPS/tightly bound (TB)-EPS ratio) and poor in chemical composition (indicated by a high proteins (PN)/polysaccharides (PS) ratio). These changes in EPS successively induced the transitions of sludge properties to be highly adhesive to gas bubbles — that is, high hydrophobicity, high viscosity and weak negative surface charge. Consequently, these factors combined to promote gas entrapment in sludge and decrease the sludge density to less than that of water, finally triggering sludge flotation. The control of sulfidogenic sludge flotation was achieved by manipulating reactor mixing in this study. The continuous gas recirculation (CGR) with γ = 4.2 s^-1, which achieved the lowest SFP (11 ± 2%, lower than the critical safe value of 20%) in the comparison, was proposed as an optimal mixing strategy for controlling sludge flotation in the SRUSB reactors. The optimal mixing strategy was then applied in a SRUSB reactor operation for 150 days. The outcomes demonstrated that i) satisfactory reactor hydrodynamics (indicated by a low short-circuiting flow fraction of 1.3 ± 0.1% and a low dead zone fraction of 0.2 ± 0.01%) were achieved in the CGR-SRUSB; ii) the energy consumption of the CGR-SRUSB was only one-third of that in the traditional hydraulic-mixing SRUSB; and iii) the CGR-SRUSB sludge was transformed into micro-granules (300–350 μm) with high sulfidogenic activity (0.62 ± 0.05 g COD/(g MLVSS·day)), low sludge flotation potential (< 20%), and high settleability (SVI₅/SVI₃₀ < 1.3) within the initial 65 days, which was stably maintained for the rest of the 150 days. The CGR mixing successfully optimized the SRUSB through maintaining small-size granules, enhancing mass transfer efficiency, and sustaining low gas entrapment potential. The operational performance of two granular sludge CGR-SRUSB reactors was further examined for 30 days under two challenging conditions: the complete stoppage of wastewater (called complete food starvation) and a decrease of the influent sulfate concentration to a very low level (called sulfate starvation). The results show that i) 61% and 65% reductions in the specific removal rate of COD and 45% and 61% reductions in the specific sulfidogenic activity were caused by complete food starvation and sulfate starvation, respectively, in the CGR-SRUSB reactors; ii) the SFP of the complete-food-starved and sulfate-starved granular sludge increased from 14% to 58% and from 10% to 35%, respectively; and these performance deteriorations were found to be highly related to iii) starvation-induced unfavorable transitions in the granular sludge characteristics, including the formation of inner cavities, decreased mass permeability, decreased porosity, and increased surface adhesion. Nevertheless, the reactor restoration was quickly achieved after 10 to 15 days of reactivation operation in the two starved CGR-SRUSB reactors. The in-depth understanding of the sludge flotation problem and the proposed effective control strategy presented in this study can not only improve the stability and efficiency of SRUSB reactors but also can shed light on the optimization of other emerging UASB-type anaerobic biotechnologies.

Date of Award	2018
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology