Increasing oxygen transfer efficiency through sorption enhancing strategies.

The link between aeration efficiency and biosorption capacity in water resource recovery facilities was extensively investigated, with special emphasis on wastewater characteristics and the development of strategies to maximize adsorption. Biosorption of oxygen transfer inhibitors (i.e., surfactants, colloidal, and soluble fractions) was examined by a series of pilot batch-scale experiments and full-scale studies. The impact of a sorption-enhancing strategy (i.e., bioaugmentation) deployed at full-scale over a five-year period was evaluated. Bench-scale experiments determined the inhibition coefficient (Ki) to measure the impact of surfactants and COD fractions as inhibitors of oxygen transfer efficiencies (αSOTE) in wastewater systems. The inhibition constant for surfactants Ki was found at 2.4 ± 0.4 mg L-1 SDS while for colloidal material was at 14 ± 1 mg L-1 (no inhibition for soluble fraction was found). Two enhancing biosorption configurations (i.e., contact stabilization and anaerobic selector) resulted in significant improvements in both aeration efficiency indicators (αSOTE) and surfactants removals. αSOTE improvements of 46% and 54% in comparison to conventional high rate activated sludge process (HRAS) were reported. Similarly, the removal of surfactants was increased by 27% and 56% using optimized enhancing-sorption strategies. Further analyses helped elucidate the underlying mechanisms of surfactants removal. Findings are expected to help full-scale applications increase their sorption potential as well as the concurrent aeration efficiency, which helps WRRFs to advance toward energy-positive wastewater treatments.

Reverse flexing as a physical/mechanical treatment to mitigate fouling of fine bubble diffusers.

Achieving energy neutrality has shifted focus towards aeration system optimization, due to the high energy consumption of aeration processes in modern advanced wastewater treatment plants. A study on fine bubble diffuser fouling and mitigation, quantified by dynamic wet pressure (DWP), oxygen transfer efficiency and alpha was carried out in Blue Plains, Washington, DC. Four polyurethane fine bubble diffusers were installed in a pilot reactor column fed with high rate activated sludge from a full scale system. A mechanical cleaning method, reverse flexing (RF), was used to treat two diffusers (RF1, RF2), while two diffusers were kept as a control (i.e., no reverse flexing). There was a 45% increase in DWP of the control diffuser after 17 months of operation, an indication of fouling. RF treated diffusers (RF1 and RF2) did not show significant increase in DWP, and in comparison to the control diffuser prevented about 35% increase in DWP. Hence, reverse flexing potentially saves blower energy, by reducing the pressure burden on the air blower which increases blower energy requirement. However, no significant impact of the RF treatment in preventing a decrease in alpha-fouling (αF) of the fine pore diffusers, over time in operation was observed.

Modelling the link amongst fine-pore diffuser fouling, oxygen transfer efficiency, and aeration energy intensity.

This research systematically studied the behavior of aeration diffuser efficiency over time, and its relation to the energy usage per diffuser. Twelve diffusers were selected for a one year fouling study. Comprehensive aeration efficiency projections were carried out in two WRRFs with different influent rates, and the influence of operating conditions on aeration diffusers' performance was demonstrated. This study showed that the initial energy use, during the first year of operation, of those aeration diffusers located in high rate systems (with solids retention time - SRT-less than 2 days) increased more than 20% in comparison to the conventional systems (2 > SRT). Diffusers operating for three years in conventional systems presented the same fouling characteristics as those deployed in high rate processes for less than 15 months. A new procedure was developed to accurately project energy consumption on aeration diffusers; including the impacts of operation conditions, such SRT and organic loading rate, on specific aeration diffusers materials (i.e. silicone, polyurethane, EPDM, ceramic). Furthermore, it considers the microbial colonization dynamics, which successfully correlated with the increase of energy consumption (r2:0.82 ± 7). The presented energy model projected the energy costs and the potential savings for the diffusers after three years in operation in different operating conditions. Whereas the most efficient diffusers provided potential costs spanning from 4900 USD/Month for a small plant (20 MGD, or 74,500 m3/d) up to 24,500 USD/Month for a large plant (100 MGD, or 375,000 m3/d), other diffusers presenting less efficiency provided spans from 18,000USD/Month for a small plant to 90,000 USD/Month for large plants. The aim of this methodology is to help utilities gain more insight into process mechanisms and design better energy efficiency strategies at existing facilities to reduce energy consumption.