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Energy optimization of secondary treatment in WRRFs via off-gas and respirometric measurements


Water resource recovery facilities (WRRFs) are responsible for a significant portion of the municipal energy consumption and related GHG emissions. Most of the energy demand for water sanitation (up to 80%) is commonly required in the secondary stage of the treatment, where aeration is provided to the tank through a range of technological solutions (mechanical aerators, coarse to fine bubbles diffusers, etc.) to ensure aerobic conditions in the aerated stage of activated sludge process (AS).

Despite the high cost of aeration, the best science describing the mechanics of oxygen transfer is affected by dynamic process conditions has not yet been fully integrated into the considerations for control, design, and modelling of secondary treatment in WRRFs. First, effluent quality and process stability are prioritized over the cost of treatment; second, the capability of existing facilities to adapt the air supply to transient oxygen demand over the daily fluctuation is often limited by dated design and/or equipment; third, whereas energy rates are often time-based, this results in the highest cost of treatment correspondent to the lowest efficiency in air use from the blowers.

Hence, the objective of the dissertation is to investigate the potential of continuous characterization of process dynamics and aeration efficiency indicators as a diagnostic tool to optimize energy efficiency in WRRFs. Extensive off-gas and respirometric measurements were used to define temporal and spatial fluctuations of aeration dynamics in secondary treatment and correlate biological oxygen uptake rate (OUR) with oxygen transfer efficiency (OTE).

Additional insights were provided by the adopted methodology, as the coupling of online respirometric and off-gas monitoring with power metering allowed to create redundancy in the measurement of OUR and power demand for aeration. This ultimately served to extract more information about oxygen supply efficiency and mechanical efficiency of the blowers, compared to the standard practice.

Overall, the results confirmed how significant reduction of power demand for aeration (20-50%) can be achieved while ensuring effluent quality by strategically modifying process operations (carbon diversion in primary treatment, DO reduction in secondary treatment, flow equalization, etc.).

In addition, this study finds that better understanding of site-specific process dynamics using a validated and optimized WRRF model can significantly reduce aeration power demand. Finally, an approach to characterize process operations and highlighting optimal and critical points through the analysis of key state variables is proposed for design, modeling and process control of WRRFs secondary treatment.

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