Making Wastewater Treatment More Energy-Efficient

Arifur Rahman

To be more cost-effective and energy-efficient, cities increasingly are shifting their wastewater treatment plants toward energy-recovery systems. And the high-rate activated sludge system (HRAS) is becoming the backbone of energy-efficient wastewater treatment processes.

But not enough research has been done on the various carbon-removal technologies available. And there is a need to better understand and improve nitrogen and phosphorous management in HRAS systems.

Carbon removal technologies include the A-stage of AB (adsorption-biooxidation) systems, contact-stabilization (CS) and conventional systems, such as step-feed, plug flow and completely mixed reactor systems.

To further explore the benefits of two of those technologies, I worked with an international team during my PhD research (The George Washington University, DC Water, HRSD, Brown and Caldwell, ARA Consult GmbH) that performed a parallel comparison of the A-stage (adsorption) and high-rate contact-stabilization (CS) technology for carbon and nutrient redirection in treating high-strength raw wastewater. The study was conducted at the Chesapeake-Elizabeth Wastewater Treatment Plant in Virginia Beach, Virginia and the plant is operated by Hampton Roads Sanitation District (HRSD)

We wanted to understand carbon redirection and carbon harvesting (from high-strength domestic raw wastewater) performance of A-stage versus CS technologies; evaluate the potential of nitrogen and phosphorous redirection using each of those technologies; and determine how those results would influence HRAS technology selection.

We found that chemically enhanced primary treatment (CEPT), A-stage, CS, primary treatment + CS and CEPT + CS could all lead to energy-neutral plants because enough carbon can be redirected to generate the energy needed to support the wastewater treatment. But the best energy gain came through a combination of CEPT and CS.

The results show that it’s possible for HRAS systems to achieve efficient carbon management and effluent quality at existing facilities. We hope the study provides insight for future design and operation of energy-neutral wastewater treatment systems.

You can read more about our study in this article, which was published in the Chemical Engineering Journal, a top peer-reviewed journal in the Environmental and Chemical Engineering discipline: “A-stage and high-rate contact-stabilization performance comparison for carbon and nutrient redirection from high-strength municipal wastewater.”