Capturing Energy in Wastewater Treatment Plants

Sept. 1, 2012
We've heard of the "water energy nexus" but few recognize its importance. If the public were fully aware of its significance, we would see many more "green" projects than we are witnessing now.

By Leelon Scott

We've heard of the "water energy nexus" but few recognize its importance. If the public were fully aware of its significance, we would see many more "green" projects than we are witnessing now. I think full comprehension would result in dramatic, concern-driven actions to address this critical connection of two incredibly valuable resources.

On the subject of water supply, we look at the amount of water in the world and breathe easy, thinking enough water is available. To whatever degree this may be true, the full cost to obtain it is not at all obvious. We have been oblivious to this cost due largely to government subsidies masking the true cost of water, and the true costs to treat wastewater. If we removed the subsidies and paid the true costs, our opinions would change drastically. Water-saving and energy-saving movements would become widespread and urgent.

On the wastewater side, the energy potential contained in wastewater and its biosolids/biogases exceeds by 10 times the energy used to treat it. (Source: Water Environment Research Foundation) Some WWTPs produce 100 percent or more of the energy they need to operate, and WWTPs collectively could potentially meet 10 percent of the national electricity demand. (Source: Water Environment Research Foundation) This represents a tremendous opportunity for the wastewater industry.

In life, we reach points where something has to change, and this nexus of (waste)water and energy is such a point. Our collective ability to understand the costs and act accordingly will be vital to assuring clean water supplies well into the future.

I've been in the municipal water and wastewater market for nearly 30 years. It amazes me how little attention energy use has received over the thousands of projects I've been involved in. We in this market understand our involvement in environmental protection, so the recent green movement pleases our psyche. The implementation of the term "green" means a lot to our mother earth and to our children's children. Our responsibility is to carry the flame and design away the unnecessary costs in wastewater treatment with innovative designs, which start mainly with awareness of energy costs.

I've had an opportunity to ask operators, "How much is your power bill?" Often times I get blank stares or replies such as, "That's the city's problem". The operator's main responsibility is safe and effective operations of their plant and to ensure clean drinking water or properly processed wastewater. Energy costs are understandably not a priority for them. But they need to be a priority for the municipalities and the engineers that design the plants.

We as environmentalists need to determine a way we can unite both the plant function and its energy management. I feel that through awareness, education and incremental measures, water resource management can be improved and energy use reduced significantly.

Few of us know that thermoelectric power plants use 41% of our available fresh water to produce electricity. (Source: National Energy Technology Laboratory) Thermoelectric generation is expected to increase 18% from 2005-2030, putting even more strain on water supplies. (Source: Water Management in 2010 and Beyond) It's also not well known that a thermoelectric power plant consumes 300—400+ gallons per megawatt of power generated (Source: U.S. Department of Energy, 2006 Report to Congress of the Independency of Energy and Water, Washington D.C.) and that power plant construction in some areas is being held up due to water scarcity. (Source: Energy Law Journal , Vol. 30:11)

A wastewater plant's largest energy consumption is in aeration, by as much as 60%. (Source: Consortium for Energy Efficiency, 2007) This being the case, it is obvious that plants' collective energy costs across the country will continue to rise with the increase in treated volumes. Thus, the crisis of the water energy nexus: that growing power production places greater strains on water supplies and more wastewater treatment consumes more energy.

WWTP effluent has been successfully used as cooling water for power plants. It's also known that 81% of power plants proposed for construction have one or two WWTPs within a 10-mile radius, and those plants could provide a sufficient cooling water supply. Further, 97% could meet their cooling needs with one to two WWTPs within 25 mile radius. (Source: U.S. DOE Study, Reuse of Treated Internal or External Wastewaters in the Cooling Systems of Coal-Based Thermoelectric Power Plants)

With awareness and knowledge of the facts, municipalities can look to site WWTPs near existing or proposed power plants, with the potential to sell the clean effluent to the plants for cooling water. Biosolids and/or biogases can be recovered and sold to those same plants as fuel. Or they could be used on site to generate power to run the WWTP. New separation technologies are capable of extracting phosphates from the waste stream, which can be sold to manufacturers. It's also possible that power plants could own and operate adjacently sited WWTPs, gaining direct access to water & fuel outputs while also relieving cities of the capital expense of building a plant. The options are numerous.

There are also many options for reducing energy usage within a plant. Whether it be simple awareness, replacing older equipment with more energy efficient units, or modifying on-off cycling of equipment, savings can be significant. Benefits from investing in energy savings can come from Federal and State assistance programs or simply the savings on a treatment plant's power bill, all without impacting the treatment quality.

Energy and water are tightly connected, and WWTPs are at the heart of that nexus. Awareness of that relationship is the starting point. We must innovate by moving toward zero discharge facilities, where zero discharge for this purpose means completely recycling/reusing a plant's outputs. We must educate plant personnel on energy usage; improve process operations; and lower a plant's carbon footprint.

Through all of this, our country's WWTPs can do much more than just save energy, they can actually become cash cows from the tremendous energy potential contained in wastewater. It's imperative that we move forward on this process.

About the Author: Leelon Scott serves on the WWEMA Board of Directors and is Director of OEM Business Development for Revere Control Systems, a Birmingham, Alabama-based independent control system integrator providing complete turnkey automation, communication, and control systems for water and wastewater equipment manufacturers.

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