In a way, wastewater treatment plants have the solution to their energy needs flowing in all the time.
Compared to its drinking water counterpart, the wastewater side’s influent is rich with nutrients and biosolids which pose a lucrative opportunity only now being tapped. It gives plants the chance to achieve energy net-zero.
A plant that has reached energy net-zero, or neutrality, generates all of the energy needed for operation solely from what’s available in the water and waste that it treats.
Leading the charge towards this closed loop is the Water Environment Research Foundation (WERF), which has embarked on several research studies for energy production and efficiency to increase the number of treatment plants that are energy neutral. WERF is so laser-focused on advancing sustainability and net-zero that its representatives adamantly refer to wastewater treatment plants as “water resource recovery facilities” or WRRFs.
“Only five or six years ago, most professionals in the water sector thought that operating a WRRF to produce enough energy to meet the plant’s energy demand was next to impossible,” said Lauren Fillmore, Senior Program Director at WERF. “Now, I am currently aware of a dozen WRRFs in the U.S. that are at energy net-zero and a few overseas that produce several times more energy than they consume. What was once considered impossible is achievable, with both business case and environmental benefits.”
The U.S. EPA estimates that electricity use accounts for 25 to 40 percent of the operating budgets for wastewater utilities. These financial costs are coupled with the less precise toll that wasteful energy consumption is taking on our planet.
“Climate change is real and the long-range impacts could be catastrophic,” warned Ralph Eschborn, PE with AECOM and a collaborator in WERF’s energy neutrality case studies. “Minimizing a facility’s carbon footprint and maximizing the production of renewable energy will be more than a good idea or the right thing to do.”
Given these financial and environmental conditions, it’s no surprise that the potential for achieving net-zero has the industry’s attention. The “how” is a different story.
A webinar titled “Net-Zero Energy Solutions for Water Resource Recovery Facilities” presented by WERF and led by Eschborn notes that there is almost five times more energy trapped in wastewater than is needed for its treatment, totaling a primary energy potential of 851 trillion BTU/year. Energy embedded in wastewater is about 80 percent thermal, 20 percent chemical and less than 1 percent hydraulic, and converting those types of energy into what’s needed is the key to net-zero.
Per a downloadable WERF factsheet, wastewater often enters a plant preheated through consumer use in bathing or washing. After utilizing this free thermal energy with a heat exchanger, it can be applied throughout a plant.
“There is considerable energy embedded in the heat in wastewater,” said Fillmore. “Recovery of this low-grade heat will enable more plants to become energy neutral.”
Through a chemical process known as anaerobic digestion, plants can convert the energy in wastewater solids into biogas. This can be used as heat for a plant building or to generate electricity when run through an engine generator.
“In our investigation, anaerobic digestion with combined heat and power was the most advantageous approach to energy recovery, reducing energy requirements by up to 35 percent at WRRFs that have anaerobic digestion,” said Fillmore.
Codigestion, a practice that feeds organic waste directly into a plant’s digester after basic preparation, is widely utilized at net-zero plants.
“Anaerobic digestion coupled with codigestion of waste organics available in a facility’s service area offers a near-term pathway to net-zero or near net-zero operation,” said Eschborn.
There are also aerobic processes being developed to convert solids into synthetic gases and biofuels. A fuel known as biohydrogen can be produced using microbial electrolysis cells. However, these processes can consume more energy than they’re worth.
“The general thrust should be to maximize anaerobic treatment and minimize aerobic treatment,” said Eschborn. “Aerobic treatment is energy intensive, anaerobic treatment is a net energy producer.”
Hydraulic energy, while representing a small portion of the potential energy in wastewater, is captured by well-established and efficient technologies. Plants that are located well above their discharge point should install a hydro turbine to take full advantage of the kinetic energy running past them every day.
Practice Makes Perfect
WERF recommends several best practices for wastewater treatment plants to follow, which it claims can result in 40 percent lower energy consumption.
Naturally, the use of high-efficiency motors and generators operating near design points is a good first step.
Plants can go further by improving primary treatment and capturing more solids to be converted via the processes outlined above. WERF also urges steps to significantly reduce electricity consumption by aeration blowers, which may constitute half of a plant’s bill.
“Here’s a quick checklist of aeration measures to consider,” said Eschborn. “Fine bubble diffusion; full-floor coverage; an established cleaning procedure and frequency; closed-loop control; most-open valve control; low-pressure-drop-distribution control valves; state-of-the-art, high-efficiency blowers; and diurnal-flow equalization.”
Above all, a concerted, team effort to cut energy use is vital.
“WRRFs that employ both energy recovery and energy efficiency aspects have the greatest chance of becoming net-zero,” said Fillmore. “A lot of energy progress can be made by motivated staff and supportive management, even at fairly small plants.”
“The energy neutrality leaders that were studied had set up an institutional framework to drive their energy programs, which included long-term planning and commitment, a champion or champions, and engaged staff,” added Eschborn.
The Old Guards
Everyone wants to cut energy consumption, but installing the necessary equipment and processes won’t be cheap.
“It is usually the lack of capital funding for energy projects that slows the progress,” said Fillmore.
Standing side-by-side with the financial challenges are regulatory ones, a pair of sentinels all too familiar to treatment plants eyeing progress.
“Currently, requirements to meet lower nitrogen limits, to have better odor control, or use safer UV disinfection technology have thwarted the progress that many WRRFs have made in reducing their energy consumption,” said Fillmore.
Still, there is reason for optimism as technological achievements create energy-saving equipment and processes that are cheaper and capable of satisfying regulatory oversight. For instance, Fillmore referenced anammox, part of an emerging technology suite that can help plants with nitrogen limits reach energy neutrality.
Eschborn echoed this enthusiasm while adding a caveat.
“Looking out a decade or more, one can foresee mainstream anaerobic processes and mainstream deammonification restoring the opportunity for most facilities to approach or achieve net-zero, with a qualifier,” he said. “Economies of scale come into play. For facilities with a 10 MGD or greater loading, net-zero will be economically viable or come with only a modest economic penalty. However, there are 10 times as many facilities that are smaller, and for which net-zero may be economically prohibitive.”
Even as complete net-zero remains out of reach for many wastewater treatment plants, it’s time to strive for better efficiency.
“Not every plant will achieve energy net-zero, but every plant can include energy efficiency goals in their decision making and experience considerable energy reductions with real cost savings,” Fillmore said.
Image credit: "The wastewater treatment plant at Joint Base Pearl Harbor-Hickam gets an upgrade," Official U.S. Navy Page © 2011, used under an Attribution 2.0 Generic license: http://creativecommons.org/licenses/by/2.0/