There are approximately 15,000 municipal wastewater treatment facilities in the US. Wastewater and municipal drinking water treatment systems account for about 3% of total US energy consumption and approximately 35% of the total energy consumed by municipalities.
Figure 1 shows the breakdown of electricity consumption at a typical activated-sludge wastewater treatment facility. Electricity accounts for 90% to 95% of the total energy consumed by these facilities. The remainder is fuel (fuel oil or natural gas) used for backup electricity generation or natural gas used to heat buildings.
Average energy-use data
Wastewater treatment plants have one or more administration buildings that could benefit from the following energy-efficiency measures. These measures are low- to no-cost, are easy to implement, and use proven technologies.
Turning things off
Turning things off seems simple, but remember that for every 1,000 kilowatt-hours (kWh) you save by turning things off, you’ll save $100 on your utility bill (assuming average electricity costs of $0.10 per kWh).
Computers and monitors Computers and other electronic equipment can contribute up to 20% of overall energy consumption. You can gain considerable significant energy savings by using computer power management settings on individual monitors and computers, forcing them to enter sleep mode after a specified period of inactivity. Effective power management settings can cut a computer’s electricity use roughly in half, saving up to $75 annually per computer. For more information, the US Environmental Protection Agency (EPA) offers detailed instructions on ENERGY STAR’s The Business Case for Power Management page.
Other office equipment and plug loads Like computers, devices such as printers, fax machines, and coffee makers often have energy-reduction settings that can yield substantial energy savings. Additionally, consider using smart power strips with occupancy sensors, which are an easy way to shut off often-forgotten energy users such as printers, monitors, desk lamps, and radios.
Lights Turn off lights when they’re not in use; many people forget to take this step. When properly installed, occupancy sensors and timers can do this for you. A no-cost option is to train your staff to turn off lights as part of their closing procedures. You can also help by posting a notice that identifies the locations of light switches.
Chilled-water drinking fountains Water fountains generally don’t need to provide ice-cold water 24 hours a day unless it’s required for health reasons. In most cases, you can turn off the cooling systems in drinking fountains.
Vending machines Refrigerated vending machines operate 24 hours a day, using 2,500 to 4,400 kWh per year, and add to cooling loads in the spaces they occupy. Timers or occupancy sensors can yield considerable savings because they allow the machines to turn on only when a customer is present or when the compressor must run to maintain the product at the desired temperature.
Walk-through audits If your facility doesn’t operate constantly, conduct a walk-through of your facility after hours to identify energy-efficiency opportunities. You may be able to save energy by switching off much of the equipment that’s left on overnight or over the weekend. Consider recruiting volunteers from each shift as monitors.
Motors Identify motors that are operating unnecessarily and shut them down. This could be as simple as ceiling fans running in unoccupied spaces or as complicated as cooling tower fans still running after target temperatures have been met.
Turning things down
HVAC temperature setbacks When your facility is closed, turn down temperature settings during heating seasons and turn them up in cooling seasons. Programmable and smart thermostats make temperature setbacks a reliable option.
Peripheral and back rooms Make sure HVAC settings in rarely used offices and other peripheral rooms are at minimum settings.
Window shades and blinds During warm weather, use blinds to block direct sunlight and reduce cooling needs. In the winter, open the blinds on south-facing windows to let in sunlight to help heat the space.
Optimizing operations and maintenance
Given all the moving mechanical parts in a wastewater treatment plant, remember that regularly scheduled operations and maintenance (O&M) can keep equipment running smoothly and delivering energy savings.
Check motors Mechanical problems are the main cause of premature failures of electric motors. Help motors achieve their full-life potential and minimize their energy consumption by:
- Routinely lubricating them
- Checking for clean and adequate ventilation
- Making sure there aren’t voltage imbalances
Inspect fans, bearings, and belts Thoroughly inspect fan blades, bearings, and belts at least once a year to prevent failure and maintain efficiency. During the inspection, clean fan blades, check bearings for adequate lubrication, and adjust belts should and change them if necessary.
Conduct a recommissioning audit Recommissioning is the process of retesting and recalibrating building systems to make sure they’re working as designed. Professionals who are trained in identifying wastewater treatment O&M and capital opportunities offer recommissioning audits similar to the walk-through audit described above. But these audits are focused on identifying a broader set of opportunities (not just turning equipment off). The audit lasts up to a day, and after, you’ll get a report detailing potential savings measures and estimated costs and benefits.
This kind of auditor will also benchmark your facility, which involves collecting data about energy use, billing, and facility comparisons. Recommissioning audits identify efficiency measures that will restore the overall plant performance to original design levels, but they don’t delve into specifics about the performance of process systems.
Longer-term solutions are energy-saving measures that require higher capital investment or longer payback periods, but they may also result in larger energy and cost savings.
Monitor energy usage
For many wastewater treatment facilities, improved energy monitoring is a great place to start when trying to reduce energy consumption. Many utilities will provide access to 15-minute-interval data from your main meter. It can also be worthwhile to install additional submeters, such as for the pumps or blowers. Energy information systems gather, process, and display energy data in a user-friendly way and perform diagnostic operations. Enel X North America, a cloud-based energy intelligence software company, has conducted research on energy monitoring and information systems for the Northwest Energy Efficiency Alliance. It found that industrial facilities (including wastewater treatment plants) can save between 5% and 20% of their total energy consumption by installing energy management and energy information systems and by training facility staff to use the information.
A wastewater treatment facility in Greeley, Colorado, installed a new system in 2014 to gather and display its electricity information in a way that’s useful for plant operators. The city of Greeley wastewater treatment plant publishes the information on its Daily Energy Cost page so the public can see how much energy the facility is saving daily.
Upgrade to LED lighting
Outdoor lighting fixtures at wastewater treatment plants are especially good candidates for LED upgrades. However, indoor lighting upgrades to LEDs will save you energy as well. In addition to reducing lighting energy costs by about 70%, LEDs result in substantial savings in maintenance costs due to their longer life. In 2013, the city of Greeley completed a comprehensive upgrade of its outside lighting. It switched from metal-halide and high-pressure sodium lights to LEDs, spending a total of $47,000. After the utility incentive, the estimated payback period was about 5.7 years. Since LED costs have come down since then, the payback period would be even lower now.
Audit aeration and pumping systems
The two largest energy-consuming end uses for wastewater treatment facilities are the aeration systems for removing organic waste and the pumping systems. You can save energy by efficiently managing the demands of oxygen in the aeration system. Strategies include using efficient and properly sized blowers, controls on blowers, and diffusers with improved oxygen distribution.
Similarly, you can improve the efficiency of your pumping systems by properly sizing equipment to match demand and replacing inefficient pumps. A detailed audit can help you identify opportunities for energy savings. Often, your local utility will offer free or subsidized energy audits of these systems, or you can work with experienced consultants who work in this field. You can also use the US Department of Energy’s (DOE’s) Pumping System Assessment Tool, a free online tool, to assess the efficiency of operations in pumping systems.
Aeration system controls Upgrading the controls on the blower system can be a significant energy-saving opportunity, especially if your system is operating at less-than-full capacity. We spoke with a facility engineer at the Fort Collins, Colorado, wastewater treatment facility. He told us that in 2012, his facility managers upgraded the controls on its blower system. They installed an automated control system with better turndown capabilities and a blow-off valve to improve system performance. Along with other blower upgrades, the improved controls helped reduce energy consumption from the aeration system by about 30%.
VFDs on pumps Pumping systems are the next-largest energy consumers at wastewater treatment facilities. An energy audit will reveal whether any pumps are often operated at partial loads. These pumps are good candidates for variable frequency drives (VFDs). VFDs can achieve notable energy savings in the right applications, and simple payback periods can be as low as three years. Utility incentives can help make these projects even more cost-effective.
Consider efficient diffuser technology Diffusers are a part of the aeration system that transfer air and oxygen into the wastewater. Wastewater treatment plants typically use fine bubble diffusers, which are capable of reducing energy usage by 25% to 75%. You can learn more about the technology in the EPA’s Wastewater Technology Fact Sheet: Fine Bubble Aeration (PDF). Researchers developed an ultrafine bubble diffuser, which improves oxygen transfer efficiency (OTE). The composite material used in the diffuser membranes doesn’t get dirty as easily, meaning you don’t have to clean it as often, and maintains the OTE. According to the ACEEE paper Evaluation of Common Practices of Adopting Energy Efficiency Technologies in Municipal Wastewater Treatment Facilities (PDF), ultrafine bubble technology has an OTE 5% to 13% higher than that of fine bubble systems (figure 2).
Figure 2: Comparison of diffuser technologies
Use turbo blowers
There are several alternative technologies for providing process air to the aeration system (figure 3). The turbo blower is a newer alternative, offering high efficiency for a range of airflows and low maintenance costs. This technology has become popular since its introduction to the wastewater treatment market in 2007, according to the EPA’s report Evaluation of Energy Conservation Measures for Wastewater Treatment Plants (PDF).
Figure 3: Comparison of blowers
The Greeley wastewater treatment facility installed six 300-horsepower high-speed turbo blowers in 2011. The installation resulted in cost savings of about $100,000 per year and reduced the facility’s total electricity consumption by about 15%. The simple payback period for investment in the turbo blowers, after rebates from Xcel Energy, was about 6.6 years.
Take advantage of combined heat and power
Most midsize or large (greater than 1 million gallons per day) wastewater treatment facilities have anaerobic treatment processes. These processes produce biogas, which consists of about 65% methane and 35% carbon dioxide and impurities. According to the DOE report Characterization of CHP Opportunities at US Wastewater Treatment Plants (PDF), rather than flaring the biogas, an internal combustion engine or combustion turbine can capture it and use it for on-site electricity generation.
You can use the waste heat from the power generation to heat buildings in cooler months. You can also use it in the anaerobic digester to enhance its performance, especially during cooler months. Combined heat and power (CHP) is the process of generating electricity and using the waste heat on-site.
The potential benefits of CHP include:
- Energy cost savings
- Improved electrical reliability
- Reduced greenhouse gas emissions (from reductions in purchased electricity, flared methane, and possibly natural gas consumption)
Depending on the amount of solids being treated through the anaerobic process and their organic loading, you may need to collect supplemental food wastes to generate an adequate and consistent quantity of biogas to supply your CHP system. You’ll also need to pretreat the biogas to remove the impurities.
The DOE report discussed a case study about a wastewater treatment facility in Minnesota. Facility managers installed a 120-kilowatt CHP system in their facility to provide temperature control to the anaerobic digester and serve about 25% of the facility’s space-heating needs. The project cost $250,000 and resulted in annual savings of $60,000 to $90,000.
Add a solar PV system
A solar photovoltaic (PV) system is another option for renewable electricity generation. This option is attractive to wastewater treatment facilities in cities that have goals for reducing greenhouse gas emissions. Many facilities are surrounded by open space that could house PV arrays. With the price of PV systems coming down and financing options readily available, an increasing number of wastewater treatment facilities are installing PV systems to supplement a portion of their electricity needs.
Boulder, Colorado installed a 1-megawatt solar PV system at a wastewater treatment plant in 2010, which you can read about on the city’s Wastewater Treatment Facility Solar Electric System page. The PV system produces 14% of the facility’s electricity requirement, saving more than $200,000 so far and 4,500 tons of emissions.
For more information
These are some of the most-common energy-saving measures for wastewater treatment plants. You can find a more comprehensive list of measures and strategies in the Water & Wastewater Industry Energy Best Practice Guidebook (PDF) from Wisconsin Focus on Energy and in Energy Efficiency Strategies for Municipal Wastewater Treatment Facilities (PDF) from the National Renewable Energy Laboratory. The EPA’s Evaluation of Energy Conservation Measures describes some innovative and emerging technologies.
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