Power play
By definition, managing wastewater facilities is an environmentally sensitive job, but climate-change awareness and rising prices for electricity and natural gas are pushing plants to produce and use renewable energy. Energy accounts for 30 percent of the operating and maintenance costs of most wastewater treatment plants (WWTPs), and demand for electricity at the facilities is expected to grow 20 percent in the next 15 years, according to the Washington-based Environmental Protection Agency.
Wastewater professionals who are taking environmentalism to the next level believe local governments have a responsibility to curb greenhouse gas emissions, and wastewater operations usually are tagged as a priority within energy initiatives. “If [a city or county government] has a large wastewater treatment plant, it should expect that plant to be producing a large percentage of its energy from its own renewable energy sources,” says San Diego Chief Mechanical Engineer Tom Alspaugh. (See p. 34 for more about San Diego’s energy projects.)
The good news is that more technology, attention and resources are being directed at ways to use wastewater treatment products as energy sources. However, without any significant federal legislation — and the subsidies that could accompany it — progress at WWTPs may move more slowly.
No need to work out the bugs
The wastewater treatment process can generate energy three ways: burning sludge or excess gas; developing a dewatered cake or dried product to burn at a power-generating station or cement plant; and anaerobic digestion, the oldest and most common source of energy. “The majority of larger wastewater treatment plants have some type of energy recovery, either with digestion or incineration,” says James Welp, chair of the Bioengineering Technology Subcommittee of the Alexandria, Va.-based Water Environment Federation’s Residuals and Biosolids Committee. Welp also is a project director for Kansas City, Mo.-based Black & Veatch. “The economic attractiveness of capturing renewable energy at wastewater treatment plants has changed in the past few years as natural gas, fuel oil and electricity costs have gone up drastically.”
While several local agencies and other groups are researching untapped sources of energy from wastewater — such as reusing effluent as its own energy source in areas where water is scarce — realistic solutions for the near future focus on emerging technologies that use digester gas. Digester gas is the methane, or biogas, captured in the anaerobic digestion process, which is used at about 19 percent of U.S. WWTPs, according to the California Energy Commission.
After the solids from reclaimed water have settled, they are fed to a digester that maintains a constant temperature of 98.6 degrees Fahrenheit. Digesters are airless environments, in which “bugs,” or bacteria, chew on the material and can consume up to 50 percent of the volatile organic solids. In addition to generating gas, anaerobic digestion also reduces the volume of biosolids that have to be disposed of or treated for other uses, such as fertilizer or landfill cover.
The gas that comes out of the digesters is comprised of methane and carbon dioxide, and can be used as a source of heat, a source of energy for direct-drive combustion engines, to make steam in boilers or to generate heat and electricity in combined heat and power systems. “The most common method of producing energy in a usable form is to digest wastewater solids to produce digester gas that has about 60 percent of the energy value of natural gas per unit volume,” Welp says.
Not just a science
The Alexandria, Va., Sanitation Authority (ASA) turned to anaerobic digestion to help power high-energy-consuming treatment processes that meet effluent quality regulations to protect the Chesapeake Bay. “We’ve done a good thing by putting in treatment processes that protect the bay, but in doing that, we also use more energy at the plant,” says Paul Carbary, director of ASA’s Green Fields Division.
ASA’s anaerobic digestion facility processes the plant’s solids, which remain in the digesters for 30 days, during which a portion of the organic material is decomposed and converted to methane gas to heat low-pressure steam boilers. The new facility cost nearly $30 million to build. “The greenhouse gas emission reduction [equates to] taking almost 400 vehicles off the road annually,” Carbary says. “This is the most renewable thing we do, and we are looking into ways to further utilize our methane.”
Baltimore also has begun investing in a comprehensive energy project that includes constructing three co-generators at its Back River Wastewater Treatment Plant. The facility currently uses about half the methane gas it produces to heat a few of the site’s buildings, flaring the rest into the atmosphere.
When the project is completed in April 2008, the co-generators will convert the previously flared gas to electricity. Two of the co-generators will run constantly, and one is a backup. The co-generation plant is expected to reduce the plant’s energy consumption by 20 percent, and, according to the Baltimore City Energy Office, using the new co-generators will equal removing 5,536 cars from the roads and 15,166 tons of carbon dioxide from the atmosphere.
The Back River Project, which includes several other energy upgrades, cost $14 million, but the city expects to save $1.8 million per year in energy costs. The plant’s upgrades were one piece of Baltimore’s multi-pronged approach to saving energy and using renewable sources in its facilities.
The city’s energy conservation projects would not have moved forward without leadership from executive managers and a strong implementation team, says Hatim Jabaji, Director of Maryland’s Office of Energy Performance and Conservation. Jabaji also was the Baltimore City Energy Office manager during the Back River Project planning. “[Former] Mayor Martin O’Malley challenged the city [energy] agency and implemented a tool to quantify the [city’s energy consumers], from fleet management, buildings operations, repairs, trash collections, etc.,” Jabaji says. “In my case, whenever we introduced a project or plan, we had tremendous support from the mayor’s staff, and things moved faster.”
While gaining support within government is essential in some areas, at other times energy plans require support from outside the bubble, especially in areas where WWTPs only can be as environmentally motivated as ratepayers are. “If you put amazing processes in your plant or you go the extra mile and use renewable energy, that’s absolutely the right thing to do for the environment,” Carbary says. “But, if your base of ratepayers does not support that, in the end, they’re the ones paying the bill. So, really, the issue is to focus on building the communication between ratepayers and the organization treating their wastewater.”
Looking forward
The federal government may have declined to sign the Kyoto Protocol, but many state and local governments are imposing their own greenhouse gas emissions limitations as well as setting energy goals. As a result, WWTP managers can expect new energy mandates and can prepare for them by forming energy teams, conducting thorough energy audits to uncover areas of energy inefficiency, such as outdated lighting systems, chillers, heating and cooling equipment.
Federal aid may be around the corner to aid in the upgrades. On Aug. 13, Congress passed a $10 million Energy Block Grant Program and tax incentives, which will add to the Department of Energy’s funds for energy-efficiency and renewable-energy projects. Some WWTP managers already are discussing where money needs to be spent to achieve efficiency goals, especially in many aging and overloaded plants, where some processes could become more automated.
To further improve energy efficiency, some WWTPs are looking for smaller equipment with an ability to switch to a larger capacity during peak flows. Larger equipment uses more energy, and in most plants, it is always running at a pace to accommodate a peak wastewater stream, even when the plant is handling a small stream. “The biggest problem in the wastewater industry is if you build a WWTP to serve the community, you have to be able to serve that community no matter how much flow is coming into the plant,” Carbary says. “Plants are sized and designed to meet those requirements at the peak, but [most facilities] only see their peak flows once a month or once a year, and it’s not efficient to operate large equipment down at a low demand. Our biggest [energy-efficiency] issue in wastewater is that it’s not always efficient to use very large equipment.”
But, change does not occur rapidly in the wastewater business. “Renewable energy is a relatively new thing for everyone in general, and wastewater treatment plants typically move a little slower,” Carbary says. “We’re a slow-reacting industry. We have to be because we can never turn off.”
Lynn Peisner is an Atlanta-based freelance writer.
San Diego charges ahead
With a renewable energy resume that would make any local government green with envy, San Diego continues to implement progressive citywide energy initiatives. The city’s Metropolitan Wastewater Department (MWWD) is leading the charge. Not only does MWWD brew all its own fuel with enough left over to sell back to the local power grid, it is about to embark on a biogas project that will benefit area businesses.
MWWD serves more than 2 million people in 15 communities in addition to the city. The department operates one biosolids processing center and three WWTPs, none of which require fossil fuels to operate. In 2006, San Diego consumed more renewable energy than any other public agency in the United States, with MWWD consuming 96 percent of that renewable energy.
The Point Loma WWTP is the jewel in the department’s crown. It is the largest of the three plants, and while it has an edge over other plants with lower electricity requirements (based on an EPA permit requiring only advanced primary treatment), it is the proving ground for numerous renewable projects.
MWWD uses methane gas from anaerobic digestion to generate electricity. After meeting all the plant’s power needs, MWWD sells electricity back to the grid. Although the power company does not give the department full market value for the electricity, MWWD Director Timothy Bertch says the department saved ratepayers $1.4 million last year through the sales. And, through a state grant, the city owns a duel-fuel peaking renewable energy system at Point Loma that allows its backup diesel generator to burn digester gas.
In addition to digester-gas technology, Point Loma capitalizes on a 100-foot drop to the ocean with a hydro-generator. As the water falls through the generator, it creates additional electricity that MWWD sells back to the grid.
In September, the San Diego City Council approved a fuel cell project that will help the department take the next step in advancing methane gas applications. Fuel cell technology has been tested and proven in places like Westchester County, N.Y., and King County, Wash., but San Diego will be the first city to truck methane gas captured from wastewater treatment to areas outside the plant’s fences.
The Beneficial Use of Digester Gas Project came about partially because the wastewater department still produces more methane than it can turn into power. A collaboration between city government and private companies, the project aims to clean and compress the gas on the Point Loma site, load it onto high-pressure trucks and transport it to 1.2 megawatt fuel cells located on the properties of private companies around the city. The tennis court-sized fuel cells will convert the methane to electricity and heat, providing businesses a green source of power. Expected completion is December 2008, at which time the fuel cells will supply a total of 3.9 megawatts of power to the city. There was no cost to the businesses that agreed to participate.
— Lynn Peisner