Effective wastewater treatment is increasingly becoming the focus of municipalities looking to save money across the United States. According to the University of Michigan?s Center for Sustainable Systems and the U.S. Environmental Protection Agency (EPA), the average life expectancy of wastewater treatment equipment is 15 to 20 years, driving nearly 17,000 public wastewater treatment plants (WWTP) to consider implementing infrastructure upgrades.
As cities assess their options to update their wastewater treatment utilities, digester gas technology is rising to the top of energy efficient improvements as the most beneficial in environmental and cost savings.
A Renewable Energy Resource That Saves
Digester gas technology has several formats, but the most popular technology converts wastewater treatment gas to electricity using anaerobic digestion. The digestion process begins during water purification with dewatering, or the removal of water from wastewater solids. The solids are directed to digesters where they are stabilized and usable methane gas is produced as a byproduct of the anaerobic digestion process.
Solids are handled and disposed, often used in agricultural applications such as a fertilizer base. The methane gas first moves to storage tanks before it is cleaned, burned in micro-turbines or fed to internal-combustion engines that run generators to produce electricity. Heat created as part of the process is used to keep digester temperatures at 98 degrees Fahrenheit to optimally manage sludge, with any excess used to support domestic heating for the building.
Electricity generated from anaerobic digestion is used as power for internal operations and equipment. Surplus electricity can be sold back to the utility to gain further cost savings. The methane gas, if properly collected, can serve as a substitute for natural gas to power equipment, including boilers, hot water heaters, reciprocating engines, fuel cells or turbines.
For digester gas technology to be a viable renewable energy source for a WWTP, the plant should have the capacity to treat 10 to 75 million gallons of wastewater per day and serve 50,000 to 500,000 people. The city of Ithaca, N.Y., was well qualified to implement this beneficial technology, with existing infrastructure and equipment in place that required a complete overhaul.
A Systems Integration Approach
Prevalent in WWTPs across the U.S., digester gas technology has levels of effectiveness. Of the 3,500 facilities that utilize anaerobic digestion to support wastewater treatment, only 2 percent use digester gas byproducts to produce electricity. Of that 2 percent, even fewer are realizing the full potential of digester gas technology systems integration than Ithaca has.
In a recent study by the University of Michigan, it was determined that WWTPs generate more than 8 million tons of sludge annually. To handle this sludge, a typical plant will spend 30 to 80 percent of its total electricity use in the treatment process. Ithaca recognized the need to upgrade its failing wastewater treatment infrastructure and implement other energy-saving measures to the facility to reduce this energy burden.
To achieve the sustainability and cost savings goals Ithaca set, the city partnered with Johnson Controls, an energy services company (ESCO) with a core competency in smart systems integration for digester gas technology and energy efficient WWTP upgrades. After a comprehensive energy audit of the WWTP, the ESCO outlined a performance contract to enable Ithaca to implement the robust upgrades it desired without high capital cost barriers.
Though digester gas solutions are not unique to Ithaca, the way the systems and equipment were integrated was. Instead of simply replacing and adding micro-turbines, which would have failed under the current state of the infrastructure in just two years, the city chose to update equipment and systems across the entire digestion process. The integration of the digestion process, paired with other facility enhancements, generated significant and long-lasting energy and cost savings.
The WWTP?s two digesters had exceeded their useful life and needed to be replaced. In addition, floating digester tank covers were added to store byproduct gas in a sealed environment, further improving the production of gas during the digestion process. Additionally, two co-generation units (one non-operational) that enable the city to bring in waste from industries and co-digest it with wastewater solids were repaired and ultimately replaced. Mixers were updated to maximize gas generation during the digestion process, and four new micro-turbines were installed as key elements to digester gas technology. The micro-turbines utilize a compressor and other devices to control combustion, feed the generator and convert gas to electricity.
The WWTP?s boilers and worn out engine generators are being replaced to better utilize the gas byproduct. A critical step for digester gas success is cleaning the methane gas byproduct, which increases the life of the equipment, makes for more efficient digestion, and processes the gas in a way that meets combustion requirements.
Finally, compression and dewatering equipment were enhanced to ensure all aspects of the anaerobic digestion process were streamlined and integrated for maximum production. The energy program and digester gas solution will enable the Ithaca WWTP to supply 65 percent of the facility?s energy requirements through renewable energy technology ? an increase from 24 percent. The improvements will also reduce the plant?s utility costs by 70 percent and lower greenhouse gas emissions by 961 tons per year.
Success Beyond Energy Savings
Ithaca?s energy savings as a result of digester gas technology will enable the city to realize cost savings and additional facility efficiency improvements, including heating, ventilating and air-conditioning enhancements, lighting updates, the installation of a solar photovoltaic array at the facility and the implementation of building automation and managements systems at the WWTP.
The overall energy program was financed through a performance contract with the ESCO. Performance contracting enables facility upgrades to be funded through existing budgets, using expected utility and operational savings to pay for some or all of the cost of the enhancements.
The city is expected to realize $9 million in energy cost savings over the course of the 20-year performance contract, giving the WWTP the opportunity to implement future facility and infrastructure upgrades and redirect savings to other municipal services.
A Showcase of Leadership
The 2008 U.S. EPA Clean Watershed Needs Survey found that national investment needed to repair and replace aging wastewater treatment infrastructure is approximately $187 billion. Ithaca?s success can serve as an example for municipalities across the country.
If implemented with intelligent design, digester gas technology?s cutting-edge engineering and advanced systems integration can generate significant energy and operational cost savings, while reducing a facility?s carbon footprint. Taking a renewable energy program and tapping into the benefits of performance contracting takes savings even further, highlighting a truly innovative way to achieve efficiency goals.
Peter Cavagnaro works for the Solutions Group within the Building Efficiency business of Johnson Controls. His area of expertise includes environmental engineering, specifically for treatment of wastewater from municipal and industrial sites. Cavagnaro has a B.S. in Civil Engineering from Northeastern University in Boston, Mass., and a M.S. in Environmental Engineering from the School of Civil and Architectural Engineering at the University of Colorado, Boulder.
