Spinning Biogas Into Gold With Combined Heat And Power

Glen Illyn, IL, is a fairly typical American suburb, with 27,000 residents living about 40 minutes from the bright lights of Chicago.

The local wastewater treatment plant, the Glenbard Wastewater Authority’s Advanced Wastewater Treatment Facility, is typical as well. It has a normal daily design flow of 16 MGD and peak design flow of 47 MGD. Its advanced treatment includes primary screenings; grit removal; primary clarification; high-purity, oxygen-activated sludge; clarification; tertiary treatment; and UV disinfection.

But in one key way, Glenbard is different than many other wastewater treatment plants around the country. It had grown interested in resource recovery and utilizing the influent it treated to help power and fund its operations and that interest has blossomed into an estimated $200,000 saving in annual energy costs.

“Glenbard was interested in finding ways to harvest existing resources, not only to save money, but also to become a leader in the wastewater industry through innovation,” said Chris Buckley, a project manager with Baxter & Woodman Inc., a local firm of consulting engineers. “When the authority was offered a significant grant to fund a combined heat and power (CHP) facility, they decided to pursue this opportunity.”

The facility was using the biogas created during treatment to fire hot water boilers and heat digesters, but the excess biogas was flared, or burned away, as it is at many treatment plants. Glenbard envisioned a CHP system that would capture excess biogas and put it toward powering its wastewater treatment process.

“A CHP system is advantageous because that biogas is used not only to generate thermal energy, but also to generate electricity,” Buckley said. “The combined energy delivery of CHP varies between 60 and 90 percent. This efficiency is much higher than the separated efficiencies of conventional electrical generation, which is 33 percent, and hot water boilers, at 80 percent. In other words, CHP produced more energy with less fuel.”

Along with Boller Construction, Baxter & Woodman was awarded the project on a design-build contract and 12 months later, it was completed.

“Now, the biogas is conditioned and used to fuel the CHP system,” said Buckley. “The heat produced by the CHPs is captured and recycled to heat the digester heat exchangers and, at the same time, generate electricity to offset the plant’s electric needs.”

Furthermore, the facility introduced a high-strength waste receiving program that takes in new sources of organic waste, like restaurant grease and food processing byproducts. This is also treated in the digesters to produce biogas and reduces waste that would otherwise head to landfills.

The project included the installation of two CHP units that consist of a high-efficiency, turbo-charged engine; an electrical generator; and a heat recovery unit. Each is capable of generating 375 kW of usable electrical energy and 1.44 million BTU/hour of usable heat. It meets about 60 percent of Glenbard’s total electrical requirements and all of its digester heating needs.

“The engine burns biogas to generate mechanical energy, which is used by the generator to produce electricity,” said Buckley. “Thermal energy produced by the engine and generator is recovered through heat exchangers and a hot water loop. The hot water loop conveys the thermal energy wherever it is needed.”

The project also required a biogas conditioning system, a step to keep uncleaned biogas from harming the CHP units. It consists of filters with absorptive media that treats the biogas, including iron oxide to remove hydrogen sulfide and activated carbon to remove siloxanes and organic chemicals. As these media are fouled, they will require replacement.

A new building was installed on the site to house the CHP engines and the conditioning system, as well as an outdoor area for the conditioning filter vessels. Hot water piping was needed to connect the CHPs to the digester boiler and heat exchangers and electrical connections were added to link the CHPs’ output to the plant’s power grid.

The project earned Baxter & Woodman a national recognition award for exemplary engineering achievement from the American Council of Engineering Companies.

“It is a great honor for this local project to receive this prestigious, national recognition,” Buckley said. “It encourages us and our clients to keep working hard toward providing innovative solutions that are cost effective, energy efficient, and make a sustainable use of our resources. Most importantly, such recognition helps the public learn and understand what our industry does and the opportunity to take pride in projects which may be in their neighborhood.”

Such recognition may encourage other wastewater treatment facilities to pursue a biogas recovery project as Glenbard did. Baxter & Woodman has installed other such projects at regional plants and touts the solution as one that can benefit any facility with electric and thermal energy needs, especially those that operate anaerobic digesters.