The sewage treatment process creates methane gas as a by-product of stabilizing wastewater solids using anaerobic digestion. The Columbia Boulevard Wastewater Treatment Plant produces up to one and a half million cubic feet of this biogas every day. The methane content of biogas is typically between 60 to 65 percent. When the treatment plant began operating in 1952, the plant used some of the biogas to heat digesters and the original administration building and flared off the rest of the gas. Environmental Services has tested several methods of converting this renewable resource to energy and heat for plant operations.
In 2009, Environmental Services installed two 850 kilowatt GE/Jenbacher engine-generators with a total generating capacity of 1.7 megawatts. A pretreatment system removes hydrogen sulfide, siloxane and moisture to prepare the biogas to be used as fuel.
The plant recovers heat from water and engine exhaust to use in its anaerobic digesters. The engine-generators supply about 40 percent of the plant's electrical needs.
Environmental Services installed a 200 kilowatt ONSI fuel cell at the treatment plant in 1999 to convert biogas into electricity to power one of the plant's buildings. At start-up, the fuel cell produced 175 kilowatts of electricity per hour.
Fuel cell maintenance proved to be much more expensive than anticipated and its generating capacity decreased over time. After several years of operation, the plant's avoided power costs from the electricity the fuel cell generated only slightly exceeded the cost of keeping it in operation. After about five years, the fuel cell's power section failed. Environmental Services decided it was not cost-effective to repair it and decommissioned the fuel cell in 2004.
In 2003, Environmental Services installed four microturbines at the Columbia Boulevard plant. Fueled by biogas, the four Capstone microturbines are each capable of producing 30 kilowatts of electricity per hour.
Because biogas is 50% to 60% methane gas, it must be treated to remove hydrogen sulfide, siloxanes and moisture to make it an effective fuel. Because of these pretreatment issues, the plant's four microturbines operated sporadically. Because the time and money required to maintain them was greater than the benefits from the electricity they produced, Environmental Services removed the microturbines in 2011.
The treatment plant also compresses approximately 20% of its biogas and delivers it via pipeline to a nearby industrial facility to use for process heating. This has been a very successful partnership since 1986.
Future Biogas Reuse Opportunities
The Columbia Boulevard plant currently flares about 23 percent of the biogas it produces. In February 2015, the Portland City Council approved a contract for final design of a facility to convert biogas into compressed natural gas vehicle fuel. Biogas is a by-product of solids treatment. This project would allow the plant to re-use nearly all of the 600 million cubic feet of biogas it produces annually.
An Environmental Services study of re-use alternatives found that the vehicle fuel option is the most economical and has the greatest environmental benefit because it will reduce diesel fuel use and cut greenhouse gas emissions. Environmental Services is exploring several possible uses for the fuel including selling it to a utility company, selling to Portland area garbage haulers to fuel trucks, fueling city vehicles, or fueling trucks that haul biosolids for land application.
It would cost an estimated $10.9-million to design and construct a biogas processing and storage facility and a vehicle fueling station at the CBWTP. Environmental Services is exploring available grants and financial incentives to help reduce project costs. Construction could start early next year and the facility could be operational by 2017.