City of Lethbridge, Alberta
Wastewater Treatment Plant – Digester Gas Utilization
The City of Lethbridge has revolutionized its wastewater treatment plant with a biological nutrient removal process called Step BioP and ultra violet (UV) disinfection. In addition, the city uses digester gas to produce some of the energy required for the treatment of wastewater. Sludge from the primary and secondary treatment processes is fed to anaerobic digesters that create digested sludge and digester gas. The sludge is applied to nearby farmlands to improve soil structure, while two 800 kW generators are powered by digester gas-fueled engines in a cogeneration plant. The city expects to save about $450,000 a year in energy costs. An added benefit of the cogeneration plant is that the city is capable of treating wastewater even during power outages. With the success of this project, the city plans to explore the possibility of capturing landfill gas.
In the mid-1990s, the City of Lethbridge upgraded its wastewater treatment plant using an innovative nutrient-removal process called Step BioP. Concurrently, the city chose to use ultraviolet disinfection of wastewater over chlorination-a more cost-effective option that eliminated a harmful pollutant. Step BioP was chosen to ensure that the city would achieve nutrient levels below the Province of Alberta's environmental guidelines.
In March 2001, the city adopted a broad-based plan that moved the city from selective or minimal compliance of provincial and federal regulations to a more sustainable community planning model that employs a triple-bottom line approach. The plan contains four strategic directions: smart growth, energy efficiency, partnerships, consultation and participation, and corporate responsibility.
This cogeneration project responds to the energy-efficiency component of the plan, and presented an opportunity for the city to make use of an underutilized resource (digester gas) and to reduce its "ecological footprint."
- CO2 emissions have been reduced by 3,780 tonnes, the equivalent amount released by 700 cars on a yearly basis.
- By reducing coal combustion and by removing sulphide from the digester gas prior to combustion in the cogeneration unit, sulphur dioxide emissions have been reduced by 74 tonnes.
- Approximately $450,000 is saved per year in electricity and natural gas costs.
- Treating the digester gas to remove sulphur compounds has eliminated corrosion in the boilers when digester gas is burned, doubling the lifespan of the boilers and reducing associated repair costs.
- No additional staff is required to operate the new cogeneration plant.
- The project influenced city administration to look at the project holistically and allow non-monetary considerations to be fairly weighted.
- Involving all project partners throughout the process, and choosing a simpler cogeneration system resulted in greater support from staff. "We gave everyone an opportunity to be heard and be part of the solution, and that developed a commitment to make the project work," said Mr. Stafford. "Staff was trained on the operation and maintenance of the system. It was new to them, but the system was not dissimilar from the types of equipment they were already running."
- Clear lines of responsibility for the city and the contractors are critical to make a project like this run smoothly. "In hindsight, we would have developed even clearer performance specifications for the contractor," said Mr. Stafford.
An independent review of the project revealed that only two requirements have not been fully met: the operation of the gas scrubbing equipment and the optimal use of the gas. "There's an opportunity to look at how we can control the units better," said Mr. Stafford. "It depends on the amount of heat or energy needed and the amount of digester gas, so we are developing a mechanism to address this."
Partners and Collaboration
- Wastewater Treatment Plant staff: All staff involved in the project attended the contractor meetings.
- The engineering firms of Brown & Caldwell and Kennedy/ Jenks of Seattle, Washington provided expertise in the use of digester gas.