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The City of Lethbridge (population 73,000) took a competitive approach to bioenergy management at their wastewater treatment plant (WWTP). Bioenergy refers to the energy associated with biosolids processing, such as biogas from anaerobic digestions and energy demand for biosolids drying. The plant currently treats 30 Ml/d (8 MGD), including a significant food processing industrial component, using a biological nutrient removal and anaerobic digestion.

A 1.6 MW internal-combustion engine based, cogeneration system has been in operation for 18 months and has met expectations in energy savings. With the cogeneration plant successfully operating, the City's attention is now focused on opportunities for increased biogas production and additional energy savings. Excess capacity already exists in the digesters and the cogeneration system to support this initiative.

The City of Lethbridge has developed a “Co-digestion Work Plan” that explored options for increasing biogas production and provided a preliminary assessment of the economic feasibility of co-digestion at the WWTP. The term “co-digestion” is used herein to refer to the anaerobic digestion of a combination of municipal sewage sludge and organic waste. The City's plan sets forth that the primary goal of their “co-digestion program” is to maximize the use of existing assets such as anaerobic digester and cogeneration units. Key elements of the co-digestion program include: collection and receiving, preliminary treatment, anaerobic digestion, residuals management, and biogas utilization with cogeneration units.

The preliminary economic analysis is encouraging. The City of Lethbridge can maximize the use of its available digestion and cogeneration assets at the WWTP by sending an additional 5,900 dry tones (6500 dry tones) per year of organic solids to the digesters to produce an additional 68,500 GJ (65,000 MBtu) of biogas per year. Using biogas to fuel the cogeneration system generates an additional 740 kW of electricity on the average. The additional generation equates to about one cogeneration set. The energy savings resulting from the avoided cost of purchasing electricity will fund the operation and maintenance cost of the cogeneration system and the estimated capital cost of 1.4 to 2.7 million Canadian to install equipment to receive and prepare organic waste for digestion. The refurbishment of the two 15 meter (49.2 ft) diameter digesters is also included in the capital cost estimate. Comparing a co-digestion system with continued operation of the existing bioenergy system shows a potential savings of between 400,000 and 2,000,000 Canadian over a 15-year period (2005 through 2020), depending on whether the required capital is on the low or high end of the estimated range. Additional costs for source separation, collection, and transportation would reduce these potential savings.

Instead of methane (a greenhouse gas) being emitted to the atmosphere, it will be beneficially used to produce power. Anaerobically digesting organic waste with wastewater solids will result in extended landfill life, reduced greenhouse gas emissions, and generation of electricity from a renewable energy source.
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Document Type: Research Article

Publication date: 2005-01-01

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