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Energy costs at wastewater treatment plants have continued to rise over the past twenty years. Electricity, natural gas, and heating oil are used as energy sources for many of the processes at wastewater treatment plants. The City of Tampa Howard F. Curren Advanced Wastewater Treatment Plant (AWTP) has taken action to curtail some of the rising costs.

The City of Tampa embarked on a program to maximize the production of electricity from biogas and to beneficially utilize available waste heat. The objective is to ensure that any energy project will have an adequate rate of return to offset the capital costs. Another objective of the program was to partner with others on any project that was in the best interest of all parties. The program was executed in several phases as the opportunities became available.

The Howard F. Curren AWTP was started up in January of 1978. The plant was a two-stage pure oxygen plant with nutrient removal. The plant was designed with aerobic digestion for waste activated sludge and anaerobic digestion for primary sludge. After a couple of years of operation, costs were analyzed and it became apparent that energy consumption at the plant was very high. The City of Tampa investigated means to reduce electrical costs. In the mid 1980's additional sludge thickening and anaerobic digestion was constructed to eliminate the need for energy intensive aerobic digestion. In addition a 2.5 MW cogeneration facility was constructed to utilize digester gas (biogas) in engine generators and to utilize the heat from the generators to heat the digesters.

In 1991 the City put a sludge heat drying facility on line. At the time the operating cost, offset by the value of the product, made the process competitive with land application of biosolids. As the price of natural gas began to rise and product values declined in the late 1990's, the City began to look for enhancements to the system to reduce production costs. The City of Tampa had a need to expand the emergency power capacity at the AWTP, which required a $3M expenditure. The local power company had a need to expand their peaking capability, which led to a partnership on a new 5.8 MW power plant on the AWTP site. The plant could be used for peaking and emergency power with the additional benefit of capturing and using the exhaust heat to preheat the air going to the sludge heat drying plant. The power company built the plant and it has been in operation for 3 years.

During the same time frame an industry adjacent to the AWTP site had a need for warm water to use in heat exchangers for the purpose of loading anhydrous ammonia into railcars. Our effluent water was identified as a source of heat. The company tied into and expanded the plant general-purpose effluent water system, which allowed them to use our effluent in their heat exchangers to provide the heat necessary to load the ammonia. The City paid no construction cost. The company pays for maintenance costs and the electricity to run the pumps. The company saves approximately $1,000,000 per year over previous heat source. The 10-degree cooler water is then returned to the plant cooling water loop, which is used to cool equipment (compressors, engines, etc.) and to provide cool water for air conditioning.

Seizing the opportunity for biogas utilization and energy efficiency has produced significant savings to the Howard F. Curren AWTP in the past 20 years. Savings from biogas utilization and heat recovery are estimated to exceed $15,000,000 since the beginning of the program. Additional savings are being realized every day.
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Document Type: Research Article

Publication date: 2005-01-01

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