Low Head Hydro Powers Wastewater Plant
A plant upgrade includes the implementation of a cogeneration system based on the use of hydropower from an adjacent river
Illinois' Kankakee River has served the city of the same name for many years as a source of drinking water as well as a receiver for the effluent discharged from the local wastewater treatment plant. Now the river has been tapped for energy in the form of hydropower to assist in running the wastewater facility.
Starting near South Bend, Indiana, the Kankakee heads in a generally western direction into Illinois and to a confluence with the Des Plaines River. Together they become a major tributary of the Mississippi, the Illinois River, which joins the big river north of St. Louis. While about two thirds of the Kankakee River's drainage basin is located in Indiana, a variety of industrial concerns was established along the river banks in the 19th century and the early part of this one. The sawmills, breweries and ice houses that were built needed mechanical power and cooling water, and the Kankakee Dam was constructed across the river in the 1880s to provide these services.
Later, the electric utility company which was the predecessor of Commonwealth Edison installed a five-unit hydroelectric plant to take advantage of the head provided by the dam. It was operated as a "run of the river" facility until it was decommissioned by Commonwealth Edison in 1956. At that time the dam and related properties were deeded to the Illinois Department of Conservation for recreational and conservation purposes. Now the dam is being used to generate power once again, this time to run the wastewater treatment plant situated two miles downstream.
Wastewater Plant Upgraded to Regional Status
Operational since 1936, the Kankakee Water Pollution Control Center was upgraded and expanded in a $43 million project started in the mid-'80s and completed in 1988. The name of the city department running it was changed to the Kankakee Metropolitan Wastewater Utility, a move made to recognize the growing, more regional nature of its service area. In addition to treating the city's wastewater, the plant now also handles what is collected in the villages of Bradley, Bourbonnais and Aroma Park. Smaller plants serving the first two were taken off line, and a new collection system was built for the previously unsewered community of Aroma Park. The modified treatment facility is handling an average flow of between 15 and 16 mgd. A major sewer separation and rehabilitation program and various interceptor projects also were undertaken. Completed for $16 million, these improvements were designed to eliminate excess infiltration/inflow to the collection system, and wet weather by-passes to the river.
Kankakee's regional treatment plant is a conventional facility based on an activated sludge system. The major processes are aeration using fine bubble diffusers; anaerobic digestion of all sludges; trickling filters for roughing; dissolved air flotation for thickening of mixed sludge streams; and a methane-fueled cogeneration system which can produce 480 kw of power. The table summarizes the key technical information (hydraulic and organic) related to plant design and actual performance. As the numbers illustrate, industrial discharges from about 30 sites into the Metro collection system make up a significant portion of the organic load entering the plant.
The 480 kw cogeneration system which was constructed as part of the upgrade/expansion project uses two Waukesha engine generators, each of which are rated at 240 kw. Normal operating practice is to run one unit at a time to consume the methane gas coming off the digestion process. Waste heat from the engine is captured by heat exchangers and recycled to raise the temperature in the digesters. Should the supply of methane fall off and be insufficient to run the engine generator, an automatic switching system activates a natural gas fuel feed.
The electricity developed by the generator is fed into the plant's power grid, reducing the quantity of electric power that has to be acquired from other sources. Operating the Kankakee plant requires 850 kwh, or 20,400 kwd. This means one 240 kw generator producing 5,760 kwd accounts for 28 percent of the total power needed. The balance of 610 kwh (or 14,640 kwd) is obtained from the electric power utility, or from the new hydroelectric plant built to recover the energy available in the refurbished dam across the Kankakee River.
Consisting of a 440-ft long, curved main crest, with a 28-ft side spillway which connects it to the powerhouse situated on the south bank, the dam is 9 ft high and constructed of concrete. It has no operating machinery or gates to regulate the flow of river water as it passes over the spillway. Flashboards 15 in. high are set on the crest of the dam each spring to raise the water level. This increases the upstream pool depth, which improves conditions for recreational use, and also maintains minimum levels at the intakes of a local water treatment plant. This facility is owned and operated by a private utility which provides drinking water to the Metro area.
While the hydroelectric plant was allowed to degrade seriously after the 1956 decommissioning, the dam was found to be in very good shape and structurally sound when the recent project was started. Only minor repairs were required to bring it back to the necessary condition.
The Hydro Project
Five manually-controlled turbines with supporting equipment were installed in the original 1912 hydroelectric facility. These units were seriously limited in that they were unable to respond to variations in flow and available head. As a result, developing maximum power was not always easy, and the only operational decision to be made was determining how many units should be run.
The new three-turbine hydro installation incorporates a number of design features which provide considerable operating flexibility and result in significantly higher efficiency. Although the power output still is governed by the head made available by the dam, as well as the flow of river water, a computer-based control system monitors the river levels above and below the dam. The data gathered is used to select automatically the number of turbines that should run under the existing conditions. Also, variable pitch turbine blades can be adjusted for optimum condition. These features mean that the hydro plant can react to natural changes in the river flow and still produce power efficiently.
Computerized control of the installation minimizes the labor content of the operation, since changes are made automatically. Also, the hydro plant is monitored from the wastewater treatment plant two miles downstream, and can be run unattended through a computer interface.
The three 400 kw turbines installed at Kankakee are somewhat unconventional in that they operate on the siphon principle, in contrast with the more common pit design. They are the first units of this type to be installed in the United States, and were supplied by the C. E. Neyrpic of France. This company sees the Kankakee project as a demonstration of its innovative siphon design and its appropriateness for low head hydro sites such as this one.
Project Costs and Financing
The city received a state grant of $1,030,000 from the Illinois Department of Energy and Natural Resources to assist in financing the project. The grant was made to encourage the utilization of an alternative energy source and to allow the ENR to use this project to demonstrate the application of low head hydroelectric technology.
In addition to the hydroelectric equipment cost of $1,550,000, powerhouse and site work by a contractor required $2,489,000, an electrical transmission line cost $344,000, and connection to the electric utility was $70,000. The engineering design fee to Stanley Consultants of Muscatine, Iowa, was $350,000. The project total was $4,803,000, which netted out to a cost to Kankakee of $3,773,000 after the ENR grant from the state was received. One condition of the grant was that a "fisherman's park" had to be included in the scope of work to permit public access.
Since the hydroelectric plant went online in mid-1991 much data has been collected and analyzed about its performance, and the power consumption relationships between it, the wastewater plant's cogeneration system, the use of natural gas as a backup fuel, and what the electric utility has provided. The scenario is complicated, and a review of the rate structure reveals that producing hydro or cogenerated power solely for sale to the electric utility is not cost-effective. There is not enough revenue to offset the costs of operation and maintenance, or to support the capital costs. However, when these readily available power sources supply the entire or a significant portion of the base load needed to operate the treatment plant, real savings are achieved.
The City of Kankakee receives a substantial benefit from the current operating strategy, since the Metro utility pays the city for the hydro power at a rate matching the electric utility's charges. The net annual income during the years 1992 through 1994 ranges from about $102,000 to $275,000. And by reconstructing the hydroelectric plant to modern technology standards, Kankakee has transformed a part of its river into an environmentally safe, revenue producing showplace.