Aerators Stop Odors in Chicago
Chicago and its surrounding 51 communities cover an area of 375 square miles and have a population of over 3,000,000 people. This metropolis is served by a combined sewer network carrying both raw sewage and stormwater in a single pipe. Since a majority of the natural porous ground surfaces has been covered over with buildings, streets and pavements, stormwater runoff into the combined sewers frequently exceeds their capacities. When this occurs, excess flows bypass the treatment plants and this rainwater, mixed with the raw sewage, spills directly into area rivers and streams at hundreds of locations throughout the city. These Combined Sewage Overflows (CSOs) result in severe pollution of the waterways as well as flooding. During particularly heavy rains, overloaded rivers contaminated with raw sewage back-flow into Lake Michigan, the region's source for drinking water.
In the late 1960s, a flood control coordinating committee consisting of the Metropolitan Water Reclamation District (District), the City of Chicago, Cook County, the Environmental Protection Agency (EPA) and the State of Illinois reviewed proposals to combat flooding and water pollution in the region. After a thorough review of over 50 alternatives and combinations of plans, the Tunnel and Reservoir Plan (TARP) or "Deep Tunnel" project was selected.
Deep Tunnel In 1972 the District's Board of Commissioners designated TARP as the Chicago area's plan for cost effectively bringing Chicago area waterways up to Federal and State water quality standards. TARP is designed to prevent CSOs and back-flows to Lake Michigan by intercepting the excess combined sewer flow and storing it until it can be pumped to water reclamation plants for treatment.
TARP consists of four tunnel systems and three reservoirs. The Mainstream and Des Plaines systems will be served by the McCook Reservoir; the Calumet System will be served by the Thornton Composite Reservoir; and the recently completed O'Hare System is served by the CUP O'Hare Reservoir.
In 1975, TARP was split into two phases because of separate and distinct sources of federal funding. Phase I, considered primarily a pollution control project, was funded by the EPA. It consists of tunnels and drop shafts needed to intercept the combined sewer overflow points along the waterways. Once intercepted, the flows remain in the tunnel system until pumped out and treated. Although the tunnels have limited storage, the system captures the "first flush" as the storm flows begin. This represents approximately 85 percent of the pollution load that would otherwise enter the waterways. This is equivalent to a pollution load of 3.9 million people. These tunnels lead to the reservoir sites.
Phase II, considered the flood control component of TARP, is being funded by the U.S. Army Corps of Engineers (Corps). As a result of Corps funding requirements, a cost effectiveness study was conducted for the project. This reduced the storage initially proposed for TARP by approximately two-thirds. This revised proposed project with reduced storage became known as the Chicagoland Underflow Plan (CUP). It consists of three reservoirs, CUP O'Hare (350 million gallons), McCook (8 billion gallons) and Thornton Composite Reservoir (4.8 billion gallons). In addition, the Thornton Composite Reservoir will provide an additional 3.1 billion gallons of storage for overbank flood control for the Little Calumet River Watershed.
As a result of tunnels already completed and operational, the quality of area waterways has dramatically improved. In addition, Lake Michigan is much less endangered by polluted waterway reversals. The aesthetic and recreational potential of the waterways is being greatly enhanced, as are property values and the water environment for fish and other wildlife. Threefold increases in the number of fish species in the local waterways have been reported since the tunnels began operations in May 1985. Today, over 50 species of fish have been identified in Chicagoland waterways.
O'Hare Reservoir The 350 million gallon CUP O'Hare Reservoir is the first of the three identified CUP Reservoirs to be completed and operational. As part of the O'Hare TARP system, the CUP O'Hare Reservoir provides relief from sewer backup flooding for more than 21,000 homes and businesses in combined sewer areas of Arlington Heights, Mount Prospect and Des Plaines. The CUP O'Hare Reservoir provides 343 million gallons of flood water storage capacity and will prevent more than $2.3 million in average annual flood damage over a 7.5 square mile area containing a population of 61,000.
The CUP O'Hare Reservoir was constructed by A. A. Conte and Sons and the Kovilic Construction Company. Conte was responsible for the civil work (digging the basin and construction of the perimeter roads) while Kovilic did the mechanical aspects of the construction. This mechanical portion included installing the distribution system, aerators and the controls.
The CUP O'Hare Reservoir acts as a safety basin attached to the sewer tunnel system as it carries the wastewater to the Kirie Water Reclamation Plant. The tunnel measures 20 feet in diameter at the point where it arrives at the water reclamation plant and is buried about 190 feet below the surface.
The basin itself is located on 100 acres of land and measures approximately 1,600 feet by 1,000 feet with a surrounding perimeter road providing access to the control building at the east end of the complex. Its depth varies from 60 feet on the east (deepest) end to 50 feet on the west. The bottom of the basin is composed of concrete with the side wall being covered with a polypropylene liner to prevent erosion. The basin also includes a ramped access to the bottom of the basin for maintenance and cleaning by a skid-steer loader and a dump truck.
The chief problem associated with this type of basin is the potential noxious odors that may emanate from it. Water turning anaerobic or septic when stored causes these odors. This means that there has been depletion of oxygen in the water causing the death of the organisms that generate carbon dioxide (CO2) as a byproduct. These organisms then are replaced by others that generate methane, mercaptin and hydrogen sulfide gas (H2S), all of which can be very unpleasant smelling.
When an event occurs (an occasion where the wastewater in the sewer system exceeds the capacity of the tunnel system), the wastewater rises through a vertical drop shaft to overflow into the basin. At this point the sewer pipe is diverted to the four tide gates (8¢ « 8¢) that are composed of creosote-treated wood timbers and are hinged at the top. Water pressure forces these gates open and allows the surging floodwater to enter the basin. As the flow diminishes, gravity closes the gates and the wastewater is contained in the basin.
During the event, this wastewater continues to flow freely into the reservoir relieving the pressure on the treatment plant and the tunnel system. As the depth in the basin increases, the aerators that have been resting on the bottom of the basin during dry periods will begin floating. When an aerator has achieved seven feet of water beneath it, a limit switch attached to the mounting arms triggers its actuation based on the angle of declination from its base point. Additional units are progressively activated as the depth increases to maintain the aerobic qualities of the wastewater.
According to David Handwerk of the Army Corps of Engineers, they looked at diffusers and cascades as alternatives to the aerators selected. Diffusers are best described as a series of pipes or plates that are placed into the water. These devices emit bubbles into the water in the same way a fish tank is oxygenated. The cascade or waterfall method uses a series of vertical drops where the wastewater is cascaded down a series of steps to naturally put air into it. Floating mechanical aspirating aerators aerate and mix the basin even with varying water levels.
Because of the force they generate, Handwerk stated that the aerators could cause gouging in the floor of the basin if they should turn on too early. If they are turned on too late, there would not be sufficient oxygen in the water to maintain its aerobic state. Besides the automatic activation caused by the limit switch, the aerators can be manually triggered by the personnel at either the control building, located at the basin, or at the sewage treatment plant. Even though the aerators are capable of working to a depth of thirty feet, it was determined that a depth of seven feet was appropriate for this project.
Aerator Operation There is a total of nine surface aspirating aerators being used in the O'Hare CUP Reservoir. These units are manufactured by Aeromix Systems, Inc., Minneapolis, Minnesota. Each aerator is mounted on a set of four pontoons with each pontoon approximately 19 inches in diameter. The overall float system measures eight feet wide by twelve feet long. For increased stability and longer life, each of the pontoons has been injected with water-resistant, closed-cell polyurethane foam.
These surface aspirating aerators carry a 100 horsepower motor (75.0 kilowatts) with vibration dampeners to prevent vortexing and fatigue failures. They are mounted at an angle to the pontoons with the motor and air intake above the water and the propeller below. The motor rotates, turning the shaft that spins the propeller. Water moves at a high velocity through and near the propeller blades, creating a low pressure zone at the hub. The low pressure draws in air through the stationary intake and down the large-diameter draft tube. The air exits into the water at the propeller hub. Turbulence and flow created by the propeller breaks up the air bubbles, mixes the basin, and disperses oxygen with surface splashing. Horizontal water movement generated by the aerator maximizes oxygen transfer by
- pulling unoxygenated water in while pushing oxygenated water away,
- minimizing the coalescence of bubbles in the plume, and
- increasing the air aspiration rate.
Each of the aerators installed in the reservoir is connected to a guy wire strung between pylons anchored to the bottom of the basin. These connections are achieved using two forty-foot-long welded steel arms that give them the ability to rise and fall with the changes in wastewater level while the aerators remain in the same relative position in the basin.
When the basin is near capacity and all nine aerators are functioning, the wastewater is being aerated and slowly circulated within the basin in a counterclockwise direction. This eddying effect allows for additional circulation of the wastewater for better aeration. The rotation and complete subsurface aeration prevents odors and keeps the wastewater from becoming septic.
Before the Kirie Water Reclamation Plant processes the water in the reservoir, water in the adjoining TARP tunnels is pumped and treated to provide available storage capacity in the tunnel system for subsequent storm events.
To empty the reservoir, there are eight sluice gates positioned at various depths at the low end of the basin. Six of these gates measure 30 inches in diameter and are easily visible in the east-end. The other two gates, measuring 16 inches in diameter, are buried below the surface at the east-end of the basin under protective steel grates. These gates are metered by the Water Reclamation District personnel to control the flow into the water treatment plant.
A typical drainage operation would take the majority of its water from the uppermost layer of the basin because it is better for the sewage treatment plant to have aerobic water going through the system. The gates at the various levels of the basin give the treatment plant personnel the ability to meter the water being conveyed to the plant from various levels of the basin while assuring that the top layer of water is constantly aerated and noxious gases are at a minimum. This layout gives the personnel another control of the flow to the treatment plant.