Plant by the Lake: A Model of Innovation
My latest field trip to gather information for our System Profile series took me to the rolling country of southern Illinois, 330 miles south of Chicago, and almost to the point where Missouri, Kentucky and Illinois come together. A special occasion on June 3 prompted the timing of my visit -- the dedication of a brand new 8 mgd water treatment plant in Carbondale, a city of some 27,000 people and home to Southern Illinois University.
Now I know all who work in this industry are proud of their own plants, especially if they are new or have been updated, but this is truly an impressive facility, designed and built to incorporate a number of significant, innovative "firsts." And the staff had made important technical contributions to the design as a result of work carried out in the years before the Carbondale City Council decided in 1990 to construct a new water plant.
The facility's location is itself impressive -- described as a picture-postcard setting by James Roth, project manager on the job for CMT (or Crawford, Murphy & Tilly, Inc.), the Springfield, Illinois, consulting engineering firm which did the design work for the $8.5 million project. It sits in a clearing with wooded surroundings next to a lake, and with no hint of the city nearby. Roth and James Swayze, the City's water operations manager, participated in the well-attended dedication ceremony, which was held on the slope between the attractive main building and the lakeshore. Swayze outlined the reasons that led the Council to vote for new construction.
First, the antiquated, landlocked existing plant in town would have had to be upgraded and expanded. Its capacity was 7.3 mgd, more than the average daily demand of 4.3 mgd. But it had hydraulic limitations, and the situation was aggravated by the requirements of the 1989 Surface Water Treatment Rule. The needs of the community, including the primary commercial customer, Southern Illinois University, could not be met comfortably.
Second, Carbondale had worked diligently to remain in compliance with the drinking water standards set by the federal and state environmental protection agencies. City officials wanted to maintain this record, and to avoid any legal actions, for instance having some form of restricted status imposed, which could hinder the City's growth.
A study revealed that the least expensive option was to upgrade the existing plant, but that approach would have required the construction of a new plant by 2005 at a projected cost of over $12 million. Also, going with a new facility now would generate annual savings of about $100,000 in operations and maintenance costs.
After CMT completed the design and specifications, the construction contract was awarded to Mautz & Oren, Inc., of Effingham, Illinois, in March of '92. Carbondale's resident engineer was Brad Fleck, and members of the Public Works Department worked closely with CMT and the contractor during the project to achieve the challenging goals that had been set.
The schematic drawing of the plant shows that the basic process sequence is not unusual. But most of the individual elements certainly are, and there are multiple chlorine injection points for good reasons. Let's consider the "firsts" outlined by CMT's Jim Roth, who in addition to being the consulting firm's point man on the project, is manager of its Hydraulics and Hydrology Department.
- First water treatment plant in Illinois to use Claricone clarifiers with rim-supported foundations to increase stability during an earthquake.
- First plant in Illinois with extended shell Claricones for added disinfection contact time.
- First plant in the nation to have clarifiers with a single helical-flow weir outlet.
- First in the nation to install underdrains with patented IMS caps in round decelerating-flow filters.
- First in the nation to employ ribbon-flow clearwell baffling to extend retention time.
- One of the first plants in the midwest to use air stripping to control the concentration of disinfection by-products (DBPs).
New THM Reduction Process
The last of the "firsts" may be the most important, especially in view of the USEPA's anticipated DBP rule. As is common knowledge today, many surface waters contain organics which react with the disinfectant chlorine to form possibly carcinogenic trihalomethanes. Carbondale's water had been tested for THMs a decade ago and found to be acceptable. But tightening regulations covering their presence later caused the City to be placed on a restricted status.
Led by Colleen Ozment, their superintendent, the plant's laboratory staff investigated a variety of THM control strategies. These included improved coagulation, carbon additions to the filter media, different chlorine feed points and contact times, the use of other disinfectants, and an aeration process. The last alternative was selected and a strategy which first encouraged formation of THMs, then removed them, was developed.
A year of testing began with one-gallon raw water samples in a common fish tank. Then a pilot aeration tower built by city workers to the lab staff's design allowed 20-gpm tests. What evolved was a process which provided sufficient THM-formation time, given the characteristics of the local water supply, and controlled chlorine residuals so that no THM re-formation would occur in the distribution system. Also, the process had to be capable of handling seasonal raw water temperature variations from 36F to 84F.
The plant-scale stripping process works as follows:
- Chlorine is introduced into the headtank and at several other points in the process scheme.
- Dosages of chlorine are maintained below 2.5 mg/l free residual prior to aeration.
- The 10-ft diam. aeration tower, with a 2,800-gpm capacity, is located near the finishing end of the process train to encourage THM formation in the earlier stages.
- Water in treatment is fed at the top of the 43-ft tower, which was charged with plastic media balls to an initial depth of 15 ft., and stripping air is introduced at the bottom, passing upwards through the cascading water.
- After aeration, ammonia is added if THM concentrations exceed the 100 µg/l limit, or chlorine is added if the chlorine residual is inadequate. High service pumps then feed the finished water to the distribution system serving the community.
The Clarifier and Filter Designs
Raw water for the plant is not taken from the adjacent lake, the shallow Carbondale Reservoir, with its algae, taste and odor problems, although an emergency intake is available. Rather, the supply is pumped from seven-mile-distant Cedar Lake, noted for its soft, low-turbidity water. The process is aimed at removing turbidity, manganese, color and microbiological contamination. The low incoming turbidity sometimes makes clarification difficult because of very light floc formation.
Cationic polymers and alum aqueous ammonia are introduced ahead of the 43-ft high head tank, which provides the gravity feed that is maintained through the plant to the high service distribution system pumps. Chlorine and lime or sodium hydroxide are added directly to the head tank.
Clarification takes place in the three parallel 51.5-ft diam Claricone reactor clarifiers. The basic design has been modified in several ways for these units. For instance, to extend retention time without a large increase in diameter, the height of the straight upper shell has been doubled to eight feet.
A notable feature is the new single orifice effluent drop box, which has replaced the circumferential V-notch weir. As clarified water reverses its helical flow path to enter the 1850-gpm-capacity drop box, any remaining heavier floc aggregates tend to continue in the original direction, slow down and begin to fall toward the sludge blanket. Also, while older units of this type used the center mix can at the bottom to support the large conical section, in these a rim support near the center of the mass is designed to improve stability during a seismic event.
In operation, water enters this clarifier tangentially through two inlet pipes. Velocity is controlled by modulating a motor-driven valve on the larger pipe, which increases the flow to the smaller one. Lime and anionic polymer are added to the influent, and fixed blades protruding from the mixing can at the bottom help to produce a high stoichiometric efficiency. A circular flow pattern develops, the water velocity slows, and a sludge blanket forms at lower levels. Clear effluent is taken off in the drop box as described, excess sludge finds its way into an adjustable concentrator, to be drawn off periodically, and chlorine is fed above the blanket to further stimulate THM production.
Next in the process sequence are the six decelerating-flow, center-feed filters, whose task is to final polish the water by reducing the turbidity to less than 0.2 NTU. They contain the porous support plates, described earlier as IMS (integral media support) caps, which replace the traditional support gravel above the filter underdrains. Made from sintered plastic beads, these are light and easy to handle, and not prone to plugging. Here they are supporting 1.5 ft of anthracite on 2.5 ft of sand.
As a result of the inverse cone shape of these filters, the downward flow rate decreases as the diameter increases, which has been shown to improve the particle capture efficiency. During the up-flow backwash part of the cycle, the velocity gradually increases, and the acceleration is said to help scour the media beds more thoroughly.
Another innovative feature at the Carbondale plant is built into the two 112-ft-diam, 1.0 million gallon steel clearwells, whose new design prevents short circuiting and adds significantly to chlorine contact time. The intent here still is to encourage maximum THM formation. A 40-ft-long baffle plate, welded at one end to the tank wall, diverts the influent to a vertical slot at its other end. This imparts a wall-hugging flow path and sets up a spiral pattern which progresses toward the middle of the clearwell. A slightly off-center baffle and slot are connected to the effluent pipe, which conveys the water to the aeration tower. Depending on conditions, and the potential for THM formation, the clearwells may be operated in series or parallel, and with or without the following aeration tower.
Earlier, the seismic-protection measures built into the clarifiers were discussed. The new baffle system in the clearwells was found to be a much more economical design solution than conventional concrete baffling built to meet seismic standards. That was projected to cost more than the tank that would contain it.
Other significant construction features are earthquake related. Carbondale sits about 80 miles north of the New Madrid seismic zone, and Jim Swayze said the City takes threats of seismic activity seriously. He estimated that 10 to 15 percent of the cost of the entire project was devoted to seismic protection.
Specifically, the plant building consists of two joined but distinct sections. The offices, laboratory, and various personnel areas are housed in a one-story portion of the Tech Center. Another higher part contains operations control, chemical storage and feed facilities, and a walkway to the outdoor clarifiers. The Process Building contains the filters, pumps and generator.
The design meets the earthquake standards developed by the American Water Works Association, and the life safety requirements of the Building Officials and Code Administrators. Although the two buildings are connected, they are separate structures which will react independently to seismic movement. A six-inch rubberized seal maintains the integrity of the internal environment, and the "designed-to-fail" junction permits minor damage so as to prevent major destruction.
Having started off my career as a chemist at a lab bench, I was particularly impressed with the lab facilities, and the beautiful view of the nearby lake enjoyed by the people who work there. In addition to handling all necessary tests for the water plant, the lab takes care of wastewater testing for the City, as well for other communities. Following up on the years of process testing and development, Colleen Ozment and her lab staff continue to seek improvements in chemicals and processes. To ease that task, one clarifier and two filters can be isolated from the mainstream process to conduct fullscale testing. The lab has separate areas for microbiology, wet chemistry, atomic absorption, and gas chromatography.
This computer-based system monitors and controls the raw and finished water flow rates, chemical feed rates, and the filters for effluent turbidity. Pre-determined setpoints are used to initiate backwash. The system also monitors water quality levels, the raw water pumping station, three remote storage tanks, the remote reservoir and pump station, and all in-plant processes for status and alarm conditions. An operator can watch over the entire water supply, treatment and distribution system on one CRT screen.
To sum up, the new plant replaces one built in 1925, expanded five times since then, and now showing scars of age, including some from periodic earthquakes. The plant by the lake should be able to withstand future seismic events much more successfully. It can treat water to all current standards of quality, and versatile enough to be expanded or modified to meet future regulatory requirements or in-house performance goals.
About the Author:
Ian Lisk is editorial director of Water Engineering & Management and Water & Wastes Digest.