The summer floods of 1993 in the Midwest were driven by repeated rains which deluged the region from Minnesota to Missouri. The flood crest on the Mississippi River was 46 feet above normal at its high point, and three feet above the highest ever recorded. For over three months the people of the region had to cope with the problems accompanying the swollen rivers, broken levees and inundated land areas. It has been estimated that the flooding killed at least 26 people, caused about $8 billion in damage, and swept over 10 million acres.
Weather is almost impossible to predict more than a week or so in advance-and even then climatic variables frequently upset these forecasts. How, then, does one plan the construction of almost any type of facility on the banks of the Mississippi and hope that those plans are sufficient to cope with the flooding that likely will occur sooner or later?
This was one of the situations that had to be considered by Assistant Director of Public Works Mark Johnson and his staff at the Isle La Plume secondary wastewater treatment facility in La Crosse, Wisconsin, when they undertook an upgrade project to improve plant performance. Teaming up with City Project Engineer Tony Hutchens, they conceived a site-specific design/construction approach to the modernization of the plant's disinfection system. Because they knew the facility's requirements better than anyone, they decided to tackle the design of the system themselves rather than hire an outside firm. The project, which is based on ultraviolet (UV) disinfection technology, was built on the flood plain of the Mississippi and included as many flood protection provisions as possible. Elapsed time to complete the installation was approximately nine months.
During its first operational disinfection season with the new system in place, from May 1 through September 30 of 1993, the Isle La Plume wastewater facility was in compliance with the monthly geometric mean 400 MPN (most probable number) per 100 ml fecal coliform limit. This period of compliance included the months of June and July, during which the average daily flows into the plant were much higher than normal as a result of the extraordinary flooding of the river. The plant uses a contact stabilization version of the activated sludge process, and discharges the treated, disinfected effluent to the Mississippi.
After investigating different equipment and process options, the City Engineer's office settled on a Fischer & Porter UV system (now supplied by Bailey-Fischer & Porter Company, which is a unit of Elsag Bailey) to replace existing chlorine disinfection hardware. This system was no longer capable of meeting the municipality's permit requirements for disinfection without major modification and expansion. Continued use of chlorine disinfection also would have required the addition of a chemical dechlorinating system to meet the residual chlorine limit specified in the discharge permit.
Peak design flow rate for disinfection at this plant was set at 25 mgd. The study phase of the project had determined that installing a UV disinfection system would be a more cost-effective option than modifying and expanding the existing chlorination facilities. The new approach also had potential safety and inventory benefits since it eliminated the storage and handling of large quantities of chlorine and dechlorination chemicals.
The UV system includes four parallel channels, three integrated bank assemblies in each channel (for a total of 12 banks), 16 racks in each bank, and 8 UV lamps in each rack. This adds up to 1536 lamps in the system, fed from 12 power distribution centers. One out-of-channel cleaning tank also was installed.
Quartz sleeves enclosing the ultraviolet lamps must be cleaned periodically as a result of the normal "fouling" that is caused by hardness of the water, iron content, and other constituents in the wastewater stream. The Isle La Plume lamp cleaning system uses a mild 2 percent muriatic or phosphoric acid solution. "We experimented with various acids," said Johnson, "and found the two selected, in mild concentrations, not only to be effective, but also could save tens of thousands of dollars a year by our not having to buy citric acid."
By means of an overhead bridge crane, an entire 128-lamp integrated bank assembly is hoisted from its place in the channel, hosed off to remove any accumulated but loose debris, then placed in an air-agitated acid solution for five to ten minutes. After this period, the bank is taken out of the solution bath, hosed off again and replaced in the channel. Average start-to-finish time for the cleaning process is about 30 minutes. At the Isle La Plume plant, cleaning the UV system is an ongoing procedure; one bank assembly per shift (or three per day) is cleaned during the disinfection season.
Johnson also commented: "Even though we were skeptical at first, power consumption at the facility has been as low as predicted for the system. The City pays about four cents per kwh of power to run the UV lamps. For the first full disinfection season, operation of the entire system (which means all UV lamps on, operating the bridge crane, lighting and heating, etc.) cost less than $100 per day for electrical power."
The flooding that occurred during the summer months of '93 added considerable stress to the treatment process, but performance of the disinfection section was such that the effluent quality remained within the permit limits under all conditions.
Would the La Crosse City Engineer's office design the project if it had to do it all over again? "Absolutely," said Tony Hutchens. "This gave us an opportunity to become more familiar with our requirements. And the planning, construction, and operation have proven to be right on target. We gained invaluable experience throughout the job. The entire retrofit project cost approximately $1,000,000, with about $625,000 going for the UV system."