Clear Lake Sanitary District (CLSD) was originally built in the early 1950s and utilized a trickling filter system. By 1988, it became evident that the treatment needs of the growing population and increasing storm flow discharges would require the plant to be upgraded. In order to meet the more stringent environmental requirements defined by the NPDES permitting authority, the plant chose to undergo a major renovation. This $23 million renovation project began in 1995 and was implemented in several phases over the course of six years. Renovation included construction of new pumping systems, a new secondary treatment system, and a new 5 million-gallon storm flow equalization basin along with renovations of eight lift stations. The trickling filter system was retrofitted with a 4-basin AquaSBR system in 1997 to meet their secondary treatment process needs. The remaining renovations were completed by 2001.
Aware of the treatment plant's major upgrade, Alliant Energy contacted CLSD in 2002 to propose a partnership. Alliant asked CLSD to supply treated wastewater to a nearby, newly constructed power generation plant for reuse purposes to meet its cooling water demand. Alliant and CLSD signed a 25-year agreement. This would be the first water reuse application of its kind in the state of Iowa. In order to meet the new reuse guidelines that accompanied the agreement, CLSD added tertiary treatment processes filtration and disinfection of the effluent before it could be discharged to the power plant. Alliant paid for this supplemental upgrade, which included a 150,000 gallon post secondary equalization basin, a new Tertiary Treatment Building that houses (3) 6-disk AquaDisk® cloth media filters (photo to the right) and an ultraviolet disinfection system. The cloth media filters have an average design capacity of 3.0 MGD with one filter out of service, but can easily handle a maximum hydraulic flow of over 9.0 MGD with all three filters during storm flow conditions.
Once the power plant receives the treated effluent from CLSD, it further disinfects it using chlorination and blends the effluent with groundwater to a ratio of 60% groundwater and 40% effluent before it is used as cooling water. The "evaporate", or 20% of water that remains after the cooling water process, is returned to CLSD for discharge. An inline probe monitors the chlorine concentration of the returned cooling water to determine if sodium bisulfite addition is needed for dechlorination. CLSD's effluent is then blended with the return cooling water prior to discharge due to the high concentration of dissolved solids in the evaporate. The blended effluent is discharged to Beaver Dam Creek and eventually to Cedar River.
CLSD has been providing reuse water to the power plant since December of 2003. Alliant's power plant has access to up to 19.2% of CLSD's dry weather hydraulic treatment capacity, which is 5.7 MGD/day.
Kevin Moler, Plant Superintendent, says “I’m proud of the plant’s accomplishments and am happy to provide onsite tours to visitors”.
SOURCE: Aqua-Aerobic Systems, Inc.