October 2006, Vol. 18, No.10

Plant Profile

Clear Lake (Iowa) Sanitary District Wastewater Treatment Plant

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Location: Clear Lake, Iowa
Startup date: 1950s
Service population: 31,000
Number of employees: 9
Design flow: 8.2 mgd (31,000 m3/d)
Average flow: 5.7 mgd (21,600 m3/d)
Peak flow: 9.0 mgd (34,000 m3/d)
Annual operating cost: $850,000

The Clear Lake (Iowa) Sanitary District (CLSD) Wastewater Treatment Plant discharges its effluent to Beaver Dam Creek, but not before the water makes a 6-mi (9.7-km) trek to a power generation plant operated by Alliant Energy (Madison, Wis.), where it is used as cooling water. After 60% of the cooling water is evaporated, the remaining 40% is pumped 6 mi (9.7 km) back to the CLSD treatment system, dechlorinated, and directly discharged (assuming the quality meets permit limits).

The agreement to use effluent for cooling water developed in 2002 just after the plant expanded from 2.2 mgd (8300 m3/d) to its current design of 8.2 mgd (31,000 m3/d). Because the service area, encompassing Clear Lake and Ventura, Iowa, are recreational destinations, the population served by the plant varies from 10,000 in the winter to as many as 25,000 in the summer. The plant’s average daily flow is 2.0 mgd (7,600 m3/d). CLSD has been providing reuse water to the power plant since December 2003. Alliant’s power plant has access to up to 19.2% of CLSD’s dry weather hydraulic treatment capacity, which is about 1.3 mgd (5,000 m3/d).

Alliant and CLSD signed a 25-year agreement to reuse the effluent. This was the first water reuse application of its kind in Iowa, and the plant received the 2006 WateReuse Award of Merit for its outstanding reuse efforts from the WateReuse Association (Alexandria, Va.).

In order to meet the new reuse guidelines that accompanied the agreement, CLSD added tertiary filtration and disinfection steps. Alliant paid for this supplemental upgrade, which included a 150,000-gal (570,000-L) post secondary equalization basin, a new tertiary treatment building that houses three six-disk AquaDisk® cloth media filters, and an ultraviolet disinfection system.

The plant uses as AquaSBR® sequencing batch reactor system for secondary treatment. Treated effluent flows by gravity to the equalization basin, then is pumped into one of the three cloth media filters. Finally, the flow passes through an ultraviolet disinfection system prior to being sent to a storage tank at the Alliant Energy power plant site. Liquid chlorination was installed to provide a residual that would protect against any biogrowth in the 6-mi (9.7-km) pipe during periods when the power plant is shut down.

 

Nearby Biosolids Application
Waste-activated sludge from the sequencing batch reactor basins is pumped to an aerobic digestion system with a 30-day solids retention time, where it is thickened to 1.7% solids. The digested sludge is mixed with organic polymer slurry and then fed to a rotary drum thickener that dewaters the sludge to 6% solids. The Class B biosolids are then pumped into covered holding basins which are void of any mixing or aeration.

The stored biosolids are pumped from the holding basins into a 4000-gal tank and applied or injected each fall to portions of a 250-ac (101-ha) tract of corn and soybean fields.

Aerobic digester contents also are land-applied to ensure 365 days storage capability. A total of 240 ton (218 tonne) of solids are applied each fall. The average number of 4000-gal (15,000-L) loads each fall is approximately 325. Fortunately, the fields are adjacent to the treatment plant site, which allows operators to obtain an average haul rate of 2.3 loads per hour.

CLSD has a biosolids easement agreement with the landowner that ensures land availability without having to own it, since both parties benefit, no money is exchanged.

Collection System Rehab
In the midst of its treatment plant upgrade, CLSD’s aging collection system also got some attention. Bottlenecks in the system that created bypasses were alleviated when a 6024-ft (1836-m) horizontal-directional bore of an 18-in.-diameter (450-mm-diameter) steel pipe conduit with a 14-in.-diameter (350-mm–diameter) high-density polyethylene force main carrier pipe within was installed 30 ft (9 m) below the bottom of Clear Lake. The project set a world record for pipe size and distance for installation of trenchless piping and was one of four finalists for the 1999 project of the year award sponsored by Trenchless Technology magazine.