Challenge

The Sheboygan Water Treatment Plant in Sheboygan, Wis. uses conventional unit treatment processes (i.e. coagulation, flocculation, sedimentation, and gravity media filtration) to treat surface water from Lake Michigan. The 35 million gallons per day (MGD) facility, originally built in 1929, serves the City of Sheboygan and several other surrounding communities.

According to Ron Marshman, filter plant foreman, lengthy detention (exceeding two minutes) in the facility’s rapid mix tank was leading to chronic floc shear prior to coagulant entering the plant’s two 2.3 mg sedimentation basins. “Due to the long detention times, the coagulant would begin the flocculation process inside the rapid mix tank. The heavy turbulence from the mix tank’s large, high-speed mixing propeller would then actually rip the floc apart.” This resulted in erratic floc formation in the settling basins.

Solution

In May 2001, the existing 30-h.p. rapid mix motor, shaft and propeller were replaced with a submersible 3-h.p. Water Champ^® Chemical Induction Unit (CIU) from USFilter Stranco Products.

• The Water Champ operates on the simple principle of applying all available energy directly to the chemical being activated.
• The unit design includes a stainless steel motor with vacuum end components consisting of a titanium vacuum body and airfoil design propeller.
• Chemical solution is injected into the body of the unit and is dispersed while being simultaneously mixed by the propeller.
• The CIU is capable of inducing the chemical into the process stream at velocities up to 60 ft/sec.
  
  

Results

The Water Champ operates 24 hours-per-day, seven days-per-week at the Sheboygan facility. Submerged in the rapid mix tank (using a stainless steel guide rail) and positioned in the center of the flow regime, the new chemical induction unit provides for countercurrent mixing and a high velocity gradient across the width of the tank.

• The submerged mixer generates high zone diffusion and a high turbulence region as alum is simultaneously dispersed. Flows passing the mixing area make contact with the turbulence and distributed chemical, providing for a high rapid mixing rate prior to entering the plant’s sedimentation basins.
• “Immediately upon start-up of the Water Champ we could see a dramatic improvement in flocculation,” says Marshman. “We are now able to maintain very low particle counts (less than 10 particles/mL) in our finished water using the same coagulant dosage we had been using prior to switching to the Water Champ.
• We are achieving uniform flocculation and our long-standing problem with floc shear has been eliminated.” Marshman says the plant is currently looking into building a third sedimentation basin.