Johnstown facility utilizes dissolved air flotation technology to increase efficiency
Johnstown, Colo., is a rapidly growing community located between Denver and Fort Collins. With the town’s water treatment plant operating its circular clarifier systems at maximum capacity to meet summer peak demand rates, utility personnel began to explore options for expanding the plant to meet current and future demand requirements.
Water pumped 12 miles from the Big Thompson River into the Lonetree Reservoir becomes the raw water source for the treatment plant. The existing treatment consisted of two center-fed circular clarifiers with tube settlers, bottom sludge draw-off and dual media filters. The source water typically contained turbidity below 5 ntu, apparent color of 50 Pt/Co, true color of 14 Pt/Co and low manganese levels of 0.035 mg/L.
During the summer months, algae growth in the reservoir required the use of potassium permanganate (KMnO4) to reduce the taste and odor resulting from the algae blooms.
At the recommendation of a consultant, utility personnel decided to increase plant capacity and use dissolved air flotation technology for the clarification process.
Two 5-mgd Leopold Clari-DAF (dissolved air flotation) systems were installed and placed in operation during the month of May, shortly before the peak algae month of July, which had historically been the most difficult time to process the raw source water.
The plant immediately achieved a significant reduction in suspended solids settling on top of the filters. Previously, the system would backwash its filters after processing 3.5 million gal, and the backwashing usually correlated with the target maximum head loss. With the new Leopold Clari-DAF system in place, the filters are still backwashed after processing 3.5 million gal, but with minimum head loss. Because the Leopold Clari-DAF system provides the filters with lower solids loading, the turbidity leaving the plant has been reduced by 42.5%.
The Clari-DAF system has also allowed the plant to reduce its chemical cost by eliminating the use of KMnO4 and Nalco 8108 polymer.
With the old circular clarifier, the sludge was blended with filter backwash water in a lagoon, allowed to thicken and then removed by a backhoe every three months. Today, the solids mechanically removed from the top of the Clari-DAF system are pumped to the sewer. The filter backwash water is still processed through the lagoon, which has not required cleaning in 18 months.
With two flotation basins, the utility has the option of operating one system during the wintertime to reduce power cost and putting the second system online during the peak summer demand season to handle the additional flow.