May 21, 2015

Attacking Algae

Florence, Colo., facility uses dissolved air flotation system to address contaminants

Attacking Algae

The city of Florence, Colo., Water Treatment Plant (WTP), located 75 miles south of Denver, uses blended surface water taken from the city’s southernmost water reservoir. All raw water sources entering the reservoirs flow into the northern reservoir and then into the southern reservoir to ensure that the water is thoroughly mixed before entering the pipeline to the plant. The source water, which comes primarily from snowmelt, is low in turbidity, alkalinity, minerals and color. Algae grow throughout the year and can appear in counts as high as 400,000 during the summer season, causing the surface water to be difficult to treat.


The treatment scheme at the plant included an up-flow contact clarifier followed by gravity filters to handle average flows of 2 mgd in winter and 2.8 mgd in summer. The chemical treatment consisted of adding copper sulfate in the reservoir for algae control; and alum fed at 30 mg/L, dry nonionic polymer fed at 0.35 mg/L and liquid cationic polymer fed at 2 mg/L for the clarification process. This produced clarified water turbidity of 2 ntu and a typical finished water turbidity of 0.2 ntu.

In the summer months, with algae counts at their highest, the utility endured filter run times as short as eight hours. Each filter backwash required 20,000 gal of water, and the contact clarifier required flushing four to six times per day, using 13,000 gal of water per flush. Frequent backwash and flushing cycles resulted in the plant producing quantities of finished water far exceeding consumer demand.

In addition to excessive water production, the treatment process resulted in poor color removal, as evidenced by the greenish color tint that is noticeable during the filling of municipal swimming pools.


The Engineering Company (TEC)—from Fort Collins, Colo.—recommended increasing plant capacity to meet future growth demands and improving the treatment process for removing algae found in the raw water source. TEC recommended testing a Leopold Clari-DAF dissolved air flotation system along with a Filterworx gravity filter. Data collected during the pilot test confirmed that the dissolved air flotation process was far more successful at removing algae than the settling clarification process.


The City of Florence began treating its raw water using this newly installed treatment scheme, which consists of a Leopold Clari-DAF system with a rated capacity of 6 mgd through two basins, followed by three dual-media Leopold Filterworx filtration systems.

Plant personnel have discontinued the use of copper sulfate for algae control, since the Clari-DAF system effectively removes 99% of the raw water algae.

Additionally, they have also switched the coagulant feed to polyaluminum chloride, with dosage reductions to 11 mg/L in winter and 17 mg/L in summer, and—only when raw water quality requires it—an additional 0.25 mg/L of nonionic polymer. This treatment has resulted in average filter run times of 72 hours, a nine-fold increase over previous treatment, producing a finished water quality through the filters of less than 0.04 ntu.

Finally, the true color in the effluent is zero, resulting in noticeably clearer water in the municipal swimming pools.

Leopold, a Xylem brand


227 S. Division Street

Zelienople, PA 16063-1313

United States

Phone: 724.452.6300

Fax: 724.452.1377

[email protected]