Water Treatment
Water from the Rainy River has the following unusual characteristics which present difficulties in water treatment.
- Daily and seasonal quality changes
- High color levels
- Low alkalinity (buffering capacity)
- Low turbidity
- Soft (low calcium and magnesium hardness)
Available treatment technologies were reviewed and after careful pilot scale treatment studies were conducted at the paper mill and Fort Frances water plants, direct filtration was selected as the treatment method.
The treatment facility design intentionally included flexibility in operations and treatment schemes to allow the city to deal with future drinking water regulation changes.
Color
The river water is highly colored (tea color) from the presence of decaying vegetation (organic material). Color levels usually rise during spring and fall rainfall periods, and drop during winter conditions.
Turbidity
Turbidity is a measure of the clarity of the water. Color in the water does not significantly affect the turbidity of the water. Turbidity increases during non ice cover periods and decreases during the winter.
Adding treatment chemicals to remove color increase the turbidity of the water during treatment. The turbidity is removed by chemical treatment and two-stage filtration.
Stability
The raw water is corrosive because of the lack of hardness and alkalinity. The treatment process includes adding chemicals to increase the alkalinity and to provide a protective coating on the inside of the water main piping to help protect the piping from corrosion.
Treatment Process
First, raw water is withdrawn from the Rainy River about 1,500 feet upstream to prevent contamination from Second Creek. A screened intake prevents large solids from entering the facility. Water is then pumped to a splitter box. Polymer is added into the water before it is distributed to the five filters. Chlorine and ammonia are added for disinfection after filtration. Soda ash and zinc polyphosphate are then added for pH and corrosion control. Fluoride is added last.
The Next Step
Treated water is pumped from the clearwell to the ground storage reservoir. The reservoir provides water when the demand exceeds the treatment capacity and provides storage when the demand is less than the plant capacity. High service pumps transfer the water from the ground storage reservoir to the distribution system.
Basis of Design
- Average design flow: 1,090,000 gallons per day
- Backwash recovery: 275,000 gallons
- Design population: 11,000 people
- Design year: 2010
- Disinfection basin / clearwell: 215,000 gallons
- Elevated storage: 1,000,000 gallons
- Filter clarifier stage: 70 square feet, 42 inches of gravel
- Final filter state: 140 square feet, 10 inches of sand, and 20 inches of coal
- Ground storage: 500,000 gallons
- High service pumps 3 – 1500 gpm @ 210 feet TDH
- Low service pumps: 3 – 750 gpm @ 210 feet TDH
- Pressure reducing / metering station services Ranier
- Raw water pumps: 3 – 750 gpm @ 50 feet TDH
- River level: 1101 to 1106 feet
- Screened intake: 0.25-inch openings
Filtration
How It Works
Particles in surface water are too small to be trapped in a filter, but they are processed to stick together to make larger particles. This process is called coagulation and flocculation. Chemicals, such as polymers, are added prior to the filters to change the stability of the individual particles.
First Stage
Flocculation, which occurs in the first stage of the filters, causes the particles to impact each other so they become attached. The first stage also acts as a roughing filter removing the larger particles. Water then flows down through a bed of granular media (anthracite and sand) trapping the smaller particles that are larger than the filter pores. Particles smaller than the pores become attached to the granular media of the filter by absorption.
Back-Wash
The filters are back-washed to clean the captured material from the filter media. This is accomplished by forcing air and treated water up through the media. The waste flows to the backwash water recovery tank where approximately 75% of the water is recovered and returned to the treatment facility with the raw water, while the solids settle out and are pumped to the sanitary sewer.
Chemical Treatment
Chemical additives are used in treating Rainy River water. Multiple application points for the chemicals are incorporated into the treatment scheme for operating flexibility and for the changing characteristics of the water supply. The chemical feed rates are set by automatic controls with manual overrides.
For more information, contact the Water Plant at (218) 283-2990. Read about the filtration process that occurs during the treatment process.
