The water treatment process begins at one of the City's three drinking water reservoirs: Lake Townsend, Lake Brandt, or Lake Higgins. Water from Lake Brandt is treated at the Mitchell Water Treatment Plant, and water from Lake Townsend is treated at the Townsend Water Treatment Plant. Water from Lake Higgins is used to fill Lake Brandt in times of drought. Steps of treatment include coagulation/flocculation, sedimentation, filtration, stabilization, fluoridation, and chlorination. View a diagram of the treatment process.

Coagulation/Floculation

In the coagulation/flocculation process, very fine suspended particles are caused to come together to form larger particles that can be settled and filtered out of the water. These include fine silts, bacteria, color causing particles and viruses that might not settle for days, months or even years. Although individual particles cannot be seen with the naked eye, their combined effect is often seen as cloudiness, or turbidity, in the water. These particles are small enough to pass through later treatment processes if not properly coagulated and flocculated. This could adversely affect not only the clarity of the water, but its taste and odor, as well as the effectiveness of chlorine disinfection.

During coagulation, the chemical Aluminum Sulfate, or filter alum, is added to the incoming raw water (untreated water from the lakes). The water is stirred vigorously in a flash mixer to assure quick, uniform dispersion of the alum. The alum reacts rapidly with compounds in the water that contain carbonates, bicarbonates and hydroxides to produce a jelly-like substance that absorbs impurities. At the same time, alum, with a positive charge, neutralizes the negative charge common to natural particles, which draws them together. Small particles called microfloc are formed. The following equation shows the reaction of alum with alkalinity:

The water moves from the flash mixer to the flocculation basins (pictured at right), which contain horizontal paddle systems. The basins provide a gentle, constant mixing of the microfloc formed during coagulation. This stirring promotes formation of larger and heavier floc. After 20 to 30 minutes, the floc particles are usually visible and will look like tiny tufts of cotton or wool, separated by clear water. Once the flow is of sufficient size and density to be settled, the water moves into the sedimentation or settling basins.

Sedimentation

Sedimentation is the removal of solids from water by gravity settling. Basins (pictured below left) are designed to hold large volumes of water for several hours and to give a smooth, even flow. This design allows the velocity and turbulence of the water to be decreased to the point that the water will no longer transport the flocculated solids and they will settle to the bottom of the basin.

The inlet side of each basin is 16 feet deep and tapers up to a depth of 10.8 feet at the outlet. As the water moves slowly through the basin, it gradually becomes clearer over a four to five hour period. The solids that settle out are periodically removed, de-watered and disposed of by landfill.

At the end of the settling basin, hydrated lime, Ca(OH)₂, is added to the water to increase its pH. This is necessary because the alum that is added during the coagulation/flocculation process is acidic. After pH adjustment, chlorine is added for disinfection and to prevent aquatic growth on the filters during the next treatment process.

Filtration

The water flows from the sedimentation basins into the filter beds (pictured at right). Filters contain material of graduated size in layers that remove solids from the water. The first layer of filter is anthracite coal and the second layer is filter sand. Under the sand is a layer of stone or plastic composite. At the bottom of the filter is an underdrain system that supports the filter.

Water is pumped onto the filters, leaving behind suspended matter as it passes downward through the filter. Solids that are larger than the pores or holes between the grains are captured by the filter. Also, some suspended matter adsorbs, or sticks to the surface of the filter or the previously deposited material.

After a period of time, the accumulated solids begin to clog the filter, so the filter is backwashed. Backwashing is the reversal of the direction of the water flow through the filter. A rapid upward flow lifts the particles and keeps them in suspension until they can be washed out.

Following filtration, the water flows underground into a storage tank called a clearwell. Water leaving the clearwell is stabilized, fluoridated and chlorinated in-line on the way to the distribution pumps.

Stabilization

Stabilization is a protective measure that reduces corrosion in water lines. Corrosive waters can cause problems related to public health, aesthetics, and economics. Polyphosphate, a chemical that prevents corrosion, is added to the finished water to prevent these problems.

Fluoridation

Fluoridation is a safe and economical process that is endorsed by public health groups worldwide to reduce tooth decay. Fluoride is added to the line leaving the storage tank so the fluoride dosage can be very precisely controlled.

Chlorination

Chlorination of public water supplies is the most important process used to produce safe drinking water. A sufficient amount of Sodium Hypochlorite, essentially a strong bleach, is added to the finished water so that a minimum amount of chlorine remains in the water until it reaches the customer's tap.

Distribution

After the final addition of hypochlorite, as well as the fluoride and a corrosion inhibitor, the water is pumped into the pipes that lead to the customer's tap. The pumping system includes electric as well as diesel pumps to ensure uninterrupted water service, even during power outages or other distribution challenges.

Because the distribution system is long (2,000+ miles) and complex, and chlorine dissipates, re-chlorination is sometimes necessary. Hypochlorite is added at several points in the distribution system as needed. This assures that a minimum amount of chlorine will remain in the water to safeguard it to the customers’ taps.