Water Treatment in Detail
Fargo's Water Treatment Plant is designed to provide presedimentation, softening, disinfection, taste and odor reduction and filtration.
The plant is equipped with two 15 MGD presedimentation trains. Each consists of a rapid mixing chamber, a two-stage tapered flocculation basin with vertical turbine flocculators and a high-rate sedimentation basin equipped with inclined plates and sludge collection equipment. Chemical feed capabilities to the presedimentation facility will include powdered activated carbon, alum, potassium permanganate, coagulant polymer, and chlorine.
Following presedimentation, the water will flow to three 10 MGD softening basin trains. Lime, soda ash, and sodium aluminate will be fed to the mixing zones at the primary softening basins. From the primary softening basins, the water will flow to the rapid mix chambers. The rapid mix chambers are provided for quick and effective dispersion of the chemicals. Chemicals fed to the rapid mix chambers are soda ash, coagulant polymer, ferric sulfate and carbon dioxide. From the rapid mix chambers, the water enters the bottom of each secondary softening basin through a 42-inch pipe centered within a mixing and flocculation zone. The treated water from these secondary softening basins will be collected by rapid effluent launders with submerged orifices and conveyed to the ozone contact basins. The softening process design is based on a consistent finished water total hardness of 120 milligrams per liter (mg/L) and a magnesium hardness not exceeding 40 mg/L.
Disinfection with Ozone
The plant uses ozone for primary disinfection instead of chlorine, which was used at the old plant. Ozone is a very powerful disinfectant and also has the capability of reducing tastes and odors in the water. Ozone must be generated on site because of its short life span.
Two multi-stage ozone contact basins were constructed. Each basin is sized for a design flow rate of 15 MGD. Each ozone contact basin contains six stages. Carbon dioxide and polyphosphate are applied in stage 1 counter current flow, followed by four minutes of detention in stage 2 to attain a plant water pH of 9.0 prior to ozonation. Ozone gas is applied using fine bubble-type diffusers in counter-concurrent stage 3 and co-current stage 4. Ozone decay occurs in stages 5 and 6. Plant water exits each basin over a concrete effluent weir designed to provide approximately three feet of free fall for ozone gas stripping and flows into the filter influent flume, where fluoride, polyphosphate, filter polymer and chlorine are applied. Excess ozone gas enters the ozone destructers and is converted to oxygen before it is released to the atmosphere. The ozone destruction system is located in an enclosed room on the top slab of the contact basins.
Six filters are provided, each having an initial rated capacity of 5 MGD. Each filter is equipped with air and water backwash facilities, fiberglass wash water troughs, dual media and underdrains. The filter media consists of 12 inches of gravel, 10 inches of sand, and 20 inches of anthracite.
Finished water from the filter clearwells is conveyed to a transfer pumping station by gravity through two 42-inch pipelines. Ammonia and chlorine is added to the filtered water in these clearwell transfer pipelines. Chlorine is used for emergency primary disinfection should the operation of the ozone system be interrupted. Four 10 MGD transfer pumps then deliver the finished water to our 6.25 million gallon below-grade storage reservoir.
Plant Control System
The plant control system is a PLC-Host type system. PLCs located throughout the plant provide control of equipment. A host computer accommodates operator interface, data collection, reporting, and other system functions. In general, all process equipment is controlled by the plant control system. The system consists of manual and automatic controls as required by the individual equipment or system. Manual controls through the plant control system operate equipment according to operator commands at the central control console. Automatic control operates equipment according to process parameters or control programs, with no significant operator intervention required.
A high service pumping station delivers the water from the storage reservoir to the distribution system. There are eight elevated water storage towers in the distribution system which provide storage for 5 million gallons of water. In addition to storage, the water towers are important to maintain a more uniform pressure in the distribution system, especially during periods of high demand.
Our water treatment plant was designed to provide a high quality finished water of sufficient quantity to meet the needs of a growing city. The plant should meet or exceed the requirements of the Safe Drinking Water Act for the foreseeable future. Considerable flexibility has also been designed into the project to allow compliance with future federal regulations.