Harris Treatment Plant
Robert A. Harris Water Filtration Plant & Water Supply Facilities
Eden's Robert A. Harris Water Filtration plant was completed in October, 1978. Expansion of the filter plant was completed in October, 1994. Its facilities now have the capacity to produce 21 million gallons per day (MGD) of treated water. In addition to the water filtration plant, the other major facilities include the following:
- An intake and raw water pumping station on the Dan River located on the south end of Bridge Street.
- Three booster pump stations located on Summit Road, Caleb Street, and Dunn Street.
- 7,000 feet of 30-inch pipeline to deliver water from the raw water pumping station to the pre-settling impoundment (lake) of 87 million gallon capacity.
- 15,500 feet of 30-inch, 22,000 feet of 24-inch, 3400 feet of 20-inch, and 36,000 feet of 16-inch service water transmission line to deliver water to the City of Eden.
- Five elevated storage tanks within the city limits.
Raw Water Intake & Pump Station
The intake pumping station is located on the north bank of the Dan River near Mohawk/Karastan Rug Mill. To be upstream of most points of local urban stormwater runoff, this location was selected thereby enhancing raw water quality.
The pumping station was excavated in granite rock with the pumping level well below the bottom of the Dan River. Tubular intake screens are located in the river. These units prevent the entrance of leaves and other debris and were also designed to minimize the entrance of sand into the pump wells. The sand that does enter is collected in the hopper bottoms of the pump well and removed daily with a water-operated educator. The pumping station is equipped with one 10 MGD and two 13 MGD vertical turbine pumps. Operating data from the station is transmitted to the filtration plant via radio frequency. The pumps are started and stopped from the filtration plant.
The intake pumps discharge into a 30-inch waterline which crosses the Smith River and delivers raw water to the pre-settling impoundment.
The pre-settling impoundment retains the raw Dan River water for a period of about five days at the present design capacity of the filtration plant. The impoundment improves raw water quality by allowing time for bacterial die-off and for reduction of turbidity. The impoundment also allows the filtration plant to operate for several days without taking water from the Dan River. In this way the extremely muddy water that occurs during and after heavy rains in the Dan River basin can be kept out of the impoundment.
The pre-settling impoundment was located in a small natural valley. An earth dam with a volume of 110,000 cubic yards was constructed in the valley to create the impoundment. Raw water flows by gravity from the impoundment to the filtration plant.
The filtration plant is designed to produce 21 MGD of treated water at the filtration rates presently allowed by the North Carolina Department of Human Resources.
The treatment system consists of the unit processes described below. The process flow schematic graphically illustrates the treatment system:
- Rapid Mixing: Chemicals, i.e., alum, PAC, polymer, and caustic are added in the influent pipe just prior to the rapid mix chambers at the beginning of the system. Alum or PAC is added to coagulate the fine clay particles which cause turbidity. Polymer is added as a coagulate enhancer. Caustic is added to control alkalinity as required for effective coagulation. The detention time in the rapid mix chambers is about 1.6 minutes. One 5.0 H.P. (Horse Power) mixer per chamber provides the high degree of turbulence required for rapid chemical mixing.
- Chemical Handling: Chemicals are purchased in liquid form and are stored in tanks outside the filtration plant building. These chemicals flow by gravity to smaller day tanks located on the ground floor level of the filtration building. Chemical feed pumps deliver the chemicals to the influent pipeline just prior to the rapid mix chambers. These pumps automatically supply the amount of chemical required in proportion to the plant flow.
- Slow Mixing (Flocculation): After the chemicals have been added in the rapid mix chambers, the water flows in a series through six slow mix units or flocculators. These slow mixers gently stir the water and thereby promote the contact of small particles of clay and chemical causing them to gradually grow into large flocculent particles which can easily be settled out. Each slow mixer (flocculator unit) is operated by a variable speed drive. In general, the speed of the units is decreased as the water flows in series through the six units. The speed decrease prevents the destruction of previously-formed large floc particles. The total detention time in the slow mix units is approximately 45 minutes at the design flow of 21 MGD.
- Settling Basins: The flocculated water flows to seven settling basins. Each basin is 141 feet-10 inches long, 37 feet-4 inches wide, and has a water depth of 13 feet-0 inches. The basins are equipped with mechanical sediment rakes which carry the settled material to hoppers located near the influent end of each tank. Automatic valves and a flow control system are utilized to remove this sludge from the basins under gravity flow. A sewer system transports the sediment to the nearby Mebane Bridge Wastewater Treatment Plant.
- Filtration: Any particles which escape removal in the settling basins are removed by granular media filters which follow the settling process. The filter bed consists of 18 inches of anthracite coal underlaid by 12 inches of silica sand. There are seven filters, one for each settling basin. Central gullet channels are provided in each filter to remove backwash water. Each filter unit has a total surface area of 512 square feet and will produce 2.95 MGD of filtered water when operating at a filtration rate of four gallons per minute per square foot (the state approved rate).
Water leaving each filter flows through a control device which maintains a constant flow through each filter unit. The water entering the plant from the pre-settling impoundment is maintained the same as the amount being filtered by controlling the water level in the settling basins. When the filters become clogged with the solid particles which have been removed, they are back-washed hydraulically. A 9,600 GPM (Gallons Per Minute) backwash pump has been provided for this purpose.
After leaving the filters, water enters a series of flumes under the floor of the filter pipe gallery. Post filtration chemicals are added in these flumes. Caustic is added to adjust the final pH and alkalinity of the water for corrosion control in the water in the distribution system, chlorine is added to ensure that the water is completely disinfected, and a small amount of fluoride is added to help prevent dental cavities. Also a corrosion inhibitor is fed to protect the distribution system pipes.
- Storage and Service Pumps: Finished water flows by gravity to two 4.0 million gallon pre-stressed concrete storage tanks and is pumped to the city distribution by a 9 MGD or one of two 12 MGD pumps. Water is stored in five city-owned water tanks. Freedom Park holds 1,000,000 gallons; Grove Street holds 100,000 gallons; Hamilton Street holds 150,000 gallons; Dunn Street and Caleb Street tanks hold 500,000 gallons each.
Service Water Transmission
Water from the treatment facilities is delivered to the distribution network through a transmission line consisting of 7,933 feet of 30-inch and 11,557 feet of 24-inch ductile iron pipe.