Narrative Description
ROBERT A. HARRIS WATER FILTRATION PLANT AND WATER SUPPLY FACILITIES
EDEN, NORTH CAROLINA
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:
1. An intake and raw water pumping station on the Dan River located
on the south end of Bridge Street.
2. Two booster pump stations located on Summit Road and Dunn
Street.
3. Seven thousand 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.
4. Seven thousand feet of 30-inch and 11,000 feet of 24-inch
service water transmission line to deliver water to the City of
Eden and Miller Brewing Company.
5. Five elevated storage tanks within the city limits.
Raw Water Intake and Pump Station
The intake pumping station is located on the north bank of the
Dan River near Fieldcrest Cannon's 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
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 eductor. The pumping station is equipped
with one 10 MGD and two 15 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.
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.
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 the unit processes described
below. The process flow schematic graphically illustrates the
treatment system.
1. Rapid Mixing: Chemicals, i.e., alum, caustic, and chlorine,
are added in the influent pipe just prior to the rapid mix chambers
at the beginning of the system. Alum is added to coagulate the
fine clay particles which cause turbidity. Caustic is added to
control alkalinity as required for effective coagulation. Chlorine
is used for disinfection and for oxidation of iron and manganese.
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.
2. 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.
3. 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.
4. Settling Basins: The flocculated water flows to seven settling
basins. Each basin is 141'-10" long, 37'-4" wide, and
has a water depth of 13'-0". 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.
5. 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 by12 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. If the settling basin level drops,
the plant influent valve opens slightly to admit more water; if
the level rises, the valve closes slightly to admit less water.
When the filters become clogged with the solid particles which
have been removed, they are backwashed 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 insure 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.
6. 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 and
one tank owned by Fieldcrest Cannon. Byrd Street tank has a capacity
of 75,000 gallons; Grove Street holds 100,000 gallons; Hamilton
Street holds 150,000 gallons; Dunn Street and Caleb Street tanks
hold 500,000 gallons each. Karastan tank (Fieldcrest Cannon) holds
250,000 gallons.
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.
Unit Process Inventory
CITY OF EDEN MEBANE BRIDGE WASTEWATER TREATMENT PLANT
Unit Process/Operation Inventory and Parameters of Design
CAPACITY
13.5 MGD Plant As per completion on July 1, 1992
INFLUENT
Receives Force Main flows from the Junction Pump Station the Convenant
Branch Pump Station, and the Industrial Park Pump Station
Influent flow is measured by a Parshall Flume
Septic Waste Receiving Pump Station
SCREENING
Two Mechanical Bar Screens with 1 inch clear openings One Manual
bar screen with 1 inch clear openings
GRIT REMOVAL
One chain and bucket type grit collector
Aeration System with two Positive displacement blowers
One collection hopper for Transport to Landfill
EXTENDED
Two Aeration Basin with a 7 MG volume
AERATION
Each Basin will have: Twelve
20 HP brush aerators
Three mechanical floating mixers
Retention Time Approximately 24 hours @ 13.5 MGD flow rate
Two, flow splitter boxes to regulate flow to clarifiers
Two, 90 Ft. diameter clarifiers Siphon feed, Peripheral effluent
and suction type sludge collectors
Two, 130 Ft. diameter clarifiers Siphon feed, Peripheral effluent
and suction type sludge collectors
Wet well/Dry well recirculation station #1 with
Three 1550 GPM centrifugal pumps
Wet well/Dry well recirculation station #2 with Three 3125 GPM
centrifugal pumps
Two 800 GPM Submersible Waste Activated Sludge Pumps
DISINFECTION
Two V-notch 500 lb/day Automatic Chlorinator w/flow proportional
controllers
Automatic Switchover w/vacuum regulator
Daul Ton Cylinder Weight Scale
One V-notch 500 lb/day Manual Chlorinator
One flash mixing chamber with turbine type mixer with 14000 GPM
Chamber flow
Three 74000 gallon baffled chlorine contact basins
CHEMICAL FEED
Storage Tank for Polymer and associated feed pumps for color
removal, fed to a post-aeration location
Storage Tank for Sodium Bisulfite and associated feed for Dechlorination
DECHLORINATION
Sodium Bisulfite is used for Dechlorination
AEROBIC SLUDGE
One 2.3 MG Basin
DIGESTION
Three 20 HP brush aerators
One 25 HP floating aerator
Wet well/Dry well decant station
Two 300 GPM Centrifugal pumps
Decant pumps to the recirculation wetwell
Wet well/Dry well Waste Sludge Pumping Station
Two 150 GPM centrifugal pumps
Pumps Sludge to Sludge Holding Lagoons
SLUDGE DEWATERING and DISPOSITION
Two 2 MG Sludge Holding Lagoons
Five Sludge Drying Beds (Currently unused)
One Gravity Decant Dewatering Line
One Pump Assisted Gravity Decant Dewatering Line Decant water
returns to recirculation wetwell
Sludge is Land Applied by Contracted Company
Robert
A. Harris Water Filtration Plant
Mebane
Bridge Wastewater Treatment Plant
Dry
Creek Wastewater Treatment Plant