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News: Plant Spotlight

Plant Spotlight: City of Albemarle, Tuckertown WTP

Monday, December 5, 2016  
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Originally printed in the Fall 2016 issue of NC Currents magazine.

 

Administrative building

 

 

Filter consoles

 

 

Lab 

 

 

 Chemical feed building

 

 

 Control room

 

 

 Pipe gallery

 

 

Sedimentation basins

 

 

 Stripping towers

 

 

 Tuckertown Reservoir

 

 

 

By Chris Maidene, Treatment Plant Supervisor and John Rutledge, Smart Cover Systems
(NC AWWA-WEA Plant Operations & Maintenance Committee)

 

General

Located in central NC, the City of Albemarle is the largest city in Stanly County, with a current population of approximately 16,000. The city was incorporated in 1857 and is the county seat. In addition to several manufacturing facilities, Albemarle is only minutes from Morrow Mtn. State Park, which is part of the Uwharrie Mountains.


The City of Albemarle currently operates two Water Treatment Plants. The Tuckertown WTP is located on Highway 49 near the Tuckertown Reservoir bridge and Stanly County line. The second plant is the Highway 52 WTP located on Highway 52 North. The source water for the Highway 52 WTP is the Narrows Reservoir. The drinking water from both of these two plants is blended in the distribution system then sent to the City of Albemarle and Stanly County Utilities. In addition to this, the water is sold wholesale to Concord/Kannapolis and the Pfeiffer/N. Stanly Water Systems.

 

Planning began in 1983 for the Tuckertown WTP and after construction the plant was put into operation in 1991. The WTP currently employs a total of eight plant personnel with an annual operating budget of approximately $4 million.

 

The plant treats raw water that is pumped from the Tuckertown Reservoir, which is part of the Yadkin/Pee Dee River Basin. The reservoir is located between Badin Lake & High Rock Lake.


The WTP has a design flow of 6.8 mgd and operates with an average daily flow of 3.2 mgd and a maximum daily flow of 5.5 mgd. The plant was originally designed such that it could be expanded to 30 mgd, but due to the loss of some local industry additional water capacity is not needed at this time.


Process Flow Description

Raw Water Intake and Pump Station

The Raw Water Intake and Pump Station were originally built in 1985 to supply water to the Highway 52 WTP. After the Tuckertown WTP was completed in 1991, the water was sent to the plant and the remainder of the Highway 52 line was converted to a finished water service main. The intake structure includes one intake screen and two vertical turbine pumps rated for 3,500 gpm at 200’. The intake screen includes a backwash and air scour system. The pumps were installed as part of the original 1985 installation. A third pump was added when the new WTP came on line.

 

Raw Water Reservoir

The Raw Water Reservoir was built to provide water supply to the plant during periods of high turbidity or contamination in the Tuckertown Reservoir. It is an earth construction type with a HDPE liner. The maximum capacity is 42 million gallons at its highest elevation. It includes three floating aerators that are located between the outlets.

 

Flash Mix Tank and Flocculation

Prior to the Flash Mix Tank, alum is fed into the raw water line. It is mixed in the 16,500-gallon tank using one 5-HP vertical turbine mixer. A raw water sample is sent from here to a raw water turbidimeter and streaming current detector (SCD). The SCD is used to manually adjust the alum feed. Raw water entering the plant flows through a 20-inch Venturi flow meter with an integral butterfly control valve.

 

Once flow leaves the flash mix tank, it enters a dual train flocculation basin. The basin consists of six cells with each cell having a capacity of 31,500 gallons. The flocculators consist of six vertical turbine slow speed mixers – one each cell, three per train.

 

Sedimentation Basins and Filters

The Sedimentation Basins consist of four 439,250-gallon conventional style rectangular basins. The flow from the Flocculator Basins enters through a central channel. Each flocculation train can be isolated to flow into any of the sedimentation basins. Sludge removal is accomplished using a vacuum sludge removal system with its primary control panel located next to the main control panel in the operations building.

 

The Filter Basins include four conventional dual cell filters that are made up of anthracite, sand, gravel and gullet. The flow from the sedimentation basins enters through a central channel allowing flow to be isolated into any of the filters. Each filter has an area of 364 SF and a flow rate of 3.05 gpm/SF and is equipped with rotary surface sweeps and washwater troughs. The pipe gallery instrumentation includes filter effluent turbidimeters and flow meters. Three dual filter consoles are located directly above the pipe gallery on the main floor of the operations building. From each console, manual backwash control can be initiated.


Clearwell and Finished Water Pumps

The 280,000 gallon clearwell is constructed of cast-in-place concrete and includes baffle walls to provide the required cycle threshold value.

 

There are two 250-HP finished water pumps that are rated at 8.2 mgd. If required, the flow can be trimmed using discharge butterfly valves. Finished water is pumped to the stripping towers to remove VOCs and then pumped into the 4 MG ground storage tank. Finally, the finished water flows into the distribution system.

 

Chemical Feed and Disinfection System

The chemical feed system consists of fiberglass reinforced plastic (FRP) bulk storage tanks and metering pumps. Bulk storage includes two 10,000-gallon tanks each for alum and caustic, one 6,000-gallon tank each for fluoride and sodium bisulfate and drum storage for polymer and orthophosphate. Disinfection is achieved by a conventional gas chlorination feed system. It consists of two manual chlorine gas vacuum feeders (two standby) rated up to 500 PPD. In the summer the system feeds 2.3 ppm and in the winter it feeds 1.8 ppm. Chloramination was determined to not be an option due to a kidney dialysis center that receives water from the WTP.

 

Stripping Tower and 4 MG Ground Storage Tank

The Stripping Tower and Ground Storage Reservoir are located on a hill overlooking the plant. The finished water pumps send water from the clearwell to the stripping tower and a portion to the 1 MG backwash tank. The stripping towers consist of two force draft, packed tower aerators for removal of volatile organic compounds (VOC). The towers are 12 feet in diameter and 24 feet in height. They each utilize two 20-HP fans at 23,600 CFM. Post chlorine is fed into the tower outlet piping and then pumped to the 4-MG ground storage tank.

 

Solids Treatment

Sludge removed from the sedimentation basins are pumped to the sludge tank. The backwash water is collected into two decant basins. Supernate decanted off the decant basins is sent to the lagoon for holding, while sludge from the decant basins goes to the sludge pit. The sludge is removed by a sludge removal company contracted by the city then land-applied to a seven-acre field adjacent to the WTP.

 

SCADA System

The original WTP was equipped with a large freestanding control panel with chart recorders, meters, indicating lights and a graphic display panel. While the panel still exists in the control room, the primary control functions of the plant have been replaced with a supervisory control and data acquisition (SCADA) system. This system consists of computer monitors, human-machine interface (HMI) software and communication network that allows the operators to view the operations of the plant. The filter backwash system is automated using programmable logic controllers (PLC), but can be placed in manual if necessary.

 

Laboratory

The laboratory staff performs a variety of analyses at the in-house lab located in the operations facility. Process and finished water are analyzed for total coliform, E. coli bacteria, hardness, iron, and fluoride on a daily basis.

 

A spectrophotometer is used for many of the analyses, most frequently for iron, fluoride, and chlorine. It is also used for nitrate and copper for the purpose of reservoir management, UV254 absorbance to quantify organic material in process water, and the estimating of total trihalomethanes (THMs) content. A Colilert test is used for bacterial analysis of water sampled at the plant as well as the 15 samples collected each month in the distribution system. All parameters that require testing only quarterly or less frequently are outsourced to an independent lab.

 

Challenges

One of the biggest challenges for the WTP has been THM and haloacetic acid (HAA) issues. In order to address this issue, it was determined that the chlorine feed rate required adjustment to minimize the THMs and HAAs while still providing the required amount of disinfection and residual to the finished water. In addition to this, prechlorine feed to the head of the plant was eliminated to further minimize THMs.

 

Unique Process Features

The most remarkable feature of the plant is that it utilizes force draft, packed stripping towers for the treatment of VOCs. Air stripping is accomplished in the packed tower when dissolved molecules are transferred from water into a flowing air stream. The water is pumped to the top of the tower and sprayed uniformly across the packing through a distributor. It flows downward by gravity in a film layer along the packing surfaces. Air is blown into the base of the tower and flows upward, contacting the water. The packing provides a very large surface area for mass transfer of VOCs from the water to the air and out the top of the column. After this process, the water is sent to the ground storage tank and then on to distribution.

 

Wildlife

The plant sits on 200 acres and has abundant wildlife living on acreage behind the facility. Wildlife such as deer  turkey, osprey, coyotes, barn swallows  and eagles nesting can often be seen. The Tuckertown Reservoir is home to countless fishermen who can be seen anytime day or night near the plant entrance or on the water. It is one of the many beautiful areas that North Carolina has to offer.

 

For additional information:
Chris Maidene, WTP ORC
PO Box 190
Albemarle, NC 28002-0190
Phone: (704) 986-9656
cmaidene@albemarlenc.gov
www.albemarlenc.gov


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