21300 Chesdin Road – S. Chesterfield, VA 23803
Phone: 804-590-1145 | FAX: 804-590-9285 | EMAIL

 

Water Treatment

The Laboratory

lab3The Laboratory at the Appomattox River Water Authority includes a bacteriological lab, a wet chemistry lab, and an odors lab. The Laboratory performs analyses for conventional contaminants, disinfection by-products, and coliform bacteria. These analyses assist the plant operators by providing water quality assurance and technical support when needed.

Federal and State regulations guide the activities of the Laboratory and the reporting of analytical results in compliance with the U.S. Environmental Protection Agency’s Safe Drinking Water Act, the Virginia Department of Health Operating Permit, as well as the Department of Environmental Quality’s VPDES permit which regulates our discharges back to the reservoir or the Appomattox River.

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The Brasfield Dam

The Brasfield Dam impounds a portion of the Appomattox River, forming Chesdin Reservoir. The Chesdin Reservoir water storage volume estimates since 1968 are:

· 1968 As-built estimate = 12 billion gallons of water

· 2001 Recalculated 1968 as-built estimate based on 2001 Source Water Study by Gannett Fleming. This was determined from digitizing original contour maps = 10.5 billion gallons of water

· 2000 Bathymetric Survey = 9.6 billion gallons of water

· 2011 Bathymetric Survey = 9.3 billion gallons of water

Raw Water Pumping Station #1 (RWPS#1):  Raw lake water passes through a bar screen in the dam that prevents large debris from entering the pump well of the dam. Water can be taken from one of four depths of the lake: four feet, 14 feet, 24 feet, or 33 feet. Water then passes through a traveling screen that removes debris down to approximately 1/2″ in size. One or more of five pumps sends the water to the treatment plant by a 42″ water line. Potassium permanganate can be added here as a pre-oxidant.

Raw Water Pumping Station #2 (RWPS#2):  Raw water can enter the system from underwater screens at 7 feet and 20 feet. Water is pumped by a combination of five pumps (2 pumps at 20 mgd and 3 pumps at 10 mgd) to the treatment plant by a 54″ water line. Potassium permanganate can be added here as a pre-oxidant.

Chlorine Dioxide:  A feed point for chlorine dioxide addition is located immediately downstream from the Raw Water Meter for feeding chlorine dioxide as a pre-disinfectant/oxidant. Chlorine Dioxide is used to reduce TTHMs and HAAs formation in the plant.

Rapid Mix Building:  Water enters the rapid mix building and pre-feed chemicals are mixed. Lime (pH adjustment and alkalinity addition) and alum (coagulant) can be added here. The water passes through a combination of mixing chamber and is mixed by one or both of the turbine mixers.

Flocculators:  Raw water with chemicals added is sent to flocculators. All flocculators are horizontal paddles and over/under baffles. As the mixture proceeds through the three chambers of the basins, stirring is done at progressively slower speeds called step flocculation. Polymer is normally added to the water to aid in coagulation.

What is floc?

floc noun: a small, loosely aggregated mass of flocculent material, suspended in or precipitated from a liquid.

flocculent adjective: having a fluffy character or appearance.
1 : resembling wool, especially in loose, fluffy organization.
2 : containing, consisting of, or occurring in the form of loosely aggregated particles or soft flakes; a flocculent precipitate.

Coagulation or Settling Basins:  Water is distributed from the floc basins into one of twelve  settling basins. The majority of the floc created by chemical addition drops to the bottom of the basins, because of the density and weight of the floc. Sludge is removed daily by siphon from the basins using a ClariVac system. The settled water is taken off the top at the ends of the basins by weir launderers and flows into a pre-filter flume from one end of the plant to the other.

Filter Building:  Water is distributed from the settling basin flume into the filters. There are 32 filters that work in pairs, #1 – 8 (1968) capable of 2.75 mgd each, 8 filters, #9-16 (1985) capable of 3.00 mgd each, and 16 filters #17 -32 (2006) capable of 3.125 mgd. Normally filters are used at less than the maximum rate of flow. All filters have Carbon Caps which consist of 24” of Granular Activated Carbon (GAC). Filters are backwashed when the first of the following conditions occurs:

  1. 100 hours of operation since back washing,
  2. 6 feet of head loss has occurred or
  3. Turbidity =/>0.10 NTU. Normally turbidity is the controlling factor.

Pipe Gallery and Post Chemical Area:  Water from filters in operation is collected by a header pipe system in the pipe gallery and finally into a 54-inch pipe, where sodium hydroxide (pH adjustment to greater than seven), chlorine (for disinfection), phosphate (for pipe corrosion protection), and fluoride (for tooth decay protection) is added.

Post-chemical Addition:  Water leaves the plant area and is piped to a mixer where the post chemicals are added and mixed. SCADA (which is Supervisory Control And Data Acquisition) control then allows a sample of water to be taken, and the proper adjustment to caustic soda, fluoride and chlorine to be made. Water goes into a clearwell (5.5 million gallons) where the required contact time with chlorine is obtained.

After leaving this clearwell, the water is sampled, additional chlorine is added, ammonia is added, and caustic soda may be added. The chlorine and ammonia mix is called chloramines, a weaker but longer-lasting disinfectant than chlorine.Water is then metered to help with the automatic chemical adjustment, and stored in two other clearwells totaling 6.5 million gallons.

Finished Water Pumping Station:  The Authority can deliver water from our clearwells to our members by gravity flow or through our pumps stations. Up to 30 mgd can be deliver by gravity flow. When additional flows are needed the Authority can pump from one of our two pumps stations on site. Finished Water Pump Station 1 has 2 pumps rate at 8 MGD and 3 pumps rated at 16 MGD. Finished Water Pump Station 2 has three pumps rated at 23 mgd.