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| The Water Purification Process |
Raw water from the Mississippi River is pumped to the
Carrollton Water Purification Plant from both the Oak Street River Station and
the Industrial Avenue River Station. The purification process begins with the
addition of a coagulant chemical, commonly called polyelectrolyte, at each
river station. Polyelectrolyte consists of long chain molecules carrying electrical
charges that cause fine suspended particles to coagulate, or gather together into
larger particles. Water treated with polyelectrolyte travels through several large
pipelines from the river pumping stations to the water plant.
As the raw water enters the Carrollton Plant, it is treated with ferric sulfate and
lime. Ferric sulfate is also a coagulant chemical, and it is used to aid the
polyelectrolyte in the raw water clarification process. Lime, also known as calcium
oxide, is used for pH adjustment, softening, and corrosion control.
After the raw water has been treated with polyelectrolyte, ferric sulfate, and
lime, it is gently mixed by large mechanical paddles in two flocculation basins
where the suspended particles gather together into larger particles. The
flocculated water then travels into two (2) primary settling basins where the
particles settle. The settled particles form a sludge layer on the bottom
of each primary settling basin. This sludge is pushed into sumps by traveling
mechanical rakes, and is then removed from the basins through a series of valves
and pumps.
The clarified water then exits the settling basins, and is disinfected by the
addition of free chlorine. Anhydrous ammonia is added shortly thereafter, producing
chloramine. Chloramine is a compound produced by reacting free chlorine with
ammonia, and is used for residual disinfection.
Chloraminated water then enters large secondary settling basins, allowing
additional settling time for suspended particles and disinfection contact time.
After the water exits the secondary settling basins, it is treated with sodium
hexametaphosphate and fluorosilicic acid. The sodium hexametaphosphate is used
as a sequestrant, which holds the lime in solution keeping it from depositing on
the filter or the media. The fluorosilicic acid is used to add fluoride to the
drinking water to aid in the prevention of dental cavities.
The final step in the purification process is filtration through 44 rapid sand
filters. These filters consist of graded gravel topped first with a layer of sand
and then with a layer of anthracite. After filtration, the purification process is
complete, and drinking water is pumped out to customers.
The Sewerage and Water
Board also operates a water treatment plant on the west bank of the Mississippi
River in Algiers. The purification process at the Algiers Water Purification Plant
is similar to that of the Carrollton Water Plant, utilizing the same water
treatment chemicals with a slightly modified application scheme for their upflow
clarifiers.
The Carrollton plant normally yields about 115 million gallons per day of finished
water for the east bank of Orleans Parish. The Algiers Plant, which serves the
predominantly residential west bank portion of the parish, purifies about 10
million gallons per day of water. Combined, the two plants treat approximately
47 billion gallons of water per year, removing 20,000 tons of solid material from
the raw river water.
The treated water at the two plants is pumped through more than 1,610 miles of
mains to more than 160,000 service connections. It is delivered to approximately
440,000 people on the east bank of Orleans Parish and approximately 57,000 people
on the west bank.
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