Step 1 - Coagulation
As the river water enters the Carrollton Plant, the purification process begins with the addition of coagulant chemicals: ferric sulfate and polyelectrolyte. These chemicals are added to the process at very precise dosages and mixed rapidly with the river water to ensure efficient and complete coagulation. Coagulant chemicals cause the very fine particles that make up the suspended solids present in the river water to clump together, or coagulate. Ferric sulfate is our primary coagulant, and polyelectrolyte is used as a coagulant aid.
Step 2 - Flocculation
After the raw water has been coagulated, it is gently mixed by large mechanical paddles in a process called flocculation. Flocculation causes the fine, light particles that were created during the coagulation process to mature into larger, denser, stable particles that will settle quickly.
Step 3 - Sedimentation
The flocculated water then travels into primary settling basins or clarifiers. In the primary settling basins, the large, dense particles formed during the coagulation and flocculation processes settle allowing the clarified water to be separated and forwarded on through the remainder of the water treatment process. The settled particles form a sludge layer on the bottom of each primary settling basin. This sludge is periodically removed from the basins and returned to the Mississippi River through a permitted discharge.
Step 4 - Disinfection
After the clarified water leaves the settling basins, the disinfection process begins with the addition of chlorine. Ammonia is added following the chlorine addition, producing chloramine. The chloramine disinfected water passes through a second set of basins to provide detention time for the disinfection process to go to completion.
Step 5 - pH Adjustment
The next step in the process is adjustment of the pH of the water. Lime, also known as calcium oxide, is added during this step to achieve the desired target pH. Adjusting the pH makes the water more basic, and less corrosive to the pipes in our water distribution system and the plumbing in our customers' homes, as well as extends the life of the disinfectant residual in the distribution system. A small amount of polyphosphate solution is also added with the lime. Polyphosphate is used as a sequestrant, which helps to keep the lime in dissolved in the water.
Step 6 - Fluoridation
After the water exits the secondary settling basins, it is treated fluorosilicic acid. A small dose of fluorosilicic acid is added at this point in the treatment process, which adds fluoride to the drinking water to aid in the prevention of dental cavities.
Step 7 - Filtration
The final step in the purification process is filtration through rapid gravity filters. This type of filter uses granular filter media (sand and anthracite at our facilities) to remove any remaining suspended particles in the water. This step in the treatment process consists of passing the water through a filter at a controlled rate. Any particles remaining in the water adhere to the filter media and are removed from the water.
After filtration, the purification process is complete.
Filtered water is collected from the many filters in service and flows to one of several pumping stations located on the plant grounds, where it is pumped and delivered to our customers to provide water for drinking and for fire protection.
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 process.
The Carrollton plant normally yields about 135 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 11 million gallons per day of water. Combined, the two plants treat approximately 54 billion gallons of water per year, removing 18,000 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 100,000 service connections. It is delivered to approximately 300,000 people on the east bank of Orleans Parish and approximately 53,000 people on the west bank.