Explain secondary treatment and tertiary treatment in wastewater treatment plant.
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Secondary Treatment
Aeration Basins: Biological
From the Primary Clarifiers, the wastewater flows into large, rectangular tanks called Aeration Basins, where a biological treatment called the “activated sludge process” occurs. The wastewater flows slowing through a series of chambers as large volumes of air are bubbled up through the water. There is so much air added that it looks as if the water is boiling. In these basins, the wastewater is mixed with the “activated sludge;” hundreds of millions of actively growing single-celled microorganisms (mostly bacteria and protozoa) referred to as “bugs.”
Aeration Basins
The air is needed to help the organisms thrive and multiply. What do they eat? The waste in the wastewater! As the bugs eat, the wastewater is cleaned. While the wastewater is in the Aeration Basins, mixed with millions of bugs and air bubbling through it, it looks like boiling hot chocolate. When all of the food (waste) is gone, after about eight hours, the wastewater leaves the Aeration Basins.
Some of the bugs are sent back to the beginning of the Aeration Basins to keep the process going. Some organisms move on with the wastewater to the next step in the treatment process, the Secondary Clarifiers. The Aeration Basins can hold 1.1 million gallons at a time.
Secondary Clarifiers: Physical
When the wastewater enters the two Secondary Clarifiers, it still contains lots of microorganisms from the Aeration Basins and looks brown and murky. The Secondary Clarifiers are identical to the Primary Clarifiers; materials in the wastewater sink and float and rotating arms remove this material from the water.
After treatment in the Secondary Clarifiers, the wastewater is now ready to be released to the Napa River, or further treated to produce recycled water. The Secondary Clarifiers each hold 800,000 gallons of water.
Tertiary Treatment
Filtration: Physical / Chemical
Wastewater leaving the Secondary Clarifiers looks as clean as drinking water! Depending on conditions, this water can go directly to the Disinfection process to produce recycled water, or it can go the Filtration Building.
The Filtration Building contains a series of sand filters that are 27 feet tall, and can filter almost 14,000 gallons of water per hour. The filters remove very tiny solids (“suspended solids”) from the wastewater before it moves on to disinfection. Polymers are added at this step to cause the suspended solids to clump together, making them easier to filter out.