The final step in the treatment process is disinfection using chlorine. The Chlorine Contact Basins are long, snake-like channels where chlorine is added to the flowing treated wastewater for disinfection. Disinfection is the process where disease-causing organisms (mostly bacteria) are killed by the chlorine. It’s the same process used in swimming pools (with the same chlorine smell), but the chemicals used are much stronger.
The long channels slow down the flow, allowing the chlorine enough time, at least two hours, to kill all the disease-causing organisms. Because chlorine can also kill fish and other aquatic organisms, the chlorine is neutralized with sodium bisulfite whenever the treated water is discharged to the Napa River. This process of removing the remaining chlorine is called dechlorination.
Where did all the solids go that were removed from the wastewater in the Primary and Secondary Clarifiers? To the Digester, a giant egg-shaped building that is sometimes referred to as “the giant stomach.” Even though it doesn’t look like a person’s stomach, it works in a similar way.
All the solids, called sludge, from the Primary and Secondary Clarifiers are pumped to the egg-shaped Digester to be digested. The sludge in the Digester is kept at 95.5 degrees Fahrenheit, about the same temperature as a human stomach. In the digester, anaerobic bacteria (bacteria that do not use oxygen) break down the sludge. The sludge remains in the digester for 16 days to as long as 50 days.
The Digester is enormous; it stands 74 feet tall and contains 1.3 million gallons of sludge. After the sludge is thoroughly digested, it’s called biosolids. As a result of digestion, gases such as methane and carbon dioxide are also produced in the Digester, just as in the human stomach. The methane gas (or biogas) is actually captured and used to produce electricity in a process called cogeneration. This energy is used to help heat the digester and power the wastewater treatment plant.
From the Digester, the biosolids are pumped to the Solids Handling Building where they’re pressed to remove extra water. The de-watered biosolids are then trucked to surrounding fields where it is plowed into the ground as a soil conditioner. The biosolids contain nitrogen, phosphorus, and potassium; three elements that plants need to grow.