Approximately 20-35% of the solids that come into the plant, settle in the primary clarifiers. From there, they are pumped to the anaerobic digesters. In the absence of oxygen (anaerobic) along with other important factors such as temperature, pH, and a combination of acid and methane producing bacteria, the solids are reduced in volume. A usable byproduct of this process is methane gas, which we use for heating the digestives and the plant.
The secondary treatment comes after primary clarification. Any suspended solids that did not settle in the primary clarifiers are carried over into the second stage of treatment. This stage is called activated sludge.
This process has been around for more than 100 years. It uses bacteria that need air, in an aeration (oxygenated) basin. It basically speeds up nature's natural process of decomposition. The suspended and dissolved solid waste is food for the bacteria. This food becomes broken down through adsorption and absorption.
After a mix and digestion period, the next step is gravity separation of the solids and the liquid in the secondary clarifiers. The solids are pumped off the bottom, and returned to the aeration basins, where they become the seed stock for the newly arriving flow to receive the same digestion process.
To keep the bacteria hungry, we keep them at a certain level in their growth cycle by removing a portion of the solids. So, part of the return activated sludge is removed and becomes waste activated sludge (WAS.) This WAS is sent to the anaerobic digesters for storage and further digestion. If you have any questions, please contact Wastewater Treatment.