As we walk from the Primary Clarifiers we come to 4 tanks, square in shape, that have mixers churning the brown water within. There is a lot of noise, but oddly, there is no bad odor. Actually, the smell is like damp earth. These are the Aeration Tanks. Each tank has a capacity to hold 200,000 gallons of water. Secondary Treatment processes commonly consist of biological processes. This means that organisms living in the controlled environment of the process are used to partially stabilize organic matter not removed by Primary Treatment and to convert it into a form which is easier to remove from the wastewater. Here, the wastewater is exposed to living organisms (such as bacteria) which eat the dissolved and non settling organic material remaining in the waste.
There are different methods for accomplishing this task. This plant uses a process called Activated Sludge. Wastewater flowing out of the Primary Clarifiers still contains some suspended solids and other solids that are dissolved in water. In a natural stream, such substances are a source of food for protozoa, fungi, algae and hundreds of varieties of good bacteria.
The Secondary Treatment stage is a highly controlled artificial environment in which these same microscopic organisms are allowed to work as fast and efficiently as they can. Oxygen is supplied to the tanks by the mixers. The 'good' bacteria and other organisms thrive as they travel through the aeration tanks. With sufficient food and oxygen they multiply rapidly. Most of the organic matter in the waste is used to produce new cells. The microorganisms biologically convert the dissolved solids in the wastewater to suspended solids which will physically settle out at the end of Secondary Treatment. The Operators at the Plant take care to make sure that the temperature, oxygen levels, and contact time in these tanks support rapid and complete consumption of the dissolved wastes. The final products are carbon dioxide, clean water…and more microorganisms!