In the grit removal channels, the velocity of the water slows down enough so that sand, gravel and rocks can settle to the bottom. From the grit removal channels, the flow passes to the primary settling basins. Here the water moves at an even slower rate. Materials that tend to either sink or float can be mechanically scraped off the bottom or skimmed off the top. These basins are designed to promote the removal of approximately 45-50 percent of the organic material from the sewage.
Primary sludge is pumped to the sludge holding tanks and de-watered using belt filter presses.
The presses convert the primary sludge, grease, and scum from a concentration of approximately 4 percent solids to a concentration of approximately 28 percent. Then the wastewater, called primary effluent at this point, enters large tanks where activated sludge is added. Activated sludge contains bacteria and other micro-organisms that consume or otherwise remove the contaminants. While this is happening large amounts of air are mixed into the water to provide for respiration and mixing. The aeration tanks promote the aerobic degradation of the wastewater.