From the grit chamber, the waste water moves very slowly through what are usually called the primary clarification tanks. Here gravity does most of the work. Some of the heavier gunk sinks to bottom. Some of the light stuff floats to the top. It's called scum.
Machines scrape or skim the results.
Typically the sludge and scum is concentrated and sent to digestor tanks where microorganisms feed on it, breaking it down into methane gas, carbon dioxide, water, and a much smaller volume of sludge. Food for Bacteria After passing through the primary clarification tanks, waste water is ready for secondary treatment. First the water flows into aeration tanks where oxygen is added, which promotes the growth of bacteria that feeds on the organic waste still in the water. The water then passes into clarifying tanks where the floc is removed, some of which is returned to the aeration tanks to keep the activated sludge process moving along as new wastewater flows into the system and some of which goes to the digestor tanks for processing. By the time this secondary treatment phase is complete, about 90 percent of the original load of pollution has been removed.