When your “black water” (from toilets) and “grey water” (from sinks and showers) enter the tank, the first thing it does is chill out. The tank is split into chambers, and the flow slows to a standstill. This is the sedimentation stage, and it’s surprisingly simple.
Heavy solids—like coffee grounds, soap scum, and you-know-what—sink to the bottom and form a thick layer called sludge. Meanwhile, lighter stuff—fats, oils, and grease—float to the top and create a scum layer. This restaurant-sized mess sits quietly, letting the middle layer, the liquid effluent, prepare for its next adventure.
Fun fact: A single household generates enough grease over a year to fill a bathtub. That’s why pouring oil down the sink is basically asking your treatment plant to take a very expensive nap.
