They're where the water rests, so dirt can settle to the bottom and grease can rise to the top. The grease gets skimmed off, and the sludge keeps settling, gets pushed down and then squeezed into dry gray chunks called sludge cake. On a cool day, none of it smells too awful once you're past the first intake well. But there's just so much waste. Debris goes to the landfill.
Grease goes to the landfill.
Ash from the burned sludge cake goes to the landfill. The treatment plant hasn't always run perfectly, having had a couple periods since the 1970s when it spectacularly failed to comply with regulations. But it's back in compliance now, and its management problems have become too easy a target for a region wrestling with wastewater. The far more critical problem is capacity. The plant has room for a bit more sewage.