A cesspool is a big tank with a porous bottom and sides that lets the liquid wastewater filter into the ground while holding the solid waste. Septic systems are somewhat similar, though the tank is connected to a drainage field so that more waste can be dispersed over a wider area. In larger communities, sewer water passes first through a screen, which filters out the larger debris. It then runs through a grit chamber, a long, shallow trough with a dip in the bottom that acts like a trap. As water moves through the trough, small, hard materials in the water drift down to the bottom and fall into the trap.
Grease floats to the surface and is skimmed off.
The trap, like a cesspool, is periodically scraped clean. To remove some of these, the sewage is trickled into a sedimentation tank, or settling basin. The water enters through a pipe, then circulates slowly while the suspended particles settle to the floor. The top layer of water continually runs out through exit holes. The sludge from sedimentation tanks may be sent through a tank called a digester, where bacteria digest it, producing carbon dioxide and methane gas and other by-products.