Down the drain

The average American uses at least 300 liters (79 gallons) of water a day. What happens to all this water? In many towns, wastewater flows through sewer pipes to a treatment facility like the one here. Follow its progress, starting with the grit chamber, above right.

1. GRIT CHAMBER Wastewater runs through a series of screens that filter out large materials.

2. PRIMARY SEDIMENTATION TANK Wastewater flow slows down and suspended solids (undissolved waste) sink to the tank's bottom.

3. SECONDARY AERATION TANK Wastewater mixes with air and microorganisms. The air and microorganisms help break down solids that did not settle out in the primary sedimentation tank.

4. SECONDARY SEDIMENTATION TANK Wastewater circulates between the secondary aeration tank and the secondary sedimentation tank until it is clear of remaining solids.

5. CHLORINE TANK Wastewater free solids is disinfected with chlorine before being released into the environment--to water a golf course, for instance.

6. SOLID TREATMENT FACILITY Solids that settled out in the primary and secondary sedimentation tanks end up here, where they are treated with heat and chemicals to kill disease-causing organisms. Then the solid waste is discarded or recycled.