Water, when it drains from our homes and factories into the sewer system, is waste water. It carries industrial and domestic wastes, including human wastes, through a complex network of pipes to a wastewater treatment plant. What we pollute, the treatment plant must unpollute. Polluted water carries harmful bacteria and viruses. It makes our lakes and rivers ugly and unhealthy. It's dead water, dangerous even to swim in. Out of necessity we must clean it. A wastewater treatment plant is a big watercleaning machine. Inside it, a series of tanks, screens, filters and other processes separate the wastes from the waters. Like other big machines, it can't run itself or repair itself, so it requires a planned program of operation and maintenance: operation-to make it work; maintenance-to keep it working.