Why tap water sometimes runs red, blue or tastes of metal
Water can leave a treatment plant perfectly clean and still reach a kitchen tinted. Rust from unlined iron mains produces complaints of red water; copper pipes can turn it bluish or give a metallic tang. Keeping water pleasant after treatment is half the job of a supply network.
A supply system runs from a catchment through a raw water intake, whether a lake, river or well tapping an aquifer, along aqueducts, tunnels or pipes to a treatment works, then into storage and out through a web of distribution mains to homes, factories and fire hydrants. Sewers are usually treated as a separate system. Treatment plants tend to sit near the customers they serve, which cuts pumping costs and reduces the chance of contamination after cleaning.
Pressure is essential. Networks are kept above the pressure of the surrounding ground so that every outlet gets enough flow and dirty groundwater cannot seep in through cracks. The usual trick is to pump water up into service reservoirs built at the highest local points, letting gravity do the rest; small domestic setups may rely on a pressure vessel instead. Tall buildings often need their own pressurised storage to lift water to the top floors.
Water chemistry keeps shifting on its journey. Besides iron and copper, lead can leach from the solder joining copper pipes or from brass fittings, and regulators limit copper and lead at the tap. Utilities fight corrosion by tuning pH and alkalinity so a thin protective layer of calcium carbonate coats pipe walls, and by adding phosphates or silicates as inhibitors. As water leaves the plant it receives a dose of disinfectant, typically monochloramine or sodium hypochlorite, and booster stations top it up in distant parts of the network.
Mains are laid out much like road or power grids, in loops, branches or a mix. Looped designs let engineers isolate a broken section without cutting off everyone else. Large systems are split into zones shaped by terrain, population density and history, usually with interconnections so one zone can cover for another during a failure.
Source: Water supply network
