Slow sand filters are used in water purification for treating raw water to produce a potable product. They are typically 1–2 m (3.3–6.6 ft) deep, can be rectangular or cylindrical in cross section and are used primarily to treat surface water. The length and breadth of the tanks are determined by the flow rate desired by the filters, which typically have a loading rate of 200–400 litres (0.20–0.40 m3) per square metre per hour.