Fresh water is produced by either an evaporator or a reverse osmosis unit. The primary use for fresh water is to provide feedwater for the reactor and steam propulsion plants. It is also available for showers, sinks, cooking and cleaning once propulsion plant needs have been met. Seawater is used to flush toilets, and the resulting "black water" is stored in a sanitary tank until it is blown overboard using pressurized air or pumped overboard by using a special sanitary pump. The blackwater-discharge system is difficult to operate, and the German Type VIIC boat was lost with casualties because of human error while using this system. Water from showers and sinks is stored separately in "grey water" tanks and discharged overboard using drain pumps.