as small as . In many areas, it is difficult to keep a roof clean enough for drinking. To reduce dirt and bad tastes, systems use a metal collecting-roof and a "roof cleaner" tank that diverts the first 40 liters. Cistern water is usually chlorinated, though reverse osmosis systems provide even better quality drinking water. In the classic Roman house ("Domus"), household water was provided from a cistern (the "impluvium"), which was a decorative feature of the atrium, the house's main public space. It was fed by downspout tiles from the inward-facing roof-opening (the "compluvium"). Often water lilies were grown in it to purify the water. Wealthy households often supplemented the rain with a small fountain fed from a city's cistern. The impluvium always had an overflow drain so it could not flood the house. Modern cisterns are usually large plastic tanks. Gravity tanks on short towers are reliable, so pump repairs are less urgent. The least expensive bulk cistern is a fenced pond or pool at ground level. Reducing autonomy reduces the size and expense of cisterns. Many autonomous homes can reduce water use below per person per day, so that in a drought a month of water can be delivered inexpensively via truck. Self-delivery is often possible by installing fabric water tanks that fit the bed of a pick-up truck. It can be convenient to use the cistern as a heat sink or trap for a heat pump or air conditioning system; however this can make cold drinking water warm, and in drier years may decrease the efficiency of the HVAC system. Solar stills can efficiently produce drinking water from ditch water or cistern water, especially high-efficiency multiple effect humidification designs, which separate the evaporator(s) and condenser(s). New technologies, like reverse osmosis can create unlimited amounts of pure water from polluted water, ocean water, and even from humid air. Watermakers are available for yachts that convert seawater and electricity into potable water and brine. Atmospheric water generators extract moisture from dry desert air and filter it to pure water. Sewage Resource Composting toilets use bacteria to decompose human feces into useful, odourless, sanitary compost. The process is sanitary because soil bacteria eat the human pathogens as well as most of the mass of the waste. Nevertheless, most health authorities forbid direct use of "humanure" for growing food. The risk is microbial and viral contamination. In a dry composting