<unk> s collect human feces in a hole in the ground. The principle of a<unk>  is that all liquids that enter the<unk> in particular urine and water used for anal cleansingseep into the ground (the only exception are fully lined<unk> s, see below). File:Pit<unk>  superstructure, Lesotho (2937020595).Well maintained<unk>  at a rural household near Maseru, Lesotho. File:Pit<unk>  for household, Lesotho (good condition) (2937872226).Pit<unk>  from the inside at a household near Maseru, Lesotho (same toilet as shown left from the outside). File:School children in Zimbabwe digging a shallow<unk>  for an Arborloo toilet (a variation of a<unk> .School children in Zimbabwe digging a shallow<unk>  for an Arborloo toilet (a variation of a<unk> , Epworth in Harare, Zimbabwe. File:Traditional<unk>  in North Kamenya, Kenya.Traditional<unk>  in North Kamenya, Kenya. File:Pit<unk>  with funny user in an amusement part in Germany.This display shows children what toilets in rural areas in Germany used to look like in the recent past. File:Abandoned<unk>  (2940958117).Abandoned<unk>  in the peri-urban area of Durban, South Africa. File:MLS Kouřim, kadibudka.Interior of an outhouse the structure usually built over the<unk>  to provide privacy. Ventilated improved<unk>  The ventilated improved<unk>  (VIP) is a<unk>  with a black pipe (vent pipe) fitted to the<unk>  and a screen (flyscreen) at the top outlet of the pipe. VIP<unk> s are an improvement to overcome the disadvantages of simple<unk> s, e.g. fly and mosquito nuisance and unpleasant odors. The smell is carried upwards by the chimney effect and flies are prevented from leaving the<unk>  and spreading disease. Despite their simplicity, well-designed single VIPs can be completely smell free, and more pleasant to use than some other water-based technologies. Flies that hatch in the<unk>  are attracted to the light at the top of the ventilation pipe. When they fly towards the light and try to escape, they are trapped by the flyscreen and die. The ventilation also allows odours to escape and minimizes the attraction for flies. The principal mechanism of ventilation in VIP<unk> s is the action of wind blowing across the top of the vent pipe. The wind creates a strong circulation of air through the superstructure, down through the squat hole, across the<unk>  and up and out of the vent pipe. Unpleasant fecal odors from the<unk>  contents are thus sucked up and exhausted out of vent pipe, leaving the superstructure odor-free. In some cases solar-powered fans are added giving a constant outwards flow from the vent pipe. Flies searching for an egg-laying site are attracted by fecal odors coming from the vent pipe, but they are prevented from entering by the flyscreen at the outlet of the vent pipe. Some flies may enter into the<unk>  via the squat hole and lay their eggs there. When new adult flies emerge, they instinctively fly towards light. However, if the<unk>  is dark inside, the only light they can see is at the top of the vent pipe. Since the vent pipe is covered by a fly screen at the top, flies will not be able to escape and eventually will die and fall back into the<unk> . To ensure that there is a flow of air through the<unk> , there must be adequate ventilation of the superstructure. This is usually achieved by leaving openings above and below the door, or by constructing a spiral wall without a door. Covering the feces with an absorbent decreases smell and discourages flies. These may include soil, sawdust, ash, or lime, among others. In developing countries, the use of absorbents in<unk>  toilets is not commonly practiced. The vent pipe should have an internal diameter of at least 110 mm and reach more than 300 mm above the highest point of the toilet superstructure. Wind passing over the top creates a suction pressure within the vent pipe and induces an air circulation. Air is drawn through the User Interface into the<unk> , moves up inside the vent pipe and escapes into the atmosphere. Care should be taken that objects, such as trees or houses, do not interfere with the air stream. The vent works best in windy areas, but where there is little wind, its effectiveness can be improved by painting the pipe black. The heat difference between the<unk>  (cool) and the vent (warm) creates an updraft that pulls the air and odours up and out of the<unk> . To test the efficacy of the ventilation, a lit cigarette can be held over the User Interface; the smoke should be pulled down into the<unk>  and up into the vent and not remain in the superstructure. The mesh size of the fly screen must be large enough to prevent clogging with dust and allow air to circulate freely. Aluminium screens, with a hole-size of 1.2 to 1.5 mm, have proven to be the most effective. Typically, the<unk>  is at least 3 m deep and 1 to 1.5 m in diameter, depending on the number of users. Deep<unk> s can last up to 20 or more years. As liquid leaches from the<unk>  and migrates through the unsaturated soil matrix, pathogenic germs are sorbed to the soil surface. In this way, pathogens can be removed prior to contact with groundwater. The degree of removal varies with soil type, distance travelled, moisture and other environmental factors and, thus, it is difficult to estimate the distance necessary between a<unk>  and a water source. A minimum horizontal distance of 30 m between a<unk>  and a water source and 2 m between the bottom of the<unk>  and the groundwater table is normally recommended to limit exposure to microbial contamination. Twin<unk>  designs Design of a twin<unk> . A further improvement is the use of a second<unk>  which is used in alternation with the first<unk> . It means that the first<unk>  can rest for the duration of time it takes to fill up the second<unk> . When the second<unk>  is also full, then the first<unk>  is emptied. The fecal sludge collected in that first<unk>  has in the meantime undergone some degree of pathogen reduction although this is unlikely to be complete. This is a common design for so-called twin-pit pour flush toilets and increases the safety for those having to enter the<unk> . Also VIP<unk> s are sometimes built with two<unk> s, although for VIP toilets one problem can be that the users may not stick to this alternation method and fill up both<unk> s at the same time. The<unk> s are of an adequate size to accommodate a volume of waste generated over one or two years. This allows the contents of the full<unk>  enough time to transform into a partially sanitized, soil-like material that can be manually excavated. Twin<unk> s are constructed 1 m apart from each other to minimize cross-contamination between the maturing<unk>  and the one in use. Also<unk> s are constructed over 1 m from any structural foundation as leachate can negatively impact structural supports. Water within the<unk>  can impact its stability. Therefore, the full depth of the<unk>  walls are lined to prevent collapse and the top 30 cm are fully mortared to prevent direct infiltration and to support the superstructure. There is a risk of groundwater pollution when<unk> s are located in areas with a high or variable water table, and/or fissures or cracks in the bedrock. As soil and groundwater properties are often unknown, it is difficult to estimate the distance necessary between a<unk>  and a water source. It is normally recommended to have a minimum horizontal distance of 30 m between them to limit exposing the water source to microbial contamination. To ensure that only one of the two<unk> s is used at any time, the idle pipe of the junction connecting to the out -of- use<unk>  is closed (e.g. with cement or bricks). Pour-flush<unk>  Pour-flush<unk>  schematic showing squatting pan with water seal In a pour-flush<unk> , a squatting or pedestal toilet with a water seal (U-trap or siphon) is used over one or two offset<unk> s. Therefore, these types of toilets do require water for flushing but otherwise have many of the same characteristics as simple<unk> s. About one to three liters of water is used per flush in that case, and they often have two<unk> s that are used one after the other ("twin<unk>  pour flush<unk> "). For this reason they are subsumed under the term<unk> ". The fecal sludge that is removed from the full<unk> s of twin-pit pour-flush<unk> s is somewhat safer to handle and reuse than the fecal sludge from single<unk>  pour-flush<unk> s. However, significant health risks for the workers who are emptying the<unk> s remain in either case. A counterweight slab (called SaTo pan) installed in a<unk>  in Rwanda, converting it to a pour flush<unk>
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