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VIP - How it works
VIP refers to the 'Ventilated Improved Pit' toilet system design, which incorporates features to enhance sanitation and health aspects of pit toilets. These features are promoted naturally (by wind, sunlight and natural decomposition processes).
The VIP system produces a continuous airflow through the ventilation pipe. The airflow vents away odours, assists waste breakdown by drying action, and acts as a very effective fly control mechanism. Despite their simplicity, well-designed VIPs can be completely smell free, and be more pleasant to use than some other water-based technologies.
Excreta deposited into the pit undergo complex chemical and biological reactions which eventually lead to innocuous humus like solids. During this process the pit contents continually reduce in volume, and relatively small pits can have a long service life or even last indefinitely.
 
History & Origins
The VIP latrine was developed during the 1970s to improve public health in developing countries through better water and sanitation technology.
Initially called the 'Blair Latrine', the design can be credited to Peter Morgan of the Blair Research Laboratory, part of the then Rhodesia Ministry of Health.
The term VIP, an acronym of 'Ventilated Improved Pit', originated with the World Bank, and the system has been universally called the VIP ever since.
Classic brick spiral design VIP toilet Bellatrines VIP public toilet in a National Park
 at a school in Zimbabwe in New Zealand (disability access)
There are many types of VIP toilet, from simple to sophisticated (see pictures above) and the millions of VIP toilets in service throughout the world today are testament to their effectiveness.

 

How it works (ventilation, vector control, breakdown of waste, destruction of pathogens)
Continuous ventilation is achieved by air movement (wind, breeze) across the top of the vent pipe causing a venturi effect, and by sunlight heating the vent pipe causing a convection effect.
The ventilation of the pit removes odours, but also ensures that any flies are attracted not to the toilet but to the odours escaping from the top of the vent pipe, where the wire mesh fly-screen prevents them from entering. Any fly-hatch are trapped in the vent pipe as they escape toward the light, ultimately dying and falling back into the pit.
Studies show 99% reduction of flies in fly affected locations (Morgan 1977). Eliminating the primary  vector (flies) greatly reduces possibility of transmission of potential public health risks.
Venting the pit dries the waste which assists natural decomposition and destruction of potential pathogens, ultimately rendering a safe humus-like waste product.
Organic soil is preferred over mineral soil, with un-lined soil pits contributing to the break-down of the waste by introducing natural soil micro-organisms. Soil micro-organisms also compete with pathogens. Small amounts of organic soil can be added periodically to lined pits for the purpose of introducing soil micro-organisms.
'Composting' will occur naturally when a health micro-organism community is present, but can be further promoted by the addition of small amounts of carbon in the form of dry organic matter like sawdust (chemical free), straw or twigs. Composting will increase pit temperature to some degree, with any increase in temperature assisting with destruction of pathogens.
In populations where disease and parasite infections are not common (i.e. developed country populations like NZ) it will also be uncommon to find parasites and pathogens present in fecal waste. None-the-less it is important to treat the waste with care, and allow the required time to pass for potential pathogens to die before the waste can be regarded as safe.
In temperate climates (varying between 4-20 degrees Celsius) most pathogenic bacterial will be destroyed within 3-4 months (Winblad and Simpson-Hebert, 2004) but it is recommended by WHO (World Health Organisation) to allow 12 months. Waste may then be safely handled or used as fertiliser.

Raising the pH (alkalinity) of the waste material assists in destroying pathogens. A pH level of 9 for a period of six months will destroy most pathogenic organisms. Periodic addition of small amounts of wood ash will raise the pH level of the waste.

Any increase in temperature assists in destroying pathogens.
The most straight-forward type of VIP toilet uses a single pit which is covered over when full and the toilet moved to a new pit nearby. Alternatively, the toilet building straddles twin adjacent pits, with the second pit used once the first pit has become full. When the second pit finally becomes full (after some years) the first pit will contain only stable and safe humus material, and can be emptied for re-use.
References
Tuff Toilet Installation Manual (pdf file, 937KB)
Tuff Toilet Pit Capacity Tables (pdf file, 14 KB)
WHO VIP Latrine Fact Sheet 3.5 (pdf file, 318 KB)
Practical Action - VIP Latrine (pdf file, 813 KB)
UN Development Program 1984 Technical Note 13 - VIP Latrines (pdf file, 3.3 MB)
WEDC On Plot Sanitation 1995 (pdf file, 345 KB)
World Bank Technical Note 6 - VIP Latrines (pdf file, 1.5 MB)
Waterlines Technical Breif - VIP Latrine Vent Pipes (pdf file, 492 KB)
EcoSanRes Fact Sheet 5 - Guidelines for safe use of fecal waste (pdf file, 67 KB)

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