Surgery and Healing in the Developing World - Dartmouth-Hitchcock

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84 Surgery and Healing in the Developing World
ent temperature of 32˚C, the daily energy consumption is only about 430 Wh. A
single medium-to-large PV module, with its own battery storage system, would
provide sufficient power. Several thousand Sun Frost refrigeration units have been
manufactured. They are now deployed in over 50 countries with mean time between
failure (MTBF) over 10 years. The approximate cost of the Model RFVB in
2004 is US$ 1700 plus shipping. This model is officially approved by the World
Health Organization. A complete Sun Frost system, with dedicated PV module, is
about US$ 2800, plus shipping. Another example is the Model SFRF-6 made by
the Simpler Solar Systems company. This is a chest type refrigerator/freezer with a
total capacity of 179 liters. It costs approximately US$ 1540 plus shipping. A large
number of these have been sold to clinics in Africa. Vestfrost (a Danish company)
also makes a popular PV-powered refrigerator.
Historically, water disinfection has been accomplished by chemical processes or
by boiling. Recently, disinfection by ultraviolet (UV) irradiation has become popular.
This method allows throughput of hundreds of liters per hour from a highly
reliable, low-power, compact system. The UV light is supplied by a mercury vapor
lamp with a strong emission at 254 nm. The lamp is similar to commercial fluorescent
glass tubes used in homes and offices, except that the glass tube is not internally
coated with phosphor which would absorb in the UV and emit visible light. It is
fortuitous that such lamps are readily available and inexpensive because micro organism
absorption and germicidal effect are maximum at about 260 nm. The World
Health Organization has developed a standard of performance for official approval
of water disinfection systems. This includes the capability of processing contaminated
water so that an input concentration of 100,000 colony forming units (CFUs)
of E. coli per 100 ml is reduced to less than 1 CFU per 100 ml at the water outlet.
Water itself is highly transparent to UV light. Absorption is by the particles and
organisms in the water. Thus, for UV water disinfection, it is appropriate to speak of
dose in terms of energy per unit area, rather than energy per unit volume. For a
sample of water a few centimeters deep, a dose of 80 milijoules per square centimeter
(80 mJ/cm 2 ) will disable 99.995% of waterborne viruses and bacteria. However,
UV treatment alone will not disable microorganism cysts with protective coverings
such as Giardia and Cryptosporidium, nor will it disable larger organisms such as
amoebae and worms. A preirradiation filter must be used to remove such organisms
and also to remove fine turbidity. This can be done with a 2-micron or 5-micron
particle filter. Sometimes a sand filter can be used for this purpose. A carbon filter
can be used if chemical contamination is also present.
There are now several UV water purification systems on the market. One example
of a very successful water disinfection system is the UV Waterworks model
manufactured by WaterHealth International, Inc, (Fig. 11). The energy requirement
for this unit is about 10 Wh per hundred liters, making it ideal for a PV-powered
environment. The standard model can provide enough drinking water for 500-1500
people per day. One of these units at a clinic can insure safe drinking water for an
entire village. After preirradiation filtering, the system works by allowing a continuous
stream of gravity-fed water to flow into a shallow stainless steel pan. The water
is exposed to the UV light for 12 sec and receives a minimum dose of 80 mJ/cm 2 .
Extensive independent laboratory testing has confirmed the effectiveness of the disinfection.
The disinfection unit has a mass of only 7 kg and the whole system is easy
to set up and use. Maintenance is minimal and includes replacing the UV lamp
every year, and the transformer every 5 years. An interesting safety feature of the UV