Environmental Engineering Reference
In-Depth Information
on a skid that can also be easily transported on the same latbed truck and requires little
external pipe work. It can take less than a month to build a WHC in a community in need.
Plant capacity is sized according to the population of the community. Using the recom-
mended 4 liters of water per person (capita) per day (LCD), and an average of ive persons
per household in developing countries, WHCs are sized to provide 20 liters of water per
household per day. Since the focus is primarily on supply of potable water, WaterHealth
International (WHI) systems are small, in contrast to water treatment systems designed
to provide water to communities in developed countries. In the developed countries, all
water supplied to households is treated to drinking water standards, whereas the percent-
age of water actually consumed for drinking constitutes only 1%-2% of the total usage. A
typical point-of-entry system at a throughput of 4 gallons per minute (GPM; 15 liters per
minute) for a household in a developed country can provide suficient drinking water for
a community of 5000 people. WaterHealth installations start at this throughput and typi-
cally go up to 12 GPM for a community of 15,000 people. For larger communities, a larger
system can be installed, but the preference is to install more systems at several locations
within the community to facilitate easy access for households. Facility wise, because WHI's
systems are based on a modular design, they can be scaled to it different needs. However,
the company is predominantly deploying two versions of its community systems, one hav-
ing a throughput of 4 GPM and the other 12 GPM. The standard WHC has a footprint of
approximately 20 ft × 30 ft (approximately 55 m
2
), plus a landscaped area for social use.
WaterHealth's business focus is on decentralized systems that often prove more eficient
and more sustainable than large municipal systems among rural/peri-urban communi-
ties. Thus, the company generally installs multiple systems to serve communities of more
than 10,000 people, resulting in facilities that are relatively closer to the homes of users.
Decentralized community water systems are a right balance between a large metropolitan
water system with piped distribution network and each household's use of a point-of-use
(POU) system for treating drinking water. Large centralized treatment plants keep the cost
of water treatment low, are well maintained, and adequately monitored to assure good
water quality leaving the treatment facilities; however, they fail to guarantee good water
quality reaching the homes because of excessive reliance on the good condition of the dis-
tribution network, the so-called last mile. Centralized treatment plants require extensive
upfront capital not only to build the water treatment plant but also to build the distribu-
tion network. A common argument against the construction of a centralized treatment
plant is that only 1%-2% of the water distributed to households is consumed for drinking;
the rest of the water is used for landscaping or other utility purposes such as washing or
bathing. All water leaving the water treatment facility has to be treated to drinking water
standards. If not maintained well, the pipe network can itself become a source of con-
tamination. Distributed water generally has chlorine added to provide residual disinfec-
tion to contain any secondary contamination arising from piped network. Added chlorine
may affect the water's taste and possibly cause the formation of by-products that may be
harmful to health. On the other hand, POU systems, as the name implies, ensure that
drinking water is treated right before its use, and in concept, provide safe puriied water
at all times. The weak links in the implementation of POU systems are the users them-
selves. In the POU systems, there is a considerable reliance on their owners to carry out
the recommended maintenance at periodic intervals. These may include change of ilters,
replacement of adsorbent media, or change of germicidal UV lamps. When not properly
maintained, POU systems can actually worsen the water quality, examples being a build-
up of a bacterial slime layer on old ilters or activated carbon becoming a breeding ground
for bacteria. When coupled with an ineffective UV lamp that has not been replaced, the
