2.2 Water as an essential commodity
Water fit for human
consumption is called drinking water or potable water. Water that is not
potable may be made potable by filtration or distillation, or by a
range of other methods.
Water that is not fit for drinking but is not
harmful for humans when used for swimming or bathing is called by
various names other than potable or drinking water, and is sometimes
called safe water, or "safe for bathing". Chlorine is a skin and mucous
membrane irritant that is used to make water safe for bathing or
drinking. Its use is highly technical and is usually monitored by
government regulations (typically 1 part per million (ppm) for drinking
water, and 1–2 ppm of chlorine not yet reacted with impurities for
bathing water). Water for bathing may be maintained in satisfactory
microbiological condition using chemical disinfectants such as chlorine
or ozone or by the use of ultraviolet light.
In the USA, non-potable
forms of wastewater generated by humans may be referred to as greywater,
which is treatable and thus easily able to be made potable again, and
blackwater, which generally contains sewage and other forms of waste
which require further treatment in order to be made reusable. Greywater
composes 50–80% of residential wastewater generated by a household's
sanitation equipment (sinks, showers and kitchen runoff, but not
toilets, which generate blackwater) (Luthy, 2012). These terms may have
different meanings in other countries and cultures.
Poor water quality and bad sanitation are deadly; some five million
deaths a year are caused by polluted drinking water. The World Health
Organization estimates that safe water could prevent 1.4 million child
deaths from diarrhea each year (WHO, 2010).
Water, however, is not a
finite resource (meaning the availability of water is limited), but
rather re-circulated as potable water in precipitation (Hoekstra, 2013)
in quantities many orders of magnitude higher than human consumption.
Therefore, it is the relatively small quantity of water in reserve in
the earth (about 1% of our drinking water supply, which is replenished
in aquifers around every 1 to 10 years), that is a non-renewable
resource, and it is, rather, the distribution of potable and irrigation
water which is scarce, rather than the actual amount of it that exists
on the earth. Water-poor countries use importation of goods as the
primary method of importing water (to leave enough for local human
consumption), since the manufacturing processuses around 10 to 100 times
products' masses in water.
In the developing world, 90% of all
wastewater still goes untreated into local rivers and streams (UNEP,
2002). Some 50 countries, with roughly a third of the world's
population, also suffer from medium or high water stress, and 17 of
these extract more water annually than is recharged through their
natural water cycles (Ravindranath et al., 2002). The strain not only
affects surface freshwater bodies like rivers and lakes, but it also
degrades groundwater resources.
2.3 Human uses of water for domestic purposes
2.3.1 Drinking
The
human body contains from 55% to 78% water, depending on body size. To
function properly, the body requires between one and seven liters of
water per day to avoid dehydration; the precise amount depends on the
level of activity, temperature, humidity, and other factors. Most of
this is ingested through foods or beverages other than drinking straight
water. It is not clear how much water intake is needed by healthy
people, though most specialists agree that approximately 2 liters (6 to 7
glasses) of water daily is the minimum to maintain proper hydration.
Medical literature favors a lower consumption, typically 1 liter of
water for an average male, excluding extra requirements due to fluid
loss from exercise or warm weather (Rhoades and Tanner, 2003).
For
those who have healthy kidneys, it is rather difficult to drink too much
water, but (especially in warm humid weather and while exercising) it
is dangerous to drink too little. People can drink far more water than
necessary while exercising, however, putting them at risk of water
intoxication (hyperhydration), which can be fatal (Noakes et al., 2005,
Noakes et al., 2007). The popular claim that "a person should consume
eight glasses of water per day" seems to have no real basis in science.
Studies have shown that extra water intake especially up to 500 ml at
mealtime was conducive to weight loss (Stookey et al., 2008, Dubnov-Raz
et al., 2011; Dennis et al., 2010; Vij and Joshi, 2013; Muckelbauer et
al., 2013). Adequate fluid intake is helpful in preventing constipation.
Specifically,
pregnant and breastfeeding women need additional fluids to stay
hydrated. The Institute of Medicine (U.S.) recommends that, on average,
men consume 3.0 liters and women 2.2 liters; pregnant women should
increase intake to 2.4 liters (10 cups) and breastfeeding women should
get 3 liters (12 cups), since an especially large amount of fluid is
lost during nursing. Also noted is that normally, about 20% of water
intake comes from food, while the rest comes from drinking water and
beverages (caffeinated included). Water is excreted from the body in
multiple forms; through urine and feces, through sweating, and by
exhalation of water vapor in the breath. With physical exertion and heat
exposure, water loss will increase and daily fluid needs may increase
as well.
Humans require water with few impurities. Common impurities
include metal salts and oxides, including copper, iron, calcium and
lead, and/or harmful bacteria, such as Vibrio. Some solutes are
acceptable and even desirable for taste enhancement and to provide
needed electrolytes (Maton et al., 2003).
