WHY SHOULD WE USE WATER WISELY?
Four things are conspiring to make fresh water one of the most
valuable commodities in the twenty-first century:
1 increasing world populations;
2 climate change;
3 man’s ever increasing interference with the natural flow of
water;
4 pollution.
In 1990 the World Health Organization estimated that 1230 million
people did not have access to adequate drinking water. By 2000
this figure was estimated to have risen by 900 million people. Add
to this already chronic problem the devastating impacts of climate
change and the results can be catastrophic, even in the most
developed countries in the world. On top of these issues comes
another: the increasing household demand for water around the
world. In England and Wales alone household water use is
predicted to increase by 10–20 per cent between 1990 and 2021
under a medium-growth scenario without climate change. Per
capita demand for domestic water is predicted to rise owing to
the projected increase in use of dishwashers and other domestic
appliances, with a further increase of 4 per cent with climate
change owing to higher use of personal showers and garden
watering. Demand for spray irrigation of crops in Britain is
predicted to rise by 115 per cent with climate change between
1990 and 2021, with most irrigation water taken from rivers and
groundwater.
This increasing demand for water can be met either by increasing
the capacity of supply (e.g. by building new reservoirs), by
reducing the consumption of water or by re-using water where we
can.
WATER CONSERVATION
Water conservation becomes increasingly important as demand for
water increases and shortfalls in supply occur. A number of water
conservation measures can be used in the home with little impact
on the every day lives of householders. These can involve the
following.
Flow restrictors
Flow restrictors are readily available and can be fitted to many
appliances, but their use has to be appropriate. Where taps can be
left open by careless users and where items are washed under
running water they are a cheap way of reducing water wastage.
However, a more effective but more expensive solution would be
to install taps operated by proximity sensors.
Showers
The average amount of water used by a conventional shower is
approximately 30 l, whilst a bath requires about 80 l. Initially, it
appears that showering is more energy and water efficient, but the
fact is that households with showers use them more frequently
than households without showers use their baths. Also, pumped
and multihead showers are not so water efficient as conventional
showers. Real savings can be made if you choose your products
wisely.
Conventional showerheads can discharge water at between 0.3
and 0.5 l s–1. Low-flow showerheads can reduce this to below
0.2 l s–1 depending on the supply pressure. Research conducted in
the USA has shown that the use of low-flow showerheads can
save approximately 27 l per day per person (for a person who
mainly showers rather than takes baths). This equates to an energy
saving in hot water of 444 kWh per person per year for water
heated by gas (or 388 kWh for water heated by electricity). The
cheaper alternative to low-flow showerheads is to fit a flow restrictor
to the supply to an existing showerhead, although this may
increase the showering time.
WCs
WC cistern water displacement devices are available in all
countries, albeit only a stone or brick in some cases. More imaginative
in the UK, with a typical flush of 6–9 l, is the use of the
plastic bags, ‘Hippo’ and yellow sponges, ‘Soggy Doggy’, thathave been distributed free by water companies to encourage
their customers to conserve water. These devices either
displace or retain water within the WC cistern to reduce the
volume of water that is flushed. Water displacement devices
such as dams and bags are very popular in the USA where
cisterns are generally much larger and less cluttered than UK
cisterns. This is because American cisterns have traditionally
flushed over 15 l of water using compact cistern outlet valves:
‘flappers’. Unfortunately it has been found that some displacement
devices may actually increase the flushing volume if they
are fitted such that they obstruct the flush volume limiting
aperture in a siphon. If the entire volume of water in the cistern
is necessary to clear the WC pan, a reduced flush volume may
not be effective, resulting in the repeated flushing of the cistern
and hence an increase in the amount of water used, rather than
a decrease.
WCs can be flushed with water using compressed air assistance.
Some such cisterns use the pressure of the mains water
supply to compress a volume of air above the stored water. When
the water is released into the bowl it has a much greater velocity
than from a conventional gravity-operated cistern. These products
are used in parts of France and the USA. To be used efficiently
these cisterns need to be matched to WC pans that can use the
higher velocity water effectively. Another type of water and
compressed air toilet uses water to rinse the bowl and
compressed air to evacuate the contents. This type is used in
many types of building in the USA.
Composting toilets
Composting toilets use no water for flushing. In its domestic
form this toilet is usually electrically powered, heating the waste
material to enable composting action to occur. The major
problem with this type of toilet is its size; the smallest domestic
model is about twice the size of a conventional WC suite.
Large (greater than 15 m3) composting toilets do not usually
require the external input of energy for the process, as the
aerobic decomposition is sufficiently exothermic to be selfsustaining.
Large composting toilets may be environmentally
acceptable as they consume only a small volume of water,
require no drainage pipe work and produce compost that can be
used in the garden. However, the questions of adequate hand
washing facilities if there is no available water supply and the
safety of children using toilets with open chutes needs to be
considered.
Waterless toilets
Waterless toilets that do not compost the waste usually require
electricity to operate. Packaging toilets seal the waste into continuous
plastic sacks that require subsequent disposal. Incinerating
toilets burn the waste to produce a sterile ash that can be disposed
of in a garden.
Urinal flushing cistern controllers
Urinal flushing cistern controllers have been widely used in the UK
for some time. Water Byelaws for such appliances have to be
checked for each area. In the UK such Byelaws state the maximum
rate at which cisterns may be filled. Since 1989 new cisterns are
required to be refilled only when the urinal is in use. There are
various methods of sensing use and operation. Some use changes
in water pressure to identify operation of taps and therefore, by
association, the use of urinals; others use passive infrared (PIR)
detectors to detect movement of persons in the room; some
sense the temperature of urine in the urinal traps; and many use
various forms of proximity detector. The essence of these devices
is they all obviate the flushing of urinals when the premises are
not being used and are usually an improvement over the use of
the traditional ‘pet-cock’ that has to be set to drip water at the
required rate into the cistern.
Waterless urinals
Waterless urinals are being increasingly used in the UK. Most
modern designs feature some form of odour suppressant that
requires regular renewal. Claims for large water and maintenance
savings are made about these devices but the pipe work must be
installed and maintained correctly if prolonged service life is to be
achieved. An incinerating urinal is available from the USA, which
produces small volumes of ash. However, at a cost of over £1000
considerable water has to be saved to make it economically viable.
Controls
The use of an occupancy detector to isolate the water supply to a
washroom when unoccupied is another application of PIR technology.
This can minimize the waste in urinal flushing and that caused
by taps being left open. Automatic leak detectors are becoming
increasingly available in the UK. These devices are fitted into the
incoming mains and close when a leak is detected, preventing boththe waste of water and damage to property. Some operate by
sensing a high flow rate and others use conductivity detectors to
activate valves. Automatic closure taps can produce water savings
in commercial and public buildings where there is a risk of taps
being left open accidentally.
Domestic appliances
Presently, 85 per cent of households in the UK possess a washing
machine and 10 per cent a dishwasher. Together, these consume
about 12 per cent of domestic drinking water. The ownership of
these previously luxury goods is increasing. Water Byelaws govern
the maximum permissible volume of water used for a wash:
between 150 and 180 l for a washing machine (depending on drum
size) and about 196 l for an average dishwasher. Modern highefficiency
washing machines use far less water than this and an
AEG washing machine uses as little as 68 l of water for a 5 kg fill
and only 1.4 kWh for a hot and cold fill. This is around one-third of
the water used in a conventional machine. The Oxford Ecohouse
dishwasher is another AEG machine that uses only 15 l of water
and 1.2 kWh electricity for a 50°C biowash cycle. That is less than
one-tenth of a conventional machine.
WASTEWATER SYSTEMS
Wastewater is used water. Wastewater may contain substances
such as human waste, food scraps, oils, soaps and chemicals. In
houses, wastewater can include the water from sinks, showers,
bathtubs, toilets, washing machines and dishwashers.
Businesses and industries also use water for a wide variety of
other purposes.
Wastewater can include stormwater (rainfall) runoff. Although
many people assume that stormwater runoff is clean, it isn’t.
Contaminants such as hydrocarbons wash off urban surfaces such
as roadways, parking lots and rooftops and can harm our rivers,
lakes and marine waters.
We also waste water when we don’t use it wisely. For instance,
when we fill a glass of water to drink, we may run the water to
make sure it’s cold. It is perfectly clean but once it disappears
down the drain it mixes with sewage and polluted water from other
households, businesses and industries.
When we pull the plug in the bathtub or flush the toilet, few of
us give much thought to where the wastewater is going but wastewater
doesn’t just disappear when it leaves our homes and
businesses. There are three types of sewer systems:
1 sanitary sewers carry wastewater from sinks, toilets, tubs and
industry;
2 storm sewers carry runoff from rainfall, called stormwater;
3 combined sewers carry wastewater and stormwater through the
same pipe.
Together, these form our wastewater collection system.
• In the USA each day, the average person produces about
220–450 l of wastewater. That’s enough to completely fill a
bathtub two times.
• We all produce sludge. An adult is responsible for about 32 kg
per year.
• If everyone installed water-saving toilets and showerheads, we
could substantially reduce domestic water consumption.
• Each day, in the USA the average person uses 260 l of water
for domestic purposes. That’s about 7 million l of water in a
lifetime.
• A leaky tap will waste in excess of 90 l of water each day.
KEY DIFFERENCES BETWEEN GREY WATER AND BLACK WATER
1 Grey water contains only one-tenth of the nitrogen of black
water. Nitrogen (as nitrite and nitrate) is the most serious and
difficult-to-remove pollutant affecting our potential drinking water.
As grey water contains far less nitrogen, it is unnecessary for it
to undergo the same treatment process as black water.
2 The medical and health professionals view black water as the
most significant source of human pathogens. Organisms that
threaten human health do not grow outside of the body (unless
incubated) but are capable of surviving especially if hosted in
human faeces. Separating grey water from black water
dramatically reduces the danger posed by such pathogens
because, in grey water the faeces that carry (and may
encapsulate) them are largely absent. However, other bacteria
are present in grey water and can cause rapid growth of any
faecal contamination present in pipes and septic systems. Care
must be taken to ensure that both grey and black water travel
rapidly through the pipes in buildings and that there are no
points in the system where they can stagnate.
3 The organic content typical of grey water decomposes much
faster than the content typical of black water. The amount of
oxygen required for the decomposition of the organic content in
grey water during the first 5 days (Biological Oxygen Demand
over 5 days or BOD5) constitutes 90 per cent of the total or
Ultimate Oxygen Demand (UOD) required for complete
decomposition. BOD5 for black water is only 40 per cent of the
oxygen required. BOD1 for grey water is around 40 per cent ofthe UOD; BOD1 for black water is only 8 per cent of the UOD.
This means that the decomposing matter in black water will
continue to consume oxygen far longer and further away from
the point of discharge than it will in grey water. This faster rate
of stabilization for grey water is advantageous for the prevention
of water pollution as the impact of grey-water discharge generally
does not travel as far from the point of discharge when
combined with wastewaters. This is especially true for sand and
soil infiltration systems. As grey and black waters are so different
it is better to separate them and, more specifically, to keep urine
and faeces out of the water altogether and to treat them
separately for the best protection of health and the environment.
Doing so also has significant savings for homeowners.
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