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Without doubt, recycling or re-using water can offer very considerable
economies. It has been said that the average water molecule entering
Chesapeake Bay from the Potomac River has been used seven times, although
this may be apocryphal. Nevertheless, it is a very real possibility. One of
the greatest obstacles to this in Cyprus is the fact that much of the waste
water is disposed of through septic tanks or other small systems of
disposal, even into the sea. Ideally, all waste water should be collected as
sewage, treated and recycled for some use or another. The obstacle to this
is that sewage treatment plants should be sited close to the sea, a
perennial river or a large lake, to receive water which has been treated but
is unsuitable for re-use. Obviously, perennial rivers and large lakes do not
exist in Cyprus. The implication is that sewage plants should be placed
close to the sea and sewage piped to them or, if they are sited inland,
pipes to the sea will need to be installed for the waste water. The latter
may be more economical if water is recycled for any purpose.
Recycling of Water in Raw Sewage
The notion of recycling sewage is often fraught with psychological
difficulties. Let it be said from the start that it is perfectly feasible to
turn raw sewage into potable water. In many countries, treated sewage is
poured into water courses which are used as a source of potable water
downstream. Sewage treatment consists of mechanical filtration to remove
solid matter which is converted to a sludge. It then passes to a digestion
chamber where aerobic acting bacteria convert most of the organic matter to
water and carbon dioxide. An anaerobic chamber removes the remaining organic
matter by converting it to methane. This process may take two, three or more
days. The final stage is usually filtration through a sand bed. The
resultant water is clear but may still contain dangerous microorganisms. If
it is necessary to recycle to potable water, these microorganisms may be
eliminated by chlorination or ultraviolet radiation.
There are two ways in which agriculture may use treated sewage. The
sludge which is formed by the collection of solid matter, when fermented,
makes an excellent compost with high nitrogen contents. It also contains
reasonably high potash and phosphorus levels and may be used for providing
humus-rich additives. If the fermentation was correctly achieved, the sludge
is without odour if it is supplied in a dried condition – sun-drying may be
sufficient, without any energy consumption. It would also be free of weed
seeds, such as are common in poorly fermented animal manure. However, the
important point is that the water may be used for irrigation with minimal
treatment. Generally speaking, spraying the water after the aerobic
digestion would provide sufficient oxygen to finish the digestion and the
water percolating into the soil will naturally perform the anaerobic
digestion, provide nutritive value to the soil and water to the crops. The
odour produced by this is usually not objectionable. It is not recommended
to spray this water on crops which are likely to be harvested within two
weeks, particularly if the crop is eaten raw, such as salads and fruit.
Swimming pools can be filled with recycled sewage. The quality of the
water should be as for potable, except that it is not necessary to kill off
microorganisms because the pool’s chlorination system will be sufficient.
The great problem in recycling sewage to potable quality is not
technical, it is the psychological barrier that most people feel in the idea
of drinking sewage. As a general rule, in a large municipal system, 50 to 60
per cent of sewage can be recycled to potable water quality. However, it is
important to note that industrial users feeding the plant must adhere
strictly to waste water regulations. An excess of heavy metals in the sewage
could affect deleteriously the bacterial digestion processes and may render
the water unfit for consumption by the presence of toxic ions. It is also
possible to recycle sewage to potable quality with small plants, producing
100 to 500 tonnes per day, generally at about the same cost as desalination,
but it may require some more space than is required for treating salt water.
The process consists of aerobic digestion in a tank with a small quantity of
air pumped through, mechanical filtration, microfiltration, single-stage
reverse osmosis with automatic declogging and ultraviolet irradiation. This
water will be pure enough to drink, but it will require remineralisation to
make it more palatable.
In 2008, potable water was supplied to farmers in the Limassol region
for irrigation, at the peak of the water crisis. Yet the purified water
from the sewage treatment works was discharged into the sea because the
farmers preferred the purer water. This illustrates the problems of
persuading people that recycled water is as good as drinking water (in
this case even better, because it still had some nutritive value).
Perhaps it may have been a good idea to tell the farmers that only
recycled water would be supplied to them!
One thing that is essential is that a great effort should be made to
connect as many villages as possible to sewage systems, covering wide
areas, with suitable treatment plants. This would have a threefold
advantage:
- Cyprus would conform to EU regulations
- more water would be available for recycling
- there would be less microbial infiltration into aquifers,
providing more potable water from boreholes.
Grey Water
Grey water is a means of reducing the consumption of water in households,
other living quarters such as hotels and a few types of industry such as
laundries. In households, it may reduce water consumption by 50 per cent.
Water which is used for washing (wash-hand basins, baths, showers, clothes
washing machines and dishwashers, but not kitchen sink or toilets) is
collected in a separate system. This "grey" water, so called because of its
cloudy aspect, can be used for filling lavatory cisterns and for irrigation,
after filtration. Experience in many countries has proven its efficacy.
There is a slight psychological adjustment to be made, because we are not
used to seeing cloudy water in toilet bowls, but this is quickly made. It
requires major alterations to fit a grey water system to existing houses but
the additional cost in new buildings represents a very small investment. In
countries where it has been made obligatory for new constructions and where
the components are cheap, the extra cost is typically of the order of a few
hundred dollars. It has the added advantage that the excess water can be
used for irrigating the garden and the presence of detergents and soap has
been proved to be beneficial to plant life. Legislators may consider the
advantages of including a compulsory grey water system in all new and
renovated constructions, in much the same way as houses must be constructed
to anti-seismic standards.
Where houses are connected to a municipal sewer, grey water systems will
reduce the volume and increase the concentration of waste. This will render
the digestion of the sewage in the public treatment plant more efficient. In
the case of connection to a septic tank, the reduced volume will render the
displacement of nauseous gases from the ventilation system less likely.
Grey water systems may be easily combined with rain water collection from
the roof of a building. This will significantly reduce the cost of separate
systems and will also have the advantage that the grey water would be
diluted by the rain water, permitting the water in the toilets to be clear
for much of the year.
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