Modern urban water supply should be
clear, odourless, wholesome, fresh and free from bacteria.
Hong Kong is fortunate in having two major factors which
contribute to the reasonably good quality of its water
resources prior to purification treatment.
Firstly, the bulk of the supply is
derived from protected upland gathering grounds where
pollution arising from human activities is very low.
And secondly, subsequent storage of water in reservoirs
permits a degree of natural self-purification due to
the effects of sedimentation, aeration and sunlight.
Furthermore, owing to the local geology, the natural
water resources are both soft and low in mineral content.
The water from Guangdong is of equally good quality.
But while most of Hong Kong's sources
of raw water are relatively clean, water still needs
to be treated in order to comply consistently with international
quality standards.
By 1939, with the exception of the
Shouson Hill area, all water on the Island and in the
Kowloon urban area was filtered. The original filters
were all of the slow sand type, requiring large areas
of level ground, which is scarce in Hong Kong. The Eastern
Treatment Works built in 1949, for instance, was situated
below Tai Hang Road near Jardine's Lookout. It occupied
more than 1.2 hectares and had to be formed at two levels
with expensive retaining walls.
In the early days, the difficulty
in finding suitable sites on the Hong Kong Island resulted
in a system of separate small filters spacing out along
the hillside at Bowen Road, the Albany and the Jardine's
Lookout, interconnected by a conduit rather than one
single filtration plant.
Since these early beginnings, many
more treatment plants have been built and by end-1996
there will be 19 with a combined capacity of 4.15 million
cubic metres per day.
Click to view the Water
Treatment Process Diagram.
Raw water comes either directly from
Guangdong or from one of the storage reservoirs in Hong
Kong by gravity or via pumps and goes through large
diameter pipelines and tunnels to the treatment works.
In the treatment works the water first passes through
the clarifiers or accelators which are used for settlement,
forming the first stage of the water treatment process.
To enable large quantities of water to be treated efficiently
in a compact plant, sulphate of alumina (alum) is added
to the incoming water to assist the suspended solids
to coagulate into large particles which settle on the
floor of the clarifiers in the form of sludge. The sludge
is collected, thickened and treated and disposed of.
The water goes from the clarifiers to the filtration
plant where more finely divided suspensions are retained
on sand filters when the water passes through the filter
media to clear water tanks.
Colour and turbidity in water, which
are normally caused by solids in suspension, are removed
by the filtering process. Thus the filtered water is
clear, but it is acidic because of the sulphate of alumina
which has been added. Hydrated lime is used to neutralise
this acidity and to give the water a slightly alkaline
characteristic to reduce corrosion of water pipes and
fittings. The water is dosed with chlorine in solution
for disinfection and a fluoride compound is added for
dental care. After these processes, the water meets
the required standard and is ready for distribution
to the consumers.
Each filter bed in a treatment plant
has to be cleaned regularly by passing air and water
sequentially through the bed in a reverse direction
to the normal flow of water. Wash water at the principal
stations is collected after use and channelled back
into the main inlet point to mix with the raw water
for processing by the clarifiers. This cycle continues,
thus reducing the quantity of water running to waste.
At Sha Tin Treatment Works, for example, the amount
of wash water used ranges from 12,000 to 16,000 cubic
metres a day - approximately one per cent of the inflow
to the plant.
Water quality throughout the entire
supply system is continuously monitored by the Water
Science Division of the Water Supplies Department with
chemical, bacteriological, limnological, biological,
and radiological examination of water samples taken
at intakes, storage reservoirs, treatment works, service
reservoirs, trunk mains and consumer taps to ensure
that the quality complies with the guideline values
recommended by the World Health Organisation for drinking
water quality.
In 1996, the water quality control
and monitoring system was further enhanced when the
Mainland East Laboratory of the Water Supplies Departmnet
was accepted by the Hong Kong Accreditation Scheme (HOKLAS)
as an accredited laboratory having complied with the
requirements of the ISO/IEC Guide 25 and the relevant
requirements of the ISO 9000 series of standards including
those described in ISO 9002, for a number of tests under
the category of environmental testing.
Complaints about the quality of water
normally relate to discolouration. When a complaint
is received, or a sub-standard sample has been discovered,
staff of the department collect samples for examination.
Discolouring in water is often caused by the presence
of iron rust due to corroded pipes at the premises,
but is quite harmless to health. To rectify this, it
is usually necessary for owners of the premises to replace
the affected pipes. Owners of the premises are advised
to take the necessary action. After this, staff of the
department visit the premises again and collect more
samples to ensure that a supply of good quality has
been restored. To eradicate the problem of discoloured
water, the use of unlined galvanised steel pipes as
fresh water inside service in new buildings was abandoned
in 1995 through the enactment of the Waterworks
(Amendment) Regulations.
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