Raw water from various sources is conveyed by pipelines to the waterworks
where it is chemically treated, filtered and disinfected. Treatment
frees the water of harmful bacteria, makes it clear, sparkling,
odourless, colourless, and safe for consumption. All treatment plants
use chemical coagulation and rapid gravity filtration. For chemical
coagulation, correct doses of suitable coagulants and coagulant-aids
are dissolved in the raw water to combine or 'flocculate' the colloidal
and larger particles of suspended matter. This causes the suspended
matter to settle more readily and then be removed in the sedimentation
tank. The water is then passed through rapid gravity filters which
remove the finer particles of suspended matter. On its way from
the rapid gravity filters to the clear water tank where it is temporarily
stored, the filtered water is disinfected with chlorine which gets
rid of all harmful bacteria. The water is then pumped into the distribution
system and service reservoirs, ready for consumption.
Purification Chemistry
Aluminium sulphate is the main coagulant. In most cases, hydrated lime is also added to adjust the pH of the raw water for the best flocculation results. Polyelectrolyte is used as a coagulant aid. For disinfection, chlorine is used to destroy the bacteria. Ozone is used, as well as chlorine, at Choa Chu Kang and Bedok Waterworks. Activated carbon is also used to remove any bad taste and odour.
Sodium silicofluoride is added to the water on its way from the filters to the clear water tank. Fluoridation is a requirement by the Ministry of Health and has been a practice since 1957. It helps in the prevention of dental caries.
How Water Gets to you
Treated water is temporarily stored in clear water tanks in the waterworks and then sent into the distribution system. Besides going direct to customers, the water is also stored in large covered service reservoirs sited on high ground, so it can flow by gravity to customers. This storage provides a reserve of purified water which may be drawn on at times when the demand is greater than the output from the waterworks and during failure of mains, pumps or other equipment.
The distribution system is made up of a large network of mains of various sizes and types. For pipes up to 300 millimetres in diameter, the pipe material is normally ductile iron with cement lining and for larger pipes, it is usually steel with cement lining. This lining helps to maintain the water quality and preserve the pipeline's carrying capacity. For the smaller mains and service pipes (28 to 54 millimetres), copper, stainless steel and unplasticised PVC pipes are used. The use of unlined galvanised iron pipes is disallowed because of their susceptibility to corrosion.
Laboratory Tests
Singapore's water is moderately soft and is safe to drink straight from the tap. To ensure that your water supply is clean and safe, water samples are regularly collected and analysed chemically and bacteriologically at the Water Testing Laboratory. Samples of water at various stages of treatment at all waterworks, raw water from all sources, treated water from all service reservoirs and selected points in the distribution network are collected for daily or periodic analysis.
Tests are conducted to ensure that the quality of treated water is within the Guidelines for Drinking-Water Quality set by the World Health Organisation (WHO). The National Environment Agency also monitors the treated water quality. Some typical values for the treated water quality parameters can be found in the table below:
Potable Water Quality (Typical Values)
Chemical & Physical
Characteristics
(In mg/l where applicable)
*Water from
Choa Chu Kang
& Bedok Waterworks
**Water from
Other Waterworks
WHO Guideline
Values (2004)
Colour (Hazen
Unit)
< 5
< 5
15
Turbidity
(NTU)
< 5
< 5
5
pH Value
7.0 - 9.0
7.0 - 9.0
@
Conductivity
(umhos/cm)
250 - 550
80 - 300
@
Taste
Unobjectionable
Unobjectionable
Unobjectionable
Odour
Unobjectionable
Unobjectionable
Unobjectionable
Nitrate
(as N03)
0.5 - 7.0
0.5 - 15.0
50
Fluoride
(as F)
0.5 - 0.7
0.5 - 0.7
1.5
Total Dissolved
Solid
200 - 350
60 - 250
1000
Total Alkalinity
(as CaCO3)
20 - 50
10 -50
@
Total Hardness
(as CaCO3)
50 - 100
20 - 100
@
Chloride
(as Cl)
35 - 100
5 - 50
250
Sulphate
(as SO4)
30 - 60
5 - 40
250
Phosphate
(as PO4)
< 0.10
< 0.10
@
Silica
(as SiO2)
1 - 10
3 - 12
@
Iron (as
Fe)
< 0.04
< 0.04
0.3
Manganese
(as Mn)
< 0.05
< 0.05
0.4
Copper (as
Cu)
< 0.05
< 0.05
2
Aluminium
(as A1)
< 0.10
< 0.10
0.2
Residual
Chlorine (as Cl, Total)***
<2.0
<2.0
5.0
Bacteriological Characteristics
E. Coli
(cfu/100 ml, 35°C, 24 hrs)
< 1
< 1
< 1
* Raw water is drawn from estuarine reservoirs. The treated water
is normally supplied to western and eastern parts of Singapore.
** Raw water is drawn from upland reservoirs and rivers. The treated
water is normally supplied to the rest of Singapore.
*** The residual chlorine present in tap water is in the form
of chloramines or free chlorine.
Typical values for potable water are only guides and may vary
with varying quality of raw water or change of source of water
supply due to PUB's operational requirements. They are well within
the WHO Guidelines for Drinking Water Quality. Customers will
have to treat potable water further if they need water of higher
quality for specific applications, such as for process use in
factories. It is not necessary to treat potable water further
for drinking purpose.
