Water Supply Note
Water Supply Note
Water Supply Note
In buildings, it is necessary to provide an adequate supply of water. The water provided must
be colourless, free from small suspended matter and harmful bacteria, pleasant to taste. Ideally,
the water should not be “hard”.
Temporary hardness is caused by bicarbonates of Calcium and Magnesium. When this water
is boiled, scales will form.
Permanent hardness is caused by Calcium sulphate, Calcium Chloride or Magnesium Chloride.
These salts cannot be removed by boiling.
Soft water should have less than 50 ppm. Moderately soft is 50-100. Slightly hard is 100-150.
Moderately hard is 150-200. Hard is 200 – 300. Very hard is over 300ppm.
2.1 The rain cycle
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Water source is a dedicated reservoir
2.2.1 Aeration
Aeration is achieved by sending water over a cascading system that causes a lot of turbulence.
Aeration oxidizes iron (Fe) and manganese (Mn). Good aeration of the water is also important
for slow, sand filtration to be effective, especially if there is not enough oxygen in the surface
water. It can also remove the dissolved CO2 in water.
2.2.2 Sedimentation
In coagulation, a liquid coagulant, such as aluminium sulfate, is added to the water to attract
suspended particles. The water is then gently stirred to allow the particles to come together and
form larger particles (flocculation), which can then be removed by sedimentation, settlement
or filtration. The amount of coagulant needed will depend on the nature of the contaminating
chemical compounds and solids.
2.2.3 Slow sand filtration
Water passes slowly downwards through a bed of fine sand at a steady rate. The water should
not be too turbid, otherwise the filter will get clogged. Pathogens are naturally removed in the
top layer where a biological film builds up.
2.2.4 Rapid sand filtration
The sand used is coarser than in slow sand filtration and the flow rate is higher. The method is
used to remove suspended solids and is effective after the water has been cleared with
coagulation/flocculation. There is no build-up of biological film, hence the water will still need
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to be disinfected. From time to time, the trapped debris can be removed by supplying an up
flow under pressure.
2.2.5 Chlorination
Chlorination is the most widely used method of disinfecting drinking-water. Deciding on the
right amount of chlorine to use can be difficult, because the effectiveness of chlorination
depends on the quality of the untreated water, which may vary according to the season. It is
advisable to maintain a residual of free chlorine in the water to deal with any subsequent
contamination.
2.3 Cold water supply to buildings
Houses would need an adequate pressure to feed the top floors. There can be two different
options identified as direct and indirect supply.
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2.3.1 Direct water supply
This can be used if the water mains has sufficient pressure all the time. However, the
appliances may have to withstand the pressure fluctuations.
2.3.2 Indirect water supply
This is the system that is more suitable for Sri Lanka. A water storage tank is needed to storey
water. The pressure fluctuations will not occur since the head is constant.
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2.3.3 Water storage requirements in buildings
This is generally for a 24 hour interruption. The recommended values are given below as given
in BS EN 806:2012.
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Water supply in tall buildings Water supply in buildings taller than 20 floors
Pillar Swan neck Bib tap Stop valve Gate valve Garden tap
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appliances. To deal with this, loading units have been allocated for each appliance. Each
loading unit is 0.1 l/s. The expected flow rate can be determined using charts developed based
on theories of probability.
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2.4.2 Friction in pipes
The friction in pipes will depend on the material of the pipe, the pipe diameter and the flow
rate.
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2.4.3 Design example
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2.5 Hot water supply
Supply of hot water could be advantageous and is becoming a common feature in Sri Lanka as
well. However, in cold climates or up country of Sri Lanka, it is possible to have properly
installed centralized hot water systems. In cold climates, it is also possible to have space
heating as well.
2.5.1 Hot water supply arrangements
The generation of hot water consumes energy. Therefore, some attention has to be paid to the
cost of generating hot water. In areas with soft water, it is possible to have hot water storage
cylinder connected directly to the boiler. This is called direct system.
In areas with hard water, direct system can form scales in pipes. Hence, indirect system is
used. In this, a heat exchanger is used within the hot water cylinder to heat up the water.
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Indirect supply of hot water
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A heat exchanger
A calorifier is a heat
exchanger which
heats water indirectly
by circulating it over a
heating coil or
multiple coils. The
source of heat can be
water or steam,
heated by an external
heat source, contained
within a pipe
immersed in the water
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2.5.3
CombinedGeneration
heating of
andhot water
hot with
water electricity
system
This is an expensive way of generating hot water unless off-peak cheap electricity is available.
It is possible to have immersion heater of sufficient capacity (2-3 kW) for generating hot water
in small quantities rapidly within the storage cylinders.
2.5.4 Generation of solar hot water
This is suitable method for Sri Lanka since it can receive plenty of sun light. This uses the
green house effect. Short wave solar radiation can pass through clear glass. Then, it would be
absorbed by the solar collector and it will be heated up. However, the heat emitted by the
heated surfaces will be of long wave radiation and hence cannot escape through the glass.
Hence, most of solar gains from the sun by the collector can be used to heat the water in the
pipe network arranged within the collector.
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2.6 Space heating
In cold climates, space heating is needed. A centralized hot water heating system has three
basic elements.
1. Boiler for heat generation
2. Heated water distribution circuit
3. Heat emitter
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already used in a radiator back to the cylinder and hence it is easier to ensure a uniform heating
effectiveness. In modern systems, the water flow is regulated by using a small pump.
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2.6.3 Heat emitters
The heat emitters are radiators. Those emit the heat as radiation and part as convection.
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