water treatment plant
Greater kandy drinking water treatment plant
Drawbacks observed in plant
water is pumped against gravity with electricity causing high electricity cost
water is not aerated assuming it gets aerated within the river.
water source is arround a densly populated area
water treatment plant supply water to a large area which may result water shortage to a greater area in case of breakdown.
manual cleaning of grit filters is not effective if huge amount of inflow
pumping station is located at a low level which may obstruct incase of a flood
Steps of water purification in water treatment plant.
Coarse screening:
• to prevent leaves, sticks,
and other large pieces of
debris from entering to the
grit chamber (clear opening
50mm)
• Manual cleaning
Fine screening:
• purpose is to collect smaller
material that has passed
through the coarse screen
(clear opening 10mm )
• Mechanically cleaned
Grit removal
• The design objective of the grit
chamber is to remove sand or silt
particles greater than 0.1 mm in
diameter.
• Sand and silt is very abrasive and will
result in excessive wear to mechanical
equipment
• Full Capacity: 115,000 m³/day
• 2 grit chambers
• Dimensions of grit chamber: 6m width x
33.5m length 3.1m depth x 2 basins
• Detention time 15min
Destribution chamber
Type : Distribution chamber with hydraulic rapid mixing by weir.
• Number: 1 unit with a capacity of 115,000 m³/day
• G value for rapid mixing : 300 -330 s-1 (>300 s-1 )
• Applied chemicals : Chlorination -Liquid chlorine (99.9%)
pH control - Lime (10% solution)
Coagulant - PAC (10% solution)
FLOCCULATION
• Type : vertical flow baffled channels
• Numbers : 4 trains
• Capacity : 100,000 m3/day
• Detention time : 30 min
Coagulation purpose is to increase the settling velocity. When size
of aggregates increases, their settling velocity automatically
increases. Coagulation is completed in two steps.
• Particle de-stabilization.(through chemical reaction
between coagulant and colloids)
• Particle transportation (Flocculation).
Sedimentation
• Type Rectangular plug flow
• Number: 4 train with hydraulic
sludge removal pipe
• Capacity :100,000m3/day
• Surface loading: 38.5mm/min
• Dimensions : 11m width x 41 m
length x 4m depth
Filtration
• Type : Rapid Sand filter
• Filtering rate:
200m3/m2/day
• Filter media : Silica
sand, 1200mm thickness
ES=1.0 -1.2,mm, UC<1.5
• Number of filters : 8
• Dimension per unit:
7.2mx9.6m (69.1m²/unit )
• Capacity : 100,000cum/day
• Backwash: 48 hrs interval
Disinfection/ Chlorination
Gas chlorine is used (900kg
cylinders)
Pre chlorination: 50 kg/day
Post chlorination:70 kg/day
• Free Residual Chlorine (OCl –) is very effective in killing
Bacteria
• Min RCL concentration at treatment plant : 1 PPM
• Min RCL concentration at distribution end : 0.2 PP
