Service reservoirs: purpose and construction

Service (distribution) reservoirs are storage structures built near the distribution area, to store treated water and supply it to the consumers at the required pressure during demand variations. A clear water reservoir is the tank at the treatment plant that stores the filtered, disinfected water.

Purposes

1. Balancing storage: supply is at a uniform rate, but demand varies hourly; the reservoir stores the surplus and supplies the deficit.
2. Emergency storage: supply continues during breakdown of pumps, power failure or repair of the main.
3. Fire reserve: keeps water for fire fighting.
4. Maintains pressure in the system (elevated or hill-top tanks give pressure by gravity).
5. Lets the treatment plant and pumps work at a steady rate, which saves cost.

Types

• Underground or ground-level (on a hill), RCC, masonry or steel
• Elevated tanks (towers) for flat towns

Construction

• Site: as high as possible and near the centre of the demand area.
• Walls and floor: RCC (M20 or richer) or brick/stone masonry with cement plaster; watertight, with floor sloping to the washout.
• Roof: RCC slab or dome, covered with earth, to prevent contamination and keep the water cool.
• Compartments: divided into two cells so one can be cleaned while the other works.
• Fittings: inlet, outlet (above the floor), overflow, washout (scour) pipe, ventilation pipes, manhole with cover, ladder, level indicator and water-level gauge.

        vent        manhole
   ___ ___|___________|____ roof
  |                          |-- overflow
  | inlet-> water level ~~~~ |
  |                          |
  |__________________________|-- outlet
            washout   floor sloped

Clear water reservoir (CWR)

It is built at the treatment works after filtration, usually partly underground, with baffles. It stores the filtered water, gives chlorine contact time (30 minutes or more), supplies the pumps for back washing and for pumping to the town, and balances the difference between plant output and pumping rate. Its construction is the same as above (RCC, roof, baffles, manholes, vents, inlet/outlet, overflow, washout).