Principle for the preparation of Water supply Scheme
1. Selection of source:- Where raw water from flowing streams, lakes , tanks ad other impoundment from contaminated catchments and ponds form the source, provision should be made in the scheme for filtering such water prior to disinfection.
2. Quality of water.- The chemical quality of supplies proposed from ground water through tube wells, wells and infiltration works should be of acceptable quality, to be adjudged wit reference to local condition, where special treatment of removal of dissolved substance like iron, sulphates and flurides is necessary, provision should be made there in the scheme.
3. Disinfections. - It is advisable to provide for continuous effective disinfections of supplies drawn through tube wells, wells and infiltration works where local conditions indicate the need.
Where surface water is the source, treatment including filtration and disinfection of the final effluent is essential.
It is equally important to ensure that the free residual chlorine of a minimum of 0.2 ppm is maintained at all points in the distribution system.
4. Simple drip-feed device.- For simple rural well supplies, disinfection should be arranged by a simple and economical drip feed of a decanted bleaching powder solution, the dosage being adjusted to suit the rate of draw from the source. A simple gravity feed arrangement for such a purpose should be followed.
5. Pressure-feed chlorinators. - There are different types of pressure-feed chlorinators available for injecting chlorine solution into force mains. The two good classification of these chlorinates are -
(a) the positive-feed type, and
(b) the vaccum type
The details of their working and operation, and the care of chlorinator equipment are set out in paras 10-6-22 and 10-6-3 on pages 132-136 of the Manual of Water supply issued by the Ministry of Health.
Common defects and handicaps experienced in operating some of the pressure-type chlorinators in the market are to be attributed to the provision of inferior rubber holders, mal-adjustments of the plunger stroke and inferior types of rubber diaphragm. Every care should be exercised in selecting a good and robust quality of chlorinator suited to the needs of each case.
6. Structure of infiltration galleries - In developing safe supplies through infiltration works the essential details to be followed in the design of infiltration galleries have been set out in the Water supply Manual .
7. Slow sand filter.- Where slow sand filtration is adopted for treatment of raw water, the design criteria to the adopted are as set out in the Water Supply Manual. A typical design calculation to determine the economical size of filters is given in the following page.
8. Type design of iron removal plant. - A typical design of an iron -removal plant should b followed. The basic factors governing the design of such plants are set out in the Water Supply
Manual. These may be adopted with suitable notification where removal of iron from ground or surface water is necessary.
9. Mechanical fitter plants. - Where full-scale treatment of raw water comprising chemical design mixing, flocculation, sedimentation and filtration followed by disinfection is necessary, the functional aspect of each component as well as the design criteria recommended in respect of each, as set out in the Water Supply Manual may be adopted with suitable modification.
10. Economical size for source mains. - the per capita rate of supply to be adopted in respect of each community, the most economical means of conveyance of the supply from the source to the service points and the design of the distribution system in each case may generally follow the guide - lines set out in the Water Supply Manual.
11. Charts for pipe flow computation. - It is recommended that Hazen and William's Formula should be adopted in the design of pressure mains while the Cutter's Formula orthe Manning's Formula should be used for free-flow conduits. Readymade charts for computing pipe size under the Hazen and William and Kutter's Formulae may be used.
The Hazen and Williams's Chart is based on a value of 100 for 'c'. For other values of 'c' applicable to different pipe materials (as recommended in the Water Supply Manual) the corresponding figures will have to be deducted.
The Kutter's Charts are for a value of 'n' =.015 which may be adopted for all pipes upto 24 ‚dia. and masonry conduits of all sizes. A value of 'n' = .013 may be used for pipes above 24? dia.