VPE 511: Environment and Environmental Hygiene (2+1)

LAYING OF A DRAINAGE SYSTEM

• When laying drain pipes and fittings in a drench care must be exercised to see that the trench has a firm bottom. If the soil is of a loose character it is well to make a solid bed of concrete for the pipes.
• The gradient of its pipes should be such as to ensure a steady and even passage of sewage. For ordinary drains from most animal habitations a speed of 10 cm/second is satisfactory and this attained in a 10 cm pipe if there is a gradient of one in forty and in a 15 cm pipe with a gradient of one in sixty is maintained .
• All joints must be made perfectly gas and water- proof and the material used for sealing the join must be able to resist the action of fluids from without and of fluids and gases from within. One part of cement and one part of sand makes a excellent jointing material for stoneware or fireclay pipes.

Defects

• Defects in drains may be due to several cause
• Blockage of the pipes or fittings through the entrance of solid objects.
• In sufficient gradient or conversely to steep a gradient.
• Imperfect laying of traps so that these become tilted out of line so reducing the depth of the seal.
• Bad workman-ship in laying drains.
• Using pipes which are too large or two small.
• Too many bends in a system.

Testing of drains

• Drains must be tested after laying to ensure that they are gas and water tight and that the water seals of the traps are satisfactory. Tests are carried out on the various sections of a drainage system during its construction and a final test on the whole system when completed. Thereafter periodical tests should be made from time to time to ascertain that the system remains satisfactory as defects may developed from various causes which if not rectified may cause serious pollution.
• The methods of testing drains are described below
  • The air and smoke tests: The air test consists of plugging the open ends of the drains and ventilating pipes and the pumping in air under a pressure sufficient to be indicated on a pressure gauge. If the gauge does not show that a fixed pressure is being mentioned this denotes a leak or leakage. These can generally be located by filling the pipes with dense white smoke afterwards plugging the pipes and applying the same pressure before. This will force the smoke out at the defective parts and enable them to be located and made good. This test should be continued until the gauge remains steady and proves that the drains are sound.
  • Hydraulic test: In the hydraulic test the outlet of a drain or a section of it is plugged with a extending rubber bag or other patent stopper, and the section to be tested then filled with water . The water is left in for two or three hours and if a leak exists its pressure will be indicated by a fall in the head of water at the point of observation.