ADVANTAGES AND DISADVANTAGES OF COMBINED AND SEPARATE SYSTEM OF DRAINS OR SEWERS

The drainage system are maintained with two principal systems, known as the combined and the separate systems. In the former system, one set of drains or sewers is provided for the removal of both the soil sewage and the rain water. In the separate system, two sets of drains are provided one for the soil sewage and the other for rain water ( or one underground sewer for sewage and one surface drain for rainwater). Both the system have their advantages and the disadvantages and in most cases a partially separate system is considered most suitable system. It is essential that all sullage drains have self cleansing velocities, as far as possible so that there are no accumulations in the sewers and the sewage does not become septic. In India sewage has been found to get septic after six hours whereas, it takes over about 10 to 12 hours in the cold countries. In cold countries a velocity of 0.61 metre per second for large sewers and 0.76 metre per sec for medium and small size sewers, has been found satisfactory. In India higher velocities are found necessary for the climate and the habits of using ashes, fabulous materials and grit for cleansing of pots and pans and should be at least 0.76 to 0.91 m/sec for open drains and 0.76 to 1.1 m/sec in sewers to prevent deposition of grit and other solid matter. Greater velocities are required for combined sewers than for sewers carrying only soil sewage. Sewer pipe lines should follow the natural slope of the ground surface so that they will always lie at the same minimum depth. The minimum slope for sewers should be not less than I in 100 for 150 mm diameter pipes and not less than 1 in 150 to 200 for 200 mm diameter pipes, however slopes should be as steep as possible in the upper lengths, however , the slopes should be adjusted according to the designed velocity: -- Er Fateh chand Guleria, Director, RTI welfare Association registered number HPCD, 3552 , Bilaspur Himachal pradesh phone number, 9459334377