PRINCIPAL SYSTEMS OF DRAINAGE, KNOWN AS THE COMBINED AND THE SEPARATE SYSTEMS

SYSTEM OF DRAINAGE:-- There are two principle drainage systems known as the combined and separate systems. In the former system, one set of drains or sewers is provided for the removal of the 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 the rain water (or one underground sewer for sewage and one surface drain for rain water) Both the system have their advantages and disadvantages and in most cases a partially separate system is considered most suitable. In the case of a totally separate system two branch drains or sewers in each street and two house connections for each building are necessary and it not becomes only 1.5 to 2 times more costly than the combined system but also more complicated. Separate system is generally suitable for the places where the average rain fall per year exceeds 750 mm and a combined system where the rainfall is small. In a. Combined sewer, which will be large and deep, it is not possible to obtain a self -cleansing velocity during the dry weather, when the flow is very small Where a suitable outfall at low level is not available and the sewage has to be pumped, the separate system has definite advantages as the storm water has not then to be treated. In the partially combined (or partially separate) system, the greater part of the rain water is passed to the surface water sewers or open drains, but the run-off from roofs, paved yards and streets is discharged into the soil sewers. As regards the rain water, it is very expensive to provide for the maximum flood water which may occur during the heavy rains.