Separate, partially separate, and combined drainage systems

The type of sewage in any given town determines the type of drainage required. If separate, two separate sets of drains are required – one to take off roof water and general paved drainage*, usually known as surface water; the other to remove the sewage and wastes from sinks, baths, lavatory basins, etc., named foul water. Separate sewers increase the building costs a little, but the system is favoured, particularly where it is desirable to keep the volume of sewage at a minimum owing to the cost of pumping.

The public surface-water sewers discharge into streams. Consequently there may be quite a number of independent long lengths of surface-water sewers with outlets at convenient positions on the route of a stream or river passing through the district. The foul-water sewers cannot be treated in a similar manner. The sewage must be treated before disposal. The partially separate system still requires two separate sets of drains, and is similar to the separate system, with the exception that part of the roof water may be drained to the foul sewer. Usually the rain from the rear of the roof is dealt with the foul drain, while the rain from the front of the roof, pavements, and street surfaces is discharged to the nearest water course. If the system is combined, one set of drains must convey all soil and surface-water drainage.

Where separate drainage systems are called for, the authorities exercise special care with regard to the supervision of the connections to the public sewers. It is most important that no mistake should be made with the identification of the sewers; otherwise a wrong connection causes endless trouble and danger to public health.

If a foul-water drain is inadvertently connected to a surface-water sewer, the faulty discharge may be easily located at the outlet of the sewer to a stream; but the chief difficulty is locating the property wrongly connected once the work has been completed. The danger of wrong connections cannot be ignored; but there is little to fear if the builder makes an inspection before confirming the drain connection to the sewer.

Each system has advantages according to the local conditions. Where two sewers are used, the foul-water can be reduced in size. The volume of foul water to be treated is considerably less than the combined system, and where pumping is essential there is a saving in the outlay and annual expense of the pumping installation.

Natural water courses running through a district often effect a great saving on drainage costs, because it is quite reasonable to run surface-water drains to any convenient outlet on the course of stream and save excess piping.

A separate drainage system almost doubles the cost of drainage as compared with the combined system. There is, however, less likelihood of flooding during times of abnormal rainfall.