The underground conduit for the collection of sewage is called sewer. A network of sewers and appurtenances for the collection and conveyance of sewage generated from each of the properties to sewage pumping station for pumping to sewage treatment and disposal is called Sewage System. There are two types of sewerage system.
1. Separate sewerage system
2. Combined sewerage system.
Separate sewerage system
In separate system of sewerage there are two collection systems or pipe network;
The sanitary sewerage systems for domestic sewage are designed for peak sewage flow expected at ultimate stage at the end design period. The storm water drainage systems are designed to carry the maximum storm runoff expected during the critical duration of rainfall.
Advantages of separate sewerage system are:
Disadvantage of separate sewerage system are:
Combined sewerage system
In Combined system of sewerage both sewage discharge and the storm runoff are collected and conveyed through a common collection system. The ratio of the maximum storm runoff to sewage flow works out to be 20 to 30. Hence during non-monsoon period only 1/20th or 1/30th of the design flow, i.e, only the sewage flow is passing through the combined system with very small velocity, resulting in clogging of the systems. Combined sewers are of special types such as egg shaped etc. Combined sewers are , therefore not recommended for Indian conditions since the rainfall occurs for a period of 3 months or less and there are poor water supplies. In India, only separate sewerage system are adopted.
Advantage of Combined sewerage system are:
Disadvantage of Combined sewerage system are: