SEWERAGE  SYSTEM

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;

1. one for collecting domestic sewage as sanitary sewerage system and
2. another for collecting storm water as storm water drainage system.

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:

• The capacity of the water treatment plant will be smaller since only domestic sewage alone is to be treated.

• Operational problems are less.

Disadvantage of separate sewerage system are:

• Storm water may always find its way into the domestic sewerage system either through wrong house sewer connections or through manholes and overload the sewage treatment plant.

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:

• Only one system is provided and therefore there will not be any confusion in giving connection, and

• Less expensive to install the system.

Disadvantage of Combined sewerage system are:

• During non-rainy days the flow will be very meager causing, salivation  requiring frequent cleaning.