Understanding Surface Runoff and Sewer Systems

Surface runoff is water from rain, melted snow, or other sources that flows over the land surface. When this water flows across impermeable surfaces like roads, rooftops, and pavements, it cannot soak into the ground. Instead, it flows into drainage systems.

The question asks about surface runoff caused by rainfall that is conveyed through a sewer system. Different types of sewer systems are designed to handle different types of wastewater.

Types of Sewer Systems and What They Convey

• Sanitary Sewage: This type of sewage comes from domestic sources (like toilets, showers, sinks) and industrial sources (like factories). It contains human waste, wastewater from cleaning, etc. Sanitary sewers are specifically designed to carry this type of wastewater to treatment plants.
• Storm Sewage (or Stormwater): This is the water from rainfall or melting snow that runs off surfaces and enters drainage systems. It is primarily surface runoff. Storm sewers are separate systems designed to collect and transport this surface runoff directly into natural waterways (like rivers, lakes, or oceans), usually without significant treatment, as it is generally less contaminated than sanitary sewage (though it can pick up pollutants from surfaces).
• Combined Sewer: This is a single system that collects both sanitary sewage and storm sewage. In combined sewer systems, both types of wastewater are transported together to a treatment plant. However, during heavy rainfall, the volume of water can overwhelm the system, leading to overflows of untreated combined sewage into waterways.
• Relief or Auxiliary Sewer: These are additional sewers built to relieve pressure on existing overloaded sewer lines, often during peak flows or storm events. They provide additional capacity or alternative routes for sewage flow.

Identifying the Correct Term

Based on the definitions, the water caused by rainfall that flows over the surface (surface runoff) and is then collected and conveyed through a sewer specifically designed for this purpose is called storm sewage.

Therefore, surface run off caused by rainfall which is conveyed through sewer is called storm sewage.