ANS:B - combined system

1. Combined System Overview:
   • In a combined sewerage system, a single set of pipes is used to convey both sanitary sewage (from toilets, sinks, etc.) and stormwater runoff from rain events.
   • The combined system was historically common in older urban areas and is still in use in some locations today.
2. Reasons for Choosing a Combined System:
   • Cost and Space Constraints: In densely populated urban areas, there may be limited space available for installing separate sewerage systems for sanitary sewage and stormwater. In such cases, combining the two systems into one can be more cost-effective and practical.
   • Efficiency during Light Rains: In areas with light and evenly distributed rainfall throughout the year, the volume of stormwater runoff is relatively low. A combined sewerage system can effectively handle this moderate flow of stormwater during light rains without significant issues.
   • Hydraulic Efficiency: During light rain events, the combined system can efficiently convey both sanitary sewage and stormwater runoff in the same pipe, minimizing the need for separate infrastructure.
   • Historical Precedent: In older urban areas where combined systems were originally installed, retrofitting to separate systems may be challenging or impractical due to existing infrastructure and limited space for expansion.
3. Challenges and Considerations:
   • Overflow Risks: One of the main challenges of combined sewerage systems is their susceptibility to overflow during heavy rain events. When the volume of stormwater exceeds the capacity of the sewer system, combined sewer overflows (CSOs) can occur, leading to the discharge of untreated sewage and stormwater into receiving waters.
   • Water Quality Concerns: CSOs can result in water pollution and pose risks to public health and the environment, particularly in areas where combined sewerage systems discharge into sensitive water bodies.