Recommend a suitable sewerage system

A sewerage system is the complete arrangement of sewers, appurtenances, pumping stations, treatment works and disposal works used to collect, convey, treat and dispose of wastewater and, when included, storm water from a community.

Types

• Separate system: one set of sewers for sanitary sewage and a different set (drains) for storm water.
• Combined system: one sewer carries both sewage and storm water.
• Partially separate system: one sewer carries sewage plus a part of storm water (roof and courtyard runoff), and the remaining storm water goes in surface drains.

Recommendation for a highly populated urban area

I recommend the separate system (with the storm drains laid as open or covered drains along roads).

Justification:

1. Rainfall in Nepal is intense and seasonal (monsoon). A combined sewer would be very large for rare peak flows, and would run almost empty in the dry season, with silting problems. In a separate system, the sewage sewer is small and always flows at reasonable velocity.
2. Treatment plant economy: only sewage reaches the plant, so the plant is smaller and works at nearly constant load. In a combined system the plant would be overloaded in rain or the overflow would carry raw sewage to rivers.
3. No overflow pollution: sewage is not discharged into rivers during storms.
4. Storm water needs no treatment and can be discharged directly into the nearest stream; its drains can be built cheaply and in stages.
5. Easier maintenance in narrow congested streets: smaller sewer pipes, easier to lay, and working conditions inside are better.
6. Pumping is cheaper where the sewage must be lifted.
7. Reuse: sewage of constant quality can be reused for irrigation or biogas.

The only disadvantage is higher total cost because two systems are laid, and illegal connection of roof drains into sewers must be controlled. For a dense urban area with heavy monsoon rain, such as Kathmandu valley, the separate system is more suitable.