What is an alternating sewer system?
An alternating sewer system (also called an improved separate sewer system) is a drainage system that uses two separate pipes — one for wastewater and one for rainwater — but allows a controlled portion of rainwater to be directed to the wastewater pipe during heavy rainfall. This hybrid approach combines the benefits of separate and combined sewer systems.
How does it work?
In a standard separate sewer system, rainwater and wastewater each have their own dedicated pipe. In an alternating sewer system:
- Normal conditions — rainwater flows through the storm drain to surface water (rivers, canals, infiltration areas), while wastewater goes to the treatment plant
- Heavy rainfall — a controlled overflow or switching mechanism diverts some of the first flush of rainwater (which carries the most pollutants from roads and roofs) into the wastewater pipe
- After the first flush — the remaining cleaner rainwater continues to the storm drain
This “first flush capture” is important because the initial rainwater runoff picks up oil, dust, heavy metals and other contaminants from paved surfaces.
Comparison of sewer systems
| System | Pipes | Rainwater destination | Advantages | Disadvantages |
|---|---|---|---|---|
| Combined | 1 | Treatment plant (with wastewater) | Simple, one pipe | Overflows pollute surface water; treatment plant overloaded in rain |
| Separate | 2 | Directly to surface water | No overflows, smaller treatment load | First flush pollution reaches surface water untreated |
| Alternating | 2 | Surface water + partial diversion to treatment | First flush treated; cleaner discharge | More complex design and control |
Components
An alternating sewer system typically includes:
- Wastewater sewer — carries domestic and commercial wastewater to the treatment plant
- Storm sewer — carries rainwater to surface water or infiltration areas
- Diversion structure — a valve, weir or switching chamber that redirects the first flush
- Overflow facility — manages excess water during extreme rainfall events
- Inspection chambers — access points for maintenance and monitoring
When is it used?
- In new residential developments where separate systems are required but first flush pollution is a concern
- During upgrades of existing combined sewer systems
- In areas near sensitive surface water that must meet strict water quality standards
Maintenance
- Regular inspection of the diversion structure to ensure correct switching
- Cleaning of both pipe systems to prevent blockages
- Monitoring of overflow frequency and water quality
Related terms
- Combined sewer system
- Separate sewer system
- Surface water drainage
- Infiltration
- First flush
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.