Sewerage (wastewater) network design conveys sewage from individual properties to treatment, primarily via gravity pipe networks supplemented by pump stations where gravity flow isn't feasible.
Design Flow Estimation
Base (Dry Weather) Flow
$$ Q_{DWF} = P \times q \times PF $$Where:
- $P$ = design population (or Equivalent Persons, EP, for non-residential contributors)
- $q$ = average dry weather flow per person (typically 150–250 L/person/day in Australian design guides)
- $PF$ = peaking factor, accounting for diurnal variation
Peak Factor (Harmon's Formula, commonly used/adapted in Australia)
$$ PF = 1 + \frac{14}{4 + \sqrt{P/1000}} $$Smaller populations experience proportionally higher peaking (less averaging across many households).
Infiltration and Inflow (I&I)
Groundwater infiltration and stormwater inflow (via defective joints, illegal stormwater connections) add to base flow, particularly during wet weather:
$$ Q_{design} = Q_{DWF} \times PF + Q_{I\&I} $$ $Q_{I\&I}$ is typically estimated as a rate per unit length of pipe or per hectare, calibrated from monitoring in older/similar networks where available.Gravity Pipe Design
Sized using Manning's equation, similar to stormwater, but with different minimum velocity/grade criteria to manage solids transport and prevent septicity:
$$ Q = \frac{1}{n} A R^{2/3} S^{1/2} $$| Requirement | Typical Value |
|---|---|
| Minimum self-cleansing velocity | 0.6–0.75 m/s (at design flow) |
| Minimum pipe diameter (reticulation) | 150 mm |
| Minimum grade | Set to maintain minimum velocity at expected flow: steeper for smaller pipes |
| Design flow condition | Typically pipe running part-full (e.g. ≤ 75–80% full) at peak flow |
Part-Full Flow Considerations
Sewers are deliberately designed to run part-full at peak flow (not surcharged) to:
- Provide ventilation and avoid septic/corrosive conditions
- Accommodate flow variability without surcharge
- Allow for some future growth capacity
Pump Stations and Rising Mains
Where gravity flow isn't achievable (flat terrain, deep trunk connections, isolated developments), a pump station lifts sewage into a pressurised rising main.
| Component | Function |
|---|---|
| Wet well | Storage/inlet chamber, sized for pump cycling and emergency storage |
| Pumps (duty/standby) | Typically at least duty + standby configuration for reliability |
| Rising main | Pressurised pipe conveying flow to the next gravity connection point |
| Emergency storage/overflow | Accommodates pump failure or power outage without uncontrolled discharge |
Minimum self-cleansing velocity in a rising main (to prevent solids settling) is typically ≥ 0.6–1.0 m/s at normal operating flow.
Corrosion (Hydrogen Sulphide) Management
Septic conditions in rising mains and downstream gravity sewers generate hydrogen sulphide (H₂S), which converts to sulphuric acid and attacks concrete pipe crowns:
| Mitigation | Approach |
|---|---|
| Chemical dosing | Iron salts, oxygen/air injection, or biocide dosing in rising mains |
| Material selection | PVC, HDPE, or lined concrete in high-risk locations |
| Design | Minimise rising main length/detention time where practical |
Network Layout Principles
- Follow natural grade where possible to minimise pump stations
- Minimise depth of cover to control excavation cost, while maintaining adequate cover for structural protection and to serve connecting properties by gravity
- Provide maintenance access (manholes) at every change of grade, direction, or pipe size, and at maximum spacing intervals
Practical Notes
- Peaking factors are most significant for small catchments (new subdivisions, isolated developments): using an average-flow-only design for a small network risks undersizing for realistic peak conditions.
- Infiltration and inflow allowances should be based on local network monitoring data where available; generic textbook rates can significantly under- or over-estimate actual conditions in ageing or well-maintained systems alike.
- Rising main and downstream gravity sewer odour/corrosion risk should be assessed at the design stage, not retrofitted after odour complaints: mitigation is far cheaper to design in upfront.