Peaking factors for sewer design: For sewer mains of 0.5 m to 1.0 m diameter, what ratio is commonly assumed for maximum daily sewage flow to average daily sewage flow?
-
A1.5
-
B2.0
-
C2.5
-
D3.0
Answer
Correct Answer: 2.0
Explanation
Introduction / Context:Sewer hydraulics must accommodate diurnal and seasonal peaks. A simple way to reflect daily peaking is to multiply the average daily flow by a peaking factor that depends on catchment size and network characteristics.
Given Data / Assumptions:
- Medium-sized mains (0.5–1.0 m diameter) serving urban populations.
- Average daily flow known from per capita contributions.
- Daily peaking factor chosen per local guidelines.
Concept / Approach:
For the stated mains, a commonly adopted planning value is 2.0 for max daily/average daily. Larger systems with population averaging may have lower ratios; smaller systems may experience higher peaks. Final design should also consider hourly peaking and infiltration/inflow allowances.
Step-by-Step Solution:
Compute average daily flow from population and unit rates.Apply daily peaking factor PF_day ≈ 2.0 → Q_max,day = 2.0 * Q_avg,day.Check hourly peak factor for capacity and surcharge risk.Verify velocities for self-cleansing under peak and low flows.Verification / Alternative check:
Compare with local standards (e.g., Harmon/Metcalf recommendations) and adjust based on observed flow monitoring where available.
Why Other Options Are Wrong:
1.5 may understate peaks; 2.5 or 3.0 are more typical for smaller basins or for hourly peaking rather than daily.
Common Pitfalls:
Applying hourly factors to daily sizing; ignoring I/I contributions; neglecting future growth.
Final Answer:
2.0