Sanitary Sewer and Wastewater
Sanitary Sewer and Wastewater
Sanitary sewer systems collect and convey wastewater to treatment facilities. Candidates must understand design flows (average dry weather flow, peak wet weather flow), pipe sizing (Manning’s equation), minimum and maximum velocities (2 fps minimum for self-cleansing, 10-15 fps maximum to avoid erosion), slopes, manholes, lift stations, and treatment processes (primary, secondary, tertiary). Key regulations include the Clean Water Act and NPDES permits for discharges.
Traps include: confusing infiltration (groundwater entering sewers) with inflow (stormwater entering via direct connections). Both increase flows and must be accounted for in design. Another trap is minimum slope for gravity sewers: typically 1% for 8-inch pipe, but actual slope depends on velocity; use Manning’s to verify. Also, Peaking factor (ratio of peak flow to average flow) is often estimated using the Ten States Standards formula: peak factor = 1 + 14/(4 + √P) where P is population in thousands. A common error is to apply the peaking factor to the wrong base flow.
Worked example: A sewer serves a population of 10,000 with an average per-capita flow of 100 gpcd. Infiltration/inflow is estimated at 20% of average flow. What is the peak design flow? Average dry weather flow = 10,000 × 100 = 1,000,000 gpd = 1 mgd. Add I/I: 1.2 mgd. Peaking factor for P=10 (thousands): 1 + 14/(4+√10) = 1 + 14/(4+3.162) = 1 + 14/7.162 ≈ 1 + 1.955 = 2.955. Peak flow = 1.2 × 2.955 ≈ 3.55 mgd. Use this for pipe sizing.
Rule of thumb: Minimum sewer velocity is 2 fps; maximum is 10 fps. For 8-inch pipe, minimum slope is often 0.4% (1 in 240) to achieve 2 fps when flowing full.