Sanitary Sewer Analysis and Design

Sanitary Sewer Sizing Tool

Peaking Factors & Partially-Full Flow

Base Generation Rates

Per Capita Flow ($q_{avg}$) L/capita/day
Infiltration Rate (I/I) L/ha/day

Peaking Factor Method

Harmon Formula Babbitt Formula Ten States Standards Constant Factor (4.0)

Max PF Cap
Min PF Cap

Why use PF Caps?

Empirical formulas can generate unrealistic extremes at boundary populations.

• Max Cap (4.0-5.0): Prevents math from shooting to infinity for very small populations.
• Min Cap (1.5-2.0): Ensures massive populations still account for standard diurnal routines (e.g., morning showers).

Hydraulic Constraints

Analysis Mode	Design (Auto-Size) Assessment (Exist)
Manning's Roughness ($n$)	Custom... PVC / HDPE Concrete Pipe Vitrified Clay Corrugated Metal
Min. Pipe Size (mm)
Max Allowable $d/D$

Why limit d/D (Depth Ratio)?

Sanitary sewers are designed to flow partially full (usually $\le 0.80$) rather than under pressure.

• Ventilation: Leaves top space for toxic/corrosive sewer gases to vent freely.
• Factor of Safety: Provides a buffer for unexpected surges or heavy I/I to prevent surcharging.
• Hydraulic Efficiency: Water actually moves fastest when a circular pipe is ~81% full due to friction at the pipe crown.

Network Topology Schematic

A Sewer Network Inputs

#	Pipe ID	Up Node	Dn Node	Local Pop. (Persons)	Local Area (ha)	Dia D (mm)	Up Invert (m)	Length L (m)	Slope S (m/m)	Act

B Wastewater Routing & Sizing Results

Network			Cumulative Flow Generation				Hydraulics (Partially-Full Flow)
Pipe ID	Up Node	Dn Node	Σ Pop (Persons)	Peak Factor (PF)	Infil (I/I) (L/s)	Design Q (L/s)	Pipe D (mm)	Capacity (L/s)	Depth Ratio (d/D)	Act. Vel V (m/s)	Status

HGL / EGL Longitudinal Profile

Select Flow Path:

Pipe Conduit Physical Water Hydraulic Grade Line (HGL) Energy Grade Line (EGL)

Peaking Factor Curve Points represent active pipe segments

Hydraulic Theory & Equations

1. Design Wastewater Flow (Q)

Sanitary sewers must convey the Peak Dry Weather Flow (PDWF) plus groundwater infiltration and inflow (I/I).

Q_design = (Pop × q_avg × PF) + (Area × Rate_I/I)

2. Peaking Factor (PF) Models

Wastewater flow is highly diurnal. Smaller populations have much higher instantaneous peaks. Where $P$ is population in thousands:

• Harmon: PF = 1 + 14 / (4 + √P)
• Babbitt: PF = 5 / P^0.2
• Ten States: PF = (18 + √P) / (4 + √P)

3. Partially-Full Flow (Camp's Equations)

Unlike storm sewers, sanitary pipes are designed to flow partially full (usually $d/D \le 0.80$) to allow ventilation of sewer gases. Actual velocity must be calculated using circular segment geometry:

A = (D²/8)(θ - sinθ)  |  P = (D/2)θ  |  d/D = (1 - cos(θ/2))/2

* Minimum self-cleansing velocity is typically 0.6 m/s (2.0 ft/s) at actual flow depth.

4. Reference Design Guidelines

Typical Manning's n

PVC / HDPE (Smooth)	0.009 - 0.011
Concrete Pipe	0.012 - 0.014
Vitrified Clay Pipe (VCP)	0.013 - 0.015
Corrugated Metal Pipe	0.020 - 0.024

Design Constraints

Min. Velocity	0.6 m/s (2.0 ft/s)
Max. Velocity	3.0 m/s (10.0 ft/s)
Max Depth Ratio ($d/D$)	≤ 0.80
Typical I/I Rate	~10,000 L/ha/d

Typical Per Capita Flow ($q_{avg}$)

Source	L/c/d	gpcd
Residential (Average)	250 - 350	65 - 90
Residential (High Use)	350 - 450	90 - 120
Commercial / Office	50 - 100	15 - 25
Industrial	Site Spec.	Site Spec.

Minimum Slopes (Ten States Standards)

Diameter (mm)	Diameter (in)	Min. Slope (m/m)
200 mm	8 in	0.0040
250 mm	10 in	0.0028
300 mm	12 in	0.0022
375 mm	15 in	0.0015
450 mm	18 in	0.0012
* Based on maintaining V ≥ 0.6 m/s (2.0 ft/s) flowing full with n = 0.013.

Typical Infiltration/Inflow (I/I) Rates

Sewer Condition / Age	L/ha/day	gpd/acre
New PVC/HDPE (Excellent)	2,000 - 5,000	200 - 500
Average Condition (Standard)	10,000 - 15,000	1,000 - 1,500
Old VCP / Brick (Poor)	20,000 - 40,000+	2,000 - 4,000+
* I/I is highly dependent on groundwater table depth and joint integrity.

Specific Commercial & Institutional Flows

Facility Type	L/unit/day	gpd/unit
Schools (w/ showers & cafe)	60 - 100 / student	15 - 25 / student
Hospitals & Medical	600 - 1000 / bed	150 - 250 / bed
Office Buildings	40 - 75 / employee	10 - 20 / employee
Restaurants / Dining	30 - 50 / seat	8 - 12 / seat

Max Manhole Spacing (Ten States Standards)

Pipe Diameter	Max Spacing (m)	Max Spacing (ft)
200mm - 375mm (8" - 15")	120	400
450mm - 750mm (18" - 30")	150	500
≥ 900mm (≥ 36")	180	600
* MH required at every change in grade, size, or alignment.

References & Sources

• Great Lakes–Upper Mississippi River Board. (2014). Recommended Standards for Wastewater Facilities (Ten States Standards). (Standard peaking factor formulation, minimum slopes, and manhole spacing criteria). [View Standards]
• Metcalf & Eddy, Inc. / Tchobanoglous, G., et al. (2014). Wastewater Engineering: Treatment and Resource Recovery (5th ed.). McGraw-Hill. (Source for typical per capita flows, specific commercial generation rates, and infiltration/inflow (I/I) estimates). [View Book]
• ASCE / WEF (2007). Gravity Sanitary Sewer Design and Construction (ASCE MOP 37 / WEF MOP FD-5). (Standard engineering practice for Manning's n values, maximum velocity constraints, and max d/D depth ratios). [View on ASCE]
• Camp, T. R. (1946). Design of Sewers to Facilitate Flow. Sewage Works Journal, 18(1), 3-16. (Formulas for partially full pipe flow, velocity, and circular segment geometry). [View on JSTOR]
• Harmon, W. G. (1918). Forecasting Sewage Discharge at Toledo. Engineering News-Record, Vol. 80. (Original Harmon Peaking Factor empirical derivation). [Search Scholar]
• Babbitt, H. E. (1953). Sewerage and Sewage Treatment. John Wiley & Sons. (Babbitt Peaking Factor formula for smaller populations). [View Archive]

Version 1.0

CivilSheets by Sothearith Seak