#1 : Size

Sanitary sewer mains shall be of adequate size to convey population generated (peak dry weather) flows, plus a general allowance for inflow and infiltration, plus an allowance for inflow allowance for manholes in sag locations. The inside diameter of sanitary sewer pipes must measure at least 200 mm.

#2 : Population Generated Flows

Residential population generated flows are determined by the following formula:

Qpdw = G x P x PF/86,400


  • Qpdw  = peak dry weather flow rate (litres/second)
  • G = per capita daily sewage flow generation (300 litres/day/person)
  • P =  design contributing population
  • PF =  residential peaking factor, where P is the population, Most used is Herman’s PF=1+14/(4+P0.5)

#3 : General Inflow/Infiltration Allowance

A general allowance of 0.28 litres/second/hectare shall be used to allow for wet weather inflow to manholes not located in sag locations.

#4 : Allowance for Manholes in Sag Locations

For manholes located in sag locations where ponding will occur or low areas subject to inundation, an inflow allowance of 0.4 litres/second shall be made for each manhole.

#5 : Minimum Slopes

All sanitary sewers should be designed with a minimum slope of 0.4% or greater. Where it is not practical to do so, the following minimum slopes will be recommended for various sewer sizes:

Sewer Size (mm) 200 250 300 375 450 >500
Minimun Slope (%) 0.40 0.28 0.22 0.15 0.12 0.10

#6 : Velocity

The velocity of flow in sanitary sewers shall be kept between 0.6 metres/second and 3.0 metres/second.

#7 : Depth

In general, sewers should be sufficiently deep to receive wastewater from basements and to prevent freezing. Insulation shall be provided for sewers that cannot be placed at a depth sufficient to prevent freezing.

#8 : Manhole

Manholes must be installed at the end of each sewer, at all changes in sewer size, grade, or alignment and at all junctions. The maximum permitted manhole spacing for all sewers less than 1200 mm in diameter is 100 metres.

#9 : Pipe Materials

Approved materials for pipe construction of sanitary sewer mains are non-reinforced concrete pipe, reinforced concrete pipe,PVC (polyvinyl chloride) and Corrugated HDPE pipe.