Foul Sewerage Design Calculations
The following calculations are in accordance with ‘Sewers for Adoption’ and BS 7524.
From BS752-4, Domestic Flow Rate= 150-300 litres/ occupant/ day.
Therefore, average taken of Domestic Flow Rate = 225 l/p/d
Domestic Peak Design Flow Rate = 6 x Domestic Flow Rate
Number of Occupants per Flat = 1.5
Number of Flats on Plot = 17
Therefore:Domestic Flow Rate = (225 x 1.5 x 17)/ (3600 x 24) = 0.04427 l/s
Domestic Design Flow Rate = 6 x 0.88542 = 0.265625 l/s
Design Steps
1. The foul water system design layout was discussed and sketched out, in order
to establish the amount of properties to flow into each pipe.
2. The flows were then added together, at the points where pipes join one another
at manholes, to calculate the cumulative flow in the system.
3. In order to achieve a self-cleansing pipe, a minimum velocity of 0.75m/s at
one third Design Flow rate is required.
4. Pipe diameters and gradients were initially set at the minimum allowable
values, in accordance with ‘Sewers for Adoption’. Clause 2.11 states that the
minimum pipe diameter, for a pipe servicing more than 10 properties is
150mm.
5. Using a roughness value of 1.5mm, Hydraulic charts from ‘Tables for
Hydraulic Design of Pipes and Sewers’ were used to determine the velocity
and discharge of the pipe when full.
6. The proportional Discharge and Depth could then be found, and used to
establish the Proportional Velocity at 2DWF. This had to be above the
minimum of 0.75m/s, otherwise the pipe had to be redesigned. Where the pipe
already met minimum diameter and hydraulic gradient requirements, a value
of under 0.75m/s was acceptable.
7. Pipe gradients and depths were checked with the topography of the land to
ensure that pipes were at a minimum depth of 1.2m below ground level and a
maximum of 6m below ground level.
8. A Rising main was required in the design, and a suitable pump and wet well
designed in accordance with this.
9. A Manhole Schedule, detailing the types and levels of Manholes to be used
was then produced.