Design of sanitary sewer systems

Design of sanitary sewer systems

Infiltration to sewer pipes

Assume specific rate of groundwater infiltration (in l/s/ ha) for sewers with their invert located below the groundwater table

Allowance for illicit inflow

Compile available sewer sizes

 Storm water quantities

The amount of storm water to be transported is determined with the rational method.

Indicate what design frequency (return period) is used

Determine the rainfall intensity - duration curve for the required frequency

Indicate runoff coefficients

Determine the hydraulic performance of selected profiles

Establish partial flow diagrams if necessary

Design of sanitary sewer systems

Public sanitary sewers perform two primary functions: Safely carry the design peak discharge, Transport suspended materials to prevent deposition in the sewer. In designing a sewer system, the designer must conduct preliminary investigations, review design considerations and select basic design data and criteria,

Design the sewers which include preparation of a preliminary sewer system and design of individual sewers, and Prepare contract drawings and specifications.

Comprehensive preliminary investigations of the area to be served are required not only to obtain the data needed for design and construction but also to record pertinent information about the local conditions before construction begins. These are Maps  and  other  drawings  of  the  area;

Locations   of   streets,andbuildings,alleys,ponds,streams,railwa drainage ditches and other features and structure which may be influenced or influence the sewer systems;

A bench mark on each block of every stre If possible contours owatpoints suitableandchangesinsurfaceinterv

slopes;

Local  rainfall  and  runoff  data,  if  any, taken;

Character of the soil in which the sewer Local wages of unskilled and skilled lab

Designing a sanitary sewer involves estimation of waste flow rates for the design data and evaluation of any local conditions, which may affect the hydraulic operation of the system; the selection of the hydraulic-design equation, alternative sewer pipe materials and minimum and maximum sizes, minimum and maximum velocities  and  slopes;  the evaluation of alternative alignments or designs.

Design flow:Peak hourly flow and peak infiltration allowances for the entire service area are used for the design

of new sanitary sewers.

Hydraulic design equation: Manning equations are commonly used.

V= 1/n . R^2/3 . S^1/2