- Design of Sewer(Chapter 3 of CPHEEO Manual)
Sewerage system may be classified as
sanitary sewers designed to receive domestic sewage and industrial waste excluding storm water.
Storm sewers designed to carry off storm water and ground water but excluding domestic sewage and industrial wastes and
Combined sewers designed to receive sewage, industrial waste and storm water.
The combined system of sewerage though may be economical initially, suffer from several disadvantages such as sluggish flow during non-stormy days, leading to deposition of sewage, solids causing foul odours and increased cost of eventual sewage treatment or pumping cost, associated with disposal of sewage. In view of this, the combined system is normally not recommended.
Hydraulics of Sewers (Para 3.4 of CPHEEO Manual)
Flow in sewers is said to be steady if the rate of discharge at a point in a conduit remains constant with time and if the discharge varies with time it is unsteady. If the velocity and depth of flow are the same from point to point along the conduit, the steady open channel flow is said to be uniform flow and non-uniform if either the velocity, depth or both are changing.
Flow friction: (Para 3.4.2 of CPHEEO Manual ) –
The available head in waste water lines is utilized in overcoming surface resistance and in small part, in attaining kinetic energy for flow. For design purpose, Mannings formula for open channel flow and Hazen William and Darcy-Wcisback formula for closed conduit or pressure flow is used for working out the head loss due to friction
For design of sewer net work the slope and diameter of sewers should be decided to meet the following two conditions: i.A self cleansing velocity is maintained at present peak flow ii.A sewer runs at 0.80 full at ultimate peak flow.
Self cleansing velocity:- To ensure that deposition of suspended solids does not take place, minimum self cleansing velocities are required to be considered in the design.
Hydraulic elements of circular sewers possess equal self cleansing properties at all depths. The self cleansing properties for different conduit are given below:
i)Sanitary Sewer: For design peak flow – 0.8 metre/sec For present peak flow – 0.6 metre/sec
ii) Open drain: – 0.75 to 0.9 metre/sec
iii)Inverted siphon: – 1.00 metre/sec
iv)Minimum velocity for force main: – 0.3 metre/sec
Maximum permitted depth of flow:
The pipes will be designed to flow at depth indicated below where the maximum permissible depth of flow in sewers for established velocity criteria:
Diameter in mm (d) Depth of flow which will convey designed quantity
Up to 400 0.50 d
400 to 900 0.67 d
Above 900 0.75 d
Velocity: (Para 3.4.3 of CPHEEO Manual)
The flow in sewer varies from hour to hour and also seasonally. But for the purpose of hydraulic design, estimated peak flow is adopted. The recommended slope for minimum velocity is given below which ensure minimum velocity of 0.60 metre/sec.
Maximum Permissible Velocity:
Description Maximum permissible velocity
Stoneware pipe 1.4 metre/sec
Brick drain 1.8 to 2.1 metre/sec
Concrete drain 2.5 metre/sec
Cemented drain 3.0 metre/sec
Cast Iron pipe 3.0 metre/sec
2. Sewer Appurtenances (Chapter 4 of CPHEEO Manual)
Sewer appurtenances are devices necessary in addition to pipes and conduits for the pipes functioning of any complete system of sanitary, storm or combined sewers.
1. Manhole: (Para 4.2 of CPHEEO Manual) •A manhole is an opening constructed in the alignment of a sewer for facilitating a person to access the sewer for the purpose of inspection, testing, cleaning and removal of obstruction of the sewer line. •Spacing : Manhole should be built at every change of alignment, gradient or diameter at the head of all sewers and branches and at every junction of two or more sewers. •The maximum distance between manholes should be 30 m.
2. Flushing Tank:
Located at the head of a sewer. They are designed for 10 minutes flow as a self cleansing velocity of 0.6 m/sec
Capacities:
150 mm sewer – 6400 litres
200 mm sewer – 11000 litres
250 mm sewer – 18000 litres
The capacity of these tanks is usually 1/10 of the cubic capacity of sewer length to be flushed.
House Service Connection (Para 4.4 of CPHEEO Manual) •For large diameter of sewers, house service connections may be given through rider sewers, which should be connected through manhole or drop manhole. Where there is no Y or T left for new connection insertion of new Y or T is not prescribed. •House service connection should be minimum size of 150 mm diameter sewer with minimum slope of 1:60 laid as far as possible to a straight line and grade.
3. Materials for Sewer Construction (Chapter 5 of CPHEEO Manual)
Factors influencing the selection of materials for sewer construction are flow characteristics, availability size required including fittings and ease of handling and installations, water tightness and simplicity of assembly, physical strength, resistance to acids, alkalies, gases, solvents etc., resistance to scour, durability and cost including handling and installation.
4.STRUCTURAL DESIGN OF BURIED SEWERS (Chapter 6 of CPHEEO Manual)
The structural design of a sewer is based on the relationship that the supporting strength of the sewer as installed divided by a suitable factor of safety must equal or exceed the load imposed on it by the weight of earth and any superimposed loads.
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