Preliminary Investigation includes gathering of data such as demographics, wastewater production estimates, maps, underground survey of existing of existing utilities such as water, telephone, gas, etc...
Gravity Sewer Collection System Design •
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The following data must be addressed in the network design: Wastewater flow rates Pipe material Design criteria Design equations System Layout
Gravity Sewer Collection System Design ‐ Pipe Material ‐ •
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Vitrified Clay pipe (VCP); It’s made of clay. It’s durable, high resistance to corrosion and abrasion. Difficult for handling due to its large weight. Not recommended for pipe diameters larger than 600mm. Polyvinyl Chloride Pipe (PVC); Its advantages are corrosion resistance and small weight. It’s subject to attack by certain organic chemicals and excessive deflection if improperly bedded. Used for pressure and vacuum sewers. Ductile Iron Pipe (DIP); used for pressurized sewers. Because WW is corrosive, the DIP should be internally lined by cement mortar. High Density Polyethylene (HDPE): Can be used for pressurized sewers. Reinforced Concrete Pipe (RCP): Available from 300mm dia. to 5m. Used for trunk lines. Its major limitations are its large weight and susceptibility to crown corrosion. Glass Reinforced Pipes (GRP): Pipes are made from glass fiber reinforcements that are embedded in cured thermosetting resin. GRP pipes combine the benefits of durability, strength, and corrosion resistance; Asbestos Cement pipe (AC); It was used for long time ago. Currently, it’s forbidden to be used due to its negative environmental impact during manufacturing.
Gravity Sewer Collection System Design ‐ Pipe Material ‐ •
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Crown Corrosion: It’s it the result of biochemical reactions With long sewer reaches and little oxygen transfer, the sewage becomes ANOXIC or ANAEROBIC This strong acid‐attacks acid soluble materials such as concrete, iron, etc…
Gravity Sewer Collection System Design ‐ Design Criteria ‐ Manning Equation V
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V = velocity (m/sec) n = coefficient of roughness (dependent upon pipe material/condition) R = hydraulic radius = area/wetted perimeter (m) S = hydraulic slope (assumed to be slope of pipe) (m/m)
Collector pipes (pipe in street) is minimum 200mm diameter (to allow cleaning) Service pipes (home or building to collector is 150mm diameter Gravity sewer pipes have no bends, manholes used to make transitions in direction and pipe size Pipe sections between manholes are at a constant grade or slope (S)
Gravity Sewer Collection System Design ‐ Design Criteria ‐ CHANGE IN PIPE SIZE •
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When a smaller pipe joins a larger one, the invert of the larger sewer should be lowered sufficiently. An approximate method is to place the 0.8 depth point of both sewers at the same location. Large pipe should not discharge into smaller one.
Gravity Sewer Collection System De ‐ Design Criteria ‐ Manholes
Are placed at: ‐ The junction of 2 or more sewers, ‐ Change in vertical or horizontal alignment, ‐ End of each line The manhole dia. is typically 1.2 m for smal sewers. A minimum access of 0.6m is provid •
All sewers shall be designed to give mean velocities, when flowing fill, of not less than 0.75 m/sec (Self ‐Cleansing Velocity) and 0.6 m/sec as an absolute minimum. The maximum velocity is determined by the erosive action of WW. Maximum mean velocities of 2.5 – 3 m/sec will not damage the pipe.
