Wastewater Drainage Systems: Design and Types
Layout of Gravity Sewer Systems
The systematic arrangement of gravity sewers must be performed to drain wastewater (WW) and concentrate it at the lowest point, the wastewater treatment plant (WWTP). The following must be taken into account:
- Terrain configuration
- Type of built-up area
- Recipient disposition
- Other eventual conditions
Generally, sewers are designed to drain wastewater into the WWTP in the most optimal way.
- Radial System: Tends to be applied for the drainage of closed hollow basins without a direct connection to the recipient.
- Branch System: This design is used in broken terrain with an irregular built-up area.
- Curtain System: Is used in flat river valleys with a gentle slope of the drained area to the watercourse.
- Zone System: Is used for the drainage of larger areas with greater elevation differences.
Pressure Sewer System
This is the most common alternative method of wastewater drainage, thanks to its acquisition costs. The basis of this wastewater transport method is the construction of pumping wells (sumps). The sewage is drained by gravity. From the sump, the sewage is transported through pressure pipes of a significantly smaller diameter into the WWTP or sewage system. The normal working overpressure of the pump in a sump is approximately 20 – 50 m water column. Pressure in the mains is about 0.5 – 3.0 MPa (50 – 300 mca).
Pneumatic Sewer System
This system uses compressed air, even over long distances. It is capable of transporting even very polluted mediums. Pipes are placed at an unfreezing depth and follow the terrain. Air in the pipes absorbs shocks, and the sewage mixture is sufficiently aerated.
Vacuum Sewer System
Vacuum draining is based on the principle of creating underpressure in the main street sewerage pipeline, into which the wastewater from the connected house is sucked through the household suction valve of house sewer connections. The speed varies around 6 – 8 m/s regardless of the pipe slope. Wastewater is not transported as a closed water roller but in individual batches (portions). A portion is made of a mixture of drops carried away by the flowing air in the direction of higher underpressure. A suction pressure of 60 – 70 kPa (0.6 – 0.7 bar) compared to static pressure is permanently maintained in underpressure vessels of the underpressure station. After opening the suction valve, the wastewater and air are sucked into the pipe system; together, it flows towards the underpressure station into vacuum vessels. From the vacuum vessel, the wastewater is pumped by pumps to the WWTP.
Low-Profile Sewer System
This system is specific due to the pipeline material used, which has great length, small clear profiles, low roughness, and integrated, perfectly waterproof joints. Revision wells are replaced by control wells.
Sewer Profiles
The following sewer profiles are preferably recommended:
Shape selection depends on the evaluation of hydraulic, operational, building, economic, geological, and other requirements and conditions. From a hydraulic point of view, the egg-shaped type is more suitable for smaller flow capacities at the sewer bottom.
- Circular:
- Advantages: The easiest prefabricate production, the best for cleaning.
- Disadvantages: Statically less convenient than egg-shaped.
- Egg-shaped:
- Advantages: The best hydraulic properties, statically most convenient.
- Disadvantages: Can be designed with sufficient hanging wall height.
- Mouth:
- Advantages: Designed in constrained geological conditions (low hanging wall).
- Disadvantages: Hydraulically and statically least convenient.