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Designing Sewer Network for managing the effluent flow

Principle element of a sewerage system: The Network of underground pipelines and sewers receiving and draining waste waters away from population centres and industrial enterprises, towards the appropriate treatment facilities.

An urban sewer network consists of pipe networks draining from city blocks, buildings and groups of buildings, and streets, sewage mains, and pressure lines. The lines of the sewer network running within individual buildings are connected to the network of that group or block of buildings through special drainage outlets. Pumping stations are built to pump the sewage water to treatment facilities. Manholes are provided for inspection and repairs. Industrial enterprises may have several sewer networks for removing effluents of different composition (for example, strong acid and strong base).

Depending on topography, ground conditions, the composition of the effluents, and sequence of construction, networks may be perpendicular, transverse, parallel, zonal, or radial. Whenever possible, sewer networks are designed to use a gravity-fed system to transport household and industrial effluents. The hydraulic calculations necessary in designing a sewer network consist in determining the diameter of the pipes and their load and the rate of flow of the effluents. The network should be deep enough (depending on the depth of soil freeze) to protect the pipes against damage by surface transport.

The choice of the pipeline material depends on the composition of the effluents and ground waters and on the purpose of the pipeline. A gravity-fed sewer network is made from ceramic, asbestos-cement, concrete, and reinforced-concrete pipe. The large-diameter mains are made from reinforced-concrete pipe or prefabricated sections of reinforced concrete. Metal, asbestos cement, and reinforced-concrete pipes are used for pressure lines. Pipes can also be made from synthetic materials. Careful fitting of the joints in laying the pipe helps ensure water tightness and long life.

Sewers and estimation

      1.      Classification of Sewers
    •     Domestic or Industrial Sewers
    •     Storm Sewers
    •     Combined Sewers

    Domestic or Industrial Sewers

    They are designed to carry wastewater generated from domestic establishments or small- and medium- sized industrial establishments in a municipal area, but not storm-water.

    Storm Sewers

    They are designed to carry off only stormwater and groundwater, but excludes sewage from domestic and/ or industrial source.

    Combined Sewers

    They are designed to carry off stormwater, domestic and industrial wastewater


      1.      Advantages and Disadvantages of Combined Sewers

    It is initially economical to set up a combined sewer rather than separately installing domestic sewers and stormwater sewers.

    During a dry season lack of stormwater causes a low flow rate. Low flow rate gives rise to the low velocity of flow. At low velocities, due to less turbulence, the deposition of sewage solids are more. The result is the situation and consequent foul odour generation due to degradation of the settled solids.

    In contrast, during wet or rainy seasons, the flow rate is very high. Therefore, pumping costs are more, causing the high operation and maintenance cost. Pumps that are designed to operate at high flow rate to tackle the wet season flow, runs in low flow condition in the dry season, which is an inefficient operation that consumes more power than usual. Due to the above reasons, combined sewers are not generally recommended by the manual of sewerage and sewage treatment.

     

      1.      Estimation of Quantity of Sanitary Sewage

    The sewers are designed to carry:

    • Spent water from a community
    • Some groundwater
    • Fraction of the storm water
    • Industrial wastewater for small establishments

    The sanitary sewers are designed to carry the waste water from the above sources to a sewage/wastewater treatment plants.

    Carrying capacity of the sewers depends on:

    1. Present and
    2. Future quantities of flow rate expected.

    Thus, it is important to estimate the design flow rate for the sewers to be constructed.


      1. Estimation of Sewage Flowrate Two Parameters:
    1. The contributing population, and
    2. Per capita (per person) flow rate of sewage

    Both of these quantities depend on the design period.

    Design period: The length of the time up to which the capacity of a sewer will be adequate is called a design period. Normally the design period for a sewerage system is considered as 30 years But, mechanical rotating equipment such as pumps are designed for 15 years.

    Procedure for Designing Sewer Network

    1.        In takes from the first Manhole are plotted in the drawings.
    2.       A layout of Manholes is then identified along the sides of existing roads or any other medium as felt suitable.
    3.    The contribution of individuals is computed based on the class or category of the person, namely officers, executives, labour….etc. These classifications may vary depending on the requirements or statutory guidelines.
    4.      Based on the analysis of the contour map in the execution area the location of the septic tank is located such that the network will be mostly based on gravity.
    5.        Any other effluents like Rain water may also be added as a contribution to the manholes, though it is not recommended.
    6.      Type of the Material is then selected based on the area of influence and the discharge at various locations.
    7.      The velocity of the effluent at various conditions is analysed such that the system does not fail at any point, this could be either at peak hours or at lean periods.
    8.       Minimum and maximum velocity is then analysed.
    9.       The diameter of the pipes is analysed.
    10.      To find the roughness of the pipe (roughness co-efficient), and the possible flow velocity, Manning’s formula or any other formula is used.
    11.     Now depth (Invert level) at each manhole is calculated in such way that the flow direction is nor reversed or shall not come to an halt even during lean periods.
    12.        All efforts are put to calculate the final discharge and final velocity of the effluent.
    13.      Longitudinal profiles are then prepared as a part of the drawing submission along with several design components. A sample out is presented below
    sewer
    sewer

    The summary of this article is, Desiging Sewer Network includes considering many inputs and design parameters. Since there are many factors involved, Interpretation of values may lead to manual errors which is really dangerous, as it could lead to failure of the system.

    Sewer network module of ESurvey Civil  Tools addresses most of these issues. Click here to read more about Sewer Network Module.