Sewer Ventilation Model

A number of factors affecting the air-movement in sewers have been identified (Pomeroy, 1945; Pescod and Price, 1982; USEPA, 1992). These include the followings:

    1. Wastewater drag – Transfer of momentum from wastewater to air due to the shear force at the air-water interface. The friction between the sewer headspace air and the flowing wastewater is the primary force driving the air flow.
    2. Wall friction – The friction between the airflow and the sewer pipe wall offers natural resistance to the air flow.
    3. Wind eduction – Differential wind speeds across sewer openings cause dynamic pressure differences leading to airflow circuits.
    4. Rise and fall of wastewater level – Rising water levels can force water out of the unsealed openings, while declining water levels can draw air into sewers.
    5. Pressure Differentials – Temporal variations in barometric pressure lead to volume expansion and compression of gases, resulting in air flows in and out of sewers.
    6. Temperature Differentials – When the temperature of the sewer headspace is different from the ambient temperature, a difference in air density is created. This induces airflow through buoyancy.

Out of the factors listed above, the effects of temperature change, change in wastewater level, and the effects of pressure differential can be dealt with analytically. However, a through understanding of the processes related to wastewater drag, wall friction and wind effect is needed.

A model that takes into account the fundamental physical principles is available.

    1. Description of the ventilation model
    2. Ventilation tool user manual
    3. Link to a tool to estimate ventilation rate in a sewer pipe

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