How is Velocity important in the flow of Wastewater in Sewage system?
Sewage contains a large volume of organic and inorganic materials that float or are suspended in the water. These sediments get deposited at the invert of the pipe and hinder the passage of sewage if the velocity of flow in the sewer is low. As a result, a minimum flow velocity must be maintained to ensure that no sediments are deposited in the sewer. Self-cleaning velocity is the flow velocity that prevents solids from settling. During peak flow, this velocity should be maintained at least once a day in all sections of the sewerage system.
A sewer should be structured in such a way that solid matter in the sewage is not deposited at the bottom of the sewer, preventing clogs. The solid matter deposition and subsequent obstruction of the sewer can be avoided if the solid matter is kept suspended in the running sewage.
Minimum Velocity of Flow
It is necessary to maintain a particular minimum velocity of sewage flow in order to keep solid matter suspended. Self-cleaning velocity is the name given to such a low flow velocity. As a result, self-cleansing velocity can be described as the minimal flow velocity at which solid particles in the sewage are retained in suspension and scour of the deposited particles occurs, ensuring that the sewer is kept clean.
The size and specific gravity of the solid particles in the sewage determine the self-cleaning velocity. Due to variations in the amount of sewage flow, it is not possible to maintain the self-cleaning velocity throughout the day. During a minimum sewage flow, the flow velocity will be lower than the self-cleaning velocity.
However, the sewer should be designed in a way that at least once a day, self-cleaning velocity is maintained in the sewer, so that any solid particles that have deposited are eroded and removed by the flowing sewage, and the sewer is rendered clean.
Maximum Velocity of Flow
Just as it is required to provide a minimum sewage flow velocity or self-cleaning velocity in a sewer to prevent blockage, it is also necessary to ensure that the sewage flow velocity in a sewer is not excessive to produce scouring or erosion of the sewer's inner surface. The abrasive impact of tougher solids like sand, grit, gravel, and other elements present in the sewage will produce scouring or erosion at greater flow rates above a specific limit, causing damage to the sewer's inner surface.
Non-scouring velocity, also known as limiting velocity, is the highest flow velocity at which no scouring or erosion of the sewer's inner surface occurs. The non-scouring or limiting velocity is determined by the materials used for the construction of sewers.
In comparison to conventional bricks or concrete, vitrified tiles and glazed bricks are more resistant to erosion among the many materials used in sewer construction. Because sand, grit, gravel, and other heavy materials pass along the bottom of the sewers, regular brick or concrete sewers are occasionally coated with vitrified tiles or glazed bricks at their bottoms, where abrasion is greatest.Non-scouring velocity is the greatest speed that can be used to avoid eroding and should be kept below 3.0 m/s.
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