Sewers velocities

The occurrence of sewer sediments is primarily determined by the physical characteristics of wastewater solids and the hydraulic conditions. Basically, sewers should be designed and operated in a way that does not result in permanent deposits. This ideal performance of a sewer is not generally observed, and sediments may be more or less temporarily accumulated in sewers. In combined sewer networks, sediments may settle under dry-weather conditions when the wastewater velocity and shear stress at the bottom are low and be... 

Most sanitary and combined sewer networks consist of pipes designed to flow as open channels, i.e., with a free water surface. The wastewater flows downstream in such pipes by the force of gravity with a velocity of flow that depends principally on the pipe slope and frictional resistance. Typically, the design velocity is between 0.6 and 3 m s-1 to avoid blockage of the pipe by sewer solids accumulated at low flow conditions and to prevent damage of the sewer at a high flow. 

When designing sewer networks, particularly gravity sewers, reaeration is the major process that should be focused on to reduce sulfide formation and the formation of organic odorous substances (cf. Section 4.4). A number of hydraulic and systems characteristics can be managed to increase the reaeration rate and avoid or reduce sulfide-related problems. The hydraulic mean depth, the hydraulic radius, the wastewater flow velocity and the slope of the sewer pipe are, in this respect, important factors that are dealt with in Section 4.4. It should be stressed that it is not necessarily the objective to avoid sulfide formation (in the sewer biofilm), but the sulfide that occurs in the bulk water phase should be at a low concentration level. Therefore, the DO concentration in the bulk water phase should not be lower than about 0.2-0.5 g02 m-3, sufficiently high to oxidize sulfide before a considerable amount is emitted to the sewer atmosphere. 

The thickness of the sewer biofilm affects sulfide formation. Reduction of the biofilm thickness by increasing the wastewater velocity may lead to reduced sulfide problems. At very low velocities in an arerobic gravity sewer, a biofilm thickness may be more than 50 mm however, it may be substantially reduced to typically 1-5 mm when the velocity is increased. The thickness of... 

In Figure 8.3, the oxygen transfer coefficient, KLa, the flow velocity, u, the bulk water DO concentration and the DO consumption rate of the biofilm, ry, are all plotted versus the flow, Q, under steady state conditions in a gravity sewer pipe under the conditions given. 

A 15-in.-diameter sewer pipe is flowing full and carrying a flow of 2.7 million gallons per day. What is the average velocity of the water in the pipe ... 

The membrane separation plant is tubular ultraflltration (UF) and the pilot-plant operation was on a batch basis with a volume reduction factor approaching 40. The UF membrane had a maximum permeate flux of around 300 L/m hr at maximum 6 kg/cm inlet pressure and 3.8 m/s fluid velocity with a clean membrane. The flux typically dropped and approached 80 L/m hr at the end of a day s operation. The retentate from UF separation was returned to the feed tank whereas the permeate was routed to the sewer. Design of a full-scale plant was performed using a flux value of 40 L/m hr and volume reduction of 20x. 

Flashing liquids in which much of the liquid is vaporized on relieving, should not be emptied to a sewer or other ground location without proper protection for personnel. The vapor will propel the liquid at high velocity and may spatter passersby with the hot liquid. 

Velocities used in the design of sewer systems should have a minimum of 3 feet per second and a maximum of 7 feet per second. 

Flow capacities, velocities and slopes (for sewers running 3/4 full) can be coordinated so that the curves shown on Figures 8-9 and 8-10 will give the size of the sewer line required for the assumed slope and velocity at the desired flow capacity in gallons per minute. 

Flow capacities, velocities and slopes of sewer lines should be contingent on the grades available at the plant site. The flat grades necessary at most plant sites will be a determining factor in the slope... 

Pomeroy, R.D. (1967). Flow velocities in small sewers. Journal WPCF 39(9) 1525-1548. Pomeroy, Johnston Bailey (1974). Process design manual for sulphide control in sanitary sewage systems. Pasadena CA. 

Camp minimum self-cleaning velocity for a sewer (m/s)... 

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