The flow rate refers to the volume of influent wastewater that enters the treatment plant per unit time (generally in hours).
In sewage piping and pumping, the flow velocity must exceed certain limits to maintain trouble free operation and avoid settling and sedimentation of solids:
• For horizontal wastewater pipe systems with organic solids the speed should exceed 0.9 m/s.
However the flow velocity in wastewater treatment systems must not exceed certain limits to reduce the potential for wear and tear due to the effects of erosion and abrasion on plant infrastructure:
• The speed in high-grit sewage handling systems should not exceed 3.6 m/s
• The flow and velocity in sewage systems with low grit concentrations should not exceed 5.4 m/s
Estimated residential flow rates need to account for both average flows rates as well as peak flow rates, in order to diagnose and act in advance of flow problems before they have a deleterious effect on plant performance.
In continuously operated wastewater treatment plants (WWTP), it is often desirable that the flow through the plant should vary as little as possible for optimal performance. Influent flow predictions would therefore be helpful in making maximum use of the buffer capacity with little or no changes in pumping rate during the predicted time periods.
During dry weather the influent flow to the various treatment basins can be considered as almost constant for short time periods. If during that period of time a large change is made in the outgoing flow a clear difference will be seen in the rate of change of the water level.
During a rain event when the flow rate exceeds the highest acceptable capacity, some influent flow is diverted to a separate side-tank. These stormwater tanks are used only during periods when the influent flow rate far exceeds the normal operating range. The tank is filled with the overflow influent, thus regulating the flow into the plant, protecting the biology. As the influent flow rate decreases all the water in the tank will be diverted into the biological treatment system. Also if a toxin is measured upstream, some of the influent water may be collected in the tank or bypassed. These tanks, or large open basins, are also called dry basins since they are normally empty.
Considerably higher flow rates than the daily average are usually the result of rain. During these events the load to the plant is lower, with exception for a brief initial period known as the first flush. The lower load and higher flow rate result in partial washout of the organisms and it will take the plant some time to recover. In extreme cases the plant capacity is severely reduced during the recovery period as new bacteria are grown.
Knowing flow rates will help to determine the parameters like food to microorganisms ration, mixed liquor suspended solids (MLSS) and mixed liquor volatile suspended solids (MLVSS) and in deciding plant aeration requirements.