(Lift Stations) Calculating Velocity
Wastewater is pumped into a 6 inch line by a 300 gal/min pump that is 85 % efficient, what is the velocity of the wastewater in the line?
The velocity can be found by using the velocity equation V = Q/A. the flow (Q) is given and solving for the area using the area of a circle for the 6 in line. First convert inches into feet and gallons into ft3.
A = 0.785 x 0.5 ft x 0.5 ft = 0.1963 ft2
(300 gal/min) / (7.48 gal/cuft) = 40.11 ft3/min
V = (40.11 ft3/min) / (0.1963 ft2) = 204.31 ft/min
Divide by 60 to get the number of ft per second. Then multiply by the efficiency.
V = (204.31 ft/min) / (60 sec/min) = 3.40 ft/sec x 0.85 = 2.89 ft/sec
Advantages of Lift Stations
Lift stations are used to reduce the capital cost of sewer system construction. When gravity sewers are installed in trenches deeper than 10 feet, the cost of sewer line installation increases significantly because of the more complex and costly excavation equipment and trench shoring techniques required. The size of the gravity sewer lines is dependent on the minimum pipe slope and flow. Pumping wastewater can convey the same flow using smaller pipeline size at shallower depth, and thereby, reducing pipeline costs.
Compared to sewer lines where gravity drives wastewater flow, lift stations require a source of electric power. If the power supply is interrupted, flow conveyance is discontinued and can result in flooding upstream of the lift station, It can also interrupt the normal operation of the downstream wastewater conveyance and treatment facilities. This limitation is typically addressed by providing an emergency power supply.
Key disadvantages of lift stations include the high cost to construct and maintain and the potential for odors and noise. Lift stations also require a significant amount of power, are sometimes expensive to upgrade, and may create public concerns and negative public reaction. The low cost of gravity wastewater conveyance and the higher costs of building, operating, and maintaining lift stations means that wastewater pumping should be avoided, if possible and technically feasible.
Wastewater pumping can be eliminated or reduced by selecting alternative sewer routes or extending a gravity sewer using direction drilling or other state-of-the-art deep excavation methods. If such alternatives are viable, a cost benefit analysis can determine if a lift station is the most viable choice.
Pump stations are complex facilities that contain a significant number of equipment and auxiliary systems. Therefore, they are less reliable than gravity wastewater conveyance but the pump station reliability can be significantly improved. A way to improve the situation is by providing standby equipment (pumps and controls) and emergency power supply systems. In addition, pump station reliability is improved by using screens to remove debris, by using non-clog pumps suitable for the particular wastewater quality, and by applying emergency alarm and automatic control systems.
In an emergency (pump malfunction, power failure, etc.) a portion of the wastewater conveyed to the pump station may overflow to nearby surface waters causing potential health risk. Emergency sewer overflows are mitigated by installation of highly reliable equipment, providing redundant control systems and installing facilities for overflow storage and/or treatment prior to discharge to surface waters.
Potential odor problems are mitigated by installation of various odor control systems, including reduction of odor release by adding chemicals upstream of the pump station and odorous gases evacuation and treatment at the pump
Cost effective lift stations are designed to:
Match pump capacity, type, and configuration with wastewater quantity and quality;
Provide reliable and uninterruptible operation
Allow for easy operation and maintenance of the installed equipment
Accommodate future capacity expansion
Avoid septic conditions and excessive release of odors in the collection system and at the lift station
Minimize environmental and landscape impacts on the surrounding
Residential and commercial developments
Avoid flooding of the lift station and the surrounding areas