(Lift Stations)

The overall performance of a lift station depends on the performance of the pumps. All pumps have four common performance characteristics: capacity, head, power, and overall efficiency. Capacity (flow rate) is the quantity of liquid pumped per unit of time, typically measured as gallons per minute (gpm) or million gallons per day (MGD). Head is the energy supplied to the wastewater per unit weight, typically expressed as feet of water. Power is the energy consumed by a pump per unit time, typically measured as kilowatt-hours. Overall efficiency is the ratio of useful hydraulic work performed to actual work input. Efficiency reflects the pump relative power losses and is usually measured as a percentage of applied power.

Pump performance curves are used to define and compare the operating characteristics of a pump and to identify the best combination of performance characteristics under which a lift station pumping system will operate under typical conditions (flows and heads).

Pump systems operate at 75 to 85 percent efficiency most of the time, while overall pump efficiency depends on the type of installed pumps, their control system, and the fluctuation of influent wastewater flow. Performance optimization strategies focus on different ways to match pump operational characteristics with system flow and head requirements.

They may include the following options:

* Adjusting system flow paths

* Installing variable speed drives

* Using parallel pumps

* Installing pumps of different sizes

* Trimming a pump impeller

* Putting a two-speed motor on one or more pumps in a lift station

While savings will vary with the system, electrical energy savings in the range of 20 to 50 percent are possible by optimizing system performance.

Lift station operation is usually automated and does not require continuous on-site operator presence. However, frequent inspections are recommended to ensure normal functioning and to identify potential problems early. Weekly pump station inspection typically includes observation of the following:

* Pumps, motors and drives for unusual noise, vibration, heating or leakage

* Check of pump suction and discharge lines for valving arrangement and leakage

* Check of control panel switches for proper position; monitoring of discharge pump rates and pump speed

* Monitoring of pump suction and discharge pressure

Lift station inspection typically includes observation of pumps, motors and drives for unusual noise, vibration, heating and leakage, check of pump suction and discharge lines for valving arrangement and leakage, check of control panel switches for proper position, monitoring of discharge pump rates and pump speed, and monitoring of the pump suction and discharge pressure. Weekly inspections are typically conducted, although the frequency really depends on the size of the lift station.

If a lift station is equipped with grinder bar screens to remove coarse materials from the wastewater, these materials are collected in containers and disposed of to a sanitary landfill site as needed. If the lift station has a scrubber system for odor control, chemicals are supplied and replenished typically every three months. If chemicals are added for odor control ahead of the lift station, the chemical feed stations should be inspected weekly and chemicals replenished as needed.

Calculate the pump rate

A circular wet well measures 9 feet in diameter. If the pump lowers the water level 2.5 feet in 6 minutes, what is the pumping rate of the pump in gallons per minute? Assume no inflow.

First we find the volume of the cylinder in gallons. Then we divide it by the time to get the pump rate.

Volume= 0.785 x 9 ft x 9 ft x 2.5 ft = 158.96 ft3

OR 3.14 X 4.5 ft X 4.5 ft X 2.5 ft = 158.96 ft3

158.96 ft3 x 7.48 gal/cuft = 1,189 gal

Pump rate = (1,189 gal) / (6 min) = 198.17 gal/min

Calculating overflow

A lift station is 17 feet by 15 feet and a depth of 100 feet. If the depth is now is at elevation of 81 feet. How many additional gallons can the lift station hold before it overflows?

First calculate the total number of gallons that the lift station can hold. Then calculate how much water is in there currently. Find the difference in the two.

Total Volume = 17 ft x 15 ft x 100 ft = 25,500 ft3

25,500 ft3 x 7.48 gal/ft3 = 190,740 gal

Current Volume = 17 ft x 15 ft x 81 ft = 20,655 ft3

20,655 ft x 7.48 gal/ft3 = 154,499.4 gal

Total volume – Current Volume = 190,740 gal – 154,499.4 gal = 36,240.6 gal