(Lift Stations)

Lift station operation and maintenance costs include power, labor, maintenance, and chemicals (if used for odor control). Usually, the costs for solids disposal are minimal, but are included if the lift station is equipped with bar screens to remove coarse materials from the wastewater. Typically, power costs account for 85 to 95 percent of the total operation and maintenance costs and are directly proportional to the unit cost of power and the actual power used by the lift station pumps. Labor costs average 1 to 2 percent of total costs. Annual maintenance costs vary, depending on the complexity of the equipment and instrumentation.

Calculating Overflow

A 75,000 gallon tank receives 270,000 gpd flow. A 195 gpm pump is attached to the tank, but it is broken. How long do they have to repair or replace the pump before the tank will overflow? Assume the tank is empty now.

First find the average flow per hour. Then find the number of hours till the overflow occurs.

Hourly Flow = (270,000 gal/day) / (24 hrs/day) = 11,250 gal/hr

Hours till Overflow = (75,000 gal) / (11,250 gal/hr) = 6.67 hrs or 6 hrs 40 min

Calculating Rising Water In a Wet Well

The wet well at a lift station receives a flow of 465 gpm. The wet well has a diameter of 18 feet. How many minutes will it take to raise the water level 6 feet in the wet well?

First calculate the volume of water it will take to fill 6 feet of the wet well. Then determine based on the gallons per minute how long it will take to rise to a level of 6 feet.

Volume = 0.785 x 18 ft x 18 ft x 6 ft = 1,526.04 ft3 OR 3.14 X 9 ft X 9 ft X 6 ft

1,526.04 ft3 x 7.48 gal/ft3 = 11,414.77 gal

Minutes to rise in 6 feet = (11,414.77 gal) / (465 gal/min) = 24.55 min

Cost of painting a tank

An open topped rectangular tank is 12 feet wide, has a total depth of 13 feet and is 50 feet long.

a) What is the total inside tank surface area?

b) What would the cost be to coat the inside of the tank at 5.25 per square foot?

First find the total surface area. Calculate the cost by multiplying the surface area by the cost.

area of wall 1 (ft2) 12 x 13 = 156 ft2

area of wall 2 (ft2) 13 x 50 = 650 ft2

area of wall 3 (ft2) 12 x 13 = 156 ft2

area of wall 4 (ft2) 13 x 50 = 650 ft2

bottom of tank (ft2) 12 x 50 = 600 ft2

total area (ft2) = 2,212 ft2

Cost = $5.25 x 2,212 ft2 = $11,613.50 / ft2

Calculate Operation and Construction Costs

Two 65 hp pumps operate a lift station. The lift station runs 15 hours per day. The power in this municipality is charged at a rate of $.11 per kW*hr. what is the cost per day for running the pumps?

First calculate the total horsepower. Then use the kilowatt hours equation to find the kilowatt hours. Then multiply by the rate to find the cost.

Hp = 2 x 65 = 130

1 horsepower = 0.746 kW

Kilowatts = hp x 0.746 kW or 130 hp X 0.746 kW = 96.98 kW

Kilowatt hrs = kW used x number of hours run = 96.98 kW x 15 hrs = 1,454.70 kW hrs

Cost = kW hrs x cost = 1,454.70 kW hrs x $0.11 = $160.02 per day

You have three lift stations, #1 has a 9 hp motor and runs for 30 minutes per hour, #2 has a 12.5 hp pump and it runs 45 minutes per hour; and #3 has a 10 hp pump that runs 30 minutes every 2 hrs. What is the yearly electrical cost to operate these three stations if each kilowatt hour cost $ 0.09?

First calculate the number of hours that the pumps run. Calculate the kilowatts, kilowatt hrs, and then the power cost.

1 horsepower = 0.746 kW

Pump 1 kW = 9 hp x 0.746 kW = 6.71 kW

Pump 1 kW hrs = 6.71 kW x 12 hrs = 80.52 kW hrs x 365 = 29,389.80 kw hrs per year

Pump 1 cost per year = 29,389.80 x $ 0.09 = $2,645.08

Pump 2 kW = 12.5 hp x 0.746 kW = 9.33 kW Pump 2 kW hrs = 9.33 kW x 18 hrs = 167.94 kW hrs x 365 = 61,298.10 kw hrs per year

Pump 1 cost per year = 61,298.10 x $ 0.09 = $5,516.83

Pump 3 kW = 10 hp x 0.746 kW = 7.46 kW

Pump 3 kW hrs = 7.46 kW x 6 hrs = 44.76 kw hrs x 365 = 16,337.40 kw hrs per year

Pump 1 cost per year = 16,337.40 x $.09 = $1,470.37 Cost = $2,645.08 + $5,516.83 + $1,470.37 = $9,632.28