Dissolved Oxygen (DO)

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Calculations (Head and Horsepower)

1. Head to Pressure

A wastewater column 1 foot high exerts a pressure of 0.433 pounds per square inch. The  total pressure can be calculated by multiplying the height (depth) of the water by 0.433 psi/foot.  The height/depth of the water is also called "head".

Example: The water in the settling tank is 15 feet deep.  What is the pressure in pounds per square inch at the bottom of the tank?


2. Pressure to Head

Pressure can also be converted into its equivalent column of water or head.  A pressure of  1 psi is the same as a column of water 2.31 feet high.

The pressure gauge on the discharge line of the pump reads 45 psi.  What is the equivalent head in feet?


3. Work

Work is defined as moving an object (weight) a vertical distance.  It is expressed as foot-pounds

Example: How much work is performed if 200 gallons of wastewater are lifted (pumped) a vertical distance of 50 feet?

4. Power

Power is the amount of work done per unit time.  It is expressed in foot pounds per minute.

How much power in foot pounds per minute will be needed to lift 500 gallons of wastewater a vertical distance of 75 feet in three minutes?


5. Head

Static Head

Head is the vertical distance the wastewater must be lifted from the supply tank to the discharge.  The total head includes the vertical distance the liquid must be lifted (static  head), the loss to friction (friction head), and the energy required to maintain the desired  velocity (velocity head).

Example: The supply tank is located at elevation 118 feet.  The discharge point is at elevation 207 feet.  What is the static head in feet?


Friction Head

Friction head is the equivalent distance of the energy which must be supplied to overcome friction.  Engineering references include tables showing the equivalent vertical distance for various sizes and types of pipes, fittings, and valves.  The total friction head is the sum of the equivalent vertical distances for each component.

Velocity Head Velocity head is the equivalent distance of the energy consumed in achieving and maintaining the desired velocity in the system.

6. Horsepower

Horsepower is a common expression for power in wastewater treatment.  One horsepower is equal to 33,000 foot pounds of work per minute.

The pump lifts 649 gpm of water a vertical distance of (head) of 105 feet.  How much horsepower is used?


7. Water Horsepower

The amount of power required to move a given volume of water a specified total head.

A pump must deliver 250 gpm to total head of 95 feet.  What is the required water horsepower?


8. Brake Horsepower

The amount of horsepower to be supplied to the pump to provide the required water horsepower.  The efficiency of the pump must be taken into account.

Under the specified conditions, the pump efficiency is 76%.  If the required water horsepower is 6.0 hp, what is the required brake horsepower?


9. Motor Horsepower

The rated horsepower of the motor necessary to produce the desired brake and water horsepower.

The motor is 94% efficient.  What is the required motor horsepower when the required brake horsepower is 7.0 Bhp?

10. Electrical Power

To determine the electrical energy required/consumed during a given period of time, the horsepower is converted to electrical energy, (kilowatts), then multiplied by the hours of operation to obtain kilowatt hours.


Example: A 50 horsepower motor operates at full load 12 hours per daily (seven days per week). How many kilowatt hours of energy does it consume per day?