Water Treatment Processes - Mathematics and Statistics for the Environment - page 606

■ EXAMPLE 23.5
Problem: A well produces 260 gpm. If the drawdown for the well is 22 ft, what is the specific yield
in gpm/ft?

Solution:

Specific yield = Well yield (gpm)/Drawdown (ft) = 260 gpm/22 ft = 11.8 gpm/ft

■ EXAMPLE 23.6
Problem: The yield for a particular well is 310 gpm. If the drawdown for this well is 30 ft, what is
the specific yield in gpm/ft?

Solution:

Specific yield = Well yield (gpm)/Drawdown (ft) = 310 gpm/30 ft = 10.3 gpm/ft

23.2.1.4 Well Casing Disinfection
A new, cleaned, or a repaired well normally contains contamination that may remain for weeks
unless the well is thoroughly disinfected. This may be accomplished by using ordinary bleach at
a concentration of 100 parts per million (ppm) of chlorine. The amount of disinfectant required
is determined by the amount of water in the well. The following equation is used to calculate the
pounds of chlorine required for disinfection:

Chlorine (lb) = Chlorine (mg/L) × Casing volume (MG) × 8.34 lb/gal (23.4)

■ EXAMPLE 23.7
Problem: A new well is to be disinfected with chlorine at a dosage of 50 mg/L. If the well casing
diameter is 8 in. and the length of the water-filled casing is 110 ft, how many pounds of chlorine
will be required?

Solution: First calculate the volume of the water-filled casing:

0.785 × 0.67 × 67 × 110 ft × 7.48 gal l /f ft 3 = 290 gal

Then determine the pounds of chlorine required using the mg/L to lb equation:

Chlorine = Chlorine (mg/L) × Volume (MG) × 8.34 lb/gal

= 50 mg/L × 0.000290 MG × 8.34 lb/gal = 0.12 lb

23.2.1.5 Deep Well Turbine Pump Calculations
The deep well turbine pump is used for high-capacity deep wells. The pump, usually consisting of
more than one stage of centrifugal pump, is fastened to a pipe called the pump column ; the pump is
located in the water. The pump is driven from the surface through a shaft running inside the pump
column. The water is discharged from the pump up through the pump column to the surface. The
pump may be driven by a vertical shaft, electric motor at the top of the well, or some other power
source, usually through a right-angle gear drive located at the top of the well. A modern version of
the deep-well turbine pump is the submersible type of pump, where the pump (as well as a close-
coupled electric motor built as a single unit) is located below water level in the well. The motor is
built to operate submerged in water.