Page 158 - Water and wastewater engineering

WELLS 4-13

pressure, the vent is essential to ensure that the calculated NPSH A represents actual conditions.
The seal should be located 0.6 m above the highest known flood level.
The well casing is never used as the discharge pipe to bring water to the surface.

Disinfection. All newly constructed wells as well as those that have been repaired should be
disinfected before being put into service. The well should be thoroughly cleaned before disinfec-
tion is initiated. Foreign substances such as sediment, soil, grease, joint dope, and scum should
be removed.
Calcium hypochlorite (Ca(OCl) 2 · 4H 2 O) containing approximately 65 percent available
chlorine is the most common disinfectant in use. Alternatively commercial sodium hypochlorite
(12 to 15 percent available chlorine) or household bleach (5.25 percent available chlorine) may
be used.
®
Calcium hypochlorite, also known as high-test calcium hypochlorite or HTH , is a granular
material that contains minor amounts of impurities. It has the advantage that it is cheaper than the
other forms of chlorine as well as being able to be handled as a solid in the field. Because it is a
solid, dissolving the material requires some care.
®
Sodium hypochlorite is a liquid. While it is more expensive than HTH , less preparation
time is required to use it.
The recommended procedure for disinfection once all foreign substances have been cleared
from the well is to prepare a sufficient amount of solution to achieve 100 mg/L of available
chlorine in the water in the well without pumping, that is, the water that has risen to the static
water level. The solution is placed in the well. A 20 to 40 L excess of solution is added to force
the chlorinated water into the formation. The well is then pumped until the odor of chlorine is
apparent. Pumping should then be shut down, valves closed, and the solution held in place for a
minimum of 4 hours with 24 hours preferred. The well is then pumped until the water is free of
chlorine odor. A field test is conducted to ensure that no chlorine is present and then a sample is
take for coliform analysis (Johnson, 1975; U.S EPA, 1973).

®
Example 4-1. Calculate the volume of solution and the amount of HTH to be added to it to
disinfect a 300 mm diameter well. The static water level in the well is 18 m above the bottom of
the well.

Solution. The volume of water in the well is
⎛ (0 30 m ⎞
2
)
.
3
V (18 m ) ⎜ ⎝ ⎟ ⎠ 1 27 m or , L
.
1 270 L
4
plus the recommended 40 L excess.
For the recommended concentration of 100 mg/L, the mass of chlorine required is
)
,
Chlorine mass (100 mg/L)(1 310 L 131 000 mg or 131 g
,

® ®
Because the HTH is only 65 percent available chlorine, the mass of HTH required is
® 131 g
HTH mass 201 5. g
.
065