Disinfection - Wastewater Operators - page 356

Solution:

Chlorine Required = 90 lb/day × 124 days = 11,160 lb

In some instances, projections for flow or dose changes are not
available but the plant operator wishes to include an extra amount of
chlorine as a safety factor. This safety factor can be stated as a specific
quantity or as a percentage of the projected usage. A safety factor as a
specific quantity can be expressed as follows:

Total Required Cl 2 = Chlorine Required (lb) + Safety Factor

Note: Because chlorine is only shipped in full containers, unless asked
specifically for the amount of chlorine actually required or used during
a specified period, round all decimal parts of a cylinder up to the next
highest number of full cylinders.

9.3 ulTravioleT irradiaTion

Although ultraviolet (UV) disinfection was recognized as a method
for achieving disinfection in the late 19th century, its application vir-
tually disappeared with the evolution of chlorination technologies.
In recent years, however, there has been resurgence in its use in the
wastewater field, largely as a consequence of concern about the dis-
charge of toxic chlorine residual. Even more recently, UV disinfection
has gained more attention because of tough new regulations on chlo-
rine use imposed by both OSHA and the U.S. Environmental Protection
Agency. Because of this relatively recent increased regulatory pressure,
many facilities are actively engaged in seeking alternatives to chlorine
for other disinfection, and improvements in UV technology have made it
an attractive candidate. Ultraviolet light has very good germicidal quali-
ties and is effective in destroying microorganisms. It is used in hospi-
tals, biological testing facilities, and many other similar locations. In
wastewater treatment, the plant effluent is exposed to ultraviolet light of
a specified wavelength and intensity for a specified contact period. The
effectiveness of the process is dependent on:

• UV light intensity

• Contact time

• Wastewater quality ( turbidity)

The Achilles' heel of UV for disinfecting wastewater is turbidity.
If the wastewater quality is poor, the ultraviolet light will be unable to
penetrate the solids, and the effectiveness of the process decreases dra-
matically. For this reason, many states limit the use of UV disinfection
to facilities that can reasonably be expected to produce an effluent con-
taining 30 mg/L or less of BOD 5 and total suspended solids.

In the operation of UV systems, UV lamps must be readily avail-
able when replacements are required. The best lamps are those with a
stated operating life of at least 7500 hours and those that do not produce