Disinfection - Wastewater Operators - page 342

Note: You might wonder why it is that chlorination of critical waters
such as natural trout streams is not normal practice. This practice is
strictly prohibited because chlorine and its byproducts (e.g., chloram-
ines) are extremely toxic to aquatic organisms.

9.2.1 Chlorination Terminology

Several terms are pertinent to a discussion of disinfection by chlo-
rination. Because it is important for the operator to be familiar with
these terms, we present them here:

Chlorine —A strong oxidizing agent that has strong disinfecting capa-
bility. It is a yellow-green gas that is extremely corrosive and toxic
to humans in extremely low concentrations in air.

Contact time —The length of time the disinfecting agent and the waste-
water remain in contact.

demand —The chemical reactions that must be satisfied before a
residual or excess chemical will appear.

disinfection —The selective destruction of disease-causing organisms.
Not all of the organisms are destroyed during the process. This dif-
ferentiates disinfection from sterilization , which is the destruction
of all organisms.

dose —The amount of chemical being added in milligrams per liter.

feed rate —The amount of chemical being added in pounds per day.

residual —The amount of disinfecting chemical remaining after the
demand has been satisfied.

Sterilization —The removal of all living organisms.

9.2.2 Wastewater Chlorination facts

9.2.2.1 Chlorine Facts

• Elemental chlorine (Cl 2 , gaseous) is a yellow-green gas that is 2.5

times heavier than air.

• The most common use of chlorine in wastewater treatment is for

disinfection. Other uses include odor control and activated sludge
bulking control. Chlorination takes place prior to the discharge of
the final effluent to the receiving waters.

• Chlorine may also be used for nitrogen removal through a process

called breakpoint chlorination . For nitrogen removal, enough chlo-
rine is added to the wastewater to convert all of the ammonium
nitrogen gas. Approximately 10 mg/L of chlorine must be added for
every 1 mg/L of ammonium nitrogen in the wastewater.

• For disinfection, chlorine is fed manually or automatically into a

chlorine contact tank or basin, where it contacts flowing wastewa-
ter for at least 30 minutes to destroy disease-causing microorgan-
isms (pathogens) found in treated wastewater.