Chlorination Types

Chlorination
Advantages:

Highly effective against most pathogens
Provides a residual to protect against recontamination and to reduce slime growth in the distribution system
Easily applied, controlled, and monitored
A very strong oxidant
The most reliable operationally
The most cost-effective

Disadvantages:

Byproduct formation (THMs, HAAs)
Will oxidize bromide to bromine, forming brominated organic byproducts
Not effective against Cryptosporidium
Requires transport and storage of chemicals

Elemental Chlorine
Transported and stored as a liquefied gas under pressure, elemental chlorine is the most commonly used form of chlorine.

Advantages:

Lowest cost of chlorine forms
Unlimited shelf-life

Disadvantages:

A hazardous gas that requires special handling and operator training
Subject to regulatory requirements for occupational safety and health

Sodium Hypochlorite
Sodium Hypochlorite, or bleach, is produced by adding elemental chlorine to sodium hydroxide. Typically, hypochlorite solutions contain from 5 to 15% chlorine.

Advantages:

Solution is less hazardous and easier to handle than elemental chlorine
Fewer training requirements and regulations than elemental chlorine

Disadvantages:

Limited shelf-life
Potential to add inorganic byproducts (chlorate, chlorite and bromate) to water
Corrosive to some materials and more difficult to store than most solution chemicals
Higher chemical costs than elemental chlorine

Calcium Hypochlorite
Calcium hypochlorite is another chlorinating chemical used primarily in smaller applications. Containing approximately 65% chlorine, it is commercially available in granular and tablet forms.

Advantages:

More stable than sodium hypochlorite, allowing longer storage
Fewer training requirements and regulations than elemental chlorine

Disadvantages:

Dry chemical requires more handling than sodium hypochlorite
Precipitated solids formed in solution complicate chemical feeding
Higher chemical costs than elemental chlorine
Fire or explosive hazard if handled improperly
Potential to add inorganic byproducts (chlorate, chlorite and bromate) to water

Onsite Hypochlorite Generation
Some municipalities have installed on-site hypochlorite generators that produce weak hypochlorite solutions (~0.8%) using an electrolytic cell and a solution of salt water, much like is used in domestic and public swimming pools.

Advantages:

Minimal chemical storage and transport

Disadvantages:

More complex and requires a higher level of maintenance and technical expertise
High capital cost
Operating costs are often higher than for commercial hypochlorite
Requires careful control of salt quality
Weak solution requires high volume chemical feed and control
Byproducts in generated hypochlorite may be difficult to monitor and control
System backup may be more difficult and costly