Types of Chlorination

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