Effectiveness

Chlorine is not as effective a disinfectant as ozone and is markedly less effective against waterborne biohazards such as viruses, spores, and cysts. Chlorine usage is designed around removing bacteria from water; these other hazards can pass through a chlorinated water treatment system and may still pose a hazard.

The data in the following tables illustrate this point. CT Values for Inactivation of Viruses[i] indicate that ozone is six times as effective at deactivating (killing) virus as free chlorine, and nearly 1,500 times stronger than chloramines, a chlorine and ammonia derivative used in an attempt to reduce chlorine’s toxicity and limit formation of trihalomethanes (THMs), which are suspected carcinogens. Similar information is presented in the second table, except the organism of interest is a cyst instead of a virus.

The “CT” value means “concentration times time” and is an EPA method for comparing the effectiveness of different disinfectants. A lower value for CT means that a lower concentration of disinfectant, or a shorter contact time, or a combination of these factors is required to achieve the cited pathogen reductions. In these tables, inactivation is expressed on a logarithmic scale, i.e. 2-log equals a 99% reduction in the number of pathogens; 3-log represents a 99.9% reduction, etc. The units of mg-min/l mean, for example, that a concentration of 6 mg/l chlorine for six minutes, 3 mg/l for 12 minutes, or 1 mg/l for 36 minutes all have the same CT value of 36 (36 = 6 x 6 = 3 x 12 = 1 x 36).

As can be seen from these tables and the literature references, ozone’s effectiveness as a disinfectant is much greater than chlorine and has been well documented.

CT Values for Inactivation of Viruses

• CT values were obtained from AWWA, 1991.
• ¹ Values are based on a temperature of 10°C, pH range of 6 to 9, and a free chlorine residual of 0.2 to 0.5 mg/L.
• ² Values are based on a temperature of 10°C and a pH of 8.
• ³ Values are based on a temperature of 10°C and a pH range of 6 to 9.

CT Values for Inactivation of Giardia Cysts[ii]

• CT values were obtained from AWWA, 1991.
• ¹ Values are based on a free chlorine residual less than or equal to 0.4 mg/L, temperature of 10°C, and a pH of 7.
• ² Values are based on a temperature of 10°C and a pH in the range of 6 to 9.
• ³ Values are based on a temperature of 10°C and a pH of 6 to 9.

Safety

Chlorine is dangerous to handle. The gaseous form is poisonous. Any facility using chlorine gas must comply with a host of OSHA requirements including 40 hour/year HAZMAT training for facility personnel, and substantial investments in safety equipment including emergency eye wash stations, self-contained respirators, confined space entry equipment, chlorine gas detectors, and alarms.

Sodium hypochlorite (chlorine bleach), another form of free chlorine, is safer to handle than the gas, but is corrosive and requires its own set of safety equipment, bulk storage tanks, day tanks, and frequent deliveries. Chlorine tablets are safer yet, but are only used in very small systems due to their high cost. Regardless of the form, chlorine can form THMs by reacting with organics present in the water.

Health

There is a growing body of scientific evidence pointing to long-term adverse health effects of consuming chlorinated drinking water. Current scientific consensus is incomplete; however, several studies have claimed to find a causative link between bladder and colon cancer and the existence of halogenated organics in drinking water. These halogenated organics are collectively referred to as THMs (for “trihalomethane”) and are by products of chlorine-based disinfection for production of potable water.

Regulatory

The 2003 Amendments to the Clean Water Act stipulate additional regulations on the use of chlorine, most notably the requirement that free chlorine be removed from treated wastewater to a concentration not exceeding 17 microgram/liter prior to discharge. This low concentration is difficult to measure and the monitoring and reporting frequency for this test is proving to be burdensome to the regulated community. These new regulations are one of the primary reasons that vendors of UV based disinfection systems have reported a phenomenonal growth in sales in the past few years.

Costs

The figure below provides a cost comparison for ozone, chlorine, or UV disinfection for a recent upgrade to a wastewater treatment plant in Hoke County, North Carolina. This plant has a permitted capacity of 200,000 GPD.

Cost of UV vs. Chlorine vs. Ozone for 200,000 GPD NPDES Plant

Disinfection using UV was selected at this plant based upon the desire to avoid new regulatory burdens associated with the use of chlorine. Data used for UV represents the actual installation cost, while the civil engineering design firm that managed this project determined the data for chlorine. Chlorine’s 20-year operating costs do not include compliance with OSHA regulations or additional monitoring associated with the use of chlorine. Ozone’s costs were estimated by AOC for this application based on the cost of a Pro line Ozone Generator and a preliminary ozone system design for this treatment plant. Ozone was not considered as an option at the time this decision was made. The actual data and calculations supporting this comparison can be found . The 20-year total cost of each of these three options is:

UV – $156,000

Chlorine– $111,000

Ozone - $ 59,000

As demonstrated in this real world example, ozone can be commercially competitive in wastewater treatment.

References