Course: BFM 203 Water Treatment and Control

Lecture

(3/5)

Disinfection

Disinfection and sterlization are both decontamination processes. Disinfection is defined as the selective destruction or inactivation of pathogenic (harmful) organisms. Sterilization kills all microorganisms in the water. Sterilization of drinking water is neither required nor desired. Multiple laws and organization address the quality of water and the disinfection and other treatment processes. In the United States, the EPA (Environmental Protection Agency) is responsible for setting drinking water standards and for ensuring their enforcement. Water quality standards are governed by the Safe Drinking Water Act (SDWA); this is supplemented in Colorado by Colorado Primary Drinking Water Regulations. Other related rules include the Surface Water Treatment Rule (SWTR), the Enhanced Surface Water Treatment Rule (ESWTR), and the Disinfectant/Disinfection Byproduct Rule (D/DBR).

Factors Affecting Disinfection

Various water characteristics affect the disinfection process. The pH of water being treated can alter the effectiveness of disinfectants used. High temperature water can be disinfected more efficiently than can cooler water. Excessive turbidity greatly reduces the efficiency of the disinfecting chemical or process because turbidity can consume excessive amounts of chlorine and may shield pathogens from the disinfectant. Organics in the water can consume great amounts of disinfectants while forming unwanted compounds trihalomethanes (TTHM's), which are potential carcinogens.

In addition, inorganic matter may impact disinfection, In the presence of ammonia, some oxidizing chemicals form side compounds, causing a partial loss of disinfecting power. Silt can also create a chemical compound. The demand for chlorine by reducing agents must be satisfied before chlorine becomes available for disinfection. This includes hydrogen sulfide, ferrous ion, manganous Ion, and nitrite ion.

Finally, the higher the concentration of microorganisms the greater the demand for a disinfectant chemical. Some microorganisms are more resistant to disinfection than others. Cysts and viruses can be very resistant to certain types of disinfection

Agents of Disinfection

Disinfection agents can be either physical or chemical. Physical agents include ultraviolet rays that disrupt organic components of the cell, causing biological change that is fatal to the microorganism; boiling water, which if applied for about five minutes will destroy essentially all microorganisms; and ultrasonic waves that destroy microorganisms by vibration. Physical methods of disinfection generally require more electrical energy and/or expensive equipment.

The photos in the following album illustrate ozonation equipment used in physical disinfection processes.

Chemical agents (besides chlorine) include iodine, which is a good disinfectant, but costly. Iodine also poses some safety concerns, including potential skin burns and physiological effects. Thus, iodine is not widely accepted as a disinfectant. Bromine is very reactive and can cause skin burns. Residuals are hard to obtain. Bases, such as sodium hydroxide and lime, may also be used as disinfectants, but high pH tastes bitter and can burn skin. Bases sterilize rather than disinfect. Ozone is used primarily for T/O control.  Use of ozone is limited due to high costs, lack of residual, difficulty in storing, and maintenance requirements.

Chlorine gas (CL_2), a well-known disinfecting agent, is greenish-yellow in color and possesses a distinctive odor. It is 2.5 times heavier than air and displays a very high coefficient of expansion. It is non-flammable and non-explosive, but it will support combustion. As temperature increases, the pressure inside a chlorine cylinder will also increase. Chlorine gas expands to approximately 454 times its volume as it changes state from a compressed liquid to gas. The use of chlorine gas tends to lower pH. Note: Questions related to these characteristics have been seen on State certification exams.

Interestingly, we do not know the exact mechanism that chlorine uses to disinfect. One theory is that chlorine destroys bacterial cells. A second theory is that toxic characteristics of chlorine inactivate enzymes, making the microorganism unable to use its food supply.

Chlorine dose: mg/L = Chlorine Demand, mg/L + Chlorine residual, mg/L

Chlorine + Water &lt;--&gt; hypochlorous acid + hydrochloric acid

When chlorine is added to water containing ammonia, chloramines are formed. Ammonia may be naturally occurring in the source water, or it may be added for the intentional formation of chloramines.

Hypochlorites (OCI-), including calcium hypochlorite (HTH) and sodium hypochlorite, are yet other disinfecting agents. The use of hypochlorite tends to raise pH.

Calcium hypochlorite + water --&gt; hypochlorous acid + calcium hydroxide

Sodium hypochlorite + water --&gt; hypochlorous acid + Sodium hydroxide

Chlorine dioxide (CIO_2), when used as a disinfecting agent, does not form carcinogenic compounds (THM's) that are formed by other chlorine compounds. It is not affected by ammonia and is effective at high pH levels. Oxidation properties help remove iron and manganese without leaving objectionable taste or odor.