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Disinfection techniques and procedures

Disinfection of potable water has arguably saved more lives than any other technology known to modern man.  There are several proven disinfection technologies, including;
  • Chlorination
  • Ozonation
  • Photo-disinfection (ultra-violet radiation)

DISINFECTION - CHLORINATION

It is unlikely that Carl Scheele had any idea how useful his discovery would prove to be back in 1774 when he discovered chlorine.

Chlorine is listed on the Period Table as a Halogen.  The Halogens are so called because they produce a salt when combined with a metal. At room temperature chlorine exists as a gas.  Of interest though is that in nature chlorine hardly ever exists as a gas.  It was the work of Scheele that unlocked chlorine through the electrolysis of salt water.

Chlorine kills pathogens such as bacteria and viruses by breaking the chemical bonds in their molecules. Disinfectants that are used for this purpose consist of chlorine compounds which can exchange atoms with other compounds, such as enzymes in bacteria and other cells. When enzymes come in contact with chlorine, one or more of the hydrogen atoms in the molecule are replaced by chlorine. This causes the entire molecule to change shape or fall apart. When enzymes do not function properly, a cell or bacterium will die.disinfection Carl Wilhelm Scheele

Factors which determine chlorine disinfection effectivity include:

  • Chlorine concentrations
  • contact time
  • temperature
  • pH
  • number and types of micro organisms
  • concentrations of organic matter in the water
Disinfection time of faecal pollutants with chlorinated water
E. coli 0157 H7 bacterium Hepatitis A virus Giardia parasite Cryptosporidium < 1 minute about 16 minutes about 45 minutes about 9600 minutes (6,7 days)

Table 1: disinfection time for several different types of pathogenic microorganisms with chlorinated water, containing a chlorine concentration of 1 mg/L (1 ppm) when pH = 7,5 and T = 25 °C

OZONATION

One common method of disinfecting wastewater is ozonation.  Ozone is an unstable gas that can destroy bacteria and viruses. It is formed when oxygen molecules (O2) combine with oxygen atoms to produce ozone (O3).

Ozone is generated by an electrical discharge through dry air or pure oxygen and is generated onsite because it quickly decomposes to elemental oxygen.  Venturi’s are often used to draw ozone into the water to be disinfected.

What are the advantages and disadvantages of using ozone disinfection? Advantages
  • Ozone is reportedly more effective than chlorine in destroying viruses and bacteria.
  • The wastewater needs to be in contact with ozone for a short time (approximately 10 to 30 minutes).
  • Ozone decomposes rapidly, and therefore leaves no harmful residual that would need to be removed from the wastewater after treatment.
  • There is no regrowth of microorganisms after ozonation, unlike ultraviolet and chlorine disinfection.
  • Ozone is generated onsite, and thus, there are fewer safety problems associated with shipping and handling.
  • Ozonation increases the dissolved oxygen (DO) concentration of the discharged wastewater. The increase in DO can improve the oxygen content of the receiving body of water.
Disadvantages
  • Low dosages may not effectively inactivate some viruses, spores, and cysts.
  • Ozonation is more complex than other disinfection technologies.
  • Ozone is very reactive and corrosive, thus requiring corrosion-resistant material, such as stainless steel.
  • Ozonation is not economical for poor quality (poorly treated) wastewater.
  • Ozone is extremely irritating and possibly toxic, so off-gases from the contactor must be destroyed to prevent worker exposure.
  • The cost of treatment is relatively high, being both capital- and power-intensive.
  • There is no measurable residual to in determine the efficacy of ozone disinfection.

PHOTO-DISINFECTION

Ultraviolet disinfection uses an ultraviolet (UV) light source enclosed in a transparent protective sleeve, mounted in such a manner that the water that passes through the flow chamber admit the UV rays and absorb them into the stream. These rays are able to destroy bacteria and inactivate many viruses. This kind of system disinfects the water without the need for adding chemicals and as a result, possesses some of the benefits of distillation.

UV radiation can be an effective viricide and bactericide. Disinfection using UV radiation is commonly used in wastewater treatment applications and is finding an increased usage in drinking water treatment. Many bottlers of spring water use UV disinfection equipment to sterilize their water. Solar water disinfection is the process of using PET bottles and sunlight to disinfect water.

It used to be thought that UV disinfection was more effective for bacteria and viruses, which have more exposed genetic material, than for larger pathogens which have outer coatings or that form cyst states (e.g., Giardia) that shield their DNA from the UV light. However, it was recently discovered that ultraviolet radiation can be somewhat effective for treating the micro organism Cryptosporidium. The findings resulted in the use of UV radiation as a viable method to treat drinking water. It has been found that protists are able to survive high UV-C (wavelength 100 – 190 nm) doses but are sterilized at low doses.

A process named SODIS [http://www.sodis.ch/] has been extensively researched in Switzerland and has proven ideal to treat small quantities of water using natural sunlight. Contaminated water is poured into transparent plastic bottles and exposed to full sunlight for six hours. The sunlight treats the contaminated water through two synergetic mechanisms: Radiation in the spectrum of UV-A (wavelength 320-400 nm) and increased water temperature. If the water temperature rises above 50 °C, the disinfection process is three times faster.

UV radiation neither creates new chemical complexes nor changes the taste or odour of the water - and - it does not remove any beneficial minerals that may be in the water. However if the water is filtered before passing through the UV flow chamber, it is more effective. Some of the bacteria that can be rendered ineffective through this process include - Leptospira interrogans (Infective Jaundice); Salmonella paratyphi (enteric fever); Salmonella typhosa (typhoid fever) Shigella dysenterai (dysentery); Shigella Flexneri (dysentery) Vibrio cholerae; Streptococcus; Staphylococcus; Escherichia coil; Hepatitis virus; Influenza virus; Poliovirus (poliomyelitis);