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)
HWT disinfection systems are bespoke and selected for ease of use and suitability
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.
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.
Disinfection on an HWT sewage treatment plant – chlorine shower for just in case and sodium hypochlorite dose pump arrangement