13 Disinfection by UV, Ozone and Chlorination

Babita Khosla

14.1 Introduction

Disinfection treatment of water including (chlorination, ozone, and ultraviolet light) combined with conventional treatment such as coagulation, flocculation, sedimentation, and filtration are used for good results in wastewater treatment.

14.2 Ultraviolet (UV) Light

UV light was discovered as part of the electromagnetic spectrum by John Ritter in 1801. UV light refers to radiation with wavelengths between 30 and 400 nanometers (nm), the portion of the electromagnetic spectrum between X-rays and visible light. UV light commonly is referred to as black light because it cannot be seen by the human eye. The UV spectrum is divided into three parts: UV-A (315 – 400 nm), UV-B (280 – 315 nm) and UV-C (220 – 280nm). The most harmful are of the UV-C part specifically absorbed by proteins, RNA and DNA and can lead to cell mutations and cancer. So, the range of 250nm to 280nm is considered to be most germicidal meaning they are capable of inactivating microorganisms, such as bacteria, viruses and protozoa. This capability has allowed widespread adoption of UV light as an environmentally friendly, chemical-free, and highly effective way to disinfect and safeguard water against harmful microorganisms.

Figure1: Electromagnetic spectrum of UV light  (http://www.radiantuv.com/wp-content/uploads/2014/01/uvc-spectrum-of-light-chart-1.jpg)

UV radiations were used first time for the disinfection of drinking water in  Marseilles, France in 1910. In 1998 Clancy found that low doses of UV inactivated cryptosporidium cysts. Low pressure mercury lamps are mostly used in disinfection process as they generate huge amount of UV energy.

14.2.1 Mechanism of microorganisms deactivation:

The mechanism was described by Bolton (2011) that when bacteria, viruses and protozoa are exposed to the germicidal wavelengths of UV light, they are rendered incapable of reproducing and infecting due to damage of their nucleic acids. The high energy associated with short wavelength UV, primarily at 254 nm, is absorbed by cellular RNA and DNA. This absorption of UV energy forms new bonds between adjacent nucleotides, creating dimers. Dimerization of adjacent molecules, particularly thymine, is the most common photochemical damage. Formation of numerous thymine dimers in the DNA of bacteria and viruses prevents replication and inability to infect. The high dosage of UV radiations causes the disruption of the membrane proteins and ultimately the death of the cell.

Photochemical damage caused by UV may be repaired by some organisms if the UV dose is too low via photo reactivation or dark repair. However, studies have shown that there is little to no potential for photo reactivation at doses higher than 12 mJ/cm2. In fact, it has been shown that some organisms, like Cryptosporidium, do not exhibit any evidence of repair under light and dark conditions following low-pressure or medium-pressure lamp irradiation at UV doses as low as 3 mJ/cm2. UV systems should be designed with enough UV dose to ensure cellular damage cannot be repaired. Sizing of a system should be based on bioassay validation (field testing) to ensure proper disinfection.

UV light has demonstrated efficacy against pathogenic organisms, including those responsible for cholera, polio, typhoid, hepatitis and other bacterial, viral and parasitic diseases.

Generally the order of susceptibility of microorganisms to UV radiation is Bacteria > Viruses > Bacterial spores > Protozoan cysts

In addition, UV light (either alone or in conjunction with hydrogen peroxide) can destroy chemical contaminants such as pesticides, industrial solvents, and pharmaceuticals through a process called UV-oxidation.

UV offers a key advantage over chlorine-based disinfection, due to its ability to  inactivate protozoa that threaten public health – most notably Cryptosporidium and Giardia. The release of these harmful microorganisms into receiving lakes and rivers by wastewater facilities utilizing chlorine disinfection increases the potential of contamination in communities that rely on these same bodies of water for their drinking water source and recreational use. Drinking water treatment plants can benefit by using UV since it can easily inactivate chlorine-resistant pathogens (protozoa), while reducing chlorine usage and by-product formation.

Disinfection by UV is considered good than chlorine or other chemicals because no chemicals are used in this process and no by products are formed. There is one limitation in using UV for disinfection is the need for frequent lamp maintenance and replacement and the need for a highly treated effluent to ensure that the target microorganisms are not shielded from the UV radiation (i.e., any solids present in the treated effluent may protect microorganisms from the UV light).

14.2.3 Advantages of UV disinfection:

This technology is capable of providing effective disinfection of WWTP effluent while reducing safety and environmental toxicity issues.

• UV is a chemical-free process that adds nothing to the water except UV light.
• UV requires no transportation, storage or handling of toxic or corrosive chemicals – a safety benefit for plant operators and the surrounding community.
• UV treatment creates no carcinogenic disinfection by-products that could adversely affect quality of the water.
• UV is highly effective at inactivating a broad range of microorganisms – including chlorine-resistant pathogens like Cryptosporidium and Giardia.
• UV can be used (alone or in conjunction with hydrogen peroxide) to break down toxic chemical contaminants with simultaneous disinfection.

14.2.4 Application of UV for on-site Systems

UV treatment is used for pretreatment of waste water for onsite sewage facilities. Treatment efficiency is maximized by minimizing the turbidity of wastewater and the water should be exposed to irradiation in a continuous flow process. The quantity of radiation to be delivered, measured in microwatt-seconds per square centimeter (mW-s/cm2), depends on factors such as the wastewater transmittance value, presence of bacteria and other pathogens, lamp, and sleeve condition  Commercial wastewater systems have as mercury vapor lamps of different  pressures, horizontal or vertical layout of the irradiation chamber, quartz or Teflon as the sleeve material and water temperature.

14.2.5 Impact of UV Disinfection Process on the Environment

UV disinfection process has two negative impacts. First it results in the discharge of altered chemicals into the environment and second is the regrowth of waterborne organisms. As UV process does not use any chemical reagent for disinfection, so it does not release any toxic chemicals into the water stream, but it has the potential for altering certain chemicals into hazardous compounds. Some microorganisms have an ability to repair their damaged DNA structures after exposure to UV radiation. So care should be taken while transporting treated water through pipelines. It should be free from all harmful microorganisms.

14.3 Ozone

Ozone (O3) is a tri atomic allotropic form of oxygen, pale blue gas with pungent odor and short half-life. It is characterized by a high redox potential of -2.07 V, as compared to that of chloric (I) (hypochlorous) acid (-1.49 V), chlorine (-1.36 V) or oxygen (-0.40 V). Ozone is a very unstable and potent oxidant having antimicrobial properties. It is formed by electric discharge method, in which oxygen is passed through a high voltage potential resulting in a third oxygen atom which forms ozone. In nature ozone is formed by photochemical and electrical processes.

Ozone act as both the strongest oxidant and strongest disinfectant applicable for potable water treatment, it has number of applications including disinfection, taste and odor control, color removal, iron and manganese oxidation, H2S removal, nitrite and cyanide destruction, oxidation of many organics (e.g., phenols, some pesticides, some detergents), algae destruction and removal, and as a coagulant aid.

14.3.1 Mode of action

An ozone molecule, owing to split of the third atom of oxygen, is a strong oxidant. It is the property which makes it very effective in destroying microorganisms even the viruses. There is a direct or indirect reaction between the ozone and the microbes. The direct reaction means ozone reacts directly with the microbes. The primary attack of the ozone occurs on the double bonds of the fatty acids present in the cell wall and membrane and there is consequent change in the cell wall  permeability and cell contents leak out and causing death of microorganism. In the  case of viruses a complete loss of virus proteins was responsible for the death virus. The high oxidation potential, ozone oxidizes cell components of the bacterial cell wall leading to penetration.

Once ozone has entered the cell, it oxidizes all essential components (enzymes, proteins, DNA, RNA). Two major mechanisms have been identified through which ozone exerts the bactericidal effect. One of them is oxidation of sulfhydryl groups and amino acids of enzymes, peptides and proteins. The second mechanism is based on oxidising poly unsaturated fatty acids (PUFA).

When the cellular membrane is damaged during this process, the cell will fall apart i.e. lysis of cell. This is the direct reaction but in indirect reaction hydroxyl radicals that are formed by decomposition of molecular ozone in water interacts with the microbes. The indirect microbial inactivation is less effective as microbial cells are composed of high concentration of bicarbonate ions, catalase, peroxidase, or superoxide dismutase to scavenge the free radicals formed due to the decomposition of ozone. The effect of ozone is also reduced in some bacteria by the presence of carotenoid and flavonoid pigments. Thus it is suggested that free radicals provide little benefit in terms of microbial destruction.

The bactericidal properties of ozone have also been demonstrated in the case of Gram-positive (Listeria monocytogenes, Staphylococcus aureus, Enterococcus faecalis) and Gram-negative microorganisms (Yersinia enterocolitica, Pseudmonas aeruqinosa, Salmonella typhimurium); in both spores and vegetative cells. In Gram-negative bacteria, the lipoprotein and lipopolysaccharide layers are the main sites of the destructive effect of ozone, which contributes to increased microorganism cell permeability and results in its lysis.

Figure2: A computer generated illustration of how ozone kills healthy bacteria (1) A bacterial cell with intact cell wall, vital to the bacteria because it ensures the organism can maintain its shape. (2) As ozone molecules make contact with the cell wall, a reaction called an oxidative burst occurs which literally creates a tiny hole in the cell wall. (3&4) A newly created hole in the cell wall has injured the bacterium. (5) The bacterium begins to lose its shape while ozone molecules continue creating holes in the cell wall. After thousands of ozone collisions over only a few seconds, the bacterial wall can no longer maintain its shape and the cell dies.(Source: http://www.ozonesolutions.com/journal/2010/how-does-ozone-kill-bacteria/)

Ozone has reactive properties due to the presence of trivalent form of oxygen and it degenerate oxygen when it is added to water as follows:

2O3  2O2 + 2O .   3O2

Because of this reaction, no concentration of ozone persists in the treated effluent (as ozone is converted to oxygen) that may require removal or demonstrate that ozone was actually used to disinfect, as is the case with chlorine residuals.

Disinfection of waste water by ozone has been used since the early 1900’s. Ozone helps in declining the load of pathogens. It reduces the aggregation of organic waste in the environment. Ozone is widely used for the reduction of wide spectrum of bacteria like E.coli and Listeria from the environment and leaves no residues. Ozone is also very effective in color removal due to its bleaching action, and it also controls the taste and odour of water.

Use of Ozone is considered to be efficient than chlorine because ozone is generated on site as needed and produces less disinfection by products but chlorine has to be stored on site which is highly poisonous and produces enormous harmful by products. Due to its high operating cost its use is limited.

14.3.2 Disinfection performance

Ozone works at low concentrations and requires less contact time than other disinfectants. With increasing pH and temperature the strength of ozone decreases. Lifetime of the ozone residual at 15°C and a pH of 7.6 is reported to be in the order of 40 minutes, but it can be as low as 10 – 20 minutes at higher temperatures. This occurs due to a decrease in the efficiency of transfer of ozone into water as temperature increases.

The five basic components of an Ozone system include

1. Gas Preparation – either drying gas to a suitable dew point or using oxygen concentrators.
2. A suitable electrical power supply.
3. A properly sized Ozone Generator(s)
4. An Ozone contacting system.
5. Ozone off-gas destruction or suitable venting system.

14.3.3 Advantages and limitations of ozonation

Ozone is a very effective disinfectant for bacteria, viruses and Giardia. It is less affected by pH variations, produces no trihalo methanes (THMs) and very effective against Cryptosporidium than other disinfectants. But with these advantages ozonation has some limitations also: Its capital cost of equipment is very high and is also expensive in operation as it is generated on site and requires high skilled maintenance.

14.4 Chlorination

Chlorination is a well-established technology, effective and cheap disinfectant from eighteen century. In 1835 doctor and writer Oliver Wendel Holmes advised midwifes to wash their hands in calcium hypochlorite (Ca(ClO)2.4H2O) to prevent a spread of midwifes fever. However, we only started using disinfectants on a wider scale in the nineteenth century, after Louis Pasteur discovered that microorganisms spread certain diseases.

Chlorination is used in various forms that will kill or inactivate most pathogenic organism that are harmful to human and animal life. It is added to wastewater in the form of gas, hypochlorites (tablets, solutions, or powder), and other compounds. Some forms of chlorine disinfectants are: elemental chlorine, sodium hypochlorite solution, calcium hypochlorite, and bromium chloride. It is used both as primary disinfectant and to maintain the residual distribution system.

14.4.1 Forms of chlorine used as disinfectants:

Chlorine Dioxide

It is first used in 1944 for the treatment of wastewater to control phenolics taste and odors at the Niagara falls waste treatment plant. At concentration more than 10 % in air, ClO2 may be explosive thus generated onsite (Doull, 1980).

Chloramines

Some municipalities use chloramine for primary disinfection, because it does not form any disinfection by products. Its effective form is dichloramine (NHCl2) form. The bactericidal effectiveness of dichloramine is more than monochloramine.

Hypochlorites

Calcium hypochlorite or sodium hypochlorite has been used as an alternatively to chlorine gas as a disinfectant.

All forms of chlorine react with water to produce hypochlorous acid (HOCl), which rapidly dissociates to form the hypochlorite ion.

14.4.2 Mode of action:

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.

When chlorine is added to water, underchloric acids form and depending on the pH value, underchloric acid partly expires to hypochlorite ions and this falls apart to chlorine and oxygen atoms as follows:

• 1) Cl2 +H2O→HOCl+H+ +Cl-:
• 2) Cl2 +2H2O→HOCl+H3O+Cl-
• 3) HOCl+H2O→H3O+ +OCl-
• 4) OCl- →Cl- +O

Underchloric acid (HOCl, which is electrically neutral) and hypochlorite ions  (OCl-, electrically negative) will form free chlorine when bound together. This results in disinfection.

Both substances have very distinctive behaviour. Underchloric acid is more reactive and is a stronger disinfectant than hypochlorite. Underchloric acid is split into hydrochloric acid (HCl) and atomic oxygen (O). This nascent atomic oxygen atom is a powerful disinfectant and the disinfecting properties of chlorine in water are based on the oxidizing power of the free oxygen atoms. HOCl inactivates the pathogens by reacting with the enzyme system of the cell through its ability to penetrate the cell wall because of its small size and its electroneutrality. The chlorine disinfection process occurs primarily through oxidation of cell walls leading to bacterial cell lysis or inactivation of functional sites on the cell surface. It also damages the DNA, rendering the organism not capable of reproduction. The disinfection effectiveness varies across the range of microorganisms. Protozoans, helminthes and viruses are the most resistant, followed by bacterial pathogens, with each species varying in resistance.

The efficacy of disinfection is determined by the pH of the water. The disinfection with chlorine will take place optimally when the pH of the water is between 5.5 and 7.5. The level of underchloric acid will decrease when the pH value is higher and at pH value of 6 the level of underchloric acid is 80%, whereas the concentration of hypochlorite ions is 20%. When the pH value is 8, this is the other way around.

Chlorine is very effective against enteric bacteria, such as E.coli, but less effective against other bacterial species. The effective chlorine disinfection depends on the correct combination of pH, chlorine concentration and contact time as well as the levels of ammonia and suspended solids.

14.4.3 Advantages of Chlorination:

Controls Disease-Causing Bacteria

Controls Nuisance Organisms: Chlorine treatment will control nuisance organisms such as iron, slime and sulfate- reducing bacteria. Iron bacteria feed on the iron in the water.

Mineral Removal: A large amounts of iron can be removed from water by adding chlorine to oxidize the clear soluble iron into the filterable reddish insoluble form. Chlorine helps to remove manganese and hydrogen sulfide in the same way.

14.4.4. Disadvantages of Chlorination:

There are several health and safety risk concerns with the use of chlorine for disinfection.

Accidental release of chlorine can occur through volatilization from  chlorine contact facilities or through leaks in the storage cylinders or feed lines. Inhalation of chlorine damages the upper and lower respiratory tracts and causes severe skin irritation upon physical contact, and can be lethal to humans.

Chlorine can adversely impact receiving streams and can adversely impact biota. The residual chlorine and chloramines from the disinfection process are toxic to many aquatic organisms, including fish and oysters. The residual concentrations as low as 0.002 milligrams per liter (mg/L) have reportedly induced toxic effects in aquatic organisms. Vegetation also can be affected by residual chlorine.

       Chlorine reacts with organic material in the environment to form disinfection byproducts. Trihalomethanes (THMs) are disinfection byproducts formed when chlorine is used to treat water supplies that contain organic material or humic compounds. These disinfection byproducts are having potentially adverse impacts to human health. Lifetime consumption of water supplies with THMs at a level greater than 0.10 milligrams per liter is considered by the Environmental Protection Agency to be a potential cause of cancer. The key disinfection byproducts of concern are the formation of trihalomethanes (THMs) and haloacetic acids (HAAs).

Causes Smell and Bad Taste: The higher residual concentration of chlorine will provide bad taste and objectionable smell. Some people are sensitive to chlorine present in water. In those cases an activated carbon or charcoal filter may be used to reduce the chlorine levels in the water.