disinfection/oxidation
Disinfection
Water disinfection means the removal, deactivation or killing of pathogenic microorganisms. Microorganisms are destroyed or deactivated, resulting in termination of growth and reproduction. When microorganisms are not removed from drinking water, drinking water usage will cause people to fall ill.
Disinfection is the final stage in drinking water treatment before its distribution. Disinfection is used to remove pathogenic micro-organisms from the water. However, it should be noted that disinfection is not the same as sterilisation (sterilisation = destruction of all germs present in a medium) and therefore a few common germs may remain in the water following disinfection
Disinfection consists in rendering inactive pathogenic organisms carried in water such as bacteria, viruses or parasites. Disinfection differs from sterilisation which aims at the total elimination of all germs. The germicidal action of disinfectants is based on oxidation reduction mechanisms. Thus, the effectiveness of a chemical disinfectant will be directly related to its oxidising capacity which is itself linked to temperature and pH.
Disinfection can be attained by means of physical or chemical disinfectants. The agents also remove organic contaminants from water, which serve as nutrients or shelters for microorganisms. Disinfectants should not only kill microorganisms. Disinfectants must also have a residual effect, which means that they remain active in the water after disinfection. A disinfectant should prevent pathogenic microorganisms from growing in the plumbing after disinfection, causing the water te be recontaminated.
The way in which a disinfectant agent works will depend on the nature of the micro-organism and on its chemical structure. As a rule:
in the case of bacteria, the oxidant action makes the cell membrane more porous and affects nucleic acid macromolecules (DNA, RNA), inhibiting all reproduction;
in the case of viruses, the oxidant penetrates the capsid and modifies the DNA or RNA proteins.
General Conditions Required For Satisfactory Disinfection
In order to be effective, disinfection must be carried out on good quality water. The suspended solids content must be kept as low as possible and equal to no more than 1 mg·L–1. In effect, bacteria and especially viruses collect on suspended solids which can protect them from the effect of disinfectants.
OM, TOC and especially AOC or BDOC levels must also be as low as possible or the water will have a higher disinfectant demand, resulting in:
the need to overdose of this reagent;
difficulty to maintain a residual level through the mains without topping up at various points throughout the mains;
bacterial to regrowth as the water is distributed;
production of harmful disinfection by-products.
However, endeavours to reduce the formation of THM must not jeopardise the effectiveness of the disinfection itself.
Chlorine Dioxide
An effective level of protection is provided by maintaining a rate of 0.2 mg/l for 15 minutes (C·T = 3). The residual effect is significant. However, it is not recommended and the use of a too high amount of CℓO2 for disinfection purposes may even be banned in some countries like France. The CℓO2 oxidising action on OM releases the CℓO2– ion that has been recognised as toxic and that imparts an unpleasant metallic taste to water.
Ozone
For the purpose of eliminating pathogenic bacteria and polioviruses, maintaining a 0. 4 mg · L–1 residual for 4 minutes (C·T = 1.6) is recommended. At 5°C, a C·T equal to 2 will be required in order to ensure that Giardia cysts are eliminated; this value must be higher than 15 in the case of Cryptosporidium oocysts. Under these conditions, it is essential to ensure that the use of this type of treatment does not create unwanted oxidation by-products, particularly bromates (BrO3–) that are regarded as dangerous at levels lower than < 10 µg · L–1. In effect, this type of observation rise to the "multiple barrier" treatment concept already mentioned: chemical disinfection against bacteria and viruses by applying customary criteria, whereas cyst removal efficiency will mainly rely on filtration effectiveness (through fine granular material or, better still, through clarification membranes) or even UV irradiation.
For ozone disinfection, the water must be free of soluble manganese (Mn2+) prior to ozonation. If this is not the case, the water will take on a pink tint. This colour will then develop into a light-dark brown as the MnO2 precipitates.
In view of the comments set out in the preceding paragraphs, it is not advisable to use ozone as the final stage disinfectant. The water then has to be filtered through granular activated carbon with the aim of reducing BDOC levels in order to limit the risk of regrowth in the distribution mains, followed by a ‘belt-and-braces’ chlorination.
Chloramines
Chloramines are virtually no longer used for their bactericidal effect (far too weak) but more as a “bacteriostatic” measure in the distribution network because of their strongly persistant residual effect, especially when distributing relatively hot water (25°C or higher) because chloramines are more stable than free chlorine at these temperatures. In countries where a high level of residual disinfectant is acceptable at the consumer tap, a greater use is being made of chloramines after disinfection using ozone or chlorine (bactericidal effect).
UV Radiation
UV disinfection has been described in the section ultraviolet disinfection along with the recommended target dosage based on the treated water transmittance, target microorganisms and the elimination performance sought.
For drinking water disinfection, in view of the excellent transmittance levels (tr > 90 % m–1) in water < 1 NTU, a dosage range of 20 to 40 mW·s·cm–2 can be recommended and the use of medium pressure lamps will become obligatory
Advantages and drawbacks :
The only disinfectant that does not create damaging by-products and that effectively inactivates all micro-organisms including protozoan cysts, UV irradiation has two major drawbacks if due care is not taken :
It is not possible to check the effectiveness of the delivered dose by measuring the residual as in the case of chemical oxidants; therefore, it is essential for the reactor to be equipped with UV intensity sensors (if possible, 1 for each lamp) to ensure that the true radiation emitted by each lamp can be continuously checked so that:
normal lamp ageing can be monitored (compensated by increasing the current applied to the lamp);
any lamp failure will be instantly indicated, so that the standby reactor can automatically be brought into service or the faulty lamp replaced (only a few minutes shutdown required);
no residual effect and, except in the case of short and particularly well maintained distribution network, UV disinfection must be combined with another disinfectant that has a residual effect (Cℓ2 – CℓO2 – chloramine). Therefore, the UV reactor (see Aquaray 40 H 20 figure in section ultraviolet disinfection) must be installed after polishing but before injecting this last reagent.
OXIDATION-REDUCTION IN WATER TREATMENT
Oxidation-reduction is a process that is used to remove impurities from water. This process is also known as redox. Oxidation-reduction reactions are chemical reactions that involve the transfer of electrons between molecules. In an oxidation reaction, one molecule loses electrons and is oxidized, while another molecule gains electrons and is reduced in a reduction reaction.
HOW IS OXIDATION-REDUCTION USED IN WATER TREATMENT?
Oxidation-reduction in water treatment removes dissolved minerals, such as iron and manganese, from water. This process can also remove organic matter, such as bacteria, from water. Oxidation-reduction is usually accomplished by adding chemicals, such as chlorine or ozone, to water. Also, a process called electrolysis can be used to induce oxidation-reduction reactions.
WHAT ARE THE BENEFITS OF USING OXIDATION REDUCTION IN WATER TREATMENT?
The main benefit of using oxidation-reduction in water treatment is that it effectively removes impurities from water. This process can also be used to disinfect water and to make it safe to drink. Additionally, oxidation-reduction can be used to remove the color from water.
It helps in disinfection. Disinfection is the process of destroying or inactivating microorganisms, including bacteria, viruses, and protozoans, that can cause disease.
It helps remove color. The process of oxidation-reduction can be used to remove color from water. During the process, the electrons that are transferred during an oxidation-reduction reaction can cause a change in the color of the molecules involved in the reaction.
It makes water safe to drink. Oxidation-reduction can be used to disinfect water and make it safe to drink because of its ability to remove bacteria and other microorganisms from water. Oxidation-reduction can also make water safe by removing dissolved minerals, such as iron and manganese.
Oxidation-reduction optimizes BNR operations. Biological nutrient removal (BNR) processes are used in wastewater treatment to remove nutrients from the water, such as nitrogen and phosphorus. BNR processes typically involve bacteria to break down organic matter and make water nutrients free. However, the efficiency of BNR processes can be reduced by dissolved minerals, such as iron and manganese, in water. The process is also crucial in removing dissolved minerals from water, which can optimize the efficiency of BNR processes.
It is a sustainable approach. Oxidation-reduction is a sustainable approach to water treatment because it does not generate hazardous waste products. Additionally, this process can recycle water back into the environment.
It is energy efficient. Oxidation-reduction is energy efficient because it uses chemicals like chlorine or ozone to induce reactions. Also, electrolysis can help achieve the induction of oxidation-reduction reactions.
WHAT ARE THE DISADVANTAGES OF USING OXIDATION REDUCTION IN WATER TREATMENT?
One of the main disadvantages of using oxidation-reduction in water treatment is that it can be expensive. This is because the process requires chemicals, such as chlorine or ozone, to induce reactions. Another disadvantage of using oxidation-reduction in water treatment is producing hazardous waste products.
Physical-chemical oxidation is used in the treatment of all types of water for a range of purposes:
for disinfection before household or industrial using in order to avoid any danger of bacterial contamination;
for precipitating dissolved compounds (iron, manganese, sulphides);
for breaking down organic compounds and especially those responsible for colour, odour and taste in water, those that are toxic and, more generally, those that contribute to the water’s chemical oxygen demand;
to eliminate ammonia nitrogen;
to convert non-biodegradable pollution into substances that can be assimilated by bacteria in a subsequent biological treatment.
The choice of oxidant to be used for the different cases considered will be dictated by:
the highest possible oxidising capacity;
ability to select the targeted pollution;
control over side-effects in terms of induced toxicity;
cost of associated treatment must not be prohibitive.