DCOM Volume I Appendix H
1 Appendix H: Methods for Disinfecting Water
1.1 Physical Disinfection
The two principal physical disinfection methods are boiling of the water, and radiation with ultraviolet rays.
Boiling is safe process which destroys pathogenic micro-organisms and is effective as a household treatment, but is not a feasible method for community water supplies.
1.1.2 Ultraviolet radiation
Light radiation is an effective disinfection method for clear water but its effectiveness is significantly reduced when the water is turbid and contains constituents such as nitrate, sulphate, and ferrous iron. In addition, this disinfection method does not produce any residual that would protect the water against any new contamination that could serve for control and monitoring purpose. Hence it is not recommended.
1.2 Chemical Disinfection
In chemical disinfection in particular, the following points need to be followed:
- Good mixing between water and disinfection agent
- Sufficient dosage compared to water quality and types of micro-organisms that are to be filled.
- Sufficient contact time between water and disinfection agent.
- Suitable water quality with regard to turbidity and organic matter.
A good chemical disinfectant should possess a number of important characteristics, including:
- Quick and effective in killing pathogenic micro-organisms present in water.
- Be readily soluble in water in concentrations required for the disinfection, and capable of providing a residual.
- Not impart taste, odour or colour to water at the concentrations used
- Be easy to detect and measure in water
- Be readily available at moderate cost.
The chemicals that have been successfully used for disinfection are: chlorine, and chlorine compounds and to a lesser extent iodine dosed in suitable form, ozone and other oxidants like potassium permanganate and hydrogen peroxide. Each one of these has its advantages and limitations.
(a)Chlorine and Chlorine Compounds
Their ability to destroy pathogens is fair and their widespread availability makes them well suited for disinfection. Their cost is moderate and they are for this reasons widely used as disinfectants throughout the world. Environmentally, the production of chlorine has negative health impacts however and certain by-products are of possible health concern such as trihalomethanes.
In spite of its attractive properties as a disinfectant, iodine 'has serious limitations. High doses (10-15 mg/l) are required to achieve satisfactory disinfection. It is not effective when the water to be disinfected is coloured or turbid. Hence it is not recommended.
This is powerful oxidizing agent and has been found to be effective against cholera vibrio but not for other pathogens. It leaves stains in the container and hence it is not a very satisfactory disinfectant for community water supplies. It is destructive to aquaculture. Hence it is not recommended.
Ozone is increasingly used for disinfection of drinking water supplies in industrialized countries as it is effective in eliminating compounds that give objectionable taste or colour in water. Like ultraviolet light, ozone normally leaves no measurable residual which could serve for monitoring the process. The absence of a residual also means that there is no protection against new contamination of the water after its disinfection. The high installation and operation costs and the need for: continuous power make the use of ozone relatively expensive. Hence it is not recommended.
1.2.1 Trihalomethanes (THMs)
Trihalomethanes can be formed when raw waters containing naturally occurring organic compounds such as humic and fulvic acids are chlorinated. They are also formed by the reaction of chlorine with some algal derivatives. Control is best achieved by avoiding pre-chlorination and only using post chlorination with the removal of as much of the organic precursors as possible before the chlorine is introduced into the water.
There is evidence that trihalomethanes pose a cancer risk and for this reason the WHO has set guideline values and it is recommended that total trihalomethanes (TTHM) in public water supplies be limited to 0.2 mg/l (200 µg/l), with any single trihalomethane limited to half of this.
However, the WHO Guidelines emphasise that the disinfection process must not be compromised, and that ‗inadequate disinfection in order not to elevate the THM level is not acceptable‘.
Choice of Chemical Disinfectant
As a result and despite a number of environmental health drawbacks in its production and sometimes in its use, disinfection is still overwhelmingly done using chlorination, and one of the following agents maybe used:
For large schemes:
- Gaseous chlorine
For medium sized schemes
- Sodium hypochlorite (NaOCL), especially for on-site production electrolytically using near-pure salt and electricity
- Calcium hypochlorite (HTH) with 65 – 70 % available chlorine
For small schemes:
- Chlorinate lime or bleaching powder Ca02.Ca(OCl)2 with up to 39 % available chlorine
Great care must always be taken when using chlorine as in gaseous form it is extremely poisonous. Only qualified and authorised personnel should be involved in mixing and dosing and under no circumstances should members of the public be allowed unaccompanied into mixing and dosing facilities.
- Chlorine solutions should wherever possible be fed into the water by means of gravity or displacement dosers, and dosing pumps should preferably be confined to medium and large plants.
- A contact period of at least 30 minutes in the clear water tank and the transmission main should be allowed before the water reaches the first consumer.
- Normally the necessary disinfectant added should be in the-range of 0.5 - 2.0 mg/l (free chlorine)
If chlorine gas is used the cylinders or drums must be stored in a cool, well-ventilated place. Valves must never be left open after use as any residual gas then combines with moisture to form hydrochloric acid. If bleaching powder is used separate mixing and dosing tanks must be provided so that solid deposits do not clog the dosing mechanism and the dose at the point of application be visible
As noted above and although a number of disinfectants have been tried and used in the past in a very few instances are they still used. Notwithstanding its disadvantages, it is still the most widely used water supply disinfectant, either in the form of a gas or one of its several compounds such as chlorine of lime or sodium hypochlorite. In all cases, the active disinfectant is chlorine. Because of cost; dependability efficiency and relative ease of handling, provided this is done with care, chlorine or chlorine compounds are almost always used. As a matter 'of fact the term "chlorination" is generally used synonymously with disinfection in water works practice.
Chlorine may be applied either as a gas or as a solution, either alone or in conjunction with other chemicals. Regardless of• the form of application, the quantity or dosage is controlled by special apparatus called chlorinators or hypochlorinators.
Selection of the equipment depends on a particular installation.
Chlorine Water Reactions
Chlorine reacts with water to form hypochlorous acid (HOCL) and hydrochloric acid (HCl) according to the equation.
Cl2 + H20 = H+ + Cl- (1)
This hydrolysis reaction is reversible. The hypochlorous acid disassociates into hydrogen ions (H+) and hypochlorite ions (OCl-) according to the equation
HOCl = H+OCl- (2)
HOCl is about 100 times more powerful as disinfection than OCl- ion. When the pH value of the chlorinated water is above 3, the hydrolysis reaction is almost complete and the chlorine exists entirely in the form of HOCl.
From a consideration of the second equation it is evident that as the pH increase more and more HOCl disassociates to form OCl ion. At pH values of 5.5 and below, it is practically 100% unionised HOCl while above a pH 9.5, it is all OCl ions. Between pH 6.0 to 8.5, there occurs a very sharp change from undissociated to completely disassociated hypochlorous acid with 96% to 100% of HOCl, with equal amounts of HOCl and OCl being present at pH 7.5. The addition of chlorine does not produce any significant change in the pH of the natural waters because of their buffering capacity. Free available chlorine may be defined as the chlorine existing in water as hypochlorous acid and hypochlorite ions.
Chlorinated lime (Bleaching Powder)
Before the advent of liquid chlorine, chlorination was• mostly accomplished by the lime and chlorine gas, with the approximate composition
CaCl2 Ca(OH)2. H20 + CaCl2 2Ca(HO)2 (3)
Fresh, chlorinated lime has a chlorine content of 33 to 37percent. However, chlorinated lime is unstable and exposure to air, light and moisture causes the chlorine content fall rapidly. The compound should therefore be stored in dark, cool and dry place; in closed, corrosion resistant containers.
It also produces waste sludge which has to be disposed of.
High Test Hypochlorite (HTH)
These are not only twice as strong as chlorinated lime (60 to 70 percent available chlorine content) but retain their original strength for more than a year under normal storage conditions. HTH may be obtained in packages of 2-3 kg, and in cans of up to 50 kg, and are also available in granular or tablet form. It should therefore be used instead of chlorinated lime.
Sodium Hypochlorite is a solution, (NaOCl) produced electrolytically and usually contains 10 to 15 percent available chlorine in the commercial form. House hold bleach solutions of sodium hypochlorite usually contain only 3 to 5 percent available chlorine. It has a very short shelf life and should be produced on site as required.
Where a waterworks requires electricity and a nearly pure salt is available locally it is a preferred method.
Chlorination practises may be grouped under two categories depending upon the desired level of residual chlorine and the point of application. When it is required to provide a residual and the time of contact is limited, it is common practice to provide for free available residual chlorine. If combined available residual chlorination is used, the chlorine is applied to water to produce, with natural or added ammonia, a combined residual, effect. pre-chlorination is the application of chlorine prior to any other treatment. This has been used for the purpose of controlling algae, taste and odour but should be avoided whenever possible because of trihalomethanes (THMs). Post chlorination refers to the application of chlorine after other treatment processes particularly after filtration and should be preferred.
This is the difference between the amount of chlorine added to water and the amount of free or combined available chlorine remaining at the end of a specified contact period.
Several methods are available to measure residual chlorine in water. Consult standard water treatment textbooks to select the most appropriate method for a particular situation.
Gaseous Chlorine Storage
Gaseous chlorine is heavier than air and lethal. Should a leak be detected, people nearby should be alerted and told to move upwind. Usually cylinders are used for chlorine storage. Their choice in terms of capacity depends upon' the chlorine requirement. Cylinders up to 67 kg capacity should be stored vertically so that a leaking container if found can be removed with the least possible handling of others. It is preferable to provide space for separate storage of full and empty cylinders. Care should be taken to prevent them from falling over or from being hit by moving objects. Dropping of containers is very dangerous. Containers should be stored in a cool ventilated area protected against external sources of heat like steam, electric heaters and away from inflammable goods.