Purification of Water - Disinfection

(contd..)

General steps in purification of drinking water includes Aeration, Sedimentation, Filtration, Disinfection.

4. Disinfection: 

Disinfection is the final step of water purification. Some of the bacteria pass through filter even after filtration and must be killed before consumption of water. Therefore, disinfection of public water supply needs to be done. 

The filtered water is finally purified by using disinfectants. Disinfectant kills pathogenic as well as other microorganism in water.  Several disinfection methods are used in water treatment. Different microorganisms have different susceptibilities to disinfectants. 

    For a given microorganism, disinfection efficiency is affected especially by disinfectant concentration and contact time, and also by the disinfectant demand of the water, pH and temperature. 

    Disinfection with chlorine is the most widely used method for large water supplies but is less common in small supplies. Solutions of sodium hypochlorite were used but in recent years, chlorination of public water supply has become popular. 

Chlorination involves the release of chlorine gas in water which gets readily mixed up with water. The amount of chlorine required ie, chlorine demand of a water body, depends on organic matter and number of microorganisms present in water, and duration of time to act upon. High concentration of chlorine quickly acts upon microorganisms and vice-versa. 

    The amount of chlorine required for disinfection is called chlorine demand. Water is chlorinated to contain about 0.1 to 0.2 ppm of residual chlorine which reaches to this concentration after 20 minutes of its addition. However, if the concentration of chlorine exceeds its demand, peculiar odour and tastes are experienced. 

    The mechanism of action of chlorine on microorganisms is by the formation of highly reactive nascent oxygen. After reacting with water, chlorine is converted into hypochlorous acid which in turn quickly releases nascent oxygen. The nascent oxygen soon oxidises the cellular components of microorganisms as well as organic matter.  

    Chlorination is performed by Breakpoint chlorination method. Break point is the point till where chlorine is added to water such that the demand for chlorine is fully met. When chlorine is added first to water, nascent oxygen liberated oxidises the organic compounds present in the water, resulting in zero chlorine residual. At the break point, free ‘available’ chlorine  is detected in water. This is "residual chlorine". The organic compounds are oxidized and the chlorine demand of the water body is fully satisfied at the break point.  

    Breakpoint chlorination requires a dose of around 10 mg/l chlorine and leave a resultant free available chlorine residual in the range 0.1 to 0.2 ppm. The actual dose depends on water quality and has to be determined for each water. It is recommended that the contact time should be at least 30 minutes.  

    Chlorination can be achieved by using liquefied chlorine gas, sodium hypochlorite solution or calcium hypochlorite granules. Chlorine gas is very reactive and highly toxic and must be carefully stored and handled. It is used for treatment of large public supplies but not recommended for treatment of small water supplies. 

*Superchlorination and Shock chlorination (see below)

Advantages

· It oxidises completely organic compounds, ammonia &other reducing compounds 

· It removes colour in water, due to organic matter. 

· It destroys completely all the disease-producing bacteria 

· It removes both odour and taste from water, due to organic matter 

· It provides residual protection against recontamination 

· Ease-of-use and acceptability 

· Proven reduction of diarrheal diseases 

· Scalability and low cost 

Disadvantages

· Relatively low protection against microbial spores, protozoa and viruses. 

· Lower disinfection effectiveness in turbid waters. 

· Potential taste and odor objections. 

· Potential long-term effects of chlorination by-products. 

    If action of chlorine prolongs in water containing high amount of organic matter, chloramines, are formed. Change in odour and taste of water is due to the formation of chlorophenols. In the presence of high organic matter, chlorine reacts with it and produces different halomethanes which are a group of potential carcinogenic compounds. 

 Ozone

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

Ozone is generated by an electrical discharge through dry air or pure oxygen and is generated onsite because it decomposes to elemental oxygen in a short amount of time. After generation, ozone is fed into a contact chamber containing the wastewater to be disinfected. Ozone disinfection is generally used at medium- to large-sized plants.  

· Ozone is more effective than chlorine in destroying viruses and bacteria.  

· The wastewater needs to be in contact with ozone for just a short time (approximately 10 to 30 minutes).  

· Ozone decomposes rapidly, and therefore, it leaves no harmful residual that would need to be removed from the wastewater after treatment.  

· Ozone is generated onsite, and thus, there are fewer safety problems associated with shipping and handling. 

Hoiwever,

· Low dosages may not effectively inactivate some viruses, spores, and cysts. 

· Ozone is very reactive and corrosive, thus requiring corrosion-resistant material, such as stainless steel.  

· Ozonation is not economical - the cost of treatment is relatively high, being both capital- and power-intensive. 

· Ozone is extremely irritating and possibly toxic, so off-gases from the contactor must be destroyed to prevent worker exposure. 

· There is no measurable residual to indicate the efficacy of ozone disinfection.

  Ultraviolet irradiation (UV) 

UV is the preferred method for disinfection of small supplies with small distribution networks or retention time. Chlorination may be more suitable for larger operations in which it is necessary to maintain a residual disinfectant during storage and distribution. 

    UV light damages the deoxyribonucleic and ribonucleic acids (DNA and RNA) and prevents the reproduction of microorganisms. Microorganisms are thereby inactivated. In general, viruses are most resistant to UV disinfection compared to protozoan cysts (e.g., Cryptosporidium) and bacteria. 

       UV disinfection efficiency is particularly affected by water quality and flow rate. The water to be disinfected must be of good quality and particularly low in colour and turbidity. 

UV devices can be scaled to fit any size or type of drinking water treatment need, from small handheld devices to large systems. 

UV produces far fewer disinfection byproducts compared to other chemical disinfectants typically used in drinking water treatment (e.g., chlorine, ozone, chlorine dioxide). UV does not leave any odour, flavor in the treated water. 

However, a significant disadvantage of using UV for disinfection is its inability to provide a residual.  

Small Scale /household disinfection Several types of particulate filters using different media to remove suspended matter from water in the range 0.5 to 50µm, or greater is used to reduce turbidity and microorganisms, or to remove specific inorganic particulates such as iron, aluminium or manganese compounds. Devices incorporating activated carbon filtration, reverse osmosis (RO) or UV disinfection can be used. 

Simplest and surest way of disinfecting water at households is boiling water at 100⁰C for 5-10 minutes. 

    After disinfection, water is pumped into reservoirs/tanks for subsequent domestic distribution. Different methods of water treatment mentioned so far can be used singly or in combination depending on the raw water quality and treatment purpose. These ensure clean and safe water for healthier living.

*Super chlorination is a water treatment process in which the addition of excess amounts of chlorine to a water supply to ensure disinfection within a short contact time. Super chlorination, also known as hyper chlorination, temporarily increases the free chlorine residual in a water distribution system.

Super  chlorination is the addition of large doses of chlorine to the water followed by de  chlorination, which is the removal of excess of chlorine after disinfection
This method is applicable to heavily polluted waters whose quality fluctuates greatly.

A high free chlorine residual (i.e. above 5 mg/L) is effective against most bacteria (including Legionella) and can be used in hot, warm and cold water distribution systems (although in hot water systems chlorine degrades rapidly). Super chlorination is best applied to cold water systems while thermal treatment is preferable for hot water systems.

Superchlorination should be undertaken in accordance with water risk management plan (WRMP) and may be required:

• While installing a new water infrastructure
• for remediation of affected infrastructure following a detection of Legionella or other microbial hazard of high risk to patients or residents
• based on the complexity of the plumbing infrastructure, in areas where biofilm growth is suspected (e.g. low flow pipe sections), on a scheduled basis (e.g. every six months).

Superchlorination is most commonly used when water has very high bacteria content and generally comes from river sources or where some form of pollution has occurred. It is also an important part of swimming pool maintenance because it keeps chlorine content at the right level to effectively kill off bacteria and other contaminants.

*Shock chlorination is an effective and safe way to remove bacteria from a domestic well and the cold water part of a household water supply system. Shock chlorination is the most widely recommended means of treating bacterial contamination in home water systems. Home water systems such as wells, springs, and cisterns are disinfected using household liquid bleach (or chlorine). The chlorine should be present in a concentration that is lethal to bacteria and disinfection should take place long enough to ensure that all bacteria are killed

Care should be taken to ensure that the dose of chlorine is adequate and that bleach and harmful chemicals are removed from the system before it is used for drinking water supply again.

Shock chlorination is recommended:

·         upon completion of a new well or when an unused well is returned to service

·         if annual water test results indicate the presence of bacteria

·         if a well system is opened for any installation, repair or maintenance

·         whenever the well is surrounded by flood waters (standing water around or covering the well casing)

·         if well water becomes muddy or cloudy after a rain

·         if the well has iron bacteria or sulfur-reducing bacteria symptoms like slime (biofilm) or odor

Shock chlorination is not a recommended method for treating recurring bacteria problems. The source(s) of such contamination should be identified and eliminated or a continuous disinfection treatment system should be installed