Purification of Water

 Consumption of unsafe water continues to be one of the major causes of the diarrhoeal disease deaths occurring annually, mostly in children. Various processes of the water purification are developed in order to overcome the problem of water pollution.

In the drinking water treatment, the water has to be collected from their original sources such as rivers and reservoirs and purifed. After the purification, that water should be healthy for human consumption and free of harmful microorganisms and organic and inorganic pollutants. 

Drinking water treatment involves various steps according to the quality of raw water. Water from different sources and places has different characteristics. Inorganic compounds such as cadmium, chromium, copper, lead, mercury, arsenic, etc., natural and synthetic organic compounds and living organisms such as bacteria, algae, viruses, etc. may be present in water contributing to turbidity, particles, color, taste, odor, etc. 

Common contaminants in tap water fall into these categories:

• Suspended solid particles

• Colloids

• Dissolved inorganic salts

• Dissolved organic compounds

• Micro-organisms

• Dissolved gasses

The appropriate method of water purification can be developed according to the composition of the water to be treated. The water purification is generally carried out by the processes such as aeration, sedimentation, coagulation, flocculation, filtration, disinfection etc.   

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

1. Aeration

This is an optional treatment. Raw water is first collected in large aeraton tank and the aerated by bubbling compressed air through perforated pipes. Aeration removes bad odors and volatile organics (e.g. solvents), carbon dioxide, some taste and odour causing compounds. It also removes metal such as iron, manganese by precipitating then as their respective hydroxides.

In groundwater, iron is usually present as dissolved ferrous compounds. Manganese is usually present as dissolved manganous compounds. Aeration converts them to insoluble hydroxides and which is removed by filtration. 

Aeration processes introduce oxygen into water and removes gases and volatile compounds. For oxygen transfer, aerators are designed so that water flows in a thin film to achieve efficient aeration.              

                  

                     

  2. Sedimentation/Settling

Water may have large sized organic materials such as leaves, and gravels which have run off from the soil. Sedimentation is done so that the suspended particles settle down depending on their size and weight and conditions of the stored water. Sedimentation can be done in a settling tank. 

 Aerated water is placed in settling tank and stored. During storage about 90% of suspended solids settle down within 24 hrs and the water becomes clear. Certain heavier toxic chemicals also settle down during storage.  Pathogenic bacteria gradually die and bacterial count decreases by 90% in about 5-7 days of storage. Sedimentation provides partial reduction of microorganisms in water but does not sterilize the polluted water. During storage organic matter present in water is oxidized by microorganisms.

        Sedimentation tanks are usually rectangular with inlet and outlet at opposite ends of the tank. The inlet distributes the incoming flow as evenly as possible across the tank width and the outlet collect the clarified water. Sedimentation tanks require cleaning occasionally for good performance.

 -Assisted sedimentation- 

Simple sedimentation can reduce turbidity and solids in suspension but take time so the process is enhanced by  physical processes such as flocculation  or chemical processes such as coagulation. These treatments are used to remove very light suspended solids that do not settle by themselves during storage. A precipitate, or floc, which entraps these impurities is formed and settles down with time. Coagulation and flocculation are used to remove colour, turbidity, algae and other microorganisms. Flocculation is gentle mechanical agitation whcih allows suspended particles to form aggregates/flocs and gradually settle down.  The rate of sedimentation is also enhanced by adding alum, iron, salts, colloid silicates which act as coagulants.  The suspended materials and microorganisms are entrapped by coagulants and settle down rapidly as flocs. This procedure is called coagulation. The floc is separated from the treated water by sedimentation and/or filtration. Sedimentation provides partial reduction of microorganisms in water due to their settling down on bottom but does not sterilize the polluted water.   

The quantity of the coagulants to be added are determined by raw water quality. Water from storage tank is placed in coagulation tank and then some precipitating agents such as alum (aluminium sulphate), lime, iron salts (ferric sulphate) etc. are added in water and mixed.

 

4. Filtration

 After sedimentation the water is further purified by filtration. There are two types of sand filters which are used in water purification such as slow sand filter and rapid sand filter:

(i) Slow Sand Filter: In slow sand filtration plants the rate of filtration of water is slow; hence the plant requires a considerable area. This plant consists of a concrete floor containing drainage tubes (for collection of filtered water). The tile is covered with rock, gravel, coarse sand and then 2 to 1 feet of fine sand. Water is passed through this plant. Water passes slowly through the filter and collected by drain pipes at the bottom which later on is pumped into a reservoir. The capacity of slow sand filter plant is to filter about 5 million of water per acre per day.

    If water is turbid, slow sand filters are clogged soon. Therefore, turbid water, which is to be filtered, should be clarified first by sedimentation, thereafter, passed through slow sand filters. 

    

          

 

Water purification is done  by straining  and action of microorganisms. In the surface of layers of fine sand, a colloidal material, consisting of bacteria, algae and protozoa, is attached. This mucilaginous material called Scmutzdecke or “dirt layer”, closes the pores between the sand grains and makes filtration more effective. Sand grains have positive charges and bacterial cell walls have negative charge. Therefore, bacteria are adsorbed on the surface of sand. Protozoa ingest bacteria. Due to intense microbial interactions, organic contents of water is reduced.

Through slow sand filter plant, the pathogenic microorganisms such as Giardia and its cysts which are not removed by any other methods can be filtered from water. 

When filtration efficiency of the plant is reduced, due to deposition of thick mucilaginous material, the plant is subjected to cleaning. It is cleaned by forcing cleaned water backward i.e. back washing through the beds of gravels and sands without disturbing the fine sand.

(ii) Rapid Sand Filter

Similar to slow sand filter, the rapid sand filter is also constructed. This plant consists of layers of sand, gravel and rock. The water is allowed to pass through rapid sand filter plant. This plant depends on physical trapping of fine particles. The pores of the plants are soon clogged. It is cleaned by forcing cleaned water backward i.e. back washing through the beds of gravels and sands without disturbing the fine sand.

About 99% bacteria are removed by this plant. But it does not remove Giardia lamblia cysts, Cryptospordium oocysts and viruses which are removed through slow sand filter. Therefore, water collected after filtration needs further treatment. 

Rapid sand filter plant operates about 50 times faster than slow sand filter plant, and can deliver about 150 to 200 million gallons of water per acre per day. It requires less land area, less cost and less maintenance.

  

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 which kills pathogenic as well as other microorganism in water.  Several disinfection methods are used in water treatment. 

    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. At the break point, free ‘available’ chlorine is detected in water. This is "residual chlorine". The organic compounds present 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. 

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 

· 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 (ozone disinfection). Ozone is an unstable gas that can destroy bacteria and viruses. It is formed when oxygen molecules (O₂) collide with oxygen atoms (O) 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 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, and extremely irritating/possibly toxic, so need proper care and equipment.   

· Ozonation is not economical - the cost of treatment is relatively high. 

· There is no residual activity 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 (e.g., chlorine, ozone, chlorine dioxide). UV does not leave any odour, flavor in the treated water. 

The disadvantage of using UV for disinfection is its inability to provide a residual activity.  

Small Scale /household disinfection 

Several types of filters are used to remove suspended matter from water and reduce turbidity and microorganisms, or to remove specific inorganic 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.