Purification of Water - Aeration, Sedimentation, Filtration

About 71% of the Earth’s surface is water covered by the oceans in which 96.54% is completely undrinkable due to the salinity. The rest of the 3.46% of Earth’s water is located at places such as icebergs, groundwater, lakes, soil water, atmosphere and etc. and among them only 1.42% is fresh water for the survival of the living beings. Due to the rapid growth of the world population and the industrialization, the availability of fresh water sources is reducing day by day. 

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.

There are two main types of water purification. They are drinking water treatment and the wastewater treatment. 

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. 

In the wastewater treatment, the wastewater from the industrial processes should be collected and purified before releasing it to different environments such as surface waters, lands for irrigation purposes, marine coastal areas and etc. After the purification, that water should fulfill the applicable quality standards of discharging. 

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 water is 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 large reservoirs or in restricted area of  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. NH₃ present is oxidized into nitrate by microorganisms during storage. 

        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- 

-Coagulation/flocculation

Simple sedimentation can reduce turbidity and solids in suspension but can take time so the process is enhanced by  physical processes such as flocculation  or chemcal 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 microorganisms remain viable for some time and get killed on prolonged storage. 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. These precipitating agents form precipitate of hydroxides. Suspended solids absorbs on the surface of precipitate/flocs, so gradually precipitate becomes heavier and finally settle down. If negatively charged colloidal impurities are present, they are neutralized by Al⁺⁺⁺ ions and settle down. 

The floc is separated from the treated water by sedimentation and/or filtration. The water with floccules is passed into the sedimentation tank (clarifier tank) to allow aggregation of the flocs, which settle out to form sludge. This sludge is periodically removed.  

The efficiency of the coagulation process depends on the raw water properties, the coagulant used and operational factors including mixing conditions, temperature, coagulant dose rate and pH value. 

4. Filtration

 After sedimentation the water is further purified by passing to filtration unit. It is the effective means of removing microorganisms and the other suspended material from the water. 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. 

    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. 

    The capacity of slow sand filter plant is to filter about 5 million of water per acre per day.

          

Water purification is done  by physical straining action 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”, makes the pores more effective by closing the pores between the sand grains. 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.

For the first time in 1852, parliament of London required that the entire water supply be passed through slow sand filters before use. This plant was installed in many countries after cholera epidemics.

(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 unfortunately 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. Therefore, many plants are constructed in a chain. If one plant is being cleaned, the others are under operation.

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