What Is Water treatment?
Life exists on earth because water is available here. There is no point in further discussing the importance of water. However, all the water available on earth isn’t fit for human consumption.
Upon contaminated water consumption, we may suffer from, cholera, diarrhea, typhoid, hepatitis, scabies and worm infections, etc. The treated water is free of germs, suspended materials, etc.
The water treatment process is a multi-stage process. Every stage has a specific purpose. Some water treatment processes remove algae and planktons, some remove suspended solids, and some remove germs and bacteria.
We shall learn about each stage of water treatment and the methods of household water treatment. Before we start, let us understand the clean water parameters. What does clean water mean?
Parameters for Drinking Water
Bureau of Indian Standards has laid down the specifications for drinking water in IS 10500:2012. The details of various types of drinking water parameters as mentioned in the IS10500:2012 are-
- Table 1 physical specifications
- Table 2 undesirable substances
- Table 3 toxic substances
- Table 4 radioactive substances
- Table 5 pesticide residues
- Table 6 bacteriological quality
The summary of some of the crucial parameters are as follows-
|Sr. No.||Parameter||Acceptable Limit||Permissible Limit (if an alternate water source is unavailable)|
|5||Total Hardness as CaCO3 mg/L, Max||200||600|
|6||E. coli||Not acceptable||Not acceptable|
|7||Iron as Fe mg/L, Max||0.3||0.3|
|8||Fluoride mg/L, Max||1||1.5|
|9||Chloride mg/L, Max||250||1000|
Processes of Water Treatment
As explained above, the drinking water treatment is a multi-stage process. The stages can be are as follows-
The physical processes involve the removal of turbidity, solid contaminants, suspended solids, etc. It improves the physical properties of the water as mentioned in Table 1 of IS 10500:2012. The physical processes involve mechanical methods such as screening, aeration, sedimentation.
The screening removes around 20-30% of the contained solids. At first, the water passes through screens that remove the plants, twigs, cloths, papers etc. the efficiency of material removal depends upon the screen size. The removed material is disposed of.
The aerator removes the odor and taste caused by volatile organics and gases. The iron and manganese present in the water are oxidized.
The aeration devices are available like gravity aerator, spray aerator, diffuser, mechanical aerator.
Aeration Tank for Iron Oxidation
The gravity settles the sand/soil particles. The water is retained in a tank or flown at a slow speed through a long channel.
The sedimentation efficiency depends upon the water retention period and the size of sand particles in the water.
The coarse particles settle early due to their higher weight. The sedimentation process removes around 30-40% of particles present in water.
After the physical treatments, the water is dosed with chemicals to remove the remaining material. The material removed in chemical processes is very tiny sized.
Some of the methods of chemical processes are described in brief. The chemical dosage is calculated beforehand as per the particle concentration in water.
It is a process of balancing the pH value of water. The ideal pH value for water is 7. However, the acceptable range for pH value is 6.5-8.5. For water having a pH value lesser than 6.5, a base like milk of lime is added to the water. Similarly, an acid like hydrochloric acid is added to the water if the pH value exceeds 8.5.
A coagulant is rapidly added to the water that coagulates the colloidal particles. The coagulated material settles at the bottom of the water tank. A coagulant cleaning mechanism removes the coagulant.
The process of coagulation & flocculation involves the addition of an external flocculating agent (usually alum). The alum mixed with water produces floc which settles at the bottom of the water tank. The turbid material and the color-causing elements are trapped in the floc. The floc is later removed from the water.
The disinfection process cleans the water of germs and pathogens. A bleaching powder is a common disinfectant.
The bleaching powder produces chlorine which kills the pathogens. Other than chlorine, ozone and ultraviolet light are also popular disinfectants.
A membrane process involves water treatment by passing water through a membrane or filter. A membrane treatment process is a final treatment to the water. The water is fit for human consumption after the membrane processes.
Some of the general methods of membrane processes are mentioned below.
In the filtration process, water passes through a filter such as sand. The filter media used in the water treatment is washable and washed after a certain period to improve efficiency.
Reverse osmosis is also a filtration method. In the RO method, water passes under pressure through a semi-permeable membrane.
The semi-permeable membrane allows the water to pass through but retains the dissolved salts. Hence, the RO process removes 95-99% of the dissolved salts.
It is similar to the reverse osmosis process. However, the pressure applied in nanofiltration is lesser than in the RO process.
The membrane uses a permeable membrane but osmosis uses a semi-permeable membrane. Nanofiltration is effective in removing heavy metal ions.
Household Methods of Water Treatment
The drinking water household treatment method is suitable for a single-family. With the increase in consumption of water, the household methods are uneconomical.
The household water treatment methods assume 20-50 liters per day for a person for drinking, cooking, laundry, and personal hygiene.
WHO recommends vigorously boiling the water to kill the microorganisms. Turbid water should be filtered before boiling.
The water is stored in the pot in which it was boiled or it can be immediately stored in a separate clean container after boiling. Herbs can also be added when boiling. The water is cooled before consumption.
Boiling is the simplest method of household treatment but expensive. It is effective in killing bacteria, removing taste and odor but does not remove the turbidity, metal, or dissolved salts.
Slow Sand Filter:
The slow sand filter purifies the water by a combination of physical, biological, and chemical action. The fine particles are trapped by sand, microorganisms growing on the sand surface consumes waterborne bacteria and organic matter.
A sand filter is easily made of a tank, sand, and gravel. A filter contains coarse aggregate at the bottom of the tank and fine aggregate at the top of coarse aggregate.
The water flows at a unit cum per sqm area of the tank per hour. A tank of 0.45m diameter yields 380 liters of clean water per day.
Upon lowering of clean water flow, the top 1-2 cm of sand is scraped off. The sand is replaced with the washed sand to maintain the sand bed thickness.
The constant water flow is essential for clean water. The microorganisms on the sand surface may die in dry conditions, and the filter may not work.
A chlorination agent produces free chlorine when mixed with water. It is stirred well and kept for 30 minutes to allow the free chlorine to react and kill the microorganisms.
The free chlorine content should be between 0.2-0.5 mg/L. A special test kit determines the free chlorine content.
Around 150-1400, cum treated water can be produced per kg of dry chemical depending upon the strength and concentration required.
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