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5.0 Water Purification Techniques
In selecting a particular water purification technique, there is no particular standard or guideline. There is actually no unique solution. In fact, it is difficult to standardize the solution of treating water from different sources. However, it is vital to consider the main objective of water purification which is making water safe for drinking and for other purposes with which it is intended for. As such, here are some technologies that are well developed and are generally used by many water utilities.
Most common municipal drinking water treatment processes:
- Pre-chlorination
- Aeration
- Coagulation
- Sedimentation
- Filtration
- Desalination
- Disinfection
5.1 Things to Consider in Choosing a Water Purification Solution
Some of the things that should be considered in choosing a water purification solution includes:
- the amount of purified water you would like to use in a day for drinking and cooking
- the contaminants that are actually found in your water
- the contaminants that you are interested in removing
- the cost of the solution that you decide upon, including the initial cost and the ongoing costs
- the value of the product
5.2 Most Common Water Treatment Methods
- Boiling
Boiling is the best way to disinfect water that is unsafe, most especially during emergencies. It should be noted that if the water appears to be cloudy, it should be filtered first before boiling.
Advantages of boiling water:
- Elimination of pathogens that might be lurking in the water
- Drives out some Volatile Organic compounds present in the water
Disadvantages of boiling water:
- cannot be used when toxic metals and chemicals like mercury, lead, pesticides, solvents, and asbestos have contaminated the water
- it may concentrate any harmful contaminants that do not vaporize as the water boils off
- needs energy to boil water
- Water Filters
Water filters prevent contaminants from moving through the filter by screening them out or by trapping them within the filter matrix. There are many types of filtration strategies used all over the world.
Two main types of filters
- Sediment
- Fiber Filters
These contain cellulose, rayon, or some other material with small pores. However, fiber filters will not remove contaminants that are dissolved in the water. These include chlorine, lead, mercury, trihalomethanes or other organic compounds.
- Ceramic Filters
These are very similar to fiber filters, only that the material is ceramic and water is forced through a ceramic filtration media. Ceramic filters can reduce asbestos, some bacteria, and fiber cysts. However, these filters will not remove contaminants that are dissolved in the water, like chlorine, lead, mercury, trihalomethanes or other organic compounds.
Fig.10. Sediment Filters
- Activated carbon
These filters consist of particles of carbon that have been treated to increase their ability to adsorb. Activated Carbon filters also have a greater ability to adsorb a wide range of contaminants.
- Granular Activated Carbon (GAC)
Water is allowed to flow through a bed of loose activated carbon granules. These granules trap particulate matters present in the water. Moreover the granules also remove some organic contaminants, chlorine, and undesirable tastes and odors. In using GAC though, one has to be critical of the chanelling, dumping, and the pore size.
Advantages of GAC filters:
- reduce particulate matter and chlorine
- improve the odor and taste of water
- do not restrict water flow
- low economic maintenance
- do not require electricity
- effective water treatment solution
Disadvantages of GAC Filters
- water can channel around carbon granules
- pockets of contaminated water can form in a loose bed of carbon granules
- cannot remove bacteria since the effective pore size of the filters is relatively large
- cannot be used for hot water
- can be a breeding ground for bacteria that they trap
- difficult to know when the filters have become saturated with contaminants
- Solid Block Activated Carbon (SBAC)
Solid Block Activated Carbon uses specially treated activated carbon which are bonded together to form a uniform matrix. The effective pore size of these kinds of filters can be very small ranging from 0.5 to 1 micron. However, like other filter cartridges, SBAC may eventually become saturated by contaminants.
Advantages of SBAC filters
- provide longer contact time with the activated carbon
- allow a larger surface area for better adsorption
- eliminate channeling and dumping problems of GAC filters
- provide small pore size for the effective trapping of particulates
- useful in emergency situations
- do not dissolve beneficial minerals like calcium, potassium, and magnesium
- do not waste water
- require simple and economical maintenance
Disadvantages of SBAC filters
- do not reduce the level of soluble salts naturally
- gradually lose effectiveness in removing contaminants as they become saturated
- cannot be used with hot water
5.3 Distillation
In the distillation process, the water is usually boiled in a chamber causing the water to vaporize. As water is vaporized, the pure steam leaves the volatile contaminants behind. The steam is then moved into another unit where it is allowed to cool and condense back into liquid water. Unfortunately, anything that will not boil or evaporate like salts, metals, and sediments remain the distiller and need to be removed.
Distillation is the process of heating the water until it boils, capturing and cooling the resultant hot vapors, and collecting the condensed vapors. The impurities will “accompany” the steam and end up in distillate.
Fig.11. Simple Water Distillation Diagram
Advantages of distillation:
- produces very pure water
- removes pathogens in the water
- does not need to replace filter cartridges
- produces consistently high quality treated water
Disadvantages of distillation:
- takes time to purify the water
- uses electricity in its operation
- relatively expensive
- cannot be used in emergency situations since it requires electricity for its operation
5.4 Reverse Osmosis
Reverse Osmosis is a water purification process which uses water pressure in forcing water molecules through a member with extremely tiny pores. As such, the larger contaminants are left behind. The reverse osmosis system consists of a sediment/chlorine profiler, the RO membrane, and an activated carbon post filter.
This system is the perfect option for people with unacceptably high levels of dissolved inorganic contaminants in their drinking water. A reverse osmosis system works is highly recommended for water coming from shallow wells in agricultural areas that contain high nitrate levels.
Advantages of Reverse Osmosis
- reduces salt and other inorganic material present in the water significantly
- removes microscopic parasites and viruses as well
- does not use electricity
- purifies more water than a sediment or carbon water filter
- less expensive to operate and maintain
Disadvantages of Reverse Osmosis
- wastes around 2 to 4 gallons of water for flushing the drain for each gallon of filtered water produced
- makes only a few gallons of treated water
- depends on relatively high water pressure; thus this would not work in emergency situations where high water pressure cannot be achieved
- many conditions dictate its effectiveness in reducing the amount of contaminant in water
- some pesticides, volatile organic materials, and solvents are not completely removed
- require maintenance
- damaged membranes are not easily detected
