Water treatment is a crucial process aimed at removing potentially harmful substances, whether they be biological, chemical, or physical, in order to ensure the safety and purity of water for human and domestic use. This comprehensive treatment not only produces water that is clear, colorless, and odorless but also guarantees that it remains non-corrosive, preventing any damage to pipework.
Stages in Water Treatment
In large-scale water treatment for urban municipal water supply, there are seven pivotal steps involved, each playing a vital role in the purification process.
1. Screening: Keeping Impurities at Bay
To safeguard the primary units of a treatment plant and ensure efficient operation, the use of screens is imperative. These screens effectively remove large floating and suspended solids present in the inflow, such as leaves, twigs, paper, rags, and other debris that could obstruct flow or damage equipment.
Coarse Screens
Constructed from corrosion-resistant steel bars spaced 5–15 cm apart, these screens prevent coarse materials, like logs and fish, from entering the treatment plant. They are positioned at a 60º angle for easy removal of collected material through mechanical raking.
Fine Screens
Following coarse screening, fine screens come into play to keep out material that may block pipework. They consist of steel bars spaced 5–20 mm apart. A variation of the fine screen, known as the micro strainer, employs a rotating drum of stainless steel mesh with an incredibly small mesh size, ranging from 15 µm to 64 µm. This allows even the tiniest suspended matter, like algae and plankton, to be trapped. The trapped solids are then dislodged using high-pressure water jets and carried away for disposal.
2. Aeration: Infusing Vital Oxygen
After screening, the water undergoes aeration, a process that involves passing it over a series of steps to absorb oxygen from the air. This helps in expelling soluble gases like carbon dioxide and hydrogen sulfide, making the water less corrosive. Additionally, it removes gaseous organic compounds that may impart an undesirable taste to the water. Aeration also facilitates the oxidation of substances like iron and manganese, which can cause peculiar tastes and staining of clothing. Once oxidized into their insoluble forms, these substances can be easily removed through filtration.
In cases of excess algae in the raw water, chlorination may be employed alongside aeration or even in its stead. This process, known as pre-chlorination, helps eradicate algae growth and also oxidizes taste- and odor-causing compounds.
3. Coagulation and Flocculation: Bringing Particles Together
Following aeration, coagulation comes into play to remove fine particles (less than 1 µm in size) suspended in the water. A coagulant, possessing a positive electrical charge, is introduced to neutralize the negative charge of these particles. This addition occurs in a rapid mix tank, where a high-speed impeller swiftly disperses the coagulant.
With their charges neutralized, the fine particles converge to form soft, fluffy aggregates known as ‘flocs.’ Common coagulants used in water treatment include aluminum sulfate and ferric chloride.
Flocculation follows, where the water is gently stirred in a basin, allowing the flocs to come into contact with each other and form larger aggregates. The basin typically comprises multiple compartments with decreasing mixing speeds to enable the formation of increasingly large flocs without disruption.
4. Sedimentation: Settling Matters
Once large flocs have formed, they need to be settled out in a process called sedimentation. The water, post-coagulation, and flocculation, is retained in a tank for several hours to facilitate this settling process. The accumulated material at the tank’s bottom, referred to as sludge, is then removed for proper disposal.
5. Filtration: Refining Clarity
Filtration is the pivotal step where solids are separated from the liquid. Any solids not separated in the sedimentation tank are removed by passing the water through sand and gravel beds. Rapid gravity filters, with a flow rate of 4–8 cubic meters per square meter of filter surface per hour, are commonly employed.
Once the filters are saturated with trapped solids, they undergo backwashing. This process involves pumping clean water and air back up the filter to dislodge the impurities, and the water carrying the dirt (known as backwash) is either directed into the sewerage system or, after a settling stage in a sedimentation tank, discharged back into the source river to remove solids.
6. Chlorination: Disinfecting for Safety
After sedimentation, the water undergoes disinfection to eradicate any remaining pathogenic microorganisms. Chlorine, in the form of either sodium hypochlorite or a gas, is the most widely used disinfectant due to its cost-effectiveness and simplicity. When added to water, chlorine reacts with any pollutants present, including micro-organisms, over a specific contact period. The residual chlorine that remains in the water safeguards it from potential microbial contamination until it reaches the consumers.
According to the World Health Organization Guidelines (2003), maximum residual chlorine of 5 mg l–1 of water is recommended. After 30 minutes of contact, water should have a minimum residual chlorine content of 0.5 mg l-1 (WHO, n.d.). While there are alternative methods of water disinfection, such as using ozone gas or ultraviolet radiation, these do not provide the same level of protection from microbial contamination post-treatment. The disinfected water is then pumped into the distribution system.
7. Supplementary Treatment: Tailoring to Population Needs
In certain cases, supplementary treatment may be necessary for the well-being of the population. One such instance is water fluoridation, where fluoride is introduced. As stated by the World Health Organization, ‘fluoridation of water supplies, where possible, is the most effective public health measure for the prevention of dental decay. The ideal fluoride level is around 1 mg per liter of water (1 mg l-1).
Frequently Asked Questions (FAQs) About Water Treatment
Q1: What is the primary goal of water treatment?
A1: The primary goal of water treatment is to remove potentially harmful substances, including biological, chemical, and physical contaminants, to ensure that water is safe, clear, colorless, and odorless for human and domestic use.
Q2: How many stages are involved in large-scale water treatment?
A2: There are seven major stages involved in large-scale water treatment for urban municipal water supply.
Q3: What is the purpose of screening in the water treatment process?
A3: Screening is essential to remove large floating and suspended solids, such as leaves, twigs, paper, and debris, which could obstruct flow or damage equipment in the treatment plant.
Q4: What are the two types of screens used in water treatment?
A4: There are two types of screens used in water treatment: coarse screens and fine screens. Coarse screens are made of corrosion-resistant steel bars and are used to exclude coarse materials, while fine screens keep out material that can block pipework.
Q5: How does aeration benefit the water treatment process?
A5: Aeration introduces vital oxygen into the water, expelling soluble gases like carbon dioxide and hydrogen sulfide. This process makes the water less corrosive and helps remove undesirable tastes and odors.
Q6: What is the purpose of coagulation in water treatment?
A6: Coagulation is a process that neutralizes the negative electrical charge of fine particles suspended in the water. This facilitates the formation of soft, fluffy aggregates called ‘flocs’ that can be easily removed.
Q7: How does sedimentation contribute to water treatment?
A7: Sedimentation allows large flocs to settle to the floor of a tank, separating them from the water. This process helps remove impurities and produces clearer water.
Q8: What is the role of filtration in water treatment?
A8: Filtration separates solids from the liquid, further refining water clarity. It is a crucial step to ensure that any remaining impurities are removed.
Q9: How is chlorination used in water treatment?
A9: Chlorination is a disinfection process that eliminates any remaining pathogenic microorganisms. Chlorine, in the form of a liquid or gas, is added to the water to safeguard it from microbial contamination.
Q10: What is supplementary treatment in water treatment plants?
A10: Supplementary treatment addresses specific needs of the population, such as fluoridation, where fluoride is added to water for dental health benefits. It ensures that water is tailored to meet the requirements of the community.
