Pathogenic and non-pathogenic microorganisms are present in sewage. Pathogenic bacteria in wastewater pose serious health risks. Thus, waterborne infections including typhoid, cholera, dysentery, etc. are widespread.
Therefore, the goal of wastewater treatment is to lessen the likelihood of this occurring.
Similar to other treatment procedures, there are a number of ways to achieve disinfection. The techniques include bromine chloride additions, UV light, ozonation, and chlorination. But, the majority of the time, chlorination is used. It is a disinfection technique used in wastewater treatment plants, which aids in the eradication of bacteria from flowing wastewater.
In this article, we will talk about the types of chlorine used to disinfect the bacteria in sewage treatment plants. We will also discuss the chemistry related to disinfection procedure, while using chlorination to kill bacteria.
What is chlorination in sewage treatment plants?
Chlorination is the process of utilising chlorine to chemically disinfect treated wastewater. The chlorination process disinfects the treated wastewater by using various chlorine forms. It has played a significant role in lowering the prevalence of waterborne illnesses.
Why is chlorination in STPs necessary?
Through the elimination of pathogens, waterborne infections are avoided, such as:
· Harmful creatures like bacteria, viruses, and protozoa.
· Waterborne diseases including typhoid, dysentery, cholera, and more.
Thus, to disinfect the water or make the water sources potable, chlorination is necessary. It is the phase just prior to the water discharging into the seas, rivers, and streams.
Measurement of residual chlorine
The quantity of chlorine present in the water after it has reached its destination, are typically used by municipal water providers as a means of demonstrating safety. Although, there are different residual needs, a typical residual aim would be for 0.2 to 1 mg/L.
How can chlorine kill bacteria?
Chlorine is a powerful disinfectant that can be used to oxidise iron, manganese, and hydrogen sulphide to make removal easier, lighten the colour of water, and support other treatment procedures, including sedimentation and filtration.
Chlorine molecules eliminate the microorganisms by oxidising their biological components. Chlorine enters the cell after oxidation weakens the cell membrane. It thus interferes with DNA function and cell respiration. Thus, chlorine kills various microorganisms by interfering with their cell activity.
Factors that affect chlorine disinfection
Certain factors must be met for chlorine disinfection to be effective, which include:
· The pH value of water, i.e., the effective disinfection occurs when the water is at a lower pH;
· Water's composition and temperature;
· Type and concentration of chlorine;
· The type and quantity of microbes;
· Contact period (the time the chlorine is in the water);
· Levels of suspended solids and ammonia;
· Reducing agents present.
Relationship between pH and chlorine
In general, chlorine is more effective as a disinfectant with the lower pH of the water. Again, speaking broadly, the fact that chlorination elevates water's pH makes accurate dosing important, because overdose frequently causes pH values at which chlorine is ineffective, as a disinfectant.
This has to do chemically with how hypochlorous acid and hypochlorite ions, two components of chlorine that are together referred to as "free chlorine," interact. A lower pH is favoured for disinfection since, hypochlorous acid is the more potent disinfectant and it dominates at lower pH levels. Yet, in order for water treatment methods that rely on chlorination to oxidise, a higher pH is required.
Types of chlorine used to disinfect bacteria in STP Plant
Using chemical feed inlets, we can introduce chlorine to the tertiary system. There are three common chlorine compounds that are available for chlorination:
1: Gaseous or vaporised chlorine (useful in larger public water treatment plants);
2: Liquid Sodium hypochlorite (bleach solution);
3: Solid Calcium hypochlorite (Dry powder or pellet).
· Gaseous Chlorine
It is a yellowish-green gas. It is incredibly effective as a disinfectant. Chlorine gas is used in several drinking water facilities. Its use has certain benefits and a number of possible drawbacks. It is also an effective oxidizer.
Advantages of using chlorine gas in wastewater disinfection
1: It is the best disinfectant for killing waterborne pathogens
2: It provides affordable disinfection
3: Moreover, it requires less storage space than chlorine solutions do
Disadvantages of using chlorine gas in wastewater disinfection
1: It can irritate the respiratory system
2: Explosions of chlorine gas are caused by specific reactions
3: It is quite corrosive to use wet chlorine
4: Special handling and storage measures must be made
Owing to these drawbacks, STPs or sewage treatment plants take into account the following alternatives:
· Sodium Hypochlorite (NaClO)
It also goes by the name liquid bleach. It is a liquid that is light yellow in colour. It is a lot safer than chlorine gas. Water can be disinfected with NaClO at lower concentrations (5–15%). Compared to calcium hypochlorite or chlorine gas, this concentration is more efficient.
Hypochlorous acid and hypochlorite ions are produced by the reaction of sodium hypochlorite (NaClO) with water. Free accessible chlorine (FAC), a disinfectant, is created when these three things come together. It is frequently employed in low-hardness water.
As it is less dangerous than chlorine gas, yet the following precautions should be taken:
1: It can irritate the oesophagus, stomach, and skin depending on the chlorine content.
2: The elements of heat, light, pH, and metal contamination all determine how unstable it is.
3: Effectiveness and relative shelf life are affected by each element.
· Calcium Hypochlorite (Ca(ClO)2)
It is produced using chlorine gas. Calcium hypochlorite comes in a dry, solid form. It is available as white granules or pellets. 65% of concentrated chlorine is found in calcium hypochlorite. Costs are greater since it contains a lot more chlorine than NaClO. In comparison to the first two disinfectants, calcium hypochlorite has lower initial expenses.
It works well and is stable. If heated or kept close to an oxidised organic material, it may be dangerous.
How does chlorination kill bacteria in STPs?
Under normal pressure and temperature, chlorine is a gas, and it dissolves in water. It functions as the most effective chemical disinfectant for cleaning treated wastewater.
1: Chlorine and water interact, which results in hydrolysis. Hypochlorous acid and hydrochloric acid are also produced by the process.
Cl2 + H2O → HOCl + HCl
2: Sodium hypochlorite reacts with water as follows
2NaOCl + H2O → NaOH + NaCl
3: Chlorine reacts with water to produce hypochlorous acid (HOCl). Then, the hypochlorous acid dissociates and forms the hypochlorite ion.
HOCl ↔ OCl- + H+
Chlorine as a disinfectant causes
· The oxidation of cell walls can lead to cell lysis.
· Inactivation of functional sites on the cell surface.
Other forms of chlorine used for wastewater disinfection are:
· Monochloramine (NH2Cl)
· Dichloramine (NHCl2)
Why do we treat wastewater/sewage with chlorine?
The benefit?s of chlorination include:
· It is simple to find chlorine as a gas, liquid, or powder.
· It is more affordable.
· Chlorine compounds are simple to use because of how well they dissolve in water.
· Chlorine residues are not dangerous to humans in lesser amounts.
· They are hazardous to numerous microbes due to their potent oxidising properties.
· It aids in the clean-up of cyanide and phenol pollutants.
· It regulates the thickening of activated sludge.
· Chlorine also stabilises waste activated sludge.
· It can purge the water of ammonia.
· Chlorine treatment in STP prevents foaming and filter flies.
· It helps clear the water of grease and sludge.
· It reduces odours and prevents septicity.
Drawbacks of chlorination
· Less efficient in combating protozoa.
· Fewer efficacies in the treatment of turbid waterways.
· There could be taste and odour issues with excessive chlorine application.
· Chlorination by-products may have long-term impacts.
Conclusion
Waterborne infections are typically eliminated by chlorination, a standard method of disinfection.
During chlorination, free and mixed chlorine residues are created, which are harmful to aquatic life. Moreover, there is a potential that organo-chlorinated derivatives will occur. Due to their toxicity, persistence, and bio-accumulative nature, these derivatives are particularly concerning.
Thus, dechlorination is crucial for getting rid of mixed chlorine leftovers from chlorination.
Are you concerned about disinfection procedure in your sewage treatment plants? You do not need to worry!
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