How does granular activated carbon (GAC) work in a water treatment facility?
In a water treatment facility, activated carbon has two purposes:
1: To adsorb (trap on its walls) the organic compounds already present in the water, and
2: To get rid of the leftover oxidant that is used to disinfect it.
Let’s understand how chlorine dioxide disinfection is achieved using activated carbon.
Organic compound adsorption on activated carbon
Organic compounds can be found at varying degrees of concentration in all natural water sources, including lakes, rivers, wells, rain, and snowmelt. Some of those are relatively new to earth due to the rise in the manufacture and use of synthetic materials. Solvents, fuels, insecticides, medicines, detergents, fats, oils, and protein are a few of them.
Even in very small amounts, several of these substances can be dangerous to both people and other animals. The potability factors have been including those who exhibit signs of risk.
Thus, GAC is an adsorbent with a high non-specific capacity to absorb the majority of organic molecules. Because of its non-specificity, it keeps all different types of chemicals as long as they fit in the pores of the carbon.
Getting rid of any remaining oxidant that was employed to sterilize the water
Every water treatment system is required to have at least one disinfection stage. The train often begins by introducing a biocidal substance to the water. Typically, an oxidant is this agent. Like all biocides, free chlorine takes some time to start working. To make the water potable, a residual must be eliminated after disinfection. GAC is more than just an adsorbent. It functions as a reducer as well. As a result, it interacts with oxidising substances.
How to Chlorine dioxide sanitization utilizing activated carbon?
Free chlorine, which is hypochlorous acid or hypochlorite ion, is converted into chloride ion by this process. The carbon involved in the process, on the other hand, creates a surface oxide that ultimately dissolves in the water as carbonic acid.
C(s) + HOCl(l) = CO(s) + H+ + Cl–
CO(s) + H2O(l) = H2CO3(l)
H2CO3(l) = H2O(l) + CO2(g)
Why should a water treatment facility disinfect GAC litter?
Free chlorine is still present in the water as it enters the tank housing the GAC bed. This water is clear of organic materials and free chlorine, after passing through the bed, as one might anticipate.
However, this water begins to exhibit a bacterial count after a few days or weeks. After that, if the bed is not cleaned, the effluent will start to transport bacteria as well as a drainage odour.
Can backwashing GAC litter with chlorinated water disinfect it?
The bacteria on the outside of the GAC particles at the bottom of the bed will be rendered inactive, by the backwash with chlorinated water. By doing this, the treated effluent's bacteria content will drop. However, because it has been neutralized on the surface, the free chlorine will not reach the bacteria residing in the cracks of the charcoal.
Additionally, it won't go to the centre of the bed. In light of all of this, backwashing the GAC litter with chlorinated water is not a very effective method of disinfection. While you might temporarily achieve disinfection, the effluent soon returns to having an undesirable bacterial level.
Super-chlorination (chlorination at greater concentrations) of backwash water is likewise ineffective. Other than free chlorine, ineffective oxidizers include ozone, chloramines, hydrogen peroxide, and potassium permanganate. Chlorine dioxide is the sole oxidant that works well.
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