Question

( Water & Westwater Treatment Curse )

a) Write the advantage, limitation and process design of any three disinfectants during the water treatment. Also, evaluate the design alternative from the comparisons. (9) b) Chlorine usage in the water treatment of 15, 000 m3/day is 5 kg/ day. The residual concentration after 10 min contact is 0,1 mg/L. Calculate the dosage and chlorine demand of the water. (6)

Answer 1

The following are specific methods for disinfecting water

1 Boiling Water

Advantage-  Boiling water is an effective method for destroying bacteria and other pathogens in the water. It does not act like a filter to remove particulate matter. In fact it does not remove anything from the water; it only destroys bacteria and pathogens such as cryptosporidia, giardia, E.coli, and typhoid. Boiling will not provide any safeguard against other things such as heavy metals, pesticides, herbicides and pharmaceuticals. It will also not remove foul tastes and odors, so it is not recommended to depend on boiling water as the main method to purify your drinking water

Process- Place water in a container over heat. Bring it to the boiling point. Hold it at this temperature for 15-20 minutes. This will disinfect the water. Perhaps you have used this technique after a flood or when a water main has burst as an emergency aid. Boiling water is an effective method of treatment because no important water-borne diseases are caused by heat resisting organisms. Although the boiling does not remove them from the water, it does kill them and thus they will cause no illness.

2 ULTRAVIOLET LIGHT

Advantage- Automatic, no taste or odor, energy efficient and low contact time. One of the few affordable technologies for the home that effectively kills the majority of bacteria, viruses, and other harmful microorganisms

Disadvantages- Low penetration power, shielding by turbidity, slime layer develops on tube, no simple test of results, no residual effect, and ultraviolet tube gradually loses power. Also requires pre-filtration to maintain effectiveness.

Process- The use of ultraviolet light is an attempt to imitate nature. As you recall, sunlight destroys some bacteria in the natural purification of water. When harmful microbes are exposed to UV light rays, their nucleic acid absorbs the UV energy which then scrambles the DNA structure of the organism causing it to die. UV light is only capable of killing microorganisms such as bacteria, viruses, molds, algae, yeast, cryptosporidium, and giardia. UV light generally has no impact on chlorine, heavy metals, and other chemical contaminants. This is why a UV light is usually combined with other various types of filtration.

3 Disinfection using Chlorine

Advantage Chlorine is a powerful oxidizing agent thereby getting rid of a lot of bacteria in water. Chlorine is commercially available Chlorine is very economical

Disadvantage - Chlorine does not exterminate all bacteria. Complex microorganisms are known to become dormant in the presence of chlorine but not exactly killed .Chlorine is not green. It is harmful to the environment. Commercially available chlorine (Hypo) disintegrates into Chlorine gas rapidly when exposed. Super-chlorination poses danger to humans over a period of time.

Process-   Chlorine readily combines with chemicals dissolved in water, microorganisms, small animals, plant material, tastes, odors, and colors. These components "use up" chlorine and comprise the chlorine demand of the treatment system. It is important to add sufficient chlorine to the water to meet the chlorine demand and provide residual disinfection.