Question (1 Viewer)

[schooly]

Hi anyone mind helping me out with this question from the 2002 hsc paper? any help or sample answers or anythign needed. this part of chemistry is my weakest

Q: Describe the physical and chemical processes needed to purify and sanitise a town water supply (5marks)

thankyou

 

[jims]

im pretty sure that info would be in any textbook you opened so ill just put down the main stages:
aeration
flocculation (and sedimentation)
filtration
chlorination

 

[Frigid]

short and quick answer:

• screening - large objects such as trees and dead animals are removed by big screens.
• flocculation - a flocculant, such as Al(OH)3 is added to force the particles to clump together and settle in the bottom of tanks.
• filtration - through anthracite (a type of coal) and sand.
• chlorination - it is further sanitised by adding hypochlorite ions into the water, which kill microbes.

marking scheme: 4 marks for describing the stages, 1 mark for extra info (such as naming the flocculant or filter).

 

[inasero]

optional: fluoridation. Adding just a teensy weensy bit to help ur whites white

 

[Frigid]

Originally posted by inasero
optional: fluoridation. Adding just a teensy weensy bit to help ur whites white

i would refrain from adding it in coz purely optional and fluoridation doesn't help 'purify' or 'sanitise' the water... that's why i didn't mention aeration either.
read and answer the question - no more, no less.

 

[+:: $i[Q]u3 ::+]

fluoridation is for that dot point where u describe the chemicals added to water.

pathways (??) um.. the orange book.. not conq. yeh i think i'ts pathways.
has an excellent summary. ah.. why not.. let's go typing:

aeration - water is sprayed into the air to increase dissovled oxygen conc. This imporves the taste of drinking water, and also oxidises hydrogen sulfide gases so that they can be removed more easily later.

flocculation - chemicals are added to coagulate small suspended particles together (eg clay colloids). Alum (aluminium sulfate) and a cationic polymer are also used. This causes gelatinous precipitates to form which trap suspended particles, and also some microbes. The polymer strengthens the flocs. the process may be aided by gently stirring the water with paddles.

Sedimentation - flocs are allowed to settle out under gravity to remove.. 90% 95% oh shoot i've forgotten.. of the suspended matter. The sludge is periodically removed from the sedimentation tanks.

Filtration - water is passed through sand/gravel filters. Anthracite or charcoal is used if water of a higher purity is required.

Chlorination - chlorine is added to form hypochlorite ions which sanitise the water, killing most bacteria. It also bleaches the water so it becomes colourless. If the pH is too low (ie too acidic) then lime is also added to correct pH.

 

[mercury]

Additional Information

CHLORINATION

Sydney Water uses chlorine for disinfection. Various forms of chlorine are used including chlorine gas, liquid sodium hypochlorite and calcium hypochlorite tablets. Sufficient chlorine is added to the filtered water to ensure effective primary disinfection as well as an appropriate chlorine residual entering the distribution system. Due to the extensive nature of some parts of the distribution system, some extra chlorine may be added within the distribution systems to maintain an effective disinfection residual. In some parts of Sydney, ammonia is added after the chlorine in a fixed ratio to form a less reactive is infectant called monochloramine. Because it is less reactive than chlorine, the monochloramine residual persists further into the
distribution system.

The addition of chlorine to water may result in the formation of compounds known as disinfection by-products. The most commonly known of these compounds are a group called Trihalomethanes. The NHMRC drinking water
guidelines require Trihalomethanes to be below 250 micrograms per litre. Sydney Water complies with this guideline.

FLUORIDATION AND CORROSITIVITY CONTROL

In accordance with the Fluoridation of Public Water Supplies Act 1957, Sydney Water adds fluoride, in the form of sodium silicofluoride or hydrofluosilicic acid to achieve a fluoride content of one milligram per litre. Lime and carbon dioxide are added at some water filtration plants where the water is very soft to adjust and buffer the pH of the treated water. The carbon
dioxide reacts with the lime to form calcium bicarbonate which buffers the water (increases the resistance to changes in pH), increases hardness and reduces the general corrosivity of the water. The addition of lime and carbon dioxide have no adverse health effects.