Water Flashcards
Distinguish between the terms temporary and permanent hardness in water referring to a compound which causes each type of hardness and a method of removal
Temporary - calcium hydrogen carbonate, boiling
Permanent - calcium sulfate, ion exchange resin
Describe what takes place in the primary treatment of sewerage
It is a mechanical process involving screening and settling
Screening - floating solids screened by passing through steel bars, pebbles and grit removed by allowing them to settle into grit channels
Settling - sewage then flows into settling tanks allowing solids to settle to the bottom
Describe what takes place in secondary treatment of sewerage
Biological process using micro-organisms
Sewage is pumped into organic waste to break it down. It is then pumped into aeration tank and the sludge provides nutrients for organisms. The tank is aerated to prevent levels of dissolved oxygen getting too low. the sewerage is moved to a settling tank and sludge is removed
What are the substances removed in the tertiary treatment process
Nitrates and phosphates
Why is tertiary treatment considered necessary
To prevent eutrophication
What term describes the process of removing suspended solids in the treatment of water for drinking
Flocculation
Name the substance that is added to water during flocculation
Aluminium sulfate
Why is it necessary to adjust the PH of water if it is too acidic / What problems would arise if the PH were outside range 6-8
Prevent corrosion of pipes
What is used to adjust the PH of drinking water
Calcium hydroxide [slaked lime]
How does boiling remove any temporary hardness caused by the presence of calcium hydrogencarbonate in a water sample
Calcium hydrogencarbonate decomposes and is converted to calcium carbonate and a precipitate forms which is insoluble
[To determine the BOD of the water in a lake, two samples were taken and one was tested immediately]
Under what conditions was the second sample stored?
20C in dark
[To determine the BOD of the water in a lake, two samples were taken and one was tested immediately]
At what time interval was it tested?
5 days
The result of the BOD test suggested that eutrophication had occurred in the lake. State one of its harmful effects
Organisms in water are deprived of oxygen and die
Suggest a possible cause of eutrophication
Slurry run off
Why is there concern about lead in drinking water?
Can be harmful to human health [toxic]
Name an instrumental method used to analyse water for heavy metals
AAS
How can lead be removed from a contaminated water supply
Precipitation reaction
How does temporary hardness arise in rainwater flowing over limestone
Rain water is slightly acidic as it has reacted with CO2 in air so therefore it reacts with carbonate of limestone and forms hydrogen carbonate which is soluble
Write a balanced equation to describe the formation of limescale when this water is boiled
Ca(HCO3)2 -> CaCO3 + H2O + CO2
Explain in words how water with temporary hardness is deionised using a cation exchange resin and an anion exchange resin
two hydrogen ions from the cation-exchange resin replace a Ca2 (positive ions) in the water
a hydroxide ion from the anion-exchange resin replaces a HCO3– (negative ions) in the water
hydrogen ion and hydroxide ion combine to form water
Explain how hard water is caused
Dissolution of calcium ions
How does hard water waste soap
Reacts with calcium and magnesium ions to form scum
How can hard water be softened by ion exchange so that it is suitable for use as deionised water in the lab?
Passes through and positive ions are replaced by H+ ions and negative ions are replaced with OH- ions
In water treatment, what is the purpose of adding a flocculating agent and state a problem that would arise if it is added in excessive quantity
Clumping together of fine particles
Corrosion and tooth decay
In water treatment, what is the purpose of adding chlorine and and state a problem that would arise if it is added in excessive quantity
Kills pathogens (micro-organisms) Toxic, strong odour is repulsive
In water treatment, what is the purpose of adding a fluorine-containing compound and state a problem that would arise if it is added in excessive quantity
Prevents tooth decay
Toxic, stains teeth
In water treatment, what is the purpose of adding calcium hydroxide and state a problem that would arise if it is added in excessive quantity
Raises PH
Too high and cause hardness
In water treatment, what is the purpose of adding sulfuric acid and state a problem that would arise if it is added in excessive quantity
Lowers PH
Corrosion and tooth decay
What is meant by biochemical oxygen demand (BOD) of a water sample
p.p.m of oxygen consumed when a sample of water is kept in th dark for 5 days at 20 degrees celcius.
Describe how BOD was reduced by about 30% in primary sewage treatment
Removal of solids by screening and settlement
Explain the processes by which BOD was further reduced in secondary sewerage treatment
Digestion and Aerobically (Aeration) involving micro organisms
What treatment is carried out to ensure low levels of micro-organisms in drinking water
Chlorination
A sample of hard water is treated for domestic use by ion exchange, assuming that the hardness is due to Ca(HCO3)2, explain in words how a cation exchange resin softens this water supply
Each calcium ion is replaced by two Na+ ions
What is meant by the term flocculation
Joining together of suspended particles
A whitish deposit is often found on the insides of kettels in hard water districts. If some of this deposit is scraped into a test tube and dilute hydrochloric acid is added a reaction is observed. Write a balanced equation for this reaction
CaCO3 + 2HCL -> CaCl2 + H20 + CO2
Give one reason why heavy metal ions may be found in rivers and lakes
From industrial effluent
Improper disposal of batteries
In measuring B.O.D of a sample of water, it is normal practice to keep a sample in the dark at 20 degrees for five days, why is this necessary?
Darkness - prevent photosynthesis as this would increase the level of oxygen in water
Temperature - Valid comparison of different water samples since the amount of oxygen dissolved in water depends on temperature. Also, the rate at which oxygen is consumed depends on temperature
Five days - Ensure a standard period of time to allow all the dissolved oxygen to be used up
