Water Flashcards
pH
-log10 [H+]. pH is thus a measure of the acidity or alkalinity of a solution.
Acid-Base Indicator
A substance, which by means of a color change, indicates the presence of another substance
Hard Water
water that will not easily form a lather with soap. Hardness in water is caused by Ca2+ or Mg2+ ions dissolved in water
Temporary Hardness
The type of hardness which can be removed by boiling the water; due to calcium hydrogen carbonates, becomes calcium carbonate on heating, leads to blocked pipes
Permanent Hardness -
The type of hardness which cannot be removed by boiling the water; it is caused by calcium sulfate or magnesium sulfate
Advantages of hard water
Provides calcium for teeth and bones
Nicer taste
Good for brewing, and for tanning
Disadvantages of hard water
Blocks pipes, leaves scale on kettles
Wastes soap
Produces scum
Ways of Removing water hardness
Distillation
Ion Exchange
boiling
what are the stages of water treatment
screening flocculation sedimentation filtration chlorination fluoridation ph. adjustment
What is Biochemical oxygen demand (B.O.D)
it is the amount of dissolved oxygen consumed by biological action when a sample of water is kept at 20C in the dark for 5 days
What is Eutrophication
its the enrichment of water with nutrients which leads to the excessive growth of algae
what are the 3 stages of sewage treatment
Primary treatment-physical
secondary treatment-biological
tertiary treatment-chemical
what is used for water analysis
ph. meter
colorimetry
atomic absorption spectrometry is used to detect heavy metals(Cd,Hg,Pb)
Define atomic absorption spectrometry
atomic absorption spectrometry is used to detect heavy metals(Cd,Hg,Pb)
What is temporary hardness caused by
Compounds of calcium and magnesium undergo a chemical reaction on being boiled,
Calcium Hydrogencarbonate Ca(HCO3)2
Magnesium Hydrogencarbonate Mg(HCO3)2
What is permanent hardness caused by
Compounds of calcium and magnesium that do not undergo a chemical reaction on being boiled
Calcium sulfate CaSO4
Magnesium sulfate MgSO4
what is a distillation
Evaporating off the water leaves the dissolved salts behind. The condensed water will no longer contain the salts and be ‘soft’.
Ion Exchange Resin
Pass water through ion exchange resin: 2 types
Cation exchange and Mixed-bed ion-exchange resin
what is cation exchange Cation exchange
swaps ions that cause hardness (Ca2+ and Mg2+) for ions that do not
Ca2+ + 2RNa 🡪 R2Ca + 2Na+
(R in RNa represents the resin)
Mixed-bed ion-exchange resin
Mixed-bed ion-exchange resin removes all ions from water (water is deionized and cannot be hard)
Often used in labs
Contains: Cation exchange (to remove + ions) and RH + Na+ 🡪 RNa + H+ Anion exchange (to remove – ions) ROH +Cl- 🡪 RCl + OH-
Ion exchange resin needs to be replaced after a certain amount of time
what are the criteria for water
colorless
odorless
safe to drink
fluorinated
elaborate on Screening
1st stage
Wire mesh
* Removes large solids and floating debris like twigs, plastics etc
elaborate on Flocculation
2nd stage Flocculating agent ( or flocculant) usually aluminium sulphate (alum) is added.
Makes smaller suspended solids coagulate or stick together in large clumps, so they are easier to remove at the next stage
Settlement
3rd stage
- Large tanks
- Water goes in at the bottom and rises slowly to the top, at < 2m/hr
- Particles settle to the bottom
- Over 90% of suspended solids removed at this stage
Filtration
4th stage
- Large beds of sand
- Removes remaining susp solids
- Sand supported on a layer of gravel
- Sand cleaned regularly
- Water now clear but may contain harmful bacteria
Chlorination
5th stage
- Cl2 gas is added
- Sterilises water
- Very small quantities 0.2 – 0.5 ppm
- Controlled carefully
- Monitored by bacteriological exam of the water
Fluoridation
6th stage
- NaF or H2SiF6 added (hexafluorosilicic acid)
- Added by law as helps to reduce dental decay by strengthening the enamel
- Small quantities ~ 1ppm
pH Adjustment
7th stage
* Optimum level is between 7 - 9
* Too Acidic may cause damage to pipes
* May be corrected by addition of Ca(OH)2 (lime)to raise the pH
* If very hard water, might be softened by addition of NaCO3 which is a base
* Too Basic may be corrected by addition of dilute H2SO4 to lower the pH
Water is now ready for consumption and use
Sewage
Waste water from homes, businesses and factories
Elaborate on Primary Treatment
The incoming sewage is screened to remove debris and non-biodegradable material.
It passes into large deep sedimentation (settlement) tanks where about 50% of the suspended solids and about 33% of BOD are removed
Liquid on top is then passed onto to secondary treatment
Elaborate on Secondary Treatment
The incoming sewage from first stage of treatment is passed into aeration tanks where mechanical stirrers aerate it.
This allows aerobic bacteria and other micro-organisms to decompose the solid matter into a harmless sludge called Activated Sludge.
Sewage then flows into settling tanks, sludge can be reused, or or recyled to make methane
About 95% of BOD removed
Elaborate on Tertiary Treatment
When the liquid passes out of the second round of settling, it is clean and inoffensive, but it may contain compounds of nitrates and phosphates
Phosphates from washing powders and washing up liquids, and nitrates from organic material in sewage
Must be removed before water discharged into river as could cause eutrophication.
Elaborate on Phosphate removal
aluminium sulphate added, aluminium phosphate precipitates out
Iron (III) chloride added, iron (III) phosphate precipitates out
Lime may also be used
Insoluble phosphate compounds allowed to settle out before discharge into waterway
Elaborate on Nitrate removal
difficult and expensive as nitrogen may be present in many forms, NH3, NO2- , NO3-, organic compounds containing N
what are pollution
The release of substances into the environment that damage the environment is called pollution
3 main types of waste
Eutrophication (also found in biology)
Organic Waste
Heavy metals
Elaborate on Eutrophication
Enrichment with nutrients (fertilizers / nitrates / phosphates)
Due to run off from land (erosion from land) / due to pollution / dispersion in water / absorption by plants
Rapid growth of plants (algae) on surface / algal blooms formed
Light blocked from (photosynthesis reduced in) plants below surface
Decay by bacteria of algae, algal blooms / increase in bacteria /
Dissolved oxygen depleted (lowered, used up) / oxygen level falls
Fish levels reduced or killed off
Principle of colorimetry
The Principle of colorimetry is that the amount of light absorbed by a coloured solution is proportional to the concentration of the solution
