Water

1
Water

• Chapter 19

2
Hard Water

• Hard water is water that does not form a lather
  easily with soap.
• There are 2 types of hardness.
• 1. Temporary hardness is referred to as hardness
  in water which can be removed by boiling.
• 2. Permanent hardness cannot be removed by
  boiling.

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Water hardness in Irelands
4
Temporary Hardness

• Temporary hardness is caused by the presence in
  the water of calcium hydrogencarbonate Ca(HCO3)2
  and magnesium hydrogencarbonate Mg(HCO3)2.
• This form of hardness arises when rain water,
  containing the weak acid carbonic acid (H2CO3),
  runs over limestone rocks(CaCO3 ) dissolving out
  Ca2 according to
• CaCO3 HCO3 H ? Ca(HCO3)2
• The dissolved Ca(HCO3)2 is now responsible for
  the temporary hardness.

5

• Scale
• When this hard water is boiled a reaction occurs
  which results in the
• calcium ions being precipitated out of solution
  in the form of CaCO3
• which is almost insoluble. This precipitate forms
  the deposit called
• scale or fur which is often clearly visible
  on the inside of kettles and
• boilers in hard-water regions.
• heat
• Ca(HCO3)2(aq). ? CaCO3(s) H2O(l)
  CO2(g)

heat Ca(HCO3)2(aq). ? CaCO3(s) H2O(l)
CO2(g)
6
Water softening.
Any process which removes or reduces the degree
of hardness of a water sample is termed water
softening. Boiling a sample of water which has
temporary hardness is an example of a water
softening method.
7
Permanent Hardness

• Permanent hardness cannot be removed by boiling .
• Permanent hardness is caused by the presence of
  dissolved calcium and magnesium sulphates and
  calcium and magnesium chlorides. CaSO4 CaCl2
• An example of how this type of hardness can be
  acquired by water is where water is flowing
  through, or over, gypsum rocks (calcium
  sulphate). The calcium sulphate is slightly
  soluble in water and so the water contains
  dissolved calcium sulphate after passing through
  the rocks, giving a hardness of up to about 500
  p.p.m., again recorded as 500 p.p.m. CaCO3.
• The sum of the temporary hardness and the
  permanent hardness is referred to as the total
  hardness.

8

• A number of methods are used for removing
  hardness.
• Distillation
• 2. Addition of washing soda

9
3. Ion Exchange resins and de-ionisers. 4.
Calgon
10
Estimation of the Total Hardness in Water

• This titration involves titrating a sample of
  water (usually but not
• always 100 cm3) with a solution of disodium
• ethylenediaminetetraacetic acid (Na2edta) in the
  presence of a
• buffer which holds the pH at about 10.
• Na2edta Ca2 ?. Caedta 2Na
• The indicator is Eriochrome Black T and it
  changes colour at the
• end-point from wine-red to blue.
• An unusual feature is that the indicator is added
  as a tiny pinch of solid.

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Total Hardness
Blue
Wine Red
12
Calculations

• The calculation involves application of the
• formula
• where V volume, M molarity, n number of
  moles in the balanced equation.
• Having got the molarity of the Ca2 ions, express
  this as moles CaCO3 per litre and then as mg per
  litre, i.e. parts per million (p.p.m.)

13
Water Treatment
14
Water Treatment

• The various stages of water treatment are.
• 1. Screening- removal of large particles
• 2. Settling
• The precipitation of the particles causing
  turbidity is achieved using
• flocullation agents. The most common
  flocullation agent is are
• aluminium sulphate, Al2(SO4)3.
• Polyelectrolytes are often added to speed up
  flocculation or
• coagulation. Aluminium contamination is probably
  the most
• controversial of these agents there has been
• ongoing speculation linking it with
  neurodegenerative diseases
• such as Alzheimers disease.

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Flocullation
floc floating at the surface of a basin
16
Mechanical system to push floc out of the water
basin
17
Filtration Chlorination

• 3. Filtration
• The purpose of filtration is to remove particles
  from the water, whether
• these exist in the raw water naturally or whether
  they have been produced
• by the coagulation process. Filtration is usually
  achieved by the downward
• passage of water through about a metre of finely
  divided inert material
• (sand or anthracite) which is on a support bed
  of coarser material
• (usually gravel). Drains at the bottom of the
  filter collect the water as it
• filters through.
• 4.Chlorination
• Elemental chlorine and compounds of chlorine are
  regularly added to
• water during the treatment of the water for
  public supplies. Chlorination is
• one of a number of possible treatments whose
  purpose is to disinfect the
• water to keep the pathogen content down to a safe
  level.

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Filtration
19
5. Fluoridation

• Fluoridation
• Over the past forty years a number of studies
  have shown
• some correlation between fluoride concentration
  in water
• and the incidence of tooth decay. It would appear
  that at
• fluoride concentrations of around 1.0 p.p.m.
  Maximum
• benefit is obtained. The chemicals added to water
  to supply
• fluoride ion include simple salts such as sodium
  fluoride,
• NaF, and calcium fluoride, CaF2.

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Sewage treatment(Waste water treatment)

• Two measuring parameters are regularly used in
  analysis of wastewater effluent. These are
  Biochemical oxygen demand (BOD) and chemical
  oxygen demand (COD)
• Biochemical oxygen demand (BOD) is taken as a
  measure of the degree of pollution of a water
  sample based on the quantity of oxygen consumed
  by the micro organisms present in a one litre
  sample stored in the dark at 20 C for five days.
  It can be expressed in milligrams of oxygen per
  litre of sample, i.e. mg l 1 or p.p.m.

21
Activated sludge
22
Primary Treatment(Physical)

• Pre-treatment
• The incoming sewage is pushed through
  mechanically raked screens to macerate the sewage
  and remove large debris.
• The pre-treated sewage flows into primary
  settling tanks. The sewage enters at the centre
  of the tank, (c. 12 m in diameter and c. 2 m
  deep), and rises allowing sludge to settle.
• The decanting liquid is transferred to the
  secondary treatment system. The settling tanks
  have a skimmer mechanism at the top to remove
  floating particles and a scraper on the settling
  tank floor (the base of the tank is
  hopper-shaped, i.e. sloping to the centre) to
  gather the settling sludge.

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Primary
Solids and large floating debris are screened
from the waste water  Remaining solids are
removed by allowing the waste to settle in
sedimentation tanks
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Secondary Treatment(Biological)

• This involves the biological degradation of the
  nutrient content of the effluent. This is usually
  done aerobically using percolating filters,
  activated sludge digestion units, aeration basins
  or biotowers.
• Percolating filters and activated sludge
  digestion units are commonly used in sewage works
  but all four methods can be found in use
  separately or in pairs industrially.

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Secondary Treatment
Activated Sludge Method The activated sludge
method operates aerobically. The sewage is fed
continuously into aerated tanks that is kept
oxygenated by mechanical agitators. The
micro-organisms break down the organic material
into Carbon Dioxide and Water.
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Waste water treatment plant
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Tertiary Treatment(Nitrates and Phosphates)

• While primary and secondary treatment of effluent
  largely
• concentrate on the reduction of COD/BOD levels
  they have a
• lesser impact on phosphate and nitrate
  concentrations under the
• conditions normally applied. High levels of
  nitrogen and
• phosphorus are environmentally harmful as they
  act as nutrients
• which give rise to algal bloom, leading to
  eutrophication.
• Ammonium salts and nitrates are the common
  sources of nitrogen
• and usually originate from animal waste or
  fertilisers.
• Phosphates are precipitated by treatment with
  lime, Ca(OH)2,
• aluminium sulphate, Al2(SO4)3, or
  iron(III)sulphate, Fe2(SO4)3. In
• each case an insoluble salt is produced which can
  be filtered off.

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Tertiary Treatment(Nitrates and Phosphates)
Removal of nitrates and phosphates   Phosphates
are removed by precipitation with Aluminium
Phosphate   Nitrates are removed by biological
nitrification. Tertiary treatment is a costly
process
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Water pollution

• Eutrophication.
• The over-enrichment of waters by nutrients, such
  as nitrate and phosphate, gives rise to a problem
  known as eutrophication.
• Added nutrients act as fertilisers and result in
  increased growth of algae and other plant matter
  in waterways. This increased growth is often very
  apparent from algal blooms and scums on stretches
  of waterways.
• When this type of algal bloom is followed by
  death and decay of
• animal and plant life in a competition for
  depleting oxygen
• supplies,the term eutrophication is used.

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Eutrophication
Fish Kill in the Salton Sea as a result of
eutrophication
Algal bloom in Orielton Lagoon, Australia,
31
Suspended and Dissolved Solids

• Suspended solids can be particles of plant and
  animal remains or silt.
• These neither sink nor float they are held in
  suspension in the liquid
• but are not dissolved.
• The amount of suspended solids in a sample of
  water can be
• determined by weighing a dried sheet of fine
  grade filter paper and
• filtering through it a known volume of water (a
  reasonably large volume of
• water will usually be needed, e.g. one litre).
  The filter paper is then
• washed with distilled water, dried carefully and
  reweighed. The increase
• in mass is the mass of solids suspended in the
  sample.
• Suspended solids are usually expressed in p.p.m.

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Total Dissolved solids

• The dissolved solids can be determined by taking
  a known volume
• of filtered water (to ensure that all suspended
  solids have been
• removed) in a previously weighed dry beaker, and
  then boiling the
• contents gently to dryness.
• The dissolved solids will remain in the beaker
  and their mass can
• be accurately found by reweighing the beaker when
  it has cooled.
• The concentration of dissolved solids should also
  be expressed in p.p.m.

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Estimation of Dissolved Oxygen in Water

• Estimation of the concentration of dissolved
  oxygen in water is
• used in the determination of the quality of
  surface waters and also
• in waste waters, particularly from biological
  treatment plants.
• The most common titrimetric procedure to measure
  dissolved
• oxygen is called the Winkler method. It relies on
  reactions involving
• manganese ions, iodide ions and oxygen. Under
  alkaline conditions
• the oxygen dissolved in the water oxidises the
  Mn2 ions to Mn3
• ions. When the mixture is acidified, the Mn3
  ions are reduced
• back to Mn2 ions by the iodide ions.
• This reaction liberates iodine whose
  concentration can be estimated
• by titration against standard sodium thiosulphate
  solution. The
• concentration of the iodine in the final solution
  is twice the oxygen
• concentration of the original solution.

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Winkler method
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Biochemical Oxygen Demand (BOD)

• Biochemical Oxygen Demand (BOD)
• The BOD test was first devised by the Royal
  Commission on Sewage in the early 1900s.
• The biochemical oxygen demand is the amount of
  dissolved oxygen consumed by biochemical action
  when a sample of water is kept in the dark at 20
  C for five days.