Water ,type of Hardness,how to remove it ,
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November 02, 2015 |
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Hard water is water
that does not form a lather easily with soap.
There are
2 types of hardness.
- Temporary hardness is
referred to as hardness in water which can be removed by boiling.
2.
Permanent hardness cannot be removed by boiling.
Temporary
hardness
Temporary
hardness is caused by the presence in the water of calcium
hydrogencarbonate Ca(HCO3)2 and magnesium hydrogencarbonate,
This form
of hardness arises when rain water, containing the weak acid carbonic acid (H2CO3),
runs over
limestone rocks dissolving out Ca2+ according
to
CaCO3 + HCO3–
+
H+
à Ca(HCO3)2
The
dissolved Ca(HCO3)2 is now responsible
for the temporary hardness.
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). à CaCO3(s) + H2O(l) + CO2(g)
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.
Permanent
hardness
Permanent
hardness is caused by the presence of
dissolved calcium and magnesium
sulphates and calcium and magnesium chlorides.
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.
Permanent
hardness
cannot
be removed by boiling and therefore alternative methods must be used to soften
the water where that is necessary.
The sum of
the temporary hardness and the permanent hardness is referred to as the
total hardness.
Methods of
Removing Hardness from Water - Water Softening
A number
of methods are used for removing hardness.
- Distillation
- Addition of
washing soda
- Ion Exchange
resins and de-ionisers.
- Calgon
Student
Experiments
1.
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+ à.
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.
Notes
1. Buffer solutions are solutions which resist
change in pH when small amounts of either
acids or
bases are added to them. In this case the buffer is a mixture of ammonium
chloride
and
aqueous ammonia and maintains a pH of just above 10 for the titration.
2. The pH
is maintained at 10 because at this pH the indicator works well (its complexes
with the
magnesium
ions present break down rapidly and allow a clearly detectable end-point).
3. The
structure of disodium ethylenediaminetetraacetic
acid is
shown in Fig. 2.7.
Fig. 2.7
5. 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.).
2.
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.
Water Treatment
Most water
treatment processes consist of several stages. These normally include storage,
screening, settling or clarification, chlorination, pH adjustment, fluoridation
and filtration. The
sequence
of the stages may vary from one water treatment plant to another and some
stages may involve more than one step. However, the basic principles always
remain the
same.
Stages of 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 Alzheimer’s
disease.
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.
5. 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.
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 is taken as a measure of the degree of pollution of a water
sample based on the quantity of oxygen consumed by the microorganisms 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.
Stages Involved in Waste Water Treatment
1. 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, Fig. 3.2. 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.
2. Secondary
treatment (Biological Oxidation)
This
involves the biological degradation of the nutrient content of the
effluent. This is usually doneaerobically 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.
3. Tertiary
treatment
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.
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.
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.
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.
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.
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.
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