Chlorides:
can be found in all kind of water. The chloride may be in combination with
one or more cations, calcium, magnesium, iron and sodium. Chlorides of these
salts are present in water because of their high solubility in water.
Excessive
chloride in water causes corrosion. It also impart taste to water. Chlorides
are prominent in crevice corrosion and pitting.
Sulphates: Most
raw water contains sulphates due to leaching and erosion of sulphate minerals
and oxidation of sulphides. They can also be due to industrial waste discharge
and farm drainage.
Sulphate also occur due to aerobic oxidation of organic
matter. Sulphate in water causes corrosion and scaling in boiler. Sulphate are
normally found as calcium, magnesium and sodium salt. Sulphate are also
aggressive to concretes.
Silica:Most
natural water contain silica up to 100 ppm. Silica is an oxide
of silicon which is a major constituents of igneous and meta morphic rocks, of
clay minerals.Silica can exist in
various form as simple silicates or as a complex polymeric material. Colloidal
silica is rarely present in Bore well water but is commonly present in surface
water.
Silica can cause deposits on boiler, cooling tower,
turbines ,pipe lines etc
2-Undissolved non ionic impurities:
These include
- Colour
- Taste
and odour
- Turbidity
or suspended solids.
- Oil.
- Organic
matter
- Colloidal
silica
Colour:
is normally expressed in Hazen units and is caused by the presence of colloidal
suspension and aquatic growth. It is also caused by dissolved organic substance
due to decomposition of vegetation. Some water may have colour due to presence
of iron.
Turbidity
is caused by the presence of suspended solids in water. It is a measure
of total weight of dry solids present whereas turbidity is an optical effect.
The suspended impurities include clay, sand, algae and precipitated iron.
Taste and odor: Organics
and some inorganic chemicals present in the water contribute taste and odour.
These chemicals can originate from municipal or industrial waste or from
natural resources such as decomposition of vegetable matter.
Organic impurities: Organic impurities can be
due to vegetable decomposition. It is also due to organic matter which comes
from animal and human fecal matter or it’s degradation products, industrial
waste and agriculture pesticides and herbicides.
3-Gaseous impurities:
Gaseous impurities mainly include
- Carbon
di-oxide,
- Oxygen,
- Hydrogen
sulphide,
- Ammonia
- Chlorine.
Carbon di-oxide: Surface water contains small amount of free CO2 but
well water may contain more than 100 ppm as Carbon-di-oxide is found in waters coming in contact with decaying organic matter or carbonaceous material.
Depending
on the pH of water carbon di- oxide may be present either as free carbonic acid (dissolved
CO2 gas) or in a semi carbide form (as
bicarbonates) or in a combined form (as carbonates), free CO2
depresses the pH and thus accelerates corrosion.
Oxygen:
is practically absent in natural water, but is usually present in surface waters
rather in high concentration. Oxygen is corrosive to metals but its absence in natural waters could produce other
obnoxious gases such as methane H2S
etc.Dissolved
oxygen is highly corrosive and should be removed either chemically or
mechanically especially in Boiler feed water.
Hydrogen
sulphide: is found in some well waters in areas where the soil contains certain
types of organic matter, decomposing under anaerobic conditions .H2S
creates unpleasant odour to water, promoters metal corrosion and causes clogging of pipes.
Ammonia:
is due to industrial and agricultural pollution. It
is corrosive to copper and brass at pH 9.0. Ammonia can be removed by various methods-Deaeration,
chlorination or by hydrogen cation exchange if in ionic form.
In order to safeguard the Boiler, heat exchangers & pipe lines, it is utmost important to carryout the water treatment process to get required quality of water as feed water,cooling water etc.
Water treatment is divided into
Internal treatment: This treatment is done in Boiler drums, cooling water etc
External water treatment : It is carried out out side the unit (Boiler, Cooling water)
External water treatment process includes following sequential sub processes & systems
- Sedimentation: Normally happens in water reservoirs
- Clarification: Removes turbidity , kills microorganisms
- Filtration: Involves multi grade filters to remove turbidity
- Ultra-filtration: Process of bringing down the water turbidity to <0.2 NTU
- Reverse osmosis process: Removes reactive silica & conductivity
- De-mineralisation: Removes Cation & Anion impurities present in water