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Other than the methods already evoked, there are several ways of purifying
water and these may be used under specific circumstances. In particular, these
methods can be used for treating waste water or water polluted with waste water,
especially from industrial sources but also from natural sources. This may allow
such water to be used for other purposes.
It must be realised that most pollutants can never be eliminated. Some may be
converted to a less polluting form. For example, biodegrading a detergent may
result in the production of less polluted water, but at the cost of oxidising
the carbon atoms, producing carbon dioxide (also methane if the degradation is
partially anaerobic). In particular,
heavy metal salts are eliminated only by ion exchange or electrowinning,
where they are converted back to their metallic form. Otherwise, all the other
processes for their elimination or reduction as a pollutant merely shift them
from one form to another, perhaps into a more concentrated waste stream.
Filtration of Particulate Matter
Filtration allows any particulate matter to be mechanically or physically
removed from water.
Mechanical filtration is the most familiar form. On a medium to large scale,
it may be done by passing the water through a clean sand bed. On a smaller
scale, various forms of wire mesh, cloth filters and treated paper filters are
all used. In time, all these filters tend to clog and need treatment or
replacement to ensure adequate throughput. Depending on the material retained by
a filter, the disposal of this material may present a problem. Correct disposal
is essential.
Microfiltration consists in passing the water through a membrane with a pore
size in the sub-micron range (less than one micrometre). This will retain
contaminants which have passed through a conventional mechanical filtration
stage, such as suspensions and sols. It will not remove substances at a
molecular level.
Ultrafiltration is similar to microfiltration, except that the membrane has a
much smaller pore size which will retain large organic molecules, particularly
natural and synthetic polymers derived from some types of detergent or cosmetic
products. Although it may superficially resemble reverse osmosis,
ultrafiltration is not the same as it is still a mechanical process.
Carbon filtration
Carbon filtration consists of passing the water through a column containing a
special form of activated carbon which can retain some non- or
poorly-biodegradable organic pollutants such as oils and solvents, by
adsorption. The type of carbon must be matched to the type of pollutant. This is
a very expensive but highly efficient method on condition that it is correctly
applied. Carbon filtration may also be used to remove some natural pollutants.
The carbon is usually derived from lampblack, bone charcoal or coconut shell
charcoal, but it is essential that the carbon type and activation treatment be
matched to the pollutant being adsorbed.
Softening
Water softening consists of the replacement of calcium and magnesium salts in
hard water by sodium salts. It is a specialised form of ion exchange which is
particularly economical to operate, by passing the water through a bed of
special mineral matter which allows the calcium and magnesium to be exchanged
for sodium, derived from common salt. As such, it is not a true purification.
Most domestic dishwashing machines contain a small water softening unit. This
prevents the formation of a white deposit appearing on glassware. Softened water
is useful to reduce the production of soap scum where the water is hard, but is
unpleasant to drink.
Ion Exchange
Ion exchange is a very efficient but expensive method of purifying water
containing salts, consisting of passing the water through one or more columns
containing a special synthetic resin. The resin may be regenerated, sometimes in
situ, sometimes in a central regeneration facility. Ion exchange produces water
of a very high purity and low conductivity, but it does not eliminate any
organic matter. It is frequently used in industry for applications where a high
purity water with a low electrical conductivity is required and it may also be
used as a method of purifying waste water containing metal pollutants that are
difficult to remove by other methods.
Polymer Filtration
Polymer filtration is a specialised method, recently developed, for the
removal of specific ions from waste water. Special polymers fix themselves on
specific heavy metal ions. These can then be removed by ultrafiltration. The
polymers can be easily regenerated, releasing the ions in a concentrated stream
which may be recycled back to the process using them. The polymers themselves
are also recycled, making this process very economical. If there is a mixture of
heavy metal ions, then more than one polymer may be required. Because of the
specificity of the process, it may also be used to extracting passively
high-value metals from sea water; much research has been done to extract
uranium, for example.
Electrowinning
Electrowinning consists of passing water containing heavy metal salts through
a cell fitted with electrodes which are connected to a d.c. supply. The heavy
metals are precipitated to the bottom of the cell or adhere to one of the
electrodes where they may be mechanically removed. The advantage of this system
is that the metal may be easily recycled, particularly important if the metals
are precious or semi- precious. With base metals, the value of the recovered
metal will never recuperate the running costs. Nevertheless, if it allows the
waste water to conform to national regulations, the running costs are relatively
small, compared to some other methods of eliminating metals from waste water.
This method may be particularly useful for removing metals from the waste
streams of other purification methods, such as reverse osmosis or the
regeneration liquids from ion exchange.
Precipitation
Many polluting metals, if the concentration is reasonably high, may be
removed from waste water by chemically converting the salts to insoluble
hydroxides or ferrites, allowing them to precipitate and filtering them out of
the water. In large plants, this process produces large amounts of sludge which
constitutes a hazardous waste and must be recycled back into the metals or
possibly disposed of correctly in certified chemically safe landfills.
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