Purification of Water

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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 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 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 specialised 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 will be required.

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 disposed of correctly in certified safe landfills.

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