While there was a time when the human race drank freely from rivers and streams or from man-made wells that tapped into natural underground aquifers, in most parts of the world, such practices are no longer considered safe. Where once nature’s own water cycle sufficed to remove impurities and provide all the potable liquid we needed to slake our thirst, a population explosion and widespread industrialisation have created the need for artificial water treatments to augment the inadequate attempts of Mother Nature to cope with the vast levels of consumption and subsequent pollution that are the legacy of life in the 21st century.
The more of this precious liquid we consume, whether for drinking, bathing, flushing toilets, or innumerable commercial and industrial purposes, the greater the need to purify it and reuse it. Whereas returning untreated wastewater to the environment might have been less of a problem in ancient times, on the scale it is produced today, water treatment has become vital whether for safe disposal and long-term recycling or immediate reuse. To achieve this, there is a variety of options and which of these is the most appropriate is determined by the condition of the wastewater and the tasks for which the treated liquid is to be used. The various processes used for this purpose can be divided into three main categories. They are either physical, biological or chemical in nature. Of these, it is the physical options that were used first and which continue to be the most widely employed. This type of water treatment is aimed at the removal of solid contaminants and employs one of two simple principles with their origins in nature. The first of these is the process known as sedimentation.
In this case, solid particles that have become suspended in wastewater and are sufficiently dense are left to settle out in tanks under the action of gravity. This allows the clear liquid remaining above the layer of sediment to be removed for further processing. As a preliminary step, sedimentation can be highly effective with heavily contaminated effluent but a more efficient form of water treatment is necessary to remove finer, less dense particles that remain in suspension. This process is mechanical filtration and is achieved by passing the effluent through a layer of sand, much like rainwater percolates through the soil. Particulate matter is captured by the sand either as the result of direct collisions, Van der Waal forces, or surface-charge attraction.
In some circumstances, it may be necessary to add special chemical agents known as coagulants or flocculants to the effluent during the filtration stage of water treatment. These substances act to make the smaller, less-dense particles clump together so that they will be more easily trapped in the filter bed. However, while sand filtration may be able to remove almost all the suspended solids, including bacteria, solids in solution present more of a problem. Where it may be necessary to remove these, the process known as reverse osmosis (RO) is widely used. RO is a form of ultra-fine filtration in which the filter medium is a synthetic membrane whose microscopic pores allow only water molecules to pass but nothing larger. The process is conducted under pressure and is often used as a final “polishing” stage of water treatment.
During the processing of sewage, large quantities of sludge are produced and even this can have its uses when suitably treated. It is at this point where biology takes over from physics. The organic matter in sludge and wastewater from industrial and municipal sources can undergo bacterial digestion both in the absence of air (anaerobic digestion) and in its presence (aerobic digestion). In each case, the goal of these water-treatment processes is to break down complex organic molecules, including pathogenic microorganisms, into smaller, harmless components. The process can be used to generate methane gas and heat while the treated sludge serves as rich, natural compost for agricultural use. In the case of a municipal water plant, there is one more requirement before its potable end-product can be delivered to consumers. The oxidative action of chlorine serves to kill residual microorganisms and inhibit any further growth within the distribution pipelines.
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