Four Processes Commonly Used in the Treatment of Waste Water
Many of the materials we routinely dispose of after use have the potential to harm the environment unless suitably treated, and water is no exception. Because it is a finite natural resource, it must not only be rendered safe before it is disposed of, but also rendered suitable for re-use as the earth’s population now consume it faster than nature can recycle it unassisted. Industrial recycling of manufactured items made from materials, such as plastics, glass, metal, paper, and cardboard is a relatively recent, but expanding, initiative. By contrast, the use of filtration by the first waste water treatment plant in the world was introduced in London during the early nineteenth century and, over the decades, was adopted across the UK, spreading later to Europe and the US..
Since then, additional purification methods have been developed and refined, but the more basic processes still in use for the reclamation of this vital commodity remain essentially unchanged and fall into one of four main categories. As exemplified by the London plant’s pioneering use of filtration, one of those categories refers to the various physical processes that are commonly applied. Together, these make up the primary treatment stage.
For example, large items of debris, such as branches and discarded refuse can be trapped by a coarse mesh in the process known as screening, while skimming is employed at waste water treatment plants to remove any small objects or scum that may be floating on the surface. Further, physical processing takes place in sedimentation tanks where suspended solids are left to settle out under the action of gravity, thus allowing the clear fluid to be separated in preparation for passage through a sand filter. Physical treatments, however, are unable to remove all of the organic matter present.
To deal with materials, such as soap residues, food, edible oils, and human waste, these must first be broken down into simpler compounds. To achieve this, the most effective options are biological in nature. This secondary stage of the waste water treatment process relies on the natural metabolic action of the microorganisms present to break down the organic matter. Depending upon the type of organism, this can be achieved by means of either aerobic or anaerobic reactions. To promote aerobic activity, the effluent must first be oxygenated. The oxygen is then consumed in the decomposition process, forming carbon dioxide to be utilised by green plants. Conversely, anaerobic activity is only possible in the absence of oxygen, and a specific temperature is required to promote a fermentation reaction.
While the primary and secondary stages in the treatment of waste water are able to remove the bulk of the solids present, trace quantities of certain soluble nutrients, such as compounds of phosphorus and nitrogen may still remain in solution when passed to the tertiary stage of the purification process. This stage involves the addition of chemical agents.
Though by now, essentially free of inanimate contaminants, the product still contains bacteria which, if left, could multiply and thus expose anyone who drinks it to the risk of enteric infections. In this stage of the waste water treatment process, the objective is to prevent the proliferation of bacteria. In most cases, this is achieved by the addition of measured doses of chlorine. As a powerful oxidising agent and solution, it is able to penetrate the surfaces of bacteria, viruses, and protozoa and kill them. After adjusting the pH to neutral, the water is ready to dispense to consumers. Residual traces of chlorine serve to prevent the risk of bacterial contamination that might occur during the distribution process. Ozone is sometimes used as an alternative sterilising agent.
One of the by-products of waste water treatment plants is sludge, a mixture composed mainly of solid waste with sufficient residual moisture to provide a consistency similar to that of mud. After further treatment, the solid content of the sludge has the potential to be used as a natural organic fertilizer. However, before the solids can be processed, as much residual water as possible must first be removed. This can be achieved with the use of a suitable solid-liquid separation process, such as centrifugation. A similar dewatering process is often used for the treatment of industrial waste water, prior to discharging it into the environment.
Water is a vital, but finite resource. Conserving it is not only important to sustain life on our planet, but to the long-term future of the earth as a whole.