The treatment of wastewater has become increasingly important since the onset of the industrial revolution, but at no time since has the practice been as crucial as it is today. It is essential, not only as an environmental conservation measure, but also to ensure that there is a sufficient reserve of this vital commodity to meet the ever-increasing demands of both industrial and domestic consumers. While both types of consumer are jointly responsible for its denaturation, only industrial consumers are bound by the law to ensure that, when it is disposed of back into the environment, its composition complies with the standards of safety set by the government, and so effluents are treated on site.
In the case of that which is produced in our homes, however, the responsibility for wastewater treatment is often left entirely to a local municipality. The processing is undertaken at large-scale purification plants that make use of a series of processes applied in sequence. As before, the purpose of the processing is to reduce the level of contamination sufficiently so that the treated liquid can be discharged safely into the surrounding environment. Rather than resulting from some industrial process that may have introduced harmful chemicals such as mercury and lead from mining operations, domestic sources tend to become contaminated purely as the result of bathing, use of the toilet, washing clothes, and rainwater runoff, so toxicity is not a major concern.
While there may be a degree of overlap in some cases, wastewater treatment plants can be broadly divided into two main types. In those used mainly for processing the effluents produced by the nation’s factories, manufacturing plants, and major industries, where the contaminants are invariably of an inorganic nature, the cleansing processes employed are primarily of a chemical and/or physical nature. By contrast, the methodology applied to the purification of effluent produced by households and offices, with its predominately organic content, is largely biological and so tends to leverage the properties of biological matter and bacteria as the means to break down the contaminants that are typically present.
The processes involved are best explained by describing those applied to the purification of domestic effluent. In these large-scale plants, the first step is to collect the wastewater prior to treatment. In this case, it is delivered to the plant via underground drainage systems consisting of pipes or tracks, and the next step is to deal with its unpleasant odours. This is achieved by the addition of chemicals designed to neutralise them. After the foul smells, the next concern is the physical detritus that is common in domestic effluents and which could cause damage to equipment used for subsequent processing. Known as screening, it is a coarse filtration procedure that removes items such as disposable nappies, cotton buds, sanitary towels and the coarse grit that is often washed into the sewage system as a result of heavy rainfall.
The next step in wastewater treatment is to separate the macrobiotic material, which is achieved by allowing it to sediment in large tanks. The sludge of solid material that is left floating on top is then scraped into other tanks to be treated further. The partially clarified liquid is also pumped off for further processing in which the action of aerobic microorganisms break down organic material. The resulting sediment may be treated in digesters to produce methane that can be used to power boilers. At this stage, the processed wastewater will still require further treatment to eliminate any residual bacteria that might present the risk of a water-borne disease. This requires the addition of a disinfectant and normally involves a mixture of chlorine and sodium hypochlorite, the same chemical used in swimming pools. To be effective, exposure to the disinfectant mixture should continue for at least 25 minutes. Thereafter, the treated product can either be safely discharged via local waterways back into the environment or subjected to additional processing to create a potable supply for direct consumption.
This, however, is not the whole story. The processing plants operate on a “waste not, want not” philosophy in which even the sludge serves a purpose. After concentration in thickening tanks and separation of residual liquid that is returned for processing, the final step in wastewater treatment is to purify the sludge for agricultural purposes.