Wastewater Treatment

Wastewater includes the sewage-bearing water that is flushed down toilets as well as the water used to wash dishes and for bathing. Processing plants use water to wash raw material and in other stages of the wastewater treatment production process. The treatment of water that exits households, processing plants and other institutions is a standard, even mandated, practice in many countries around the world. The purpose of the treatment if to remove compounds and microorganisms that could pollute the water to which the wastewater is discharged. Particularly with respect to microorganisms, the sewage entering a treatment plant contains extremely high numbers of bacteria, viruses, and protozoa that can cause disease if present in drinking water. Wastewater treatment lowers the numbers of such disease-causing microbes to levels that are deemed to be acceptable from a health standpoint. As well, organic matter, solids, and other pollutants are removed.

Wastewater treatment is typically a multi-stage process. Typically, the first step is known as the preliminary treatment. This step removes or grinds up large material that would otherwise clog up the tanks and equipment further on in the treatment process. Large matter can be retained by screens or ground up by passage through a grinder. Examples of items that are removed at this stage are rags, sand, plastic objects, and sticks.

The next step is known as primary treatment. The wastewater is held for a period of time in a tank. Solids in the water settle out while grease, which does not mix with water, floats to the surface. Skimmers can pass along the top and bottom of the holding tank to remove the solids and the grease. The clarified water passes to the next treatment stage, which is known as secondary treatment.

During secondary treatment, the action of microorganisms comes into play. There are three versions of secondary treatment. One version, which was developed in the mid-nineteenth century, is called the fixed film system. The fixed film in such a system is a film of microorganisms that has developed on a support such as rocks, sand, or plastic. If the film is in the form of a sheet, the wastewater can be overlaid on the fixed film. The domestic septic system represents such a type of fixed film. Alternatively, the sheets can be positioned on a rotating arm, which can slowly sweep the microbial films through the tank of wastewater. The microorganisms are able to extract organic and inorganic material from the wastewater to use as nutrients for growth and reproduction. As the microbial film thickens and matures, the metabolic activity of the film increases. In this way, much of the organic and inorganic load in the wastewater can be removed.

Another version of secondary treatment is called the suspended film. Instead of being fixed on a support, microorganisms are suspended in the wastewater. As the microbes acquire nutrients and grow, they form aggregates that settle out. The settled material is referred to as sludge. The sludge can be scrapped up and removed. As well, some of the sludge is added back to the wastewater. This is analogous to inoculating growth media with microorganisms. The microbes in the sludge now have a source of nutrients to support more growth, which further depletes the wastewater of the organic waste. This cycle can be repeated a number of times on the same volume of water.

Sludge can be digested and the methane that has been formed by bacterial fermentation can be collected. Burning of the methane can be used to produce electricity. The sludge can also be dried and processed for use as compost.

A third version of secondary treatment utilizes a specially constructed lagoon. Wastewater is added to a lagoon and the sewage is naturally degraded over the course of a few months. The algae and bacteria in the lagoon consume nutrients such as phosphorus and nitrogen. Bacterial activity produces carbon dioxide. Algae can utilize this gas, and the resulting algal activity produces oxygen that fuels bacterial activity. A cycle of microbiological activity is established.

Bacteria and other microorganisms are removed from the wastewater during the last treatment step. Basically, the final treatment involves the addition of disinfectants, such as chlorine compounds or ozone, to the water, passage of the water past ultraviolet lamps, or passage of the water under pressure through membranes whose very small pore size impedes the passage of the microbes. In the case of ultraviolet irradiation, the wavelength of the lamplight is lethally disruptive to the genetic material of the microorganisms. In the case of disinfectants, neutralization of the high concentration of the chemical might be necessary prior to discharge of the treated water to a river, stream, lake, or other body of water. For example, chlorinated water can be treated with sulfur dioxide.

Chlorination remains the standard method for the final treatment of wastewater. However, the use of the other systems is becoming more popular. Ozone treatment is popular in Europe, and membrane-based or ultraviolet treatments are increasingly used as a supplement to chlorination.

Within the past several decades, the use of sequential treatments that rely on the presence of living material such as plants to treat wastewater by filtration or metabolic use of the pollutants has become more popular. These systems have been popularly dubbed "living machines." Restoration of wastewater to near drinking water quality is possible.

Wastewater treatment is usually subject to local and national standards of operational performance and quality in order to ensure that the treated water is of sufficient quality so as to pose no threat to aquatic life or settlements downstream that draw the water for drinking.

See also Biodegradable substances; Biofilm formation and dynamic behavior; Disinfection and disinfectants; Disposal of infectious microorganisms; Economic uses and benefits of microorganisms; Growth and growth media; Public health, current issues; Radiation mutagenesis; Water pollution and purification; Water quality