Sewage, also written sewerage, is the word used to describe any substance that is disposed of into the sewage system. This includes any substance that goes down the drain or is flushed down a toilet. The largest percentage of sewage is water (99%). There are different classifications of sewage domestic, commercial, industrial, agricultural and surface run off. All these types of sewage are channeled and collected in a sewage treatment facility where it is either treated or disposed of.
The Treatment of Sewage
The simplest way that sewage is treated is in sewage lagoons. Treatment plants will have a number of large concrete holding tanks. The wastewater is channeled into each holding tank, which resemble very large swimming pools, until it has reached capacity. Once a tank has reached capacity the sewage will be channeled to the next tank. The full tanks are left to age or mature. During this process any lumps or large pieces of sewage will settle to the bottom of the holding tank and the bacteria that are naturally part of sewage will break it down into gray water. This treated sewage is then released into specific waterways such as rivers, lakes and oceans.
Most cities and towns choose to use a more complex process to ensure that the harmful bacteria contained in sewage are almost completely removed before returning it to waterways. In these sewage plants the sewage will undergo a three part process.
In the preliminary stage sewage is collected in large tanks. The large lumps settle to the bottom whilst the lighter material floats to the surface. Fine mesh screens are passed through the sewage to remove items such as paper, plastics and other large objects. This material is washed off and incinerated or taken to the local waste management facility. The sewage is also screened to remove grit and other smaller solids that many not have been picked up by the first screening.
The sewage is then pumped into sedimentation tanks for the first of the three stages. The sludge that collects on the top of the sewage is skimmed off and sent to other part of the plant or a another plant for treatment.
Next the sewage undergoes aerobic biological treatment. This is where it comes into contact with micro-organisms which remove and oxidize most of the remaining organic pollutants. In smaller plants the sewage is run through a bed packed with graded mineral materials. Larger plants will mix and aerate the sewage for several hours to allow the micro-organisms to do their work. Larger plants can also remove the nitrogen and phosphorus to further reduce the environmental impact. This treated sewage is then sent to secondary sedimentation tanks.
Some plants will release the water after the secondary process, but many larger plants complete a final treatment stage in which the sewage is further filtered to remove more of the organic components, odors, viruses and very fine particulates. Tertiary treatment can be done in a number of different ways that can include sand filters, mechanical filtration or by passing the effluent through a constructed wetland such as a reed bed or grass plot. Some plants are also used to disinfect the sewage, now called effluent, before releasing it into the selected waterway.