Sewage treatment is the reduction of contamination in a water pollution effluent, usually considered for the class of liquid wastes comprised by municipal sewage and sometimes admixed by urban stormwater runoff. In most cases the purpose of sewage treatment is to produce an effluent stream that is suitable for discharge into natural receiving waters, such that the resultant discharge is not deleterious to humans or to the natural environment. Depending on the financial resources available, the process of sewage treatment may include one or more of the several successive steps: (a) Pre-treatment; (b) Primary treatment; (c) Secondary treatment; (d) Tertiary treatment. There is a great variation in percentage of sewage treated by world region, with developed countries such as the USA, New Zealand and Great Britain treating virtually all of their municipal sewage, while lesser developed countries such as Pakistan, China and Venezuela treating a relatively small fraction of the total municipal sewage. Inadequate sewage treatment is one of the chief causes of waterborne disease, with particlarly acute impacts on imfant mortality in developing countries. An alternative to the construction of expensive sewage treatment plants is the use of natural systems such as wetlands to perform one or more of the treatment steps.
The nature of sewage is often limited to the concept of household or sanitary sewage, which is considered the combination of kitchen and bathroom liquid wastes. However, the more generalized concept of municipal sewage includes the collection of liquid wastes discharged from commercial and industrial land uses. Commercial uses are usually not dissimilar to domestic sanitary discharge, except for restaurants, where a disproportionate quantitiy of food wastes and kitchen grease can be discharged. Industrial sources may have any number of exotic chemicals in their waste streams, although large industrial facilities often have their own self contained and specialized treatment plants. Depending on the collection systems used for aggregating the entirety of municipal wastes, often some of the urban stormwater runoff becomes captured and combined with the sanitary and commercial sources. While such a combined flow often reduces the concentration of some pollutants, it can aggravate the treatment process by ly expanding the volume of wastewater and also by introducing new pollutants into the mix, such as herbicides and pesticides.
The chief pollutants in sewage include: suspended solids, nutrients (especially nitrogen compounds, bacteria, petroleum hydrcarbons. A number of pathogenic organisms may be present besides bacteria, including viruses and waterborne parasites; in many cases, parasites and disease organisms may be present as a oocyst or larval life stage of the ultimate or adult that can develop in a human or animal host. Another important concept of sewage is called biological oxygen demand, meaning the composite oxygen within the water that would be used up by the biological agents polluting the waste stream.
Pre-treatment can be considered an inexpensive coarse separation of clearly extraneous elements from the wastewater stream. Screening and grit removal are the two chief features of a pre-treatment system. Screens are typically of three distinct mesh sizes. The largest grate size, about five centimeters, is intended to remove large pieces of cloth, wood or other foreign objects. For larger plants, these grates are usuallly mechanically operating bar rakes that apply a continuous motion to separate and remove high volumes of foreign objects. A medium size mesh of about .5 to 3. 0 s used to separate smaller floating objects that elude coarse filtration, while the finest mesh of about 1.5 to 3.0 millimeters finishes the screening process. In some of the smaller treatment systems only one or two of these screens may be applied. Grit removal is aimed at gravity separation of sand and gravel elements of the wastewater stream, most of which content arises from the combined collection system that includes street runoff. The grit is normally removed by travel along a gently sloping V shaped channel, where there is gravitational separation of the grit from the balance of the effluent. The chief purpose of pre-treatment is protection of the hardware in the succeeding treatment steps. Virtually no chemical or biological improvement of the waste stream is provided by removal of these bulky inert objects. Another method of pretreatment is use of skimming devices or grease traps to separate floatables or naturally stratify8ng grease and oil.
Primary treatment employs large tanks where sewage has a sufficient residence time to allow settling of significant amounts of suspended solids. There is typically a slow continuous flow rate in and out of the settling (or clarifier) tank, but the average residence time of liquids is sufficient to accomplish considerable fallout of particulate matter. The material settling out at the tank bottom is called sludge, which may be anaerobically digested to produce methane with the residual solids disposed of in appropriate landfills. Additional grease and oil skimming may also be performed at this stage, which is purely a mechanical treatment. Approximately two thirds of the suspended solids may be removed in the primary treatment process, but only about one third of the biological oxygen demand is typicallly removed.
Secondary treatment consists of biological processes that can degrade a variety of organic compounds present in sewage. The majority of these processes utilize aerobic biological processes, requiring a substrate for the organic material and oxygen to abet the chemical transformations. In each of the methods, bacteria and protozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, organic short-chain carbon molecules) and bind some of the less soluble fractions into floc. Secondary treatment systems can be subdivided into two broad categories: fixed-film and suspended-growth. Fixed-film techniques include trickling filters and rotating biological contactors. Trickling filters provide for movement of primary treated sewage across rock beds with microbial media to allow conversion of organic material. Rotating biological contactors provide surfaces where the biomass grows on media, with the input sewage stream passing across this surface.
For suspended-growth systems, such as activated sludge, the biomass is thoroughly mixed with the sewage and can be operated in a more limited volume than fixed-film systems. Fixed-film systems are more adaptable to fluctuations in concentrations of biological load and can provide more effective removal of organic material and suspended solids. Often a secondary clarifier or settling tank is used as a finishing step to secondary treatment.
Tertiiary treatment is sometimes used to attain a higher quality of effluent prior to discharge into natural water systems. The chief techniques used in this final stage of treatment include: sand filtration, lagooning, removal of phosphorus and nitrogen, odor reduction and disinfection (using various methods including ozonation, UV, chlorination etc). Tertiary treatment is performed in a very small number of instances, where effluent quality is critical for some type of direct human use or to protect sensitive ecosystems; for example, tertiary treatment has been provided for much of the treated effluent released into Lake Tahoe in California and Nevada to inhibit excessive eutropication.
Sewage treatment capability varies substantially across the globe, with the USA, Canada, United Kingdom, Israel, New Zealand and Australia providing notable examples of a high level of service for installed sewage treatment. In the European Union, approximately two thirds of households have access to sewage treatment connections, with the Netherlands presenting a high point and Greece being one of the lower installed bases. In Africa the great majority of communities have no sewage treatment capability, and 60 percent of households even lack toilets. In Kenya, for example, approximately 17 percent of the communities have some form of sewage treatment, while in Sudan the number is far lower still. Regarding Asia, only about ten percent of the sewage outfalls reaching marine waters receives any form of sewage treatment; Japan has the highest extent of sewage treatment, with India, China, Pakistan and other countries far behind. In South and Central America, the picture is somewhat better than Africa and southern Asia, but there are many countries such as Venezuela and St. Lucia that lag consderably behind the region.
Benefits of sewage treatment
There are significant advantages to sewage treatment accruing both to humans and the natural environment. With regard to people, the most significant benefit is a major decrease in incidence of waterborne disease, since most effluent streams have the potential to enter drinking water supplies as well as dermal contact. Pathogens in untreated wastewater include bacteria, viruses and protozoa; helminthes (intestinal worms and worm-like parasites); it is important to note that not all pathogens are removed in secondary treatment, especially in the case of viruses and protozoa. There are also tangible improvements in the aesthetics of natural receiving waters. leading to more odor free and clearer waters for recreational and aesthetic enjoyment by people. With regard to favorable impacts to ecosystems as a whole, there are appreciable benefits to retaining the metabolic health of aquatic systems receiving effluent discharge. Removal of large quantities of suspended solids preserves the ability of sunlight to reach parts of the water column necessary for normal aquatic ecosystem productivity; furthermore, sewage treatment also prevents premature siltation of water channels and also protects downstream ecosystems, including marine coral reefs from sediment damage. Economic consequences of the lack of sewage treatment pose colossal losses; for example, in the year 2006, the loss of shellfish productivity cost fishermen an estimated $12 billion USD, due to lack of sewage treatment. Minimization of nutrient input to natural waters awoids eutrophic effects from potential algae blooms. Sewage treatment also removes materials that create biological oxygen demand, and thus preserve adequate oxygen in the receiving waters to support health of fish and other aquatic fauna.
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