Wastewater treatment is a process used to remove
contaminants
Contamination is the presence of a constituent, impurity, or some other undesirable element that spoils, corrupts, infects, makes unfit, or makes inferior a material, physical body, natural environment, workplace, etc.
Types of contamination
...
from
wastewater and convert it into an
effluent that can be returned to the
water cycle
The water cycle, also known as the hydrologic cycle or the hydrological cycle, is a biogeochemical cycle that describes the continuous movement of water on, above and below the surface of the Earth. The mass of water on Earth remains fairly co ...
. Once returned to the water cycle, the effluent creates an acceptable impact on the environment or is reused for various purposes (called
water reclamation). The treatment process takes place in a wastewater treatment plant. There are several kinds of wastewater which are treated at the appropriate type of wastewater treatment plant. For domestic wastewater (also called municipal wastewater or
sewage), the treatment plant is called a
sewage treatment plant
Sewage treatment (or domestic wastewater treatment, municipal wastewater treatment) is a type of wastewater treatment which aims to remove contaminants from sewage to produce an effluent that is suitable for discharge to the surrounding en ...
. For industrial wastewater, treatment either takes place in a separate
industrial wastewater treatment plant, or in a sewage treatment plant (usually after some form of pre-treatment). Further types of wastewater treatment plants include
agricultural wastewater treatment plants and
leachate treatment plants.
Processes commonly used in wastewater treatment include phase separation (such as sedimentation), biological and chemical processes (such as oxidation) or polishing. The main by-product from wastewater treatment plants is a type of sludge that is usually treated in the same or another wastewater treatment plant.
Biogas can be another by-product if anaerobic treatment processes are used. Treated wastewater can be reused as
reclaimed water. The main purpose of wastewater treatment is for the treated wastewater to be able to be disposed or reused safely. However, before it is treated, the options for disposal or reuse must be considered so the correct treatment process is used on the wastewater.
The term "wastewater treatment" is often used to mean "sewage treatment".
Types of treatment plants
Wastewater treatment plants may be distinguished by the type of wastewater to be treated. There are numerous processes that can be used to treat wastewater depending on the type and extent of contamination. The treatment steps include physical, chemical and biological treatment processes.
Types of wastewater treatment plants include:
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Sewage treatment plants
Sewage treatment (or domestic wastewater treatment, municipal wastewater treatment) is a type of wastewater treatment which aims to remove contaminants from sewage to produce an effluent that is suitable for discharge to the surrounding en ...
*
Industrial wastewater treatment plants
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Agricultural wastewater treatment plants
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Leachate treatment plants
Sewage treatment plants
Industrial wastewater treatment plants
Agricultural wastewater treatment plants
Leachate treatment plants
Leachate treatment plants are used to treat leachate from
landfills. Treatment options include: biological treatment, mechanical treatment by
ultrafiltration, treatment with
active carbon filters, electrochemical treatment including
electrocoagulation by various proprietary technologies and reverse osmosis membrane filtration using disc tube module technology.
Unit processes
The unit processes involved in wastewater treatment include physical processes such as settlement or flotation and biological processes such oxidation or anaerobic treatment. Some wastewaters require specialized treatment methods. At the simplest level, treatment of most wastewaters is carried out through separation of
solid
Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structur ...
s from
liquids, usually by
sedimentation. By progressively converting dissolved material into solids, usually a biological floc or biofilm, which is then settled out or separated, an effluent stream of increasing purity is produced.
[.]
Phase separation
Phase separation transfers impurities into a
non-aqueous phase. Phase separation may occur at intermediate points in a treatment sequence to remove solids generated during
oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...
or polishing. Grease and oil may be recovered for fuel or
saponification. Solids often require
dewatering of
sludge in a
wastewater treatment plant. Disposal options for dried solids vary with the type and concentration of impurities removed from water.
Sedimentation
Solid
Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structur ...
s such as
stones, grit, and sand may be removed from wastewater by
gravity when
density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...
differences are sufficient to overcome dispersion by
turbulence. This is typically achieved using a grit channel designed to produce an optimum flow rate that allows grit to settle and other less-dense solids to be carried forward to the next treatment stage.
Gravity separation of solids is the primary treatment of
sewage, where the unit process is called "primary settling tanks" or "primary sedimentation tanks." It is also widely used for the treatment of other types of wastewater. Solids that are denser than water will accumulate at the bottom of quiescent
settling basin
A settling basin, settling pond or decant pond is an earthen or concrete structure using sedimentation to remove settleable matter and turbidity from wastewater. The basins are used to control water pollution in diverse industries such as agri ...
s. More complex
clarifier
Clarifiers are settling tanks built with mechanical means for continuous removal of solids being deposited by sedimentation. A clarifier is generally used to remove solid particulates or suspended solids from liquid for clarification and/or thi ...
s also have skimmers to simultaneously remove floating grease such as soap scum and solids such as feathers, wood chips, or
condoms. Containers like the
API oil-water separator are specifically designed to separate non-polar liquids.
Biological and chemical processes
Oxidation
Oxidation reduces the
biochemical oxygen demand of wastewater, and may reduce the toxicity of some impurities.
Secondary treatment converts organic compounds into
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
,
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
, and
biosolids
Biosolids are solid organic matter recovered from a sewage treatment process and used as fertilizer. In the past, it was common for farmers to use animal manure to improve their soil fertility. In the 1920s, the farming community began also to use ...
through oxidation and reduction reactions. Chemical oxidation is widely used for disinfection.
=Biochemical oxidation (secondary treatment)
=
=Chemical oxidation
=
Advanced oxidation process
Advanced oxidation processes (AOPs), in a broad sense, are a set of chemical treatment procedures designed to remove organic (and sometimes inorganic) materials in water and wastewater by oxidation through reactions with hydroxyl radicals (·OH) ...
es are used to remove some
persistent organic pollutants and concentrations remaining after biochemical oxidation.
Disinfection by chemical oxidation kills
bacteria and microbial
pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a g ...
s by adding
hydroxyl radicals such as
ozone
Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the l ...
,
chlorine or hypochlorite to wastewater.
These hydroxyl radical then break down complex compounds in the organic pollutants into simple compounds such as water, carbon dioxide, and
salts.
Anaerobic treatment
Anaerobic wastewater treatment processes (for example
UASB
Upflow anaerobic sludge blanket (UASB) technology, normally referred to as UASB reactor, is a form of anaerobic digester that is used for wastewater treatment.
The UASB reactor is a methanogenic (methane-producing) digester that evolved from the ...
,
EGSB
An expanded granular sludge bed (EGSB) reactor is a variant of the upflow anaerobic sludge blanket digestion (UASB) concept for anaerobic wastewater treatment. The distinguishing feature is that a faster rate of upward-flow velocity is designed f ...
) are also widely applied in the treatment of industrial wastewaters and biological sludge.
Polishing
Polishing refers to treatments made in further advanced treatment steps after the above methods (also called "fourth stage" treatment). These treatments may also be used independently for some industrial wastewater.
Chemical reduction or
pH adjustment minimizes chemical reactivity of wastewater following chemical oxidation.
Carbon filtering
Carbon filtering is a method of filtering that uses a bed of activated carbon to remove impurities from a fluid using adsorption.
Mechanism
Carbon filtering works by adsorption, in which pollutants in the fluid to be treated are trapped inside the ...
removes remaining contaminants and impurities by chemical absorption onto activated carbon.
Filtration through sand (calcium carbonate) or fabric filters is the most common method used in municipal wastewater treatment.
See also
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List of largest wastewater treatment plants
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List of wastewater treatment technologies
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Water treatment
References
{{Authority control
Sanitation
Water pollution
Environmental engineering