stages of the wastewater treatment process in Detail

Wastewater treatment is a series of physical, chemical and biological processes, the purpose of which is focused on the purification of such...

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wastewater treatment

Wastewater treatment is a series of physical, chemical and biological processes, the purpose of which is focused on the purification of such water, before being discharged into its natural channel, being one of the main strategies for the care of the environment and the use of natural resources.

The wastewater is any kind of water resources, whose quality has been affected by anthropogenic activity of man. These waters are the result of the development of the daily activities of human subsistence, being able to contain from simple polluting residues to toxic polluting agents that can represent a danger of mortality for those who ingest or make inappropriate use of such water.

wastewater treatment

The reuse of wastewater is a very attractive approach for the protection of the environment, since it allows the use of natural resources for human development and prevents the indiscriminate waste of such a vital resource in human life.


The shortage of fresh water as a consequence of demographic growth and climate change, has led to the implementation of such a laudable ecological proposal in the economic and industrial development of life in society, with agriculture and aquaculture being those human subsistence activities that has bet on the reuse of wastewater with a low degree of contamination and / or the presence of toxicological elements as a water saving measure , considering that good management practices must be applied so that this measure produces a real positive impact on health public.

wastewater treatment process

The treatment of wastewater is of utmost importance, in the development of environmental strategies that safeguard and protect the existing water resources as well as the ecosystem that surrounds them, since the optimal purification of such waters is essential before being channeled back into the different reserves. natural hydrological conditions in the best possible conditions.

The discharge of wastewater into hydrological channels without the respective treatment can lead to the total destruction of aquifer and / or marine fauna and flora, due to the prevailing presence of chemicals and organic matter in them, causing severe damage to the environment. as well as the quality of life of the population, since it would not only mean the permanent intentional disabling of natural spaces for recreation, but also the loss of millions of liters of water that could have been destined for human consumption for the development of various economic and human subsistence activities.


In the 21st century with the imposing technological advances in wastewater sanitation , there are still corners of the global geography where such unorthodox and harmful practices are applied.

Wastewater treatment consists of a series of physical, chemical and biological processes, which aim to eliminate undesirable pollutants in effluent water for human use, which are carried out through treatment plants in hydraulic installations. and industrial.

This set of processes, methods and operations must be carried out under the supervision of the local authorities, who must ensure under any circumstances that these processes are developed with the greatest kindness towards the environment and in the most optimal way for the benefit of public health. , considering that these waters must have the highest quality, depending on the application in human development.

wastewater treatment

This process of treatment or purification of wastewater is divided into 4 stages such as preliminary or pre-treatment, first, secondary and tertiary, highlighting that the wastewater is transferred through the network of pipes, sewers and public drains to such facilities, where it is left to rest for 24 to 48 hours, before receiving the respective treatment. These stages are as follows:

Preliminary stage:

It is defined as preliminary because it is the “prelude” to the purification treatment that the wastewater will receive because it fulfills the functions of measuring and regulating the flow of water that enters the plant as well as extracting large floating solids, sand and fat, highlighting that the elimination of these undesirable agents occurs through a filtration process, which is necessary for the normal development of this phase.

During this stage, the residual water is conditioned to facilitate said treatment, in order to preserve the installation from erosions and plugging.

Likewise, a pre-aeration can be carried out, which allows the elimination of volatile compounds present in the wastewater , which are characterized by being malodorous and increases the oxygen content in the water , which helps the decrease in the production of unpleasant odors in the following stages of wastewater treatment .

wastewater treatment

Primary stage:

The purpose of this stage is to eliminate suspended solids through a simple sedimentation process by gravity or assisted by chemical substances such as coagulants and flocculants.

The residual water is left deposited in large decanter ponds and remains retained for 1 to 2 hours, where chemical compounds such as iron salts, aluminum and flocculating polyelectrolytes will be added to complete the process, as well as produce precipitation of phosphorus, suspended solids. very fine or those in a colloid state up to 70%. This process is developed through the use of a hydraulic machine, based there on recognition as a mechanical treatment.

machine process

This phase is subdivided into several stages, which are the following:

  • Solids removal or screening:

It is carried out through screening, where large solids such as bottles, sticks, bags, balls, tires and other elements discarded in human action, are removed, avoiding problems of breakdowns in treatment plants because if they are not removed, they can cause clogging of pipes and severe damage to equipment.

wastewater treatment

  • Sand removal:

This phase, also known as maceration or scanning , includes a sand channel where the speed of the wastewater is carefully controlled , allowing the sands and stones of these to take up particles, even when most of the organic material remains with the flow, being used a sand collector for this purpose.

As in the solids removal phase, the sand and stones must be removed in time to prevent damage to the pumps and other equipment remaining in the treatment. In some cases, there are sand baths also known as sand classifiers, which, followed by a conveyor, transfer the sand to a container for disposal, the content of the sand collector being fed into the incinerator by a processing plant of mud, but usually such sand is sent to an embankment.

wastewater treatment

  • Research and Maceration:

The abrasive-free liquid is passed through rotating screens to remove floating material and large matter as well as small particles such as peas and corn. Such wastes are collected and may be returned to the sludge treatment plant or disposed of outside to fields or incineration.

Likewise, in maceration the solids are cut into small particles through the use of rotating knives mounted on a rotating cylinder, being used in plants that can process this waste into particles.

  •  Sedimentation:

It is carried out in rectangular or cylindrical tanks, where between 60% and 65% of the sedimentable solids and between 30% and 35% of the suspended solids in the wastewater are removed , being a flocculent type process, in which sludge is made up of organic particles.

Such tanks are called primary clarifiers or primary settling tanks, being large enough so that faecal solids can settle out and floating material like grease and plastics can rise to the surface, denaturing there.

wastewater treatment

Secondary stage:

The elimination of organic matter in solution and in a colloidal state by means of an oxidation process of a biological nature as well as the degradation of substances from the biological content of the residual water originated by human waste, are the primary objectives that arise during the performance of this stage. of the wastewater treatment process.

The secondary stage is defined as a natural biological process, where the microorganisms present in the wastewater participate and that develop in a reactor or aeration tank, not counting those that also develop to a lesser extent in the secondary decanter.

Aerobic and anaerobic biological and physical-chemical processes such as flocculation are part of this stage of the wastewater purification process , which reduce most of the biological oxygen demand as well as remove additional amounts of settled solids. Tale processes are developed in 6 phases and are as follows:

  • Roughing:

It consists of the retention of the coarse solids of the residual water through a screen, manual or self-cleaning, or a sieve, usually with a smaller pitch or mesh size. This operation produces a reduction in the pollutant load at the inlet, which allows the preservation of equipment such as pipes, pumps and valves, in contrast to the deposits and obstructions caused by solids, which can generally be very fibrous.

wastewater treatment

  • Activated sludge:

Active sludge plants or active sludge, as this aerobic process of suspended biomass is also called, use various mechanisms to make use of dissolved oxygen and promote the growth of biological organisms that substantially remove organic matter as well as trapping material particles.

This phase puts the residual water in contact with previously formed biological flocs, in which the organic matter is absorbed and where it is degraded by the bacteria present, maintaining a certain concentration of aerobic organisms, all this is carried out in an aerated pool or aeration tank.

wastewater treatment

  • Filter beds:

Drip filter beds are those in which wastewater is sprayed on the surface of a deep bed composed of coal, limestone or specially made of plastic media, which must contain high surfaces to support the biofilms that are formed.

Its use in the treatment of wastewater is developed in older plants or plants receiving variable loads, giving rise to a distribution of water through rotating perforated arms that radiate from a central pivot, such water content drips into the bed and is collected by the drains at the base, which also provide an air resource that infiltrates up the bed, maintaining an aerobic environment. Biological films of bacteria, protozoa and fungi are formed on the surface of the medium, reducing or eating the organic contents.

wastewater treatment

  • Rotating plates and spirals:

Slowly revolving plates or spirals are used for partial submergence in the water, creating a biotic floc that provides the required substrate.

  • Moving bed bioreactor:

MBBR, according to its acronym in English, assumes the addition of inert media in existing activated sludge vessels to provide active sites, with the intention of gathering biomass, maintaining a high density of the biomass population and increasing the efficiency of the system without the need to increase the concentration of mixed liquor of solids.

This technological process employs millions of carriers of the polyethylene biofilm, which works with a movement of mixed air within an aerated wash from wastewater treatment .

wastewater treatment

  • Biological aerated filters:

Bio anoxic filters combine filtration with biological carbon reduction, nitrification or denitrification, usually including a reactor filled with filter media. The purpose of this medium is the high support of the active biomass that binds to it and to the suspended solids in the filter.

Carbon reduction such as ammonia conversion occurs in an aerobic environment and is sometimes achieved in a single reactor, while nitrate conversion occurs in an anoxic manner.

  • Membrane biological reactors:

It is that system with a semi-permeable membrane barrier or in conjunction with a sludge process; This is composed of 2 unilaterally integrated parts, being on the one hand, the biological reactor responsible for biological purification and on the other hand, the physical separation of biomass and water through a membrane filtration system.

Its application in the wastewater treatment process is diverse, highlighting that this technology guarantees the removal of all suspended pollutants and dissolved solids.

wastewater treatment

It also allows the elimination of the chemical demand for colloidal oxygen, since by not crossing the membrane it has a longer contact time with the biomass.

Likewise, MBR systems of the high biomass concentrations used in the biological reactor as a consequence of the presence of a physical barrier (membrane) that does not let bacteria escape, which allows perfect control over the age of the sludge. and the main operating parameters of the system.

  • Secondary sedimentation:

It is constituted as the final step of the secondary stage of the wastewater treatment process, where biological flocs are removed from the filter material, producing treated water with low levels of organic matter and suspended matter.

wastewater treatment

Tertiary stage:

It is the final phase of wastewater treatment, in which a series of processes are practiced to increase the quality of the water to standards required for its discharge into rivers, seas, lakes, fields and other hydrographic basins, such processes are the following:

  • Filtration:

Sand filtration retains much of the suspended matter residues and residual toxins, they are retained by the excess carbon from the filtration.

wastewater treatment

  • Lagoon:

This treatment of lagoons provides sedimentation and additional biological improvement by storage in artificial ponds and lagoons, being an imitation of the self-purification processes that a river or a lake submits to wastewater in a natural way.

Such lagoons are highly aerobic and are often colonized by native microphytes, especially reeds. This process can be carried out in large lagoons with long retention times between 1 and 3 days that makes them practically insensitive to load variations, but that require very extensive fields. Agitation must be sufficient to keep the sludge in suspension, except in the area most immediate to the effluent outlet.

water type

  • Artificial wetlands:

They include reed beds or a series of similar methods that provide a high degree of aerobic biological matter and can often be used instead of secondary treatment for small populations, also for phytoremediation.

grass innwater

  • Nutrient removal:

 Wastewater can contain high levels of presence of the nutrients nitrogen and phosphorus, which can be toxic to marine fauna species such as fish and invertebrates in low concentrations, as well as creating unhealthy conditions in the receiving environment. Algae can produce toxins, their death and by consumption of bacteria, being able to deplete the oxygen in the water, suffocating fish and other species of aquatic life.

Given this, it is necessary to point out that when there is a discharge from rivers to lakes or low seas, the added nutrients can cause severe entropic losses, causing the death of many fish sensitive to contamination in the water.

dead fish

Nitrogen removal is effected by the biological oxidation of nitrogen from ammonia to nitrate, and through reduction, nitrate is converted into nitrogen gas which is released to the atmosphere. Such conversions require carefully controlled conditions to allow proper formation of biological communities.

Sand filters, lagoons and sheet beds are used to carry out this work to reduce nitrogen, sometimes suggesting that the conversion of toxic ammonia into nitrate is done only as a tertiary treatment.

Likewise, nitrogen removal is carried out biologically in a process called biological phosphorus removal, in which specific bacteria called polyphosphate accumulating organisms are selectively enriched and accumulate large amounts of phosphorus within their cells. This removal can also be carried out by chemical precipitation with iron or aluminum salts.

  • Disinfection:

The purpose of this last phase of tertiary treatment is to substantially reduce the number of living organisms, which will be discharged back to their natural source. The effectiveness of this process will depend on the quality of the treated water, the type of disinfection that is applied, the dose of disinfectant and other environmental variables, being verified in the case of cloudy water, which is treated with less success since the Solid matter can shield organisms, especially from ultraviolet light or if contact times are short.

wastewater treatment

The most used methods for disinfection are ozone, chlorine and UV light, considering that the chloramine used for drinking water is different from that used for wastewater treatment due to its persistence.

wastewater treatment

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