Different processes are available to you for the different stages of water treatment preparation :

  • Physical and mechanical treatment processes, such as aeration, sedimentation or thermal influences. The use of screens, filters and sieves are also included.
  • Biological processes, such as anaerobic wastewater treatment, biochemical oxidation or sludge digestion.
  • Chemical processes, such as neutralization, disinfection, flocculation and sedimentation.
  • Membrane processes, such as filtration, permeation and nanofiltration.

It is in municipal wastewater treatment plants that we see the largest volumes of water to treat, so we need to provide more diversify solutions and more efficient procedures. The prefer procedures depend on the type of wastewater treatment plant.


We consider that the treatment processes in wastewater treatment plants are divide into different phases.


In this first phase, the wastewater will undergo a mechanical treatment which will make it possible to eliminate approximately 20 to 30% of the suspend solids. To do this, the system will return the wastewater to a screening installation, where a screen or a grid or a drum-screen will take care of filtering coarse impurities, such as leaves, paper or textiles. The water will flow through different screens, ranging from coarse screens several centimeters wide to small mesh screens of only a few millimeters, flowing at different speeds, and thus gradually filtering the coarse materials. The mechanically collect debris is drain, transport, and then dispose of in an incineration plant.

The pre-purify water then passes through what is call a sand trap. In a wastewater treatment system, a settling pit is use to remove coarse particles, such as stones, shards of glass, or sand, as well as organic material that has not been pre-filter by the screens. The flow flows at a relatively high speed of about 0.3 m / s. There are sand traps long non-aerategrit chambers ventilate long – also call cylindrical grit – and finally the round sand traps.

wastewater treatment plant pic

 Aerate grit trap

The aerate grit trap removes grease and other oils from wastewater using the following process: the introduce process air generates a rolling movement in the water, which will lift lighter substances, such as oils and greases, to the area. On the surface, they are then relatively easy to extract.

 Round grit separates

round grit separates substances from wastewater by centrifugal force before removing them by suction. Once clean in the sand trap, the latter’s debris is subject to a second cleaning and are free of all organic substances. This procedure improves the dehydration of the collect inorganic materials, which can then recycle for road construction. If it is not possible to recycle these residues, they must be dispose of in accordance with the regulations in force, namely, be landfill or destroye in waste incineration plants.

Primary wastewater treatment basinis

The primary wastewater treatment basinis the next phase of wastewater treatment. The sewage velocity is approx. 1.5 cm / s, significantly slower than in the sand trap. It suffices to enlarge the diameter of the basin to reduce the flow speed. The process involves a low flow speed to allow the finer particles to settle, depending on their nature, to the bottom or to the surface of the water. The sludge produce by sedimentation (settling at the bottom of the tank) is call primary sludge. They are generally made up of organic matter. The primary sludge is collect by a scraper to a fresh sludge hopper. The floating substances are transfer to a floating slurry pipe. A pump directs the fresh sludge to a digester.

This digesterwill allow the release of methane in four phases (hydrolysis, acidogenesis, acetogenesis and methanogenesis); this methane is then convert into electricity in a heating plant and can thus be use on site to supply a power plant with energy. The digestion process in the digester takes place in about four weeks. The residue consists of an odorless sludge, which is often use for agricultural purposes after dewatering by centrifuge or filter.

This is where the mechanical cleaning phase ends. On average, about 30 to 40% of pollutants are remove from wastewater during this phase. At this stage in the treatment plant, the wastewater then reaches the next treatment phase.


In most sewage treatment plants, pre-purify water during the mechanical treatment phase is rout to aeration basins, which often take the form of circulation basins . This is where the biological cleansing will take place.

The water is circulate by the supply of oxygen and with the aid of agitators. The objective is to create more or less ventilate areas in order to establish various conditions for bacteria and microorganisms. These microorganisms will be able to feed on residual organic contaminants and convert them into inorganic substances. The bacteria form flakes of activate sludge that float freely in the water. The supply of oxygen stimulates the multiplication of bacteria, thus promoting the formation of activate sludge. Thus, this biological wastewater treatment process also takes the name of activate the sludge process.

Sludge Water

The wastewater containing the activate sludge is then discharge into the secondary waste water treatment in Pakistan basin. Again, the system here reduces the flow velocity of the wastewater stream. Sedimentation can then take place: The activate sludge is deposite at the bottom of the purified water, where it can be extract by mechanical cleaning devices positioned at the end of the basin. A part is then transfer to the digester, thus supplied with additional biomass.

The other part of the sludge also refer to as “return sludge”, is return to the aeration basin to ensure a sufficient level of microorganisms in the aeration basin, for the decomposition of impurities. Once the biological treatment is complete, nearly 90% of the wastewater has been clean of biodegradable substances. The biological cleaning step involves the supply of a large quantity of oxygen, it is, therefore, consumes the most energy throughout the process. Once the water has reach regulatory quality, it can reintegrate into clean water channels, for example, to a river.

But in many other cases, biological cleaning is insufficient. In such cases, other wastewater treatment processes are required, and for example, chemical treatment will take over. We will therefore use chemical additives to increase water purification.


At this stage of wastewater treatment, chemical processes use for the treatment of wastewater. These chemical compounds use in order to achieve the values ​​prescribe by law. Chemical treatments use in sewage treatment plants include neutralization, disinfection, phosphate precipitation, the removal of nitrogen, and manganese by the cold.


 is use to reach the prescribe pH value, where we will add an acid, for example, hydrochloric acid, or a base, for example milk of lime.

During disinfection , pathogens are eliminate by adding chlorine or chlorine dioxide. UV irradiation of wastewater is a good alternative to adding chemicals, although it is less frequent use. The elimination of phosphates: our wastewater is often contaminate with phosphates from detergents, fertilizers, food additives, and other feces. If they are still present in wastewater, they lead to over-fertilization of water bodies and nutrient enrichment which can lead to plant growth (eutrophication) that is harmful to the ecosystem.

wastewater treatment plant

Phosphates are remove by a process of chemical precipitation or flocculation. The precipitation of phosphates will be partially trigger by the addition of aluminum or iron salts in the heart of the grit trap, or in the secondary wastewater treatment basin. During this secondary clarification, the metal-phosphate flakes thus form will then be evacuate from the wastewater with the activate sludge. Depending on the configuration, it will be possible to “fish” for the phosphate using microorganisms drawn from the wastewater. This scenario corresponds to a biological elimination of phosphorus, a technique still rarely use.

Chemical water purification

Chemical water purification also includes the removal of nitrogen : through this process, nitrogen compounds harmful to water, such as ammonia and ammonium, are remove from the wastewater. When release into water bodies, nitrogen compounds absorb vital oxygen from the water, causing fish to die. Nitrogen is eliminate by nitrification and denitrification processes: In the nitrification phase , ammonia is oxidize to nitrite by a reaction with anaerobic bacteria and oxygen; then in a second step, the nitrite is oxidize to nitrate. Denitrification followingis also trigger by the addition of anaerobic microorganisms. They break down nitrate into nitrogen gas via enzymatic activities; the latter is then release into the atmosphere.

Iron removal : to reduce the iron content of wastewater and comply with threshold values, oxygen is add to trigger the oxidation of iron (II) cations. To start the oxidation process, it is necessary to add caustic soda to the wastewater.

Itching : In general, manganese is present in wastewater as manganese bicarbonate. The addition of oxygen forms poorly soluble IV manganese compounds which are easy to remove from water.


The fourth and final phase of treatment will involve processes using membranes and filters. This cleaning step combines in part with the chemical processes of precipitation and flocculation. This is refer to as a flocculation filtration method. Precipitants and flocculants are add to the waste water, thus causing the flocculation of the substances which one seeks to separate. The wastewater with the flocculate material is then channel to a membrane or a sand filter.

It slowly seeps through the filter layer. Even the smallest organic suspend solids are remove.

Nanofiltration works in a very similar way. Unlike normal filtration, water is pressurize through a membrane that retains the smallest dissolve particles, such as heavy metal molecules or ions. This is an identical phenomenon that is found with reverse osmosis, where high pressures and thin membranes are use.

Contaminants collected during the filtration, nanofiltration and reverse osmosis stages are filtered by the primary clarifier in the form of filtration sludge during sludge treatment.


The water finally reaches the last section of the treatment plant, the treate water storage tank. This is where we will take water samples, the quality of which we will analyze. Purified water is only transfer to clean drinking water channels when the parameters prescribe by law are respect.

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