An Overview of Wastewater Treatment Methods, Levels, And Processes

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1. Wastewater Treatment Levels

  • Primary Treatment (including primary enhanced treatment)
  • Secondary Treatment (including secondary enhanced treatment)
  • Tertiary Treatment or Advanced Treatment

2. Composition of Wastewater Treatment Processes

  • Physical Treatment: Methods that remove suspended solids without altering the chemical composition of pollutants, such as filtration, sedimentation, and flotation.
  • Biological Treatment: Uses microorganisms to break down organic pollutants in wastewater.

3. Principles for Selecting Treatment Processes

The choice of treatment process should be based on:

  • Cost efficiency: Includes investment per unit of water treated, cost of reducing pollutants, and energy consumption.
  • Reliability: Ensures the process can reliably handle varying water qualities and flow rates.
  • Environmental Impact: The overall environmental benefits, including pollution reduction and resource recovery.
  • Area and space requirements: Considering the available land area.
  • Maintenance and management: Ensuring ease of operation and upkeep.
  • Local conditions: Factors like wastewater characteristics, receiving water body function, and regulatory standards.

4. Major Wastewater Treatment Methods

1) Physical Treatment Methods

These methods rely on physical processes to remove contaminants without altering their chemical structure. Common techniques include:

  • Filtration: Uses media (like screens, cloths, or granular materials) to remove suspended particles from water.
    • Grates and Screens: Prevent debris from entering the treatment system by using a series of parallel bars or filters.
  • Sedimentation: Relies on gravity to allow suspended solids to settle out of wastewater.
    • Commonly used in primary treatment, where heavier particles settle in a clarifier.
  • Flotation: Involves introducing air bubbles into the wastewater, allowing lighter particles to float to the surface for removal.

2) Chemical Treatment Methods

These methods use chemicals to alter the pollutants, facilitating their removal through processes like coagulation, flocculation, or precipitation.

3) Physical-Chemical Treatment Methods

Combining physical and chemical processes to treat water. This could involve combining sedimentation with chemical dosing to facilitate flocculation and coagulation, or using membranes for separation.

4) Biological Treatment Methods

These include processes where microorganisms degrade organic pollutants in wastewater. Common biological treatment systems are:

  • Activated Sludge Process
  • Trickling Filters
  • Rotating Biological Contactors

This step is part of wastewater pretreatment, aimed at recovering valuable substances, initially clarifying wastewater to facilitate subsequent treatment, and reducing the load on settling tanks or other treatment equipment. It also protects pumping machinery to prevent clogging from particulate matter, thereby avoiding equipment failure. The effectiveness of the grating depends on the wastewater quality and the size of the grating openings. Sludge removal methods include both manual and mechanical options. The grating sludge should be cleaned and processed promptly.

Screens are mainly used to intercept fine suspended solids ranging from a few millimeters to several tens of millimeters in size, such as fibers, pulp, algae, etc. They are typically made of metal wire, synthetic fibers, or perforated steel plates, with openings usually smaller than 5mm, and as small as 0.2mm. Screen filtration devices include drum-type, rotating-type, disc-type, and fixed vibrating inclined screens. Regardless of the structure, they must both capture debris and allow for easy removal and cleaning of the screen surface.

Grain Medium Filtration (also called bed filtration) involves wastewater passing through a granular filter medium (such as quartz sand), where fine suspended particles are trapped in the surface and internal voids of the filter media. Common filter media include quartz sand, anthracite coal, and garnet. During filtration, the filter media physically trap, settle, and adsorb the suspended solids. The effectiveness of the filtration depends on factors such as the filter media’s pore size, layer thickness, filtration rate, and the characteristics of the wastewater.

When wastewater flows from top to bottom through the granular filter bed, larger suspended particles are first intercepted in the upper layer of the filter media. This reduces the void size in this layer, gradually forming a “filter cake” composed mainly of trapped particles, which performs the primary filtration function. This effect is known as resistance interception or sieving.

As wastewater flows through the filter media, the large surface area of the grains provides effective space for suspended solids to settle, creating numerous “mini-settling tanks,” where suspended solids easily settle down. This effect is known as gravity sedimentation.

Due to the large surface area of the filter media, there is significant physical adsorption between the media and the suspended solids. Additionally, sand grains often carry a negative surface charge, which allows them to adsorb positively charged substances such as iron and aluminum. This forms a positively charged film on the filter media surface, which then adsorbs negatively charged colloids and various organic materials, leading to contact flocculation on the sand particles.

Sedimentation: Sedimentation is based on the principle that suspended solids in wastewater have different densities compared to water, and gravity sedimentation is used to separate solids from water. According to the concentration of suspended particles and their flocculation characteristics (i.e., the ability to form aggregates), sedimentation can be classified into four types:

  1. Separation Sedimentation (or Free Sedimentation): In this process, particles do not aggregate and settle individually. The particles are only influenced by their own weight in the water and the water flow resistance. Their shape, size, and mass do not change, and their settling speed remains constant.
  1. Coagulation and Sedimentation (or Flocculation Sedimentation): Coagulation sedimentation refers to the process where coagulants are added to wastewater, causing colloidal particles and fine suspended solids to aggregate into flocculent particles that can be separated. These flocs are then removed by gravity sedimentation. The characteristic of coagulation sedimentation is that during the sedimentation process, particles collide and aggregate to form larger flocs. As a result, the size and mass of the particles increase with depth, and their settling velocity also increases.

Common inorganic coagulants include aluminum sulfate, ferrous sulfate, ferric chloride, and polyaluminum chloride. Common organic flocculants include polyacrylamide, among others. Coagulant aids such as water glass and lime can also be used.

  1. Regional Sedimentation (also called Crowded Sedimentation or Stratified Sedimentation): When the suspended solids content in wastewater is high, the distance between particles is small, and the cohesive forces between them cause the particles to aggregate into a single mass and settle together. The positions of the particles relative to each other do not change, and thus a distinct interface between clarified water and mixed water is formed, moving downward over time. This type of sedimentation is known as regional sedimentation. It commonly occurs in high-turbidity wastewater in sedimentation tanks and secondary clarifiers (in the later stages of sedimentation).
  2. Compression Sedimentation: When the concentration of suspended solids in the slurry is very high, particles contact and compress each other. Under the gravitational force of the upper particles, water in the gaps between the lower particles is squeezed out, and the particle clusters are compressed. Compression sedimentation occurs at the bottom of sedimentation tanks, sludge hoppers, or sludge concentration tanks, where the process is very slow. Depending on the nature of the suspended solids in the water, sedimentation tanks or sand traps are used for different purposes in this type of process.

(3) Dissolved Air Flotation (DAF): In this method, air is introduced into the wastewater, forming small air bubbles that rise to the surface. The micro-sized particles in the wastewater, which have a density close to that of water (such as emulsified oils), attach to the air bubbles and float to the surface. These floating particles are then skimmed off mechanically, effectively separating them from the wastewater. Hydrophobic substances are more likely to float, whereas hydrophilic substances do not. To improve flotation efficiency, flotation agents are often added to alter the surface properties of pollutants, making hydrophilic substances hydrophobic, which can then be removed by flotation. This is known as “flotation.”

For effective flotation, the bubbles must be small and well-dispersed, and there must be an adequate amount of them to improve the flotation process. The stability of the foam layer is important— it must be stable enough to allow the scum to remain on the surface, while also facilitating the removal and dehydration of the scum. There are two primary methods for generating bubbles:

  1. Mechanical Method: Air is passed through microporous tubes, microporous plates, or perforated disks to create fine air bubbles.
  2. Pressure Dissolved Air Method: Air is dissolved in water under pressure until saturation is reached. When the pressure is suddenly released, the supersaturated air escapes as small bubbles.

The main advantages of flotation are:

  • It performs better than sedimentation tanks, typically completing solid-liquid separation in just 15–20 minutes, leading to less land requirement and higher efficiency.
  • The sludge generated by flotation tends to be drier, less prone to decomposition, and easier to manage through scraping.
  • The process increases the dissolved oxygen level in the water, which improves the removal of organic matter, algae, surfactants, and odors, providing favorable conditions for subsequent treatment and utilization.

The main disadvantages include:

  • Higher power consumption.
  • Increased maintenance and management workload, with potential for blockages in moving parts.
  • The floating scum on the surface can be affected by weather conditions like wind and rain.

Aside from these methods, electrolytic flotation is another commonly used flotation technique.

(4) Centrifugal Separation: In this method, wastewater containing suspended pollutants is subjected to high-speed rotation. Due to the difference in centrifugal force acting on the suspended particles (such as emulsified oil) and the water, separation occurs. Common centrifugal devices used include cyclone separators and centrifuges.

Chemical Treatment Method

This method involves adding chemical reagents to the wastewater to separate and recover pollutants or to transform them into harmless substances through chemical reactions. Chemical treatment can achieve a higher level of purification than physical methods by not only separating pollutants from water but also changing the nature of the pollutants—such as neutralizing pH, removing metal ions, or oxidizing toxic and harmful substances. Common chemical methods include chemical precipitation, neutralization, redox reactions, and coagulation.

Limitations of Chemical Treatment:

  • High cost due to chemical reagent consumption.
  • Disposal of chemical sludge produced during the process.
  • May require further treatment after chemical reactions to meet discharge standards.

Chemical Treatment Methods (continued):

  1. Chemical Precipitation
    Chemical precipitation involves adding certain chemical reagents to wastewater, which react with dissolved pollutants to form insoluble salts (precipitates) that can be removed from the water. This method is commonly used to remove calcium ions, magnesium ions, and heavy metals such as cadmium, copper, lead, and zinc.
    Chemical precipitation can be classified based on the type of precipitant used, including:
    • Lime Method (also known as hydroxide precipitation): Lime is added to remove hardness in water by precipitating calcium (Ca²⁺) and magnesium (Mg²⁺) as calcium carbonate (CaCO₃) and magnesium hydroxide (Mg(OH)₂).
    • Sulfide Method: This method is used for removing heavy metals by adding sulfide reagents to form insoluble metal sulfides.
    • Silver Salt Method: Involves using silver salts to precipitate certain pollutants from the wastewater.
    When the wastewater contains high concentrations of calcium and magnesium, which contribute to hardness, lime is often used for pretreatment before other methods like ion exchange softening, to reduce operational costs.Example for removing heavy metals:
    • ZnSO₄ + Na₂CO₃ → ZnCO₃↓ + Na₂SO₄
    Advantages:
    • Economical and simple, with widely available reagents.
    • Commonly used for treating heavy metal-contaminated wastewater.
    Challenges:
    • Labor-intensive and poor working conditions.
    • Pipes may get clogged and corroded.
    • Large volumes of sludge are produced, which are difficult to dewater.
  2. Neutralization
    Neutralization treatment relies on the chemical principle of acid-base reaction, which converts acidic or alkaline wastewater to a neutral pH. This method is typically applied to wastewater with concentrations greater than 3% for either acids or alkalis. For lower concentrations, neutralization is an effective method.
    • Acidic wastewater is usually neutralized by adding lime, caustic soda, sodium carbonate, or limestone.
    • Alkaline wastewater is neutralized by adding acids like sulfuric acid or hydrochloric acid, or by introducing carbon dioxide.
    For cases where both acidic and alkaline wastewater need treatment, the two can be neutralized against each other.
  3. Oxidation-Reduction (Redox) Methods
    Redox methods use chemical agents that undergo oxidation-reduction reactions to convert harmful pollutants into less toxic substances. This method is especially effective for treating inorganic pollutants like heavy metals and oxides.
    • Oxidizing agents: Common oxidants include ozone, chlorine, hypochlorite, and oxygen, which can break down harmful substances in the wastewater.
    • Reducing agents: Common reducing agents like ferrous sulfate, sodium bisulfite, or iron filings can reduce harmful pollutants to non-toxic or less harmful forms.
    • Ozone oxidation is often used for decolorization, disinfection, and deodorization of wastewater.
    • Air oxidation can be used to treat sulfur-containing wastewaters, while reduction methods are often employed to treat wastewater from electroplating containing heavy metals.
  4. Coagulation (Flocculation)
    Coagulation involves adding electrolytes to wastewater containing fine suspended solids or colloidal particles to destabilize them and promote aggregation (flocculation). The flocs can then be removed by sedimentation or filtration. Common coagulants include:
    • Aluminum sulfate (alum), ferric sulfate, ferric chloride, and polyaluminum chloride.
    • Coagulants may be complemented by flocculants such as lime, activated silica, or gelatin, which help to further enhance the coagulation process.
    This method is widely used to remove fine particles from wastewater, including colloids and emulsions, that are difficult to settle under normal conditions.

1. Chemical Precipitation Method
Chemical precipitation involves adding chemical reagents to wastewater that react with dissolved pollutants to form insoluble salts (precipitates), which are then removed from the water. This method is commonly used for removing calcium ions (Ca²⁺), magnesium ions (Mg²⁺), and heavy metals like cadmium (Cd²⁺), copper (Cu²⁺), lead (Pb²⁺), and zinc (Zn²⁺). Precipitation methods can be classified based on the type of reagent used, such as:

  • Lime Method (Hydroxide Precipitation): Lime (Ca(OH)₂) is added to precipitate calcium and magnesium as calcium carbonate (CaCO₃) and magnesium hydroxide (Mg(OH)₂). This method is effective for reducing hardness in water.
  • Sulfide Method: This method involves using sulfide reagents to precipitate heavy metals as insoluble metal sulfides, making it suitable for treating wastewater contaminated with metals like cadmium, zinc, and copper.
  • Silver Salt Method: This method uses silver salts to precipitate certain metals from wastewater.

For treating hard water, especially when calcium and magnesium concentrations are high, lime-softening is often used as a pre-treatment before ion exchange, helping to reduce operational costs. In the case of removing heavy metals, carbonates are commonly used to form insoluble metal carbonates, which are easier to recover.

Example of Heavy Metal Precipitation: ZnSO₄ + Na₂CO₃ → ZnCO₃↓ + Na₂SO₄

Advantages:

  • Economical and easy to implement.
  • Reagents are widely available.
  • Often used for treating wastewater contaminated with heavy metals.

Challenges:

  • Poor working conditions (due to labor intensity).
  • Pipes can become clogged or corroded.
  • Large volumes of sludge are generated, which are difficult to dewater.

2. Neutralization Method
The neutralization method uses the acid-base reaction principle to adjust the pH of wastewater to near neutral by adding acidic or alkaline substances. It is used to treat wastewater with high concentrations of either acid or base. For wastewater with concentrations greater than 3% of acid or base, recovery is typically recommended before neutralization. For low-concentration acid or base wastewater, neutralization treatment is commonly applied.

  • Acidic wastewater is neutralized by adding lime, caustic soda, sodium carbonate, or using limestone or marble as neutralizing agents.
  • Alkaline wastewater is treated by adding acids such as sulfuric acid, hydrochloric acid, or by introducing carbon dioxide gas.

In some cases, neutralization can be achieved by mixing acidic and alkaline wastewater.

3. Oxidation-Reduction Method
Oxidation-reduction (redox) methods involve the use of chemical agents that undergo redox reactions with pollutants in wastewater to transform harmful substances into less toxic or non-toxic substances. This method is mainly used for treating inorganic pollutants, such as heavy metals and oxides.

  • Oxidizing agents: Strong oxidizing agents like chlorine, ozone, and oxygen are used to oxidize and break down harmful pollutants in wastewater.
  • Reducing agents: Reducing agents such as iron filings, ferrous sulfate, and sodium bisulfite are used to reduce harmful substances to less toxic forms.

Common applications include:

  • Ozone oxidation for decolorization, disinfection, and deodorization.
  • Air oxidation for treating sulfur-containing wastewater.
  • Reduction methods for treating electroplating wastewater containing heavy metals.

Common oxidants include ozone, chlorine, hypochlorite, and oxygen. Common reducing agents include ferrous sulfate, sodium bisulfite, and iron filings.

4. Membrane Separation Methods

1) Electrodialysis Method
Electrodialysis is a water treatment method that uses direct current (DC) electrical fields to drive ions through selective ion-exchange membranes. The cation exchange membranes allow cations (positively charged ions) to pass, while the anion exchange membranes allow anions (negatively charged ions) to pass. This process results in the separation of ions from water, allowing for concentration, purification, and separation of various solutes in the solution. Electrodialysis is a newer method developed from ion-exchange technology and is primarily used in wastewater treatment, desalination of seawater, and the production of deionized (pure) water.

  • Applications: Electrodialysis is effective for removing salts and small organic molecules. It is commonly used in brackish water desalination, wastewater treatment, and producing ultra-pure water.
  • Advantages:
    • Highly efficient in separating ionic contaminants.
    • Can concentrate and purify water simultaneously.
    • Useful for separating both cations and anions.
  • Limitations:
    • Requires a DC power supply.
    • Limited to treating waters with low to medium salinity.

2) Reverse Osmosis Method
Reverse osmosis (RO) is a widely used technology for water purification and desalination. It is applied to treat wastewater, remove heavy metal ions, and desalinate seawater. In reverse osmosis, water is forced through a semi-permeable membrane under pressure, separating dissolved ions, salts, heavy metals, and organic molecules from the water. The process essentially “reverses” the natural osmosis process, where water typically moves from an area of lower solute concentration to higher solute concentration.

  • Applications:
    • Heavy Metal Wastewater Treatment: RO is effective in removing dissolved heavy metals from wastewater, including lead, mercury, cadmium, and others.
    • Wastewater Depth Treatment: RO is often used as a final step in treating wastewater to remove organic pollutants, salts, and other contaminants.
    • Desalination: It is used in seawater desalination to provide potable water in regions facing freshwater scarcity.
    • Ultra-Pure Water Production: RO can be used in combination with ion-exchange systems to produce ultra-pure water, which is vital in industries such as pharmaceuticals, electronics, and food production.
  • Types of Membranes Used:
    • Cellulose Acetate Membranes: These are one of the oldest types of membranes used for RO systems.
    • Polyamide Composite Membranes: These are more durable and are now commonly used in most reverse osmosis applications.
  • Common RO System Configurations:
    • Tubular: Cylindrical membrane elements are used, and water flows through the tubes.
    • Spiral-wound: Membranes are wound around a central core, offering compact designs and efficient performance.
    • Hollow-fiber: A high surface area, often used in portable or modular systems.
    • Plate-and-frame: Membranes are arranged in a flat panel configuration and are typically used for smaller systems.
  • Advantages:
    • High Efficiency: Can remove a wide range of contaminants, including salts, heavy metals, and organic compounds.
    • Energy Efficiency: Compared to other desalination methods, reverse osmosis is relatively energy-efficient, especially when coupled with energy recovery devices.
    • Reusable Permeate: The treated water (permeate) can be reused or directed to further treatment processes.
  • Limitations:
    • Membrane Fouling: Membranes can become clogged by suspended solids or organic matter, requiring periodic cleaning or replacement.
    • Wastewater Generation: The process generates concentrated brine (reject water) that must be managed.
    • High Initial Costs: Reverse osmosis systems can be expensive to install, particularly large-scale systems.

Biological Treatment Method

The biological treatment method utilizes the biochemical activity of microorganisms in the natural environment to oxidize and decompose organic pollutants dissolved in wastewater or in colloidal form, as well as certain inorganic toxins (such as fluorides and sulfides), converting them into stable and harmless inorganic substances, thereby purifying the wastewater. This method has the advantages of low investment, good effectiveness, and low operating costs, making it widely used in both urban and industrial wastewater treatment.

Modern biological treatment methods are divided into aerobic biological treatment and anaerobic biological treatment based on whether microorganisms require oxygen in the biochemical process.

(1) Aerobic Biological Treatment

Aerobic biological treatment is a process that relies on the biochemical activity of aerobic bacteria and facultative anaerobes to treat wastewater under aerobic conditions. This method requires the supply of oxygen. Based on the state of aerobic microorganisms in the treatment system, it can be divided into activated sludge method and biofilm method.

1) Activated Sludge Method

The activated sludge method is currently the most widely used biological treatment method. This method involves continuously supplying air (aeration) to a wastewater tank containing organic pollutants and bacteria. After a certain period, suspended floc-like particles are formed, which are aggregates of organic matter adsorbed by aerobic bacteria (and facultative aerobic bacteria) and the products of bacterial metabolic activity. These aggregates are known as activated sludge, which has a strong ability to decompose organic matter.

The mixed liquor of wastewater and activated sludge is separated in a settling tank, where the clarified water is discharged, and the sludge is returned as seed sludge to the aeration tank for continued operation. This biological treatment method, which is based on activated sludge, is called the activated sludge process. Wastewater stays in the aeration tank for 4-6 hours, which can remove about 90% of the organic matter (BOD₆). There are various types of activated sludge processes, such as conventional activated sludge, completely mixed surface aeration, and adsorption regeneration methods.

2) Biofilm Method

The biofilm method involves continuously passing wastewater through solid packing materials (such as gravel, coal slag, or plastic packing), on which microorganisms proliferate and form a sludge-like gel film known as a biofilm. The wastewater treatment method that utilizes biofilms is called the biofilm method. The biofilm mainly consists of large amounts of bacterial aggregates, fungi, algae, and protozoa.

The microorganisms on the biofilm perform the same purification function as activated sludge by adsorbing and degrading organic pollutants in the water. The aging biofilm detaches from the packing materials and flows into the settling tank with the treated wastewater. After settling, the wastewater is purified. Common biofilm methods include biological filter beds, biological contact oxidation tanks, and biological rotating discs.

(2) Anaerobic Biological Treatment

Anaerobic biological treatment is a process that utilizes anaerobic microorganisms to decompose organic matter in wastewater under anoxic conditions, thereby purifying the wastewater. In recent years, global energy shortages have driven the development of energy-efficient and energy-recovering wastewater treatment methods, thus promoting the growth of anaerobic microbial treatment technologies. A large number of high-efficiency, novel anaerobic bioreactors have emerged, including anaerobic biofilters, upflow anaerobic sludge blankets, anaerobic sulfide beds, and others.

These systems share the common characteristics of having a high concentration of microbial communities in the reactor, and a long sludge retention time, which significantly enhances their treatment capacity. As a result, anaerobic biological treatment methods demonstrate the following advantages: low energy consumption, energy recovery potential, low residual sludge production, stable and easy-to-handle sludge, and high treatment efficiency for high-concentration organic wastewater. After years of development, anaerobic biological treatment has become one of the primary methods for wastewater treatment.

Phosphorus Removal and Nitrogen Removal

(1) Phosphorus Removal:
The main sources of phosphorus in municipal wastewater are feces, detergents, and certain industrial wastewaters, existing in the forms of orthophosphate, polyphosphate, and organic phosphates dissolved in water. Common methods for phosphorus removal include chemical methods and biological methods.

  1. Chemical Phosphorus Removal:
    This method involves the reaction of phosphate with iron salts, lime, aluminum salts, etc., to form precipitates such as iron phosphate, calcium phosphate, and aluminum phosphate, thus removing phosphorus from the wastewater. The advantage of the chemical method is its high phosphorus removal efficiency, stable treatment results, and the prevention of secondary pollution caused by the release of phosphorus from the sludge during treatment and disposal. However, the drawback is the relatively large volume of sludge produced.
  2. Biological Phosphorus Removal:
    Biological phosphorus removal uses microorganisms to absorb excess dissolved phosphate in wastewater under aerobic conditions, followed by precipitation and separation to remove the phosphorus. The process consists of two stages: anaerobic phosphorus release and aerobic phosphorus uptake.

Wastewater with excessive phosphorus and phosphorus-rich activated sludge enter an anaerobic state. In this state, polyphosphate stored in the microorganisms breaks down into inorganic phosphorus, which is released back into the wastewater. This is known as “anaerobic phosphorus release.” The energy produced by polyphosphate-degrading bacteria is partly used for their own survival, while the rest is used for absorbing organic matter from the wastewater, which is then converted to acetic acid under the action of anaerobic fermentation acid-producing bacteria. This acetic acid is further converted to PHB (polyhydroxybutyrate), which is stored inside the cells.

In the subsequent aerobic state, polyphosphate bacteria aerobically decompose the PHB stored inside them, releasing a large amount of energy. Some of this energy is used for the bacteria’s reproduction, while the rest is used for absorbing phosphate from the wastewater, accumulating it as polyphosphate inside the cells. This is known as “aerobic phosphorus uptake.” During this phase, the activated sludge continuously proliferates. Apart from some phosphorus-rich activated sludge that is returned to the anaerobic tank, the rest is discharged as residual sludge, achieving the goal of phosphorus removal.

(2) Nitrogen Removal:
In domestic wastewater, the proportions of various forms of nitrogen are relatively constant: organic nitrogen 50%-60%, ammonia nitrogen 40%-50%, and nitrogen in nitrites and nitrates 0%-5%. These nitrogen compounds primarily come from proteins in human food. Methods for nitrogen removal include both chemical and biological methods.

  1. Chemical Nitrogen Removal:
    This includes ammonia absorption and chlorination methods.
    • Ammonia Absorption: The pH of the wastewater is first adjusted to above 10, and then ammonia is absorbed in a stripping tower.
    • Chlorination: Chlorine is added to wastewater containing ammonia nitrogen. By controlling the amount of chlorine added, ammonia nitrogen can be completely removed from the water. To reduce the chlorine dosage, this method is often combined with biological nitrification, where nitrification is carried out first and then the residual ammonia nitrogen is removed.
  2. Biological Nitrogen Removal:
    Biological nitrogen removal is the process where organic nitrogen and ammonia nitrogen are converted into nitrogen gas by microbial action. This process involves two reactions: nitrification and denitrification.
    • Nitrification: In an aerobic environment, ammonia nitrogen in the wastewater is converted into nitrites and nitrates by nitrifying bacteria (nitrite bacteria and nitrate bacteria).
    • Denitrification: In an anoxic environment, denitrifying bacteria reduce nitrate nitrogen (NO₃⁻) and nitrite nitrogen (NO₂⁻) into nitrogen gas. Therefore, the entire nitrogen removal process requires both aerobic and anoxic stages.
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