Wastewater treatment plants are classified into five categories: WWTP (Wastewater Treatment Plant), STP (Sewage Treatment Plant), ETP (Effluent Treatment Plant), DM (Demineralization), and RO (Reverse Osmosis). WWTPs treat industrial and domestic wastewater; STPs specialize in municipal sewage; ETPs handle highly chemically contaminated industrial wastewater; DM is used for deionization to meet high-purity water requirements; and RO membrane filtration is used in the boiler, food, and metal industries. Three factors determine the appropriate type: wastewater source, contaminant concentration, and intended use of the treated water.
The following will describe the five types of water treatment plants, how they work, and the industry applications for each type.
Wastewater treatment plant (WWTP)
Wastewater is water that originates from domestic, agricultural, commercial and industrial activities where it is used.
Basically, wastewater can be divided into two categories.
- Domestic wastewater—This is wastewater from domestic activities such as toilets, showers, etc.
- Industrial wastewater—This comes from commercial, industrial manufacturing activities and has a completely different composition than sewage.
How does it work?
| Key Data | Value / Fact | Source |
|---|---|---|
| U.S. wastewater treated daily | Approx. 34 billion gallons per day | EPA |
| Global wastewater treatment rate | Approx. 52%; high-income countries 74%, low-income countries 4.3% | UNU-INWEH / Utrecht University |
| Industrial wastewater treatment data | Available data is limited; 22 countries reported calculable industrial wastewater treatment ratios | UN-Water 2024 |
| Main nutrient pollutants | Nitrogen and phosphorus from human waste, food, soaps and detergents | EPA |
| Typical treatment stages | Preliminary → Primary → Secondary → Tertiary/Advanced → Disinfection | LACSD / wastewater treatment references |
| Main by-products | Sludge and biogas from anaerobic digestion; biogas commonly contains about 50–75% methane | Biogas reference |
Table of WWTP
- The wastewater treatment process begins when wastewater flows into the treatment plant through the main sewer system by gravity. The first stage is pretreatment, where large debris, sand, and grit are removed to protect downstream equipment and improve system efficiency.
- During pretreatment, the wastewater first passes through a grit chamber or grit classifier, where sand, gravel, and heavy inorganic particles settle and are removed. The water then flows through coarse bar screens to remove large objects, followed by fine screens that capture smaller debris.
- After pretreatment, the wastewater enters the primary treatment stage. In primary settling tanks, also known as primary clarifiers, the flow slows down, allowing heavier suspended solids to settle at the bottom as sludge while lighter materials float to the surface for removal. This process reduces the amount of suspended solids and organic load entering the next treatment stage.
- The secondary treatment stage, also called biological treatment, uses microorganisms and aeration to break down biodegradable organic matter, including carbon and phosphorus compounds. During this process, bacteria consume pollutants and form biological sludge.
- The mixed liquor then moves to secondary settling tanks, where activated sludge settles and separates from the treated water. Part of the settled sludge is returned to the biological process, while excess sludge is pumped to sludge treatment systems.
- In the sludge digestion stage, the sludge is heated and mixed inside storage tank. Anaerobic digestion produces biogas, which can be reused by the treatment plant for electricity or heat generation.
- After digestion, the sludge enters storage and dewatering systems, where excess water is removed through mechanical dewatering equipment such as filter presses or centrifuges. The remaining sludge is then transported for disposal or further reuse.
- The final stage is tertiary treatment and inspection. Additional filtration and disinfection methods such as UV, ozone, or chlorination may be used to improve water quality. The treated water is then tested to ensure it meets environmental discharge or water reuse standards for municipal or industrial applications.
Applications
Most petrochemical and chemical industries generate large volumes of wastewater and require on-site wastewater treatment plants. Other industries such as paper and pulp production also generate large amounts of wastewater.
Sewage treatment plants (STP)
These treatment plants remove sewage waste from homes, commercial buildings, and sometimes even into commercial wastewater. It can also receive storm water and debris from sewers.
STPs clean wastewater through a series of chemical, physical and biological processes before it is discharged into the environment.
Difference from WWTP: STPs do not treat high-concentration industrial chemical wastewater.
How does it work?
When wastewater arrives at a wastewater treatment plant, it goes through a filtration process that acts as a pretreatment process. Here the wastewater flows through a filter and enters a settling tank, which removes a large amount of debris.
| Key Data | Value / Fact | Source |
|---|---|---|
| Typical STP Process | Preliminary Treatment → Primary Treatment → Secondary Biological Treatment → Tertiary/Advanced Treatment & Disinfection | Seven Seas Water / wastewater treatment references |
| Activated Sludge Process | Microorganisms in the aeration tank break down organic matter, followed by sludge-water separation in the secondary clarifier | Seven Seas Water / Activated Sludge references |
| RAS (Return Activated Sludge) | A portion of settled sludge is returned from the secondary clarifier to the aeration tank to maintain microbial concentration | Activated Sludge references |
| Filtration & Disinfection | Sand or multimedia filtration removes remaining fine particles, while UV, ozone, or sodium hypochlorite is used for final disinfection | Seven Seas Water |
| Typical Applications | Municipal wastewater treatment, residential communities, commercial buildings, hotels, industrial parks, and decentralized STP systems | Seven Seas Water |
As mentioned above, it occurs as pretreatment in three stages – primary, secondary and tertiary treatment – before.
Primary treatment
In this stage, the wastewater enters a clarifier. The effluent flows slowly through the settling tanks. It is the design of these tanks that causes settling, where organic solids collect at the bottom of the tank and lighter material floats to the top becoming easier to remove.
The organic material that settles at the bottom is called the primary sludge layer. After a few hours, the sludge that settles in the clarifier enters the aeration tank for another process called the activated sludge process.
Secondary Treatment
This treatment stage includes aerobic aeration. As the air flows through the aerators, small holes appear, turning them into bubbles and mixing them with the water column. This interaction of oxygen with the bacteria in the wastewater causes the bacteria to digest the organic matter, giving the wastewater its characteristic appearance and odor.
At the end of aeration, the wastewater flows into the next tank, the secondary clarifier. It is in this case that the bacteria settle at the bottom for a day or two and form a sludge layer, which can then be pumped out by the water treatment plant.
Once the sludge bed has settled completely, it becomes return activated sludge (RAS). This RAS is returned to the primary clarifier, where bacteria help break down any organic matter in the effluent.
Once the RAS has passed through the primary and secondary clarifiers in complete succession, it becomes waste activated sludge (WAS). The WAS then does not return to the primary clarifiers, but moves to covered tanks, also called aerobic sludge digesters. In these tanks, the bacteria do not digest the organic matter present in the wastewater, but the activated sludge begins to digest each other, causing most of the sludge to disappear.
Finally, the remaining sludge moves to a dewatering facility containing dewatering tanks, where the plant uses a filter press (usually a belt filter press or plate and frame filter press) to squeeze any remaining water out of the sludge.
Tertiary Treatment
Tertiary treatment follows the primary and secondary processes, but also involves mechanical and photochemical processes. This is a more advanced treatment method and is useful for domestic wastewater containing microbial contaminants that require disinfection.
In this treatment stage, the wastewater is passed through a sand filter to remove very fine particulate matter. The photochemical process takes place after this and the water flows under ultraviolet (UV) light, which will eliminate any bacteria and viruses, and remove any infection. Nowadays, sodium hypochlorite generators and ozone generator systems are also used for disinfection.
Thus, wastewater from a wastewater treatment plant is completely safe to discharge into the environment after the three stages mentioned above.
Applications
A good example of an STP is the treatment plants you find in large cities. Residential dwellings, commercial buildings, municipal wastewater, etc.
Effluent treatment plant (ETP)
As mentioned earlier, sewage treatment plants provide preliminary treatment for domestic or municipal purposes. Sewage treatment plants, on the other hand, are the exact opposite, as they treat industrial wastewater, i.e., wastewater that results from the flow of industrial by-products.
How does an ETP work?
| Key Data | Value / Fact |
|---|---|
| Pretreatment | Screening, grit removal, and removal of large solids |
| Primary Treatment | Sedimentation, clarification, pH adjustment, and chemical coagulation |
| Secondary Treatment | Biological treatment for BOD and COD removal |
| Tertiary Treatment | Filtration, nutrient removal, and disinfection |
| Typical Industries | Pharmaceutical, chemical, textile dyeing, food processing, and industrial wastewater |
Like STPs, ETPs involve processes of preliminary, primary, secondary and tertiary treatment.
Preliminary treatment: e.g. screening, sedimentation, filtration, clarification, etc. The aim is to remove as much solid physical matter as possible before sending the wastewater for further treatment.
Primary treatment: This stage involves the removal of solid waste and organic matter. Chemicals are added here to break down any solid and chemical wastes. This is accomplished by chemical coagulation, chemical precipitation or the addition of sodium carbonate or hydrochloric acid to control pH (done with polymer dosing systems).
Secondary treatment: Here suspended particles and any biodegradable materials are removed with the help of many chemical and biological processes.
Tertiary treatment: Tertiary treatment combines all three physical, chemical and biological processes.
Applications
ETP is used in a wide range of industries with high levels of chemical contamination in the wastewater, such as pharmaceutical manufacturing and chemical industries such as textile and dye manufacturing.
Demineralization (DM) treatment plants
In water treatment for industrial purposes, demineralization usually involves the complete removal of dissolved solids or minerals from the feed water or process stream.
How does it work?
The water treatment system operates based on the ion exchange process. Special ion exchange resins are used to remove dissolved minerals and ions from the feed water by exchanging unwanted ions with hydrogen and hydroxide ions.
The combination of cation and anion exchange can remove virtually all dissolved ionic contaminants, producing high-purity deionized water through a process known as deionization.
| Key Data | Value / Fact |
|---|---|
| Working Principle | Cation exchange + anion exchange for deionization |
| Removal Target | Removes most dissolved ionic contaminants / TDS |
| Main Applications | Boiler feedwater, food & beverage, electronics manufacturing, laboratory water |
| Water Quality | Produces deionized or high-purity water |
Applications
Demineralization results in the complete removal of minerals from water and is commonly used in industries requiring high purity water, such as make-up or feed water in high-pressure boilers, the food and beverage industry, and in the manufacture of process flow electronics for industrial use.
Reverse osmosis (RO) water treatment
The principle of Reverse Osmosis (RO) acts as a filtration method to remove large amounts of contaminants and impurities from wastewater by applying pressure to the wastewater when it is located on one side of the membrane.
How does it work?
The water treatment plant works with a high-pressure pump that increases the pressure on the salt side of the RO, forcing the water through the semi-permeable RO membrane. In other words, when the wastewater enters the reverse osmosis membrane and pressure is applied, the water molecules pass through the semi-permeable membrane while other salts are not allowed to pass and are discharged to the waste stream.
| Key Data | Value / Fact |
|---|---|
| Working Principle | High-pressure pump pushes water through a semi-permeable RO membrane, while salts and impurities are rejected into the concentrate stream |
| Core Equipment | High-pressure pump, RO membrane modules, pretreatment, cartridge filter, and control instruments |
| Typical Applications | Boiler feedwater, food & beverage, electronics manufacturing, metal processing, industrial pure water, and desalination |
| Energy Feature | Requires high pressure; usually higher energy use than DM/ion exchange, but lower than thermal evaporation or crystallization |
Applications
Reverse osmosis water treatment plants are widely used in industries such as machinery, boiler feed water, food and beverage, and metal processing.
Wastewater treatment plant common system
The wastewater treatment capacity of different types of wastewater treatment plants varies, and four systems commonly used in various types of water treatment plants are listed below.
Activated sludge plant (ASP)
Activated sludge treatment plant, also known as activated wastewater treatment, is one of the most commonly used biological treatment methods. In this method, treatment conditions are provided by mixing the wastewater and a large number of active microorganisms in a tank and providing the necessary oxygen.
Carousel system
Wastewater treatment systems with carousel systems are a powerful and reliable solution for providing high quality wastewater. These systems are suitable for all applications and can be converted to existing wastewater management systems.
Submerged air filtration systems
Submerged drainage basin filtration systems are one of the common methods of wastewater treatment. The system also requires less maintenance due to the use of fewer moving parts.
With no need for regular operators and high effluent quality, submerged aeration systems are suitable for domestic and industrial wastewater treatment.
Sequential batch reactor (SBR)
Purification systems with continuous intermittent reactors are known as systems that use a specific sequence for purification. These systems have a high degree of flexibility and can treat a wide range of wastewaters.
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