One of the major challenges of modern society is managing water resources responsibly. As populations grow and industries expand, the volume of wastewater produced continues to increase. Treating this wastewater efficiently is essential to protect the environment, conserve water, and safeguard public health.
Wastewater treatment plants, also known as WWTPs, play a crucial role in this process. They collect, clean, and purify used water before returning it to the environment or reusing it for agriculture and other non-potable purposes. The treatment process involves a series of physical, chemical, and biological steps that remove pollutants and ensure that the water meets strict quality standards.
Below, we explore the four main stages of wastewater treatment and how each one contributes to transforming wastewater into a cleaner, reusable resource.
What processes take place in wastewater treatment plants?
In a typical wastewater treatment plant, water goes through a sequence of carefully designed stages. These stages are commonly known as preliminary, primary, secondary, and tertiary treatment. Each phase plays a different role in cleaning the water and removing contaminants.
At the beginning of the process, large debris, sand, and grease are removed to prevent damage to equipment. The next stages focus on settling out solids, breaking down organic matter, and disinfecting the water before it is released or reused. Together, these steps ensure that treated wastewater complies with environmental and safety regulations.
1. Preliminary Treatment
Preliminary or pretreatment is the first step in the wastewater treatment process. Its main purpose is to prepare the incoming water for the next stages by removing coarse solids and materials that could obstruct or damage pumps, pipes, and other plant equipment.
Roughing filtration and screen removal
At this point, the water passes through screens or metal grates that trap large objects such as plastics, wood fragments, or other debris carried by the flow. These items are mechanically removed and disposed of separately. This step is essential to prevent blockages and maintain the smooth operation of the plant.
Grease and sand removal
After the coarse solids have been removed, the next phase focuses on eliminating sand, gravel, and oils. Desanders and degreasers separate these materials by using gravity or flotation. Sand and grit settle at the bottom, while grease and oils float to the surface and are skimmed off. This step not only protects the plant’s machinery but also improves the efficiency of later treatments.
2. Primary Treatment
The aim of primary treatment is to remove fine suspended solids and organic matter that remain in the wastewater after preliminary treatment. This stage relies on physical and chemical processes to separate particles from the water.
Suspended solids removal
In this process, wastewater flows into large settling tanks, often called primary clarifiers or decanter centrifuges. Here, the water is held for around one to two hours, allowing heavier solids to sink to the bottom by gravity. The collected material forms what is known as primary sludge. At the same time, lighter materials such as oils or fats float to the top and are removed by skimming.
Chemical additives and sedimentation
To enhance the separation process, chemicals like coagulants and flocculants can be added. These substances cause fine particles to clump together into larger masses, which then settle more easily. Bases or acidic agents may also be introduced to balance the pH of the water, creating ideal conditions for subsequent treatment. The result is a significant reduction in suspended solids and organic load. Our expertise in water and waste treatment ensures sustainable solutions that protect the environment and comply with strict industry standards.
3. Secondary Treatment
Secondary treatment is one of the most important phases of the wastewater purification process. It focuses on removing dissolved organic matter and nutrients such as nitrogen and phosphorus, which can cause pollution if released into natural water bodies.
Biological treatment (activated sludge)
This stage uses biological processes that rely on naturally occurring bacteria and microorganisms. In an aeration tank or reactor, air is introduced to promote microbial growth. These microorganisms feed on organic compounds and nutrients, converting them into more stable substances such as carbon dioxide, water, and biological biomass.
The most common method used is the activated sludge process. It involves mixing wastewater with a culture of microorganisms under controlled oxygen levels—ranging from aerobic to anaerobic conditions—depending on the specific treatment requirements. This step significantly reduces biochemical oxygen demand (BOD) and nutrient concentration.
Secondary settling process
After biological treatment, the mixture passes into a secondary clarifier or settling tank. In this tank, the heavier biomass, now known as biological sludge, settles at the bottom while clarified water rises to the surface. The settled sludge is partly returned to the biological reactor to maintain microbial activity, while the excess is sent for further sludge treatment.
Clarified water generation
The clear water that leaves the secondary clarifier is known as clarified effluent. It meets most discharge standards for many uses, but additional treatment may still be required if the water is to be reused for irrigation, industrial processes, or environmental discharge.
4. Tertiary Treatment
Tertiary or advanced treatment improves the final quality of the effluent, ensuring it meets high standards for reuse or discharge into sensitive ecosystems such as rivers, lakes, or coastal areas.
Filtration (sand beds or other media)
Filtration removes any remaining suspended particles that were not eliminated in earlier stages. The water passes through sand filters or other porous materials that trap fine impurities. This process enhances water clarity and prevents microbial growth in later uses.
Disinfection (chlorine or UV)
Disinfection is essential for destroying any remaining pathogens or bacteria. Common disinfection methods include chlorination, often using sodium hypochlorite, and ultraviolet (UV) radiation. Both methods ensure the treated water is safe for its intended purpose, whether for environmental release or reuse.
Preparation for discharge or reuse
Once disinfected, the water can either be safely discharged into natural water bodies or reused. In many modern treatment plants, the treated wastewater is used for agricultural irrigation, industrial cooling, or urban landscaping. This not only conserves freshwater resources but also supports sustainable water management practices.
Sludge Treatment – WWTP By-products
Throughout the different stages of treatment, solid waste known as sludge is produced. This material contains organic and inorganic components that must be processed before disposal or reuse.
The sludge is first thickened to reduce its water content, followed by biological digestion. During digestion, microorganisms break down organic matter, reducing volume and odour while producing biogas that can be captured for energy. The digested sludge is then dewatered using centrifuges or filters to achieve the required dryness.
If the final product meets safety standards for heavy metals and pathogens, it can be reused in agriculture as fertiliser or disposed of safely. Managing sludge efficiently is a vital aspect of wastewater treatment, contributing to environmental protection and circular economy practices.
Wastewater: A Key Resource to Address Current Challenges
Wastewater is no longer seen as waste but as a valuable resource. It can provide renewable water, energy, and nutrients if treated and managed correctly. This approach aligns with Sustainable Development Goal 6 of the United Nations, which calls for clean water and sanitation for all.
Across the world, advances in technology are improving the efficiency of wastewater treatment plants, making them more sustainable and cost-effective. Digital monitoring systems, automation, and smart data analysis help optimise operations, reduce energy use, and improve water quality outcomes.
However, global figures show that a significant portion of wastewater is still discharged untreated. Expanding wastewater treatment infrastructure and adopting innovative technologies are essential to ensure that more regions can benefit from water reuse and environmental protection. Through our procedure engineering approach, we optimize processes to enhance productivity, safety, and overall operational efficiency.
FAQs
What are the main stages of wastewater treatment?
The four main stages are preliminary, primary, secondary, and tertiary treatment. Each one focuses on different types of pollutants, from large debris to dissolved nutrients and microorganisms.
What is the purpose of secondary treatment?
Secondary treatment removes organic matter and nutrients through biological processes. Microorganisms break down pollutants, making the water safe for further treatment or discharge.
How is wastewater disinfected before reuse?
Disinfection is usually achieved using chlorine-based solutions such as sodium hypochlorite or ultraviolet light, both of which eliminate harmful bacteria and pathogens.
What happens to the sludge from treatment plants?
Sludge is processed separately through thickening, digestion, and dewatering. It can be reused as fertiliser or disposed of in an environmentally safe manner.
Can treated wastewater be reused?
Yes. Depending on its final quality, treated wastewater can be reused for agriculture, industry, or urban landscaping, reducing pressure on freshwater resources.
