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Views: 222 Author: Carie Publish Time: 2025-05-13 Origin: Site
Content Menu
● Overview of Sewage Treatment Stages
>> 1. Preliminary Treatment / Screening
>> Objectives of Tertiary Treatment
● How Does Tertiary Treatment Work?
>> Filtration
>> Disinfection
● Emerging and Beyond Tertiary Treatment: Quaternary Treatment
>> Technologies Used in Quaternary Treatment
● Benefits of Tertiary Treatment
● Challenges and Considerations in Tertiary Treatment
● FAQ
>> 1. What is the difference between secondary and tertiary sewage treatment?
>> 2. Why is nutrient removal important in tertiary treatment?
>> 3. Can tertiary treated water be reused?
>> 4. What are common disinfection methods used in tertiary treatment?
>> 5. What is quaternary treatment in sewage treatment?
Sewage treatment is a critical process for protecting public health and the environment by removing contaminants from wastewater before it is released back into nature or reused. Among the various stages of sewage treatment, tertiary treatment represents the highest and most advanced level of purification. This article explores the entire sewage treatment process, with a detailed focus on tertiary treatment, its technologies, benefits, and emerging advanced methods beyond tertiary treatment.
Sewage treatment typically involves multiple stages, each designed to progressively remove solids, organic matter, nutrients, and pathogens from wastewater. The main stages are:
This initial step removes large solids and grit from the sewage using screens and grit chambers. Solids larger than 6 mm are filtered out to protect downstream equipment. This stage prevents damage and clogging in pumps and pipes by removing debris such as sticks, plastics, and rags.
Primary treatment involves sedimentation tanks where suspended solids settle out by gravity. This process removes about 70% of total suspended solids (TSS) and 30% of biochemical oxygen demand (BOD), which measures organic pollution. The settled solids form sludge that is removed for further treatment or disposal. Primary treatment is mainly physical and does not significantly reduce dissolved pollutants or pathogens.
Secondary treatment uses biological processes to degrade dissolved and suspended organic matter. Microorganisms consume organic pollutants, reducing BOD and nutrient levels substantially. Common secondary treatment methods include activated sludge systems, trickling filters, and oxidation ponds. The treated water typically meets standards of 30 mg/L TSS and 20 mg/L BOD, suitable for discharge into many water bodies but still containing nutrients and pathogens.
Tertiary treatment, also known as advanced treatment or effluent polishing, is the highest level of sewage treatment. It further removes residual nutrients, suspended solids, pathogens, and micropollutants to produce water safe for discharge into sensitive ecosystems or reuse. This stage is essential when the receiving environment is vulnerable or when water reuse is planned.
Tertiary treatment is the final stage of wastewater treatment designed to provide a high level of purification beyond primary and secondary processes. It is particularly important when treated water is discharged into ecologically sensitive areas such as estuaries, coral reefs, or low-flow rivers, or when water reuse is intended for agriculture, industry, or even potable uses.
- Removal of residual suspended solids and fine particles
- Nutrient removal, especially nitrogen and phosphorus, to prevent eutrophication
- Disinfection to eliminate pathogens
- Removal of micropollutants such as pharmaceuticals and household chemicals (in some advanced forms)
- Improvement of water clarity and color
Tertiary treatment ensures that the effluent meets stringent regulatory standards, protecting aquatic ecosystems from harmful nutrient loading and reducing risks to human health.
Tertiary treatment employs a combination of physical, chemical, and biological processes. Common methods include:
- Sand filtration: Water passes through sand beds to remove fine particulate matter. This is one of the oldest and most widely used filtration methods.
- Activated carbon filters: These filters adsorb dissolved organic compounds, toxins, and micropollutants, improving water taste and odor.
- Membrane filtration: Includes microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. These membranes provide a physical barrier to remove pathogens, suspended solids, and even dissolved substances at the molecular level.
Excess nutrients like nitrogen and phosphorus in wastewater can cause eutrophication in receiving waters, leading to oxygen depletion and harmful algal blooms. Tertiary treatment focuses heavily on removing these nutrients.
- Biological nutrient removal (BNR): This process uses specialized bacteria in controlled aerobic and anoxic conditions to convert nitrogen compounds through nitrification and denitrification. Phosphorus is removed biologically by bacteria that uptake and store it intracellularly, which is then removed with the sludge.
- Chemical precipitation: Chemicals such as alum (aluminum sulfate), ferric chloride, or lime are added to precipitate phosphorus, which is then removed by sedimentation or filtration.
Disinfection is a critical step in tertiary treatment to eliminate pathogenic microorganisms, reducing risks of waterborne diseases.
- Chlorination: The most common chemical disinfectant, effective against bacteria and viruses. However, it can produce harmful disinfection by-products if not managed properly.
- Ultraviolet (UV) irradiation: Uses UV light to inactivate microorganisms by damaging their DNA. It is chemical-free and leaves no residuals.
- Ozonation: Ozone is a powerful oxidant that disinfects and breaks down organic pollutants. It is effective but more expensive and complex to operate.
- Lagooning: Large, aerated ponds where natural biological activity further cleans water. This method is cost-effective for small communities but requires large land areas.
- Advanced oxidation processes (AOP): These involve generating highly reactive species like hydroxyl radicals to degrade micropollutants and organic contaminants that are resistant to conventional treatment.
Quaternary treatment refers to cutting-edge processes designed to remove micropollutants that conventional treatments miss, such as pharmaceuticals, pesticides, personal care products, and environmentally persistent pharmaceutical pollutants (EPPP). These contaminants can pose risks to aquatic life and human health even at very low concentrations.
- Advanced oxidation processes (AOP): Combining ozone, UV light, and hydrogen peroxide to generate reactive radicals that degrade micropollutants.
- Membrane bioreactors (MBR): Combining biological treatment with membrane filtration for superior solids and pathogen removal.
- Activated carbon adsorption: Granular or powdered activated carbon to adsorb trace organic contaminants.
- Nanotechnology-based treatments: Emerging methods using nanoparticles to catalyze degradation of pollutants.
While quaternary treatment offers promising results, it is currently limited by high operational costs and energy demands. Research continues to improve efficiency and reduce costs for future widespread adoption.
- Environmental Protection: Prevents nutrient pollution that causes harmful algal blooms and oxygen depletion in water bodies, protecting aquatic biodiversity.
- Public Health: Removes pathogens to reduce disease transmission through contaminated water.
- Water Reuse: Produces high-quality effluent suitable for irrigation, industrial processes, groundwater recharge, and potentially potable reuse, helping to alleviate water scarcity.
- Regulatory Compliance: Meets stringent discharge standards required by environmental agencies worldwide, avoiding fines and environmental damage.
- Odor and Aesthetic Improvement: Removes color, turbidity, and odor-causing compounds, improving the quality of discharged water.
While tertiary treatment offers significant benefits, it also presents challenges:
- Cost: Advanced treatment technologies require significant capital investment and operational costs, including energy consumption and chemical usage.
- Complexity: Operation and maintenance require skilled personnel and monitoring to ensure consistent performance.
- Sludge Management: Nutrient removal processes generate sludge that must be treated and disposed of safely.
- By-products: Chemical disinfection can produce harmful by-products if not carefully controlled.
Despite these challenges, the benefits of tertiary treatment in safeguarding water resources and public health make it a necessary investment, especially in densely populated or ecologically sensitive areas.
Tertiary treatment is the highest conventional level of sewage treatment, providing advanced purification that protects ecosystems and human health. It builds upon primary and secondary treatments by removing residual solids, nutrients, pathogens, and micropollutants through filtration, biological nutrient removal, chemical processes, and disinfection. Emerging quaternary treatments push the boundaries further by targeting micropollutants that threaten water quality. As water scarcity and environmental concerns grow, tertiary and beyond treatments will become increasingly vital for sustainable wastewater management.
Secondary treatment primarily uses biological processes to remove organic matter and some nutrients, while tertiary treatment further polishes the effluent by removing residual suspended solids, nutrients, pathogens, and micropollutants to achieve higher water quality.
Excess nutrients like nitrogen and phosphorus can cause eutrophication in receiving waters, leading to algal blooms and oxygen depletion that harm aquatic life. Tertiary treatment removes these nutrients to protect sensitive ecosystems.
Yes, tertiary treated water is often clean enough for reuse in agriculture, industrial processes, groundwater recharge, and in some cases, after further treatment, for potable uses.
Chlorination, ultraviolet (UV) irradiation, and ozonation are commonly used to disinfect treated wastewater and eliminate pathogens before discharge or reuse.
Quaternary treatment is an advanced stage beyond tertiary treatment that targets micropollutants like pharmaceuticals and pesticides using specialized technologies such as advanced oxidation and membrane filtration. It is not yet widely adopted due to high costs.
