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Views: 222 Author: Carie Publish Time: 2025-05-19 Origin: Site
Content Menu
● Overview of Standard Sewage Treatment Processes
● Pollutants Typically Removed by Standard Treatment
● Water Pollutants Not Removed by Standard Sewage Treatment
>> 1. Micropollutants and Emerging Contaminants
>> 2. Heavy Metals and Toxic Chemicals
>> 3. Dissolved Salts and Nutrients in Some Cases
>> 4. Microplastics and Nanoparticles
● Why Are These Pollutants Difficult to Remove?
● Advanced and Emerging Treatment Technologies
>> Advanced Oxidation Processes (AOPs)
>> Activated Carbon Adsorption
>> Constructed Wetlands and Bioaugmentation
● Regulatory and Policy Developments
● FAQ
>> 1. What are micropollutants in wastewater?
>> 2. Why can't standard sewage treatment remove pharmaceuticals?
>> 3. How do advanced oxidation processes remove pollutants?
>> 4. Are heavy metals removed during sewage treatment?
>> 5. What regulations exist for removing micropollutants?
Sewage treatment plants play a crucial role in protecting public health and the environment by removing pollutants from wastewater before it is released back into natural water bodies. However, despite advances in treatment technologies, standard sewage treatment processes cannot remove all types of pollutants effectively. This article explores the water pollutants that typically evade removal by conventional sewage treatment, the reasons behind their persistence, and emerging solutions to address these challenges.
Standard sewage treatment generally involves three main stages:
- Primary Treatment: Physical removal of large solids and sedimentation of suspended particles.
- Secondary Treatment: Biological degradation of organic matter using microorganisms.
- Tertiary Treatment: Advanced treatment to remove nutrients such as nitrogen and phosphorus, and further polishing of the effluent.
Together, these steps remove a significant portion of organic matter, suspended solids, nutrients, and pathogens, producing water that is much cleaner than raw sewage.
In primary treatment, wastewater flows through screens and grit chambers to remove large debris such as plastics, rags, and sand. The wastewater then enters sedimentation tanks where heavier solids settle to the bottom as sludge. This process removes about 30-40% of suspended solids and 25-30% of biochemical oxygen demand (BOD), which is a measure of organic pollution.
Secondary treatment uses biological processes to degrade dissolved and suspended organic matter. Aerobic bacteria consume organic pollutants, converting them into carbon dioxide, water, and more bacterial biomass. Common methods include activated sludge systems, trickling filters, and rotating biological contactors. This stage typically removes 85-90% of BOD and suspended solids.
Tertiary treatment is an additional polishing step that targets nutrients such as nitrogen and phosphorus, which can cause eutrophication (excessive algae growth) in receiving waters. Techniques include chemical precipitation, biological nutrient removal, filtration, and disinfection through chlorination or ultraviolet (UV) light.
- Suspended solids: Removed mostly during primary and secondary treatment through sedimentation and biological processes.
- Organic matter: Broken down primarily in secondary treatment by aerobic bacteria.
- Pathogens: Reduced through disinfection processes such as chlorination or UV treatment.
- Nutrients (Nitrogen and Phosphorus): Partially removed in tertiary treatment through biological and chemical methods.
These processes have been highly successful in improving water quality worldwide, reducing disease transmission and environmental degradation. However, many modern pollutants remain problematic.
Despite these processes, several pollutants are not effectively removed or degraded by conventional sewage treatment plants. These pollutants pose significant environmental and health risks due to their persistence and bioaccumulation potential.
Micropollutants are trace organic compounds that enter wastewater from various sources such as pharmaceuticals, personal care products, pesticides, and industrial chemicals. They are typically present at concentrations ranging from nanograms to micrograms per liter, making them difficult to detect and remove.
- Pharmaceuticals and Personal Care Products (PPCPs): Medications, hormones, fragrances, and sunscreen agents often pass through treatment plants largely intact. Examples include antibiotics, painkillers, antidepressants, and synthetic musks.
- Endocrine Disrupting Chemicals (EDCs): Chemicals such as bisphenol A (BPA), phthalates, and certain pesticides interfere with the hormonal systems of wildlife and humans, even at very low concentrations.
- Persistent Organic Pollutants (POPs): These include chlorinated solvents, polychlorinated biphenyls (PCBs), and dioxins, which are highly resistant to degradation and tend to bioaccumulate in the food chain.
- Antibiotics and Antibiotic Resistance Genes: The presence of antibiotics in wastewater can promote the development and spread of antibiotic-resistant bacteria, a major global health concern.
Heavy metals such as mercury, lead, cadmium, arsenic, and chromium are toxic and non-biodegradable. They can accumulate in sediments and aquatic organisms, causing long-term ecological damage and health risks to humans through the consumption of contaminated fish and water.
Industrial discharges often contain these metals, and although some metals may settle with sludge during primary treatment, significant amounts remain dissolved in the water phase and are not removed by biological treatment.
While nitrogen and phosphorus are targeted in tertiary treatment, complete removal is challenging and often incomplete. Ammonia, in particular, can be difficult to eliminate without specialized processes such as nitrification-denitrification or ammonia stripping. Excessive nutrients in effluent can cause eutrophication in receiving waters, leading to oxygen depletion and fish kills.
Additionally, salts such as chlorides and sulfates from industrial or domestic sources are not removed by standard sewage treatment and can increase salinity in freshwater bodies, affecting aquatic life.
Microplastics, defined as plastic particles less than 5 millimeters in size, have become a significant emerging pollutant. They originate from the breakdown of larger plastics, synthetic fibers from laundry, and microbeads in personal care products. Standard treatment plants are not designed to filter out these tiny particles effectively, allowing them to enter rivers and oceans.
Nanoparticles, engineered for various industrial and medical applications, also pose challenges due to their small size and unknown long-term environmental effects. Their removal requires advanced filtration technologies.
Several factors contribute to the persistence of these pollutants through standard sewage treatment:
- Chemical Stability: Many micropollutants are chemically stable and do not break down easily during biological treatment. For example, synthetic hormones and pharmaceuticals have molecular structures resistant to microbial degradation.
- Low Concentrations: These substances are often present in very low concentrations, making their removal by conventional methods inefficient and economically challenging.
- Complex Mixtures: Wastewater contains a complex mixture of chemicals that can interfere with treatment processes. Some compounds may inhibit microbial activity or react to form more persistent substances.
- Limitations of Biological Treatment: Microorganisms used in secondary treatment cannot metabolize many synthetic or halogenated compounds. Biological processes are optimized for natural organic matter, not man-made chemicals.
- Physical Size and Solubility: Microplastics and nanoparticles are too small to be effectively trapped by sedimentation or filtration used in conventional plants.
Because of these limitations, some countries have begun implementing or testing advanced treatment stages beyond tertiary treatment, sometimes called "quaternary" or "fourth stage" treatment. These technologies aim to target micropollutants, heavy metals, and other persistent contaminants.
AOPs use highly reactive species such as hydroxyl radicals generated from ozone, hydrogen peroxide, or UV light to break down persistent organic pollutants into less harmful substances like carbon dioxide and water. These processes are effective against pharmaceuticals, pesticides, and EDCs.
- Ozonation: Ozone is bubbled through the wastewater, oxidizing organic contaminants.
- UV/Hydrogen Peroxide: UV light activates hydrogen peroxide to produce hydroxyl radicals.
- Fenton Reaction: Uses iron catalysts and hydrogen peroxide for oxidation.
Activated carbon has a high surface area and can adsorb a wide range of organic micropollutants from water. It is often used as a polishing step after biological treatment.
- Powdered Activated Carbon (PAC): Added directly to the treatment process.
- Granular Activated Carbon (GAC): Used in filtration beds.
Membrane technologies such as nanofiltration (NF) and reverse osmosis (RO) physically separate contaminants based on size and charge.
- Nanofiltration: Removes divalent ions, organic molecules, and some micropollutants.
- Reverse Osmosis: Provides the highest level of filtration, removing salts, metals, and nearly all organic contaminants.
Constructed wetlands use plants and microbial communities to naturally degrade pollutants. Bioaugmentation involves adding specialized microorganisms capable of degrading specific contaminants.
Electrochemical oxidation and coagulation can remove heavy metals and degrade organic pollutants by applying electrical currents.
Recognizing the limitations of conventional treatment, several countries have introduced regulations to address micropollutants and emerging contaminants:
- Switzerland: Pioneered mandatory removal of micropollutants in wastewater treatment plants, requiring advanced treatment technologies.
- Germany: Enforces strict limits on pharmaceutical residues and industrial chemicals.
- European Union: Updated the Urban Waste Water Treatment Directive in 2025 to include requirements for micropollutant removal.
- United States: The EPA is evaluating new guidelines for emerging contaminants such as PFAS (per- and polyfluoroalkyl substances).
These regulations drive innovation and investment in advanced treatment infrastructure worldwide.
Standard sewage treatment processes are highly effective at removing organic matter, suspended solids, pathogens, and some nutrients from wastewater. However, they fall short in eliminating micropollutants such as pharmaceuticals, endocrine disruptors, persistent organic pollutants, heavy metals, and microplastics. These substances pose significant environmental and health risks due to their persistence and toxicity. Advanced treatment technologies, including oxidation processes, adsorption, membrane filtration, and constructed wetlands, are essential to address these challenges. Regulatory frameworks are evolving to require such advanced treatments to safeguard water quality and ecosystem health. Continued research, innovation, and policy support will be critical to ensuring safe and sustainable water resources for future generations.
Micropollutants are trace organic compounds such as pharmaceuticals, personal care products, pesticides, and endocrine-disrupting chemicals that are present in very low concentrations but can have harmful effects on aquatic life and human health.
Pharmaceuticals are chemically stable and often not biodegradable by the microorganisms used in secondary treatment, allowing them to pass through treatment plants largely unchanged.
Advanced oxidation processes generate highly reactive species like hydroxyl radicals from ozone or hydrogen peroxide, which can break down persistent organic pollutants into less harmful substances.
Standard biological treatment does not remove heavy metals effectively. Special chemical or physical treatments are required to remove metals like mercury, lead, and cadmium.
Some countries, such as Switzerland and Germany, have enacted laws mandating the removal of micropollutants through advanced treatment stages. The European Union updated its Urban Waste Water Treatment Directive in 2025 to include such requirements.
