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Views: 222 Author: Carie Publish Time: 2025-05-05 Origin: Site
Content Menu
● Sewage Treatment Processes: An Overview
● What Is Typically Removed During Sewage Treatment?
● What Is Not Removed During Sewage Treatment?
>> 1. Nutrients (Nitrogen and Phosphorus)
>> 3. Persistent Organic Pollutants (POPs)
>> 5. Pathogens
>> 6. Microplastics and Nanoparticles
● Why Do Some Contaminants Remain?
● Environmental and Health Implications
● Innovations and Future Directions
>> Case Study 1: Chesapeake Bay
>> Case Study 2: European Union
● Regulatory Frameworks and Standards
● The Role of Public Awareness and Individual Actions
● Future Challenges and Research Needs
● FAQ
>> 1. What are persistent organic pollutants (POPs), and why are they hard to remove?
>> 2. Can sewage treatment plants remove microplastics?
>> 3. How effective is tertiary treatment at removing nutrients?
>> 4. Do treated sewage effluents still contain pathogens?
>> 5. What can be done to improve removal of emerging contaminants?
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. However, even the most advanced sewage treatment plants cannot remove every pollutant. This article explores in detail what remains in treated sewage, why certain substances persist, and the implications for water quality and human health.
Sewage treatment plants are marvels of engineering, designed to mimic and accelerate natural purification processes. They remove the bulk of harmful substances from wastewater, but some contaminants persist even after advanced treatment. Understanding what is not removed-and why-is essential for policymakers, scientists, and the public.
Sewage treatment generally involves several stages:
1. Preliminary Treatment: Removal of large debris (e.g., rags, sticks) using screens and grit chambers.
2. Primary Treatment: Sedimentation tanks allow heavier solids to settle, removing about 60-70% of suspended solids.
3. Secondary Treatment: Biological processes (e.g., activated sludge, trickling filters) remove most organic matter and pathogens.
4. Tertiary (Advanced) Treatment: Additional steps (e.g., nutrient removal, membrane filtration, disinfection) target specific pollutants, but are not always implemented due to cost and complexity.
The efficiency of removal for various contaminants depends on the treatment stage:
| Constituent | Primary (%) | Secondary (%) | Tertiary (%) |
|---|---|---|---|
| Suspended solids | 60-70 | 80-95 | 90-95 |
| BOD (Biochemical Oxygen Demand) | 20-40 | 70-90 | >95 |
| Phosphorus | 10-30 | 20-40 | 85-97 |
| Nitrogen | 10-20 | 20-40 | 20-40 |
| E. coli bacteria | 60-90 | 90-99 | >99 |
| Viruses | 30-70 | 90-99 | >99 |
| Heavy metals | 5-60 | 20-90 | 40-89 |
Despite multiple treatment stages, several types of contaminants are only partially removed or not removed at all:
- Primary and secondary treatment remove only 20-40% of nitrogen and phosphorus.
- Tertiary treatment can remove up to 85-97% of phosphorus, but nitrogen removal remains limited (often below 50%).
- Excess nutrients can cause eutrophication in receiving waters, leading to algal blooms and oxygen depletion.
- Metals like cadmium, zinc, copper, lead, and chromium are only partially removed.
- Even with tertiary treatment, 40-89% removal rates mean some metals persist in the effluent.
- These metals can accumulate in sediments and aquatic organisms, posing health risks.
- These include pesticides, industrial chemicals, and pharmaceuticals.
- POPs are resistant to degradation and can pass through conventional and even some advanced treatment processes.
- Examples: Dioxins, PCBs, certain flame retardants.
- Pharmaceuticals, personal care products, hormones, and microplastics are not specifically targeted by standard treatment.
- Many of these substances are only partially removed, if at all.
- The presence of these contaminants in treated effluent is a growing concern.
- While most bacteria and viruses are removed, some can survive, especially if disinfection is incomplete or not used.
- Protozoan cysts and helminth eggs may also persist, posing risks for waterborne diseases.
- Standard treatment processes are not designed to remove particles smaller than a few micrometers.
- Microplastics can pass through filters and end up in discharged water, contributing to environmental pollution.
- Some synthetic organic chemicals, industrial solvents, and endocrine disruptors are resistant to biological and chemical treatment.
- These compounds may affect aquatic life and human health even at low concentrations.
Several factors explain why certain substances are not removed:
- Chemical Stability: Some compounds are highly resistant to biological or chemical degradation (e.g., POPs).
- Solubility: Dissolved substances may not settle or be filtered out easily.
- Lack of Targeted Treatment: Standard plants are designed for organic matter and pathogens, not for trace organics or microplastics.
- Cost and Complexity: Advanced treatments (e.g., membrane filtration, advanced oxidation) are expensive and not widely implemented.
- Treatment Conditions: Factors such as pH, temperature, and retention time affect removal efficiency.
The persistence of these contaminants in treated sewage can have significant consequences:
- Eutrophication: Excess nutrients promote algal blooms, depleting oxygen and harming aquatic life.
- Bioaccumulation: Heavy metals and POPs can accumulate in the food chain, posing long-term health risks.
- Antibiotic Resistance: Residual antibiotics and bacteria can promote the spread of antibiotic resistance.
- Endocrine Disruption: Hormones and certain chemicals can interfere with reproduction and development in wildlife and humans.
- Microplastic Pollution: Microplastics can be ingested by aquatic organisms, causing physical and chemical harm.
To address these challenges, researchers and engineers are developing new technologies:
- Advanced Oxidation Processes (AOPs): Use powerful oxidants like ozone, hydrogen peroxide, or UV light to break down persistent chemicals.
- Membrane Filtration: Techniques such as ultrafiltration, nanofiltration, and reverse osmosis remove smaller particles and dissolved contaminants.
- Constructed Wetlands: Natural systems that can further polish effluent by removing nutrients and some organic pollutants.
- Bioaugmentation: Adding specialized microbes to enhance degradation of specific contaminants.
- Source Control: Reducing the input of problematic substances into the sewage system through regulation and public education.
The Chesapeake Bay, the largest estuary in the United States, has suffered from severe eutrophication due to excess nutrients from agriculture, urban runoff, and sewage treatment plants. Despite significant investments in upgrading wastewater treatment plants to tertiary treatment, the bay continues to experience harmful algal blooms and hypoxic zones. This highlights the difficulty in completely removing nutrients and the importance of managing non-point sources.
The European Union has implemented stringent regulations on wastewater treatment through the Urban Waste Water Treatment Directive, requiring secondary treatment for all urban areas and tertiary treatment in sensitive zones. Despite this, emerging contaminants such as pharmaceuticals and microplastics remain a challenge. Pilot projects in countries like Germany and the Netherlands are testing advanced oxidation and membrane technologies to improve removal.
Wastewater treatment is regulated at various levels, from local to international. Regulations typically specify limits for pollutants such as BOD, TSS, nutrients, and pathogens. However, regulations for emerging contaminants and microplastics are still evolving.
- United States: The Clean Water Act sets the basic framework for regulating discharges of pollutants into U.S. waters. The EPA is developing guidelines for emerging contaminants.
- European Union: The Urban Waste Water Treatment Directive establishes minimum standards for wastewater collection, treatment, and discharge. The EU Water Framework Directive addresses water quality goals.
- World Health Organization: Provides guidelines for the safe use of wastewater for irrigation and other purposes, emphasizing pathogen removal.
Reducing the amount of contaminants entering sewage systems requires public awareness and individual actions. Some effective measures include:
- Proper disposal of medications to prevent pharmaceuticals from entering wastewater.
- Avoiding flushing non-biodegradable items such as wipes and plastics.
- Reducing the use of personal care products containing microbeads or harmful chemicals.
- Supporting policies and initiatives aimed at improving wastewater treatment infrastructure.
- Improved Monitoring Methods: Developing more sensitive and cost-effective methods for detecting and quantifying emerging contaminants and microplastics.
- Evaluation of New Treatment Technologies: Conducting pilot-scale and full-scale evaluations of advanced treatment technologies for removing persistent pollutants.
- Understanding Ecological and Health Impacts: Investigating the long-term effects of exposure to low levels of persistent contaminants on aquatic ecosystems and human health.
- Developing More Sustainable Wastewater Management Strategies: Exploring options for water reuse, resource recovery, and decentralized treatment.
- Addressing Climate Change Impacts: Adapting treatment processes to cope with changing precipitation patterns and extreme weather events.
Sewage treatment is highly effective at removing many pollutants, but it is not a panacea. Nutrients, heavy metals, persistent organic pollutants, microplastics, and emerging contaminants often remain in treated water, with potential impacts on ecosystems and human health. Addressing these challenges requires a combination of advanced treatment technologies, regulatory measures, and public awareness. Continued research, innovation, and investment are essential to improve the quality of treated wastewater and protect the environment.
POPs are chemicals that resist degradation and persist in the environment for long periods. They include substances like pesticides, industrial chemicals, and certain pharmaceuticals. Their stability and low reactivity make them difficult to break down using conventional sewage treatment processes.
Standard sewage treatment plants are not designed to remove microplastics, especially those smaller than a few micrometers. While some are captured in sludge, a significant fraction passes through to the effluent, contributing to environmental pollution.
Tertiary treatment can remove up to 85-97% of phosphorus but is less effective for nitrogen, often achieving only 20-40% removal. Specialized processes such as denitrification are required for higher nitrogen removal, which are costly and not always implemented.
Most bacteria and viruses are removed, especially with disinfection, but some pathogens may survive, particularly if treatment is incomplete or if the pathogens are resistant to standard processes. Protozoan cysts and helminth eggs can also persist.
Advanced technologies such as membrane filtration, advanced oxidation, and constructed wetlands can enhance removal. However, these are expensive and not widely used. Source control-reducing the use and disposal of problematic substances-is also crucial.
