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Views: 222 Author: Carie Publish Time: 2025-04-27 Origin: Site
Content Menu
● Understanding Sewage Treatment Levels
>> What is Secondary Sewage Treatment?
>> The Three Main Levels of Sewage Treatment
● Historical Progress and Federal Mandates
● Current Fraction of Americans with Secondary Sewage Treatment
>> On-site Wastewater Treatment Systems (OWTS)
● How Secondary Treatment Works
>> Biological Treatment Technologies
● Environmental and Public Health Benefits of Secondary Treatment
● Challenges and Opportunities
● Case Studies: Secondary Treatment in Action
>> California's Water Reuse Initiatives
● Future Trends in Sewage Treatment
>> Water Reuse and Resource Recovery
>> Smart Infrastructure and Monitoring
● FAQ
>> 1. What is the difference between primary, secondary, and tertiary sewage treatment?
>> 2. Why do some Americans not have secondary sewage treatment?
>> 3. How does secondary treatment benefit the environment?
>> 4. Are U.S. wastewater treatment plants keeping up with population growth?
>> 5. What are the future trends in U.S. wastewater treatment?
● Citation
Wastewater treatment is a cornerstone of public health and environmental protection in the United States. As urbanization and industrial activities increase, so does the need for effective sewage treatment. Among the various treatment levels, secondary sewage treatment is the federally mandated minimum for most municipal discharges, designed to remove at least 85% of organic matter and suspended solids from wastewater. But what fraction of Americans benefit from this level of treatment? This article explores the answer, delving into the history, current statistics, treatment processes, and future challenges of secondary sewage treatment in the U.S.
Secondary sewage treatment refers to the biological treatment of wastewater following primary treatment (physical removal of solids). This stage uses microorganisms to break down organic matter, significantly reducing pollutants before the water is released into the environment or undergoes further treatment.
Key Processes:
- Activated sludge systems
- Trickling filters
- Rotating biological contactors
Typical Removal Rates:
- Organic matter (BOD5): ≥85%
- Suspended solids: ≥85%
To understand secondary treatment, it's important to know the other levels:
1. Primary Treatment: This is the initial phase where large solids, grit, and floating materials are physically removed through screening and sedimentation. It typically removes about 30-40% of suspended solids and organic matter.
2. Secondary Treatment: The biological phase where microbes consume dissolved and suspended organic matter. It significantly improves water quality by reducing biochemical oxygen demand (BOD) and suspended solids by approximately 85%.
3. Tertiary (Advanced) Treatment: This optional stage further removes nutrients such as nitrogen and phosphorus, pathogens, and other pollutants to meet stricter environmental standards or for water reuse purposes.
The Clean Water Act (CWA) of 1972 marked a turning point by requiring all municipal sewage treatment plants to provide at least secondary treatment, or more stringent treatment where necessary for water quality. Federal funding and regulations have driven a dramatic expansion of treatment infrastructure.
- 1972: Only about 85 million Americans served by secondary treatment
- 2008: 223 million Americans (72% of the population) served by at least secondary treatment
- 2020s: Further improvements and expansions continue, with increasing emphasis on nutrient removal and water reuse
The CWA's implementation led to the construction of thousands of new treatment plants and upgrades to existing ones, drastically improving the quality of U.S. waterways.
- As of 2008: 223 million Americans (72% of the U.S. population) are served by centralized sewage treatment plants providing at least secondary treatment.
- About 3.8 million are served by facilities providing less than secondary treatment.
- Roughly 79 million rely on on-site septic systems, not centralized municipal treatment.
- Almost 15,000 publicly owned treatment works (POTWs) treat and discharge over 34 billion gallons of wastewater daily in the U.S..
- 37.5% of POTWs had advanced (tertiary) treatment as of 2022, exceeding secondary standards.
- The vast majority of centralized municipal plants provide at least secondary treatment, as required by law.
Chart: U.S. Population by Wastewater Treatment Type
| Treatment Type | Population Served (Millions) | % of U.S. Population |
|---|---|---|
| Secondary or Higher Treatment | 223 | 72% |
| Less than Secondary | 3.8 | 1.2% |
| On-site Septic Systems | 79 | 25% |
Recent studies suggest that about 25% of U.S. households use on-site systems like septic tanks, a higher estimate than previous surveys. These systems are not regulated as strictly as municipal plants and may not provide secondary treatment.
On-site systems typically involve a septic tank that provides primary treatment (settling solids) and a drain field for soil-based treatment. While effective in low-density areas, they can fail or cause groundwater contamination if poorly maintained or installed in unsuitable soils.
1. Primary Treatment: Physical removal of large solids and sedimentation.
2. Secondary Treatment: Biological degradation of dissolved and suspended organic matter.
3. Disinfection: Often follows secondary treatment to kill pathogens.
4. Advanced/Tertiary Treatment: Additional removal of nutrients, metals, or contaminants if required.
- Activated Sludge Process: The most common method, where air is pumped into aeration tanks to stimulate microbial growth that digests organic pollutants.
- Trickling Filters: Wastewater passes over a bed of media covered with microorganisms that consume organic matter.
- Rotating Biological Contactors: Large rotating disks partially submerged in wastewater support microbial biofilms that treat the water.
[Infographic: Activated Sludge Process]
- Wastewater enters aeration tanks.
- Microorganisms consume organic pollutants.
- Mixture settles in secondary clarifiers.
- Treated water is discharged or sent for further treatment.
Secondary treatment plays a crucial role in:
- Reducing Biological Oxygen Demand (BOD): Lower BOD means less oxygen depletion in receiving waters, protecting aquatic life.
- Removing Suspended Solids: Reduces turbidity and sedimentation, improving water clarity and habitat quality.
- Pathogen Reduction: Though disinfection is often separate, secondary treatment reduces pathogen loads.
- Preventing Eutrophication: By reducing organic matter, secondary treatment helps limit nutrient-driven algal blooms.
Many U.S. treatment plants are decades old, requiring upgrades to maintain compliance and efficiency. According to the American Society of Civil Engineers (ASCE), the U.S. wastewater infrastructure received a "D+" grade in recent years, highlighting the need for investment.
- Corroded pipes and outdated equipment increase the risk of failures and pollution.
- Funding gaps at federal, state, and local levels hinder modernization efforts.
Increasing population and urban sprawl put pressure on existing facilities, driving the need for expansion and modernization.
- Metropolitan areas often face capacity challenges.
- New developments require connection to existing systems or new treatment plants.
Stricter EPA standards for water quality and nutrient removal are prompting upgrades from secondary to advanced treatment in many regions.
- Nutrient pollution (nitrogen and phosphorus) is a major cause of dead zones in bodies like the Gulf of Mexico.
- Many states require enhanced nutrient removal beyond secondary treatment.
Septic systems, while prevalent in rural areas, can fail or pollute groundwater if not properly maintained, and typically do not provide secondary treatment.
- Failing septic systems contribute to localized water quality problems.
- Upgrading or connecting rural areas to municipal systems is costly and complex.
- NYC's wastewater treatment plants serve over 8 million people.
- All plants provide at least secondary treatment, with many upgraded to advanced nutrient removal.
- The city discharges treated water into the Atlantic Ocean and surrounding waterways.
- Many small towns rely on lagoon systems providing secondary treatment.
- Lagoon systems use natural processes but require large land areas.
- Some communities still use primary treatment or septic systems due to cost constraints.
- California is a leader in upgrading secondary treatment plants to tertiary or advanced treatment.
- Treated wastewater is reused for irrigation, groundwater recharge, and industrial uses.
- This reduces freshwater demand and helps manage drought conditions.
- Increasingly, wastewater is seen as a resource for water, energy, and nutrients.
- Technologies like membrane bioreactors and nutrient recovery systems are gaining traction.
- Sensors and AI help optimize treatment processes and detect failures early.
- Real-time data improves operational efficiency and compliance.
- Treatment plants must prepare for extreme weather events that can overwhelm systems.
- Flood-resistant designs and decentralized treatment options are being explored.
What Fraction of Americans Have Secondary Sewage Treatment?
As of the latest comprehensive data, about 72% of Americans are served by centralized sewage treatment plants providing at least secondary treatment, as required by federal law. This fraction has grown steadily since the 1970s, reflecting significant investments in public health and environmental protection. However, about a quarter of Americans rely on on-site systems, which may not provide the same level of treatment. Continued upgrades, stricter regulations, and better data collection are essential to ensure safe and effective wastewater management for all Americans.
The future of sewage treatment in the U.S. lies in modernizing aging infrastructure, expanding advanced treatment technologies, and integrating resource recovery and water reuse to meet growing environmental and public health challenges.
Primary treatment removes large solids and sediment through physical processes. Secondary treatment uses biological processes to degrade organic matter and suspended solids. Tertiary (advanced) treatment further removes nutrients, pathogens, and contaminants to achieve higher water quality standards.
About 25% of Americans use on-site systems like septic tanks, which are common in rural areas without access to centralized sewage infrastructure. These systems may not provide secondary treatment and are regulated differently from municipal plants.
Secondary treatment significantly reduces organic pollution and pathogens, improving water quality in rivers, lakes, and coastal areas. This protects aquatic life, public health, and recreational water uses.
While most urban areas are served by secondary or higher treatment, aging infrastructure and rapid urbanization challenge the capacity and effectiveness of many plants. Continued investment and modernization are needed to keep pace.
Trends include upgrading to advanced treatment technologies, integrating water reuse, improving nutrient removal, and expanding centralized treatment to underserved areas. Regulatory pressure and public awareness are driving these improvements.
[1] https://www.statista.com/statistics/1473528/distribution-wastewater-treatment-plants-usa-by-type/
[2] https://css.umich.edu/publications/factsheets/water/us-wastewater-treatment-factsheet
[3] https://www.grandviewresearch.com/industry-analysis/us-wastewater-secondary-treatment-equipment-market
[4] https://www.epa.gov/nutrientpollution/sources-and-solutions-wastewater
[5] https://www3.epa.gov/npdes/pubs/npdes_secondary_treatment_report_march2013.pdf
[6] https://www.everycrsreport.com/files/20141030_98-323_440beacb2b941a29c1c27a784b1f11a70aa3f459.pdf
[7] https://iwaponline.com/wp/article/25/9/927/97569/Household-level-wastewater-management-and-disposal
[8] https://www3.epa.gov/npdes/pubs/mstr-ch3.pdf
