Understanding Sewer Overflows: Risks to Environment and Public Health

Sewer overflows are more than a nuisance. They can flood streets, back up into homes, and contaminate local waterways, posing serious risks to both environmental and public health. From residential sanitary sewer overflow events to combined sewer overflows (CSOs) in urban areas, untreated sewage often enters rivers, streets, and drainage systems.

Understanding the impacts of sewer overflows on environmental health helps communities, policymakers, and homeowners take action. This guide explains why sewers overflow, the resulting environmental and health effects, and strategies to prevent and respond to overflows.

‍

Causes of Sewer Overflows

Overflows typically occur when wastewater systems cannot handle excess volume or large volumes of flow. Seattle’s frequent rainstorms and surface runoff can overwhelm the system, resulting in storm water drain overflow and increased pressure on combined sewer systems.

Blockages are another major cause. Debris, grease, and tree roots can restrict wastewater flow in sewer pipes, causing sewer line blockage and forcing backups. These may result in overflowing sewage drains in streets, basements, or rivers.

Key contributors:

• Heavy rain and surface runoff exceeding system capacity
• Aging sanitary systems prone to infiltration during wet weather or dry weather
• Grease, oil, and improper disposal of fats causing clogs in sewer lines
• Malfunctioning lift stations or broken pipes
• Industrial waste entering the system from businesses and households

Repeated overflows can lead to sewer damage, which increases the risk of environmental contamination and costly infrastructure repairs.

‍

‍

Environmental Consequences

The environmental impact extends beyond immediate contamination. Sewer discharge into waterways introduces nutrients, pathogens, and chemicals that can harm aquatic life and destabilize ecosystems. High nutrient levels often trigger algal blooms, reducing oxygen and threatening fish and plants.

Soil quality can also be affected. Contaminated soil may hinder plant growth, impact local agriculture, and contribute to long-term damage. Repeated overflows, even minor ones, can gradually harm ecosystems and increase environmental contamination.

Environmental impacts include:

• Water contamination from raw sewage and untreated stormwater
• Harm to aquatic ecosystems, including fish kills and plankton disruption
• Increased turbidity and reduced dissolved oxygen
• Soil degradation and vegetation damage near overflow sites
• Potential property damage from flooding and backups

Even a single event can have cascading environmental consequences if left unaddressed.

‍

Public Health Risks

Overflows pose direct public health concerns for Seattle residents. Overflowing drains carry bacteria, viruses, and parasites that can infect humans upon contact. Children, elderly individuals, and immunocompromised people are particularly vulnerable.

Both minor and heavy overflows can expose communities to pathogens, contaminating streets, homes, and recreational areas. Vulnerable populations, like children and the elderly, face the greatest risk.

Health risks include:

• Gastrointestinal illnesses from contact with contaminated water
• Skin infections and respiratory issues
• Increased risk during beach closures and after flooding
• Exposure to hazardous substances from industrial wastewater

Residents should avoid contact with standing water, report incidents promptly, and stay informed about local sewer conditions.

‍

Sewer System Types and Overflow Mechanisms

Understanding sewer systems explains how overflows occur:

• Separate sanitary sewer systems: They carry sanitary sewage to wastewater treatment plants separately from stormwater.
• Combined sewer systems: Often found in older urban areas, they carry both sewage and stormwater in one single pipe. During heavy rainfall, excess combined flow can trigger a combined sewage overflow through relief points to prevent property damage.
• Lift stations and collection systems: They move wastewater through the network but can fail or become overwhelmed, contributing to spills.

‍

Mitigation and Response

Effective management requires proactive and reactive strategies.

Proactive measures:

• Regular inspections and clearing sewage blockage, especially of grease and debris in sewer pipes
• Grease interceptors to reduce fats, oils, and grease from businesses and households
• Improved drainage to minimize stormwater overflow
• Infrastructure upgrades to handle peak flow and reduce incidents, including repairs to sewer lines and treatment plants
• Monitoring equipment to detect problems early and reduce overflow events

Reactive measures:

• Knowing how to fix overflows in minor incidents
• Municipal emergency protocols for spills
• Prompt reporting of sewer overflows to authorities for cleanup and repair

Balancing prevention and response reduces environmental harm and public health risks, helping to minimize incidents of sewer system overflow.

‍

Building Resilience Against Sewer Overflows

Preventing sewer overflows requires investment in infrastructure and community practices. Modernizing systems, implementing effective stormwater management, and maintaining regular inspections reduce sewage pollution.

Communities benefit when residents monitor and report problems, and when municipalities maintain their infrastructure. Long-term planning and awareness help mitigate environmental and health risks.

Long-term strategies include:

• Upgrading combined sewer system infrastructure and sewage treatment plants
• Implementing green infrastructure to manage runoff and reduce untreated stormwater
• Encouraging community reporting to identify issues early
• Enforcing proper waste disposal regulations, including avoiding flushing anything but toilet paper and disposing of grease and oil properly
• Promoting public education on what to flush and what to discard, to prevent clogs and protect sewer lines

‍

Keeping Our Environment Safe from Sewer Overflows

Sewer overflows have significant impacts on both the environment and public health. From contamination of waterways to threats to human well-being, communities must understand the causes and take action to reduce risks.

Aces Four provides guidance and expertise on addressing sewer infrastructure challenges and minimizing environmental impacts. For advice, support, or information on local solutions, contact us to learn how your community can respond effectively and protect both people and ecosystems.

‍

FAQs

Why do sewers overflow?

Sewers overflow when the system cannot handle the volume of wastewater due to blockages, heavy rainfall, or aging infrastructure. Debris, grease, and tree roots can restrict flow, while stormwater can overwhelm combined systems. Understanding these causes helps communities prevent repeated incidents and reduce risks to health and the environment.

What are the effects of sewer overflows?

Overflow events contribute to pollution and sewer spills, contaminating waterways and soil. Repeated incidents can disrupt local ecosystems, harm aquatic life, and damage vegetation. Human exposure to pathogens is also a concern, particularly for vulnerable populations.

What happens if an overflow occurs?

Untreated wastewater can enter streets, homes, or rivers, contaminating ecosystems, drinking water sources, and recreational areas. Prompt reporting helps authorities reduce environmental and health impacts.

How can communities reduce overflows?

Communities can reduce overflows by maintaining sewer lines, clearing blockages, and upgrading outdated infrastructure. Public awareness and timely reporting are also key, as residents play an important role in preventing damage and protecting public health.

How do overflows affect human health?

Contact with contaminated water can transmit bacteria, viruses, and parasites. This can cause gastrointestinal illness, skin infections, or respiratory issues, especially among children, the elderly, and immunocompromised individuals. Avoiding exposure and following safety guidance is essential.

How can minor overflows be addressed?

Residents should know how to fix sewer overflow safely in minor incidents: stop the backup, avoid contact with contaminated areas, and notify authorities promptly.

What is the role of stormwater?

Excess rainfall can trigger stormwater overflow, adding pressure to combined sewer systems. When the system exceeds capacity, untreated wastewater may enter waterways. Managing runoff effectively through proper drainage and green infrastructure helps reduce contamination and associated health risks.

What should residents avoid flushing or pouring down drains?

To prevent blockages, only flush toilet paper. Items like wipes, sanitary products, grease, oil, or chemicals should go in the trash. Proper disposal protects sewer systems, prevents overflows, and reduces contamination.