Combined Sewer Overflows (CSOs) are a major issue in cities with older infrastructure, posing risks to both human health and the environment. This blog explains what CSOs are, why they happen, and their impacts, while exploring solutions being implemented to create sustainable urban water systems.
What Are Combined Sewer Overflows?
CSOs occur in older urban areas with combined sewer systems. These systems are designed to collect rainwater runoff, domestic sewage, and industrial wastewater in a single pipe. Typically, this combination of water is sent to a treatment plant where pollutants are removed before being released into water bodies.
However, during heavy rainfall or snowmelt, the system can become overwhelmed. When this happens, excess water—containing untreated sewage and stormwater—is discharged directly into rivers, lakes, or coastal waters. These discharges, meant to prevent urban flooding, are known as Combined Sewer Overflows.
How Common Are Combined Sewer Systems?
Combined sewer systems are most prevalent in older cities, particularly in the United States, Canada, and parts of Europe. According to the United States Environmental Protection Agency (EPA), there are approximately 860 communities in 32 states with combined sewer systems. Cities like New York City, Chicago, and San Francisco rely on this outdated infrastructure, making them more prone to the challenges associated with CSOs.
What’s Inside a CSO Discharge?
CSO discharges often contain:
- Untreated human waste
- Industrial waste contaminants
- Harmful bacteria and pathogens
- Nutrients like nitrogen and phosphorus
- Garbage and debris
Because this mixture bypasses treatment plants during overflow events, it can wreak havoc on ecosystems and public health.
Why Do Combined Sewer Overflows Happen?
Several factors contribute to the occurrence of CSOs in urban areas:
1. Aging Infrastructure
Many combined sewer systems were built in the late 19th or early 20th centuries when cities were much less populated. These systems now struggle to handle the increasing volume of wastewater and stormwater generated by modern urban areas.
2. Urbanization
The growth of impervious surfaces like roads, parking lots, and buildings means less stormwater is absorbed naturally into the ground. Instead, it rapidly flows into sewer systems, overloading them during storms.
3. Climate Change
With climate change causing more frequent and intense rainfall, combined sewer systems are facing unprecedented stress. The rising severity of storms means more overflow events.
4. Lack of Separation
Unlike modern sewer systems, which separate wastewater from stormwater, combined systems funnel everything into a single pipe. This design makes overflows inevitable under certain conditions.
The Impact of CSOs on the Environment and Public Health
The consequences of Combined Sewer Overflows go far beyond the temporary inconvenience of dirty water in city streets. Their impact is felt at both local and global levels.
Environmental Impacts
- Water Pollution
CSO discharge introduces harmful substances, like raw sewage and toxins, into natural water bodies. This depletes oxygen levels in the water, harming fish and aquatic ecosystems.
- Algal Blooms
The excessive nutrients (like nitrogen and phosphorus) in CSOs encourage the growth of algae. When algae die and decompose, they further reduce oxygen levels, leading to “dead zones” where marine life cannot survive.
- Ecosystem Disruption
Debris and contaminants from CSO events can accumulate in waterways, disrupting natural habitats and reducing biodiversity.
Public Health Risks
- Exposure to Pathogens
CSO water can contain harmful bacteria and viruses, increasing the risk of diseases like hepatitis, gastroenteritis, and respiratory infections.
- Recreational Limitations
Waterways affected by CSOs often become unsafe for activities like swimming, fishing, or boating. This not only limits community enjoyment but can also impact local tourism.
- Drinking Water Contamination
If untreated CSO water reaches drinking water sources, it can pose severe health risks for residents, particularly in communities with limited water treatment capacity.
How Are Cities Addressing Combined Sewer Overflows?
Thankfully, many cities and governments are taking proactive steps to mitigate the impact of CSOs. Here’s how they’re tackling the issue:
1. The Installation of Green Infrastructure
Green infrastructure solutions are designed to manage stormwater before it enters the sewer system, thereby reducing overflow risks. Examples include:
- Rain gardens that absorb runoff.
- Green roofs that capture rainfall.
- Permeable pavements that allow water to seep into the ground naturally.
2. Upgrading Sewer Systems
Cities are gradually replacing combined systems with separated sewer systems, which direct stormwater and wastewater into separate pipes. While expensive and time-intensive, this solution, much like SWPPP in Idaho, is highly effective in eliminating CSOs.
3. Building Storage Tanks and Tunnels
Many cities are constructing underground storage solutions to temporarily hold excess water during storms. After the storm subsides, the water is sent to treatment plants.
4. Implementing Real-Time Sewer Monitoring
Advanced technology allows cities to monitor sewer flow in real-time. This enables operators to better predict and manage overflow events, often preventing them altogether.
5. Enforcing Regulations and Permits
Environmental policies and permits are critical for reducing CSO-related hazards. Agencies like the EPA have mandated communities to reduce CSO discharges through its CSO Control Policy, which prioritizes water quality standards.
What Can Residents and Businesses Do to Help?
While governments and utilities play a significant role in addressing CSOs, individual actions can also make a difference. Here are some simple ways to contribute:
- Use Water Wisely
Avoid unnecessary water use during storms. For example, delay laundry, dishwashing, or other activities that send excess wastewater into the system.
- Install Rain Barrels
Collecting rainwater reduces runoff from your property, easing the workload of sewer systems.
- Support Green Projects
Advocate for your community to invest in green infrastructure initiatives.
- Educate Yourself and Others
Understanding CSOs and their impacts can lead to better community-supported solutions.
Conclusion
Combined Sewer Overflows remain one of the most pressing infrastructure challenges in aging urban areas. Yet, the steps outlined here provide a roadmap for reducing their occurrence and impact. Whether it’s upgrading sewer systems, investing in green infrastructure, or contributing on a personal level, every effort counts toward a cleaner, healthier future.
