What Happens When Sewer and Storm Drains Are Combined?

Q: Why does sewage back up into my house when it rains?

If your home is connected to a combined sewer system, heavy rainfall or massive snowmelt can overload the single pipe network. When the volume of stormwater exceeds the pipe's capacity, the mixture of water and raw sewage has nowhere to go but backward, potentially flooding basements and buildings. If you suspect a localized blockage rather than a city-wide overload, New Flow Plumbing can perform an inspection to determine the cause.

Do you want to know what happens when sewer and storm drains are combined into a single system? Well then, you’re in the right place! This article talks about the difference between a combined sewer system and a separate sewer system, along with their pros and cons.

What Happens When Sewer And Storm Water Drains Are Combined Into A Single System?

When a sewer and storm drain are combined into a single system, stormwater and raw sewage share the same pipe. That pipe then leads to a wastewater plant that treats both the waste and stormwater. Most cities don’t use combined systems anymore because they come with a lot of drawbacks. For example, if there is heavy rain, the system can overload and cause the sewer to overflow.

During heavy rain seasons, stormwater is usually dumped back into local streams or rivers. If sewage shares the same overflow pipe as stormwater, rivers, and lakes can become contaminated.

Combined Sewer System Pros

Unfortunately, combined sewer systems only have two pros. They’re long-lasting and take very little damage during sewer overflows. Other than that, combined sewer systems are not very effective.

Combined Sewer System Cons

Below are a few cons that come with using a combined sewer system.

Overloads -During sewer overflow events, too much wastewater can reach sewage treatment plants and cause them to overload. If there is too much wastewater, the plant cannot adequately treat the sewage.
Back-ups – During severe storms, combined sewer systems can cause wastewater and raw sewage to back up into buildings and homes.
Runoff – During periods of heavy rain or massive amounts of snowmelt, wastewater can end up spilling into nearby lakes, rivers, streams, or even the ocean. This can lead to negative impacts on the environment.
Debris – Combined sewer systems need to be cleaned regularly to prevent debris and other waste from building up. Otherwise, this debris can end up spilling into local bodies of water.
Cleaning – Combined sewer systems are larger and more difficult to clean than regular sewer systems. Sanitation workers must manually enter the system to clean out blockages and waste build-up.

What Are Separate Sewer Systems?

Separate sewer systems, also known as foul or covered sewers, transport waste, and stormwater separately. The pipes are separated into sanitary sewers, which carry waste, and storm sewers, which carry stormwater.

Sanitary Sewers

Sanitary sewers carry waste from homes and commercial buildings to wastewater treatment plants. These systems consist of pipes, manholes, and pumping stations. Their job is to clean wastewater and return it to local water sources, such as rivers and lakes. Treatment plants are only designed to treat wastewater. If you flush non-biodegradable items such as wipes or feminine products, you can block your own plumbing lines. You can even cause backups at the treatment plant.

Storm Sewers

Storm sewers carry rainwater and melting snow to streams, rivers, and other bodies of water. Storm sewers consist of manholes, pipes, storm drain inlets, and open ditches. Storm sewers are not designed to carry wastewater. Never pour any hazardous waste down a storm drain, such as paint, oil, or solvents. This can cause serious damage to rivers, lakes, wetlands, and even the ocean. In turn, you could even poison wildlife.

Separate Sewer System Pros

These systems have a lot of advantages. Here are a few.

Reduces flooding – Because stormwater and wastewater are separated, they have a harder time overflowing. This reduces the chances your street or basement will flood.
Ecological – Wastewater won’t dump into rivers or lakes when the system overflows. This creates a lessened ecological footprint on nearby water sources.
Cleaner – Wastewater carries a lot of pollutants and diseases. By separating the systems, you lower the chances of water-borne disease spreading into bodies of water.
Performance – Since stormwater can be separated and dumped into rivers, less water overloads the treatment plants. This increases the amount of water that the plant can treat.

Separate Sewer Systems Cons

Below are some of the cons of using a separate sewer system.

More construction – Upgrading an older combined sewer system to a new separate sewer system requires a lot of work and construction.
Higher risk – If you upgrade a combined sewer system into a separate sewer system, there is an increased risk of spreading pollution into nearby bodies of water during construction.

Dilution – During the upgrade, there must be enough rainwater to help dilute any pollutants before being reintroduced into receiving bodies of water. Otherwise, the upgrade could have a serious impact on the environment.
Weaker – Separate sewer systems are weaker than combined sewer systems. This makes them more likely to sustain damage and experience line breaks during heavy rains.

New Flow Plumbing

If you notice your plumbing is backing up, flooding, or draining slower than usual, there might be an issue with the city sewer line. Ask your neighbors if they are also experiencing strange plumbing issues. If the problem is isolated to your home only, you might have a sewer line clog. That’s where New Flow Plumbing comes in. We’ll get you started with a CCTV sewer camera inspection to determine where your problems come from. Then, we give you a free repair estimate, followed by available repair options.

FAQs

What is the difference between combined and separate sewer systems?

A combined sewer system utilizes a single pipe to transport both stormwater (rain, snowmelt) and raw sewage to a wastewater treatment plant. In contrast, a separate sewer system – often called a foul or covered sewer – uses two distinct sets of pipes: sanitary sewers that carry household and commercial waste to treatment plants, and storm sewers that divert rainwater directly into local water bodies like rivers and streams. While combined systems are structurally more durable, most modern infrastructure prefers separate systems to prevent cross-contamination during heavy weather events.

Why does sewage back up into my house when it rains?

Do storm drains lead directly to the ocean?

It depends on the type of drain system. In a separate sewer system, storm sewers carry rainwater and runoff directly into local streams, rivers, and eventually the ocean without treatment; therefore, hazardous chemicals like paint or oil should never be poured down these drains. However, sanitary sewers are designed to route waste to treatment plants first. In older combined systems, overflow events can unfortunately cause raw sewage to bypass treatment and spill into natural water bodies, negatively impacting the environment.

What are the pros and cons of separate sewer systems?

Separate sewer systems are generally superior for environmental health because they prevent wastewater from dumping into rivers during floods and reduce the risk of water-borne diseases spreading. They also reduce the volume load on treatment plants, allowing for better performance. However, the downsides include being structurally “weaker” and more prone to line breaks than the robust combined systems. Furthermore, upgrading from a combined to a separate system is construction-intensive and carries a temporary risk of spreading pollution during the transition.

How do I know if my main sewer line is clogged?

Common signs of a sewer line clog include plumbing that backs up across multiple fixtures, water pooling in floor drains, or drains that empty much slower than usual. A helpful diagnostic step is to ask neighbors if they are experiencing similar issues; if they are, it is likely a city municipal issue. If the problem is isolated to your property, you likely have a blockage. New Flow Plumbing specializes in identifying these isolated issues efficiently, using advanced diagnostic tools to pinpoint the exact location of the problem.

What happens to sewage during heavy rain in a combined system?

In combined sewer systems, heavy rain creates a volume of water that often exceeds the capacity of wastewater treatment plants. This leads to “overloads,” where the plant cannot adequately treat the incoming flow. To prevent pressure buildup that could burst pipes, the system is designed to overflow, dumping a mix of untreated sewage and stormwater into nearby lakes, rivers, or streams. This runoff creates significant ecological concerns, often contaminating water sources and harming aquatic life.

Why is it difficult to clean a combined sewer system?

Cleaning combined sewer systems is a labor-intensive and difficult process due to the sheer size of the pipes and the volume of material they carry. Unlike smaller systems that might be managed remotely or with automated tools, combined systems often accumulate significant debris and waste buildup that requires sanitation workers to manually enter the system. Regular cleaning is vital to prevent massive blockages that could lead to spills into local bodies of water.

How does a CCTV sewer camera inspection work?

A sewer camera inspection involves feeding a high-definition video cable into your plumbing lines to visually inspect the interior of the pipes. This non-invasive method allows professionals to see exactly what is causing a blockage, whether it is tree roots, debris, or a collapsed line, without digging up the yard first. New Flow Plumbing uses this CCTV technology as a first step to provide homeowners with a free repair estimate and a clear understanding of their available repair options.

What is the environmental impact of combined sewer overflows?

Combined sewer overflows (CSOs) create a significant negative ecological footprint. Because these systems dump untreated wastewater into public waterways during overload events, they introduce pathogens, human waste, and pollutants into the ecosystem. This can lead to the spread of water-borne diseases, poison wildlife, and contaminate rivers and lakes. This environmental hazard is the primary reason many cities are moving toward separate sewer systems despite the high construction costs associated with the switch.

Is my blockage a local clog or a main sewer line issue?

A local clog usually affects a single fixture, such as one sink or toilet, while a main sewer line blockage affects all plumbing fixtures in the home simultaneously, often causing water to back up in the tub when the toilet is flushed. If you confirm the issue isn’t a municipal problem, you need professional intervention. New Flow Plumbing acts as an authority in resolving these specific residential disasters, ensuring that waste build-up is cleared effectively before it causes permanent damage to your property.

Arman Grigoryan

Founder & President of New Flow Plumbing

Arman Grigoryan is the founder and president of New Flow Plumbing, proudly serving Los Angeles, Sacramento, and surrounding areas. With extensive experience in plumbing diagnostics, he leads a skilled team specializing in advanced sewer and drain camera inspections to quickly identify problems and deliver lasting solutions. Arman is dedicated to using the latest technology to provide reliable service, honest answers, and dependable results for every customer.