Which sewer system carries both sanitary waste and stormwater? A closer look at combined sewers.

Understanding sewer systems isn’t just for engineers in hard hats. It’s a practical topic that touches daily life—how rainy days suddenly feel a tad messier, or why a city’s pipe network matters when you flush the toilet after a storm. If you’ve ever wondered how a single underground network can carry both sanitary waste and rainwater, you’re in the right place. Let’s unpack the idea behind combined sewers and why they show up the way they do in real cities.

What the main sewer types actually do

First, a quick tour of the main players in the sewer world. Think of wastewater infrastructure as a city’s plumbing that never sleeps.

• Sanitary sewers: These pipes carry only wastewater from bathrooms, sinks, and kitchens. It’s the clean, predictable stream that heads toward a treatment plant after it’s been collected.

• Storm sewers: These are dedicated to rainwater and melted snow. They channel runoff away from streets and properties, usually straight to streams or rivers, bypassing wastewater treatment.

• Separation sewers: Some cities invest in keeping sanitary waste and stormwater totally apart. It’s two parallel systems—two pipes, two paths—designed to reduce the chances of overloading treatment facilities during storms.

• Combined sewers: Here’s the “two-for-one” option. A single pipe carries both sanitary waste and stormwater. It’s a shared artery that moves wastewater and rainwater toward a treatment facility, or, in heavy rain, toward an overflow point to prevent street flooding.

If you’re keeping score, the one that conveys both sanitary waste and stormwater is the combined sewer. It’s a design choice that makes urban sense in certain contexts, especially where space and cost constraints make it attractive to consolidate systems.

Why combined sewers exist—and what they’re good at

Combined sewers aren’t a new invention. In many older cities, space inside the city core is precious, and laying a second, separate network can be prohibitively expensive. A single set of pipes that handles both flows can be a practical, cost-conscious approach. Here’s why cities have leaned into this model:

• Economic efficiency: One pipe, one trench, one treatment pathway. Fewer assets to install and maintain can be appealing in dense urban areas.

• Flow management during moderate rain: In steady rain, a single pipeline can accommodate the increased volume without immediately overwhelming different systems. It’s like sharing a highway during rush hour—one lane can do the job when traffic isn’t spiking.

But there are trade-offs to chew on, too.

The flip side: challenges and concerns with combined systems

When rain comes hard and fast, combined sewers can face a headache. The same pipe that moves clean wastewater also has to absorb an influx of stormwater. If the stormwater volume overwhelms the capacity of the treatment plant, you can get combined sewer overflows (CSOs). That means untreated or partially treated wastewater is discharged into nearby water bodies, which is not ideal for environmental or public health reasons.

This is where the real-world balancing act shows up. On one hand, you gain cost and space efficiency. On the other hand, you’re weighing a potential risk to receiving waters during major rainfall events. Cities manage this through a mix of strategies, from implementing storage tanks and underground tanks to gradually separating some neighborhoods from the main combined system over time, where feasible.

Overflow management—the practical tightrope

CSOs are a familiar term in wastewater circles. They are not a badge of honor; they’re a symptom of a system that’s under stress during big storms. Utilities mitigate the risk with a few common approaches:

• Storage: Large underground tanks or advanced storage facilities can hold excess stormwater and sanitary flows until treatment capacity becomes available.

• Real-time controls: Modern systems use sensors and software to adjust flows, trying to slow down the surge before it hits the plant.

• Hybrid strategies: In some cases, a city will continue to use the combined system but create targeted separations or green infrastructure in high-risk zones to reduce the combined load.

• Overflow points: Some CSOs are designed as controlled relief valves—intentional, monitored releases that minimize street flooding, even if they trade a little short-term water quality for public safety during extreme events.

All of this matters because it isn’t just about pipes. It’s about protecting water quality, reducing flood risks, and making sure communities can function smoothly when skies unleash their worst.

A city’s footprint and the utility’s toolkit

Every urban area has its own story. A river bend, a centuries-old street layout, and a budget all help determine whether a combined system makes sense. In cities with hilly terrains, or where late-stage development happened without modern stormwater controls, mixed approaches often emerge. You might see a core of combined sewers accompanied by newer separate systems in the suburbs or newer districts. It’s a mosaic—one that’s continually evolving as cities grow and climate patterns shift.

A practical way to think about it is this: the sewer network is the city’s sustainability backbone. It’s not just about moving waste; it’s about anticipating rain events, protecting rivers and bays, and keeping neighborhoods safe and dry. The choice of system—combined, separated, or hybrid—reflects a balancing act between cost, space, capacity, and environmental goals.

What this means for wastewater treatment fundamentals

Understanding the types of sewer systems isn’t just trivia. It anchors a bigger picture about how treatment works and what operators need to monitor. Combined sewers, by their nature, push a broader range of flows toward treatment facilities. That means plant design and operation must consider peak wet-weather conditions in addition to dry-weather loads. The idea is to keep the water as clean as possible by the time it leaves the plant, even if occasional overflows can’t be completely avoided during severe rain events.

For students and professionals following GWWI WEF Wastewater Treatment Fundamentals, this topic helps frame several core principles:

• Flow management: Knowing whether a system is combined or separated informs how you model peak flows and plan for storage or treatment capacity.

• Interactions with stormwater: Stormwater isn’t the same as sanitary waste, but in a combined system, they collide in the same pipe. This shapes treatment strategies and environmental protection planning.

• Overflow considerations: CSOs aren’t just a nuisance; they’re a signal that a broader system design or operational adjustment is needed. Studying them builds a mindset for risk mitigation and resilience.

• Infrastructure aging: Many combined sewer systems are remnants of earlier urban planning. Recognizing their limitations invites thoughtful upgrades that balance cost, performance, and environmental safeguards.

A few analogies to make the concept stick

If you’re listening to this on a rainy afternoon while you’re sipping coffee, picture a busy highway during a downpour. The road carries both cars and rainwater. In light rain, the road handles the extra flow without issue. In a heavy storm, you need lanes that can close, diversions that keep traffic moving, and perhaps a few temporary chicanes to prevent pileups. A combined sewer works like that road: it’s efficient in the right conditions, but extreme weather tests its limits. Separating the flows is like building an alternate route for the rainwater and letting the cars keep to a familiar lane. Both approaches have their place, depending on geography, budget, and risk tolerance.

Real-world flavors and credible sources

Cities around the world manage these dynamics with varying strategies. In some historic cores, combined sewers survive as a pragmatic legacy system that still serves millions of people. In newer districts, authorities might push toward separation to improve water quality outcomes and reduce overflow risks during storms. For readers curious about standards and best practices, organizations like the Water Environment Federation and related global water networks provide a wealth of guidelines, case studies, and technical notes that illustrate how these decisions play out in practice.

A tiny but important note about terminology

You’ll hear the term CSO (Combined Sewer Overflow) thrown around in policy discussions and field reports. It’s not a victory lap—it’s a reminder that even well-engineered networks face weather-driven stress. Understanding CSO mechanisms helps engineers design smarter controls, better storage, and smarter green-infrastructure overlays that can relieve pressure on the system during downpours.

Bringing it back to fundamentals

So, which type of sewer system conveys both sanitary waste and stormwater? Combined sewers. They embody a practical, historically common approach to urban piping—one set of pipes, one treatment pathway, one shared fate during heavy rain. They’re a reminder that infrastructure is often a balancing act: cost versus capacity, simplicity versus resilience, and efficiency versus environmental stewardship.

If you’re studying the big picture of wastewater treatment fundamentals, keep these threads in mind:

• How flow characteristics influence treatment design

• The difference between stormwater and sanitary waste, and how a single system handles both

• The environmental and operational implications of overflows

• The evolving role of green infrastructure and gradual retrofits in modern cities

A closing thought

Cities are living systems, and their pipes tell part of that story. As populations grow and climate patterns shift, the conversation around sewer systems becomes more about resilience and thoughtful planning than about any single perfect solution. By understanding why some places rely on combined sewers—and how they manage the challenges that come with them—you gain a clearer view of how wastewater treatment fundamentals translate into real-world outcomes. It’s not just about pipes; it’s about safeguarding water quality, ensuring reliable services, and keeping communities healthy and dry when the skies open up. And that’s a conversation worth having—whether you’re new to the field or brushing up on the essentials for coursework and professional life alike.