Two ways to carry water away
Every city must deal with two kinds of used or unwanted water. One is wastewater from toilets, sinks, showers and industry, which needs treatment before it is released. The other is stormwater, the rain and snowmelt that runs off roofs, streets and paved areas. How a city handles these two flows depends heavily on when and how its sewers were built.
There are two basic designs.
- Separate systems use two pipe networks. A sanitary sewer carries wastewater to a treatment plant. A separate storm drain carries rain runoff, usually to a nearby stream, river or other water body, often with little or no treatment.
- Combined systems use a single pipe for both. Wastewater and stormwater mix and flow together toward a treatment plant.
Many older cities, especially those with a long history of post-industrial cities growth and decline, built combined sewers, partly because a single pipe was cheaper and partly because of the practices of the time. Newer areas are generally built with separate systems, and many regulations now require it for new construction.
What happens in a combined system when it rains
In dry weather a combined sewer carries only wastewater, and the treatment plant handles it. During a storm, the volume can multiply because runoff from streets and roofs enters the same pipe. Treatment plants and pipes are limited in how much they can handle. To prevent flooding in streets and basements, combined systems usually include relief points called combined sewer overflows. When flow exceeds capacity, a mix of stormwater and untreated or partially treated sewage may be released into a nearby waterway.
Overflows are intended as a safety valve, and in many places they occur only a few times a year, but even occasional events can affect water quality and recreation. Regulators in many countries require cities with combined systems to reduce overflows, and meeting those requirements can cost very large sums. The related problem of waterway pollution is a reason that river revival projects often start with the sewers.
What happens in separate systems
Separate sewers avoid mixing, so sewage does not overflow due to storms in the same way. They have their own issues, though.
- Stormwater is not treated. Runoff picks up oil, litter, fertilizer, pet waste and other pollutants from streets and lawns and carries it directly to waterways in many systems.
- Cross-connections and leaks. Rainwater can enter sanitary sewers through cracks, faulty joints, or improper connections such as roof drains tied to the wrong pipe. This extra flow is often called inflow and infiltration, and it can overload sanitary sewers and treatment plants during wet weather even in separate systems.
- Storm drains can flood if they are too small for heavy rain or are blocked by debris.
So separation helps but does not guarantee that sewers will never back up or overflow.
Fixes cities consider
Reducing problems in combined systems generally involves some mix of the following. Which is best depends on local geography, cost and space.
- Separation. Building new pipes so that stormwater and wastewater travel separately. It can be effective but is expensive and disruptive, because it means digging up streets and sometimes work on private property.
- Storage. Large tanks or tunnels that hold excess flow during storms and release it to the treatment plant later when capacity returns.
- Treatment upgrades. Expanding plant capacity or adding treatment at overflow points.
- Reducing runoff at the source. Using green infrastructure such as rain gardens, permeable pavement, green roofs and street trees to absorb water before it reaches pipes. These measures can help, but how much they reduce overflow depends on scale and design, and they require maintenance.
- Operational measures. Real-time monitoring and control that use the existing pipes more effectively.
Cities often combine approaches because no single one is sufficient. Green measures can reduce the size of the gray infrastructure needed, but seldom eliminate it.
Why this matters for redevelopment
Sewer type shapes what new buildings do to the system. In a combined sewer area, adding paved surface and more residents can increase both wastewater and runoff entering the same pipe, so a project can worsen overflows unless it compensates. This is why many cities ask projects to manage rain on site, through retention, infiltration or detention tanks, and may limit the rate at which runoff leaves the property.
Capacity is also checked downstream, often well away from the site, since a pipe that is fine at the project’s doorstep may be constrained further along. Where a street is being rebuilt anyway, it can be a chance to separate sewers or add storage, tying the work into coordinated street projects.
Flooding in basements is another practical concern. When a sewer surcharges during heavy rain, wastewater can back up into low-lying buildings. Backflow prevention devices and maintenance can reduce risk for individual owners, but the underlying cause lies in the public system.
Cost, climate and fairness
Wet-weather improvements are costly and usually paid through sewer rates, which means households share the burden. Because rate increases hit lower-income households harder in proportion to income, many cities discuss assistance programs, though their design and scope differ widely.
Climate also matters, and it links to broader climate resilience planning. In regions where heavy downpours are becoming more frequent, older systems designed for earlier rainfall patterns may be pushed past their limits more often. Planners treat this as a reason to build flexibility, but forecasts are uncertain and local data is essential.
Reading your local system
A resident can often find out what kind of sewer serves a street by asking the local sewer or public works agency, or by reading its published plans and permit documents. Knowing whether a neighborhood is on a combined or separate system helps in understanding why rules about roof drains, pavement and tree planting exist, and why a city may care so much about the rainfall that falls on a single lot.