Why Do We Treat Stormwater, and How Do We Do It?

Welcome to the Urbanista blog where we discuss water management challenges of Nordic cities. From safe drinking water distribution and stormwater collection, to building sustainable urban living environments. The Urbanista blog is based on the Urbanista podcast episodes. 

Key topics
 

Why is stormwater treatment important?

Stormwater treatment is essential for maintaining clean water systems, protecting public health and preserving the environment.
As cities expand, and more land is covered with impervious surfaces such as roads, roofs or parking lots, rainwater cannot infiltrate the ground. Instead, it flows rapidly over surfaces, picking up a wide range of pollutants before being discharged into rivers, lakes or oceans. If left untreated, stormwater can cause severe water pollution, pose risks to human health, and endanger aquatic ecosystems.
The pollutants carried by stormwater runoff include metals, nutrients, pesticides and others, all of which can degrade water quality and lead to environmental and economic consequences. In conclusion, one of the primary reasons to treat stormwater is to prevent water pollution.
Additionally, treatment is also crucial for reducing flooding and erosion in urban areas. When rainwater cannot infiltrate to the ground, it quickly accumulates, overwhelming storm drains and increasing the risk of flash floods. In many systems, stormwater drainage has not been designed to manage the extreme weather events that are becoming more frequent. As a result, untreated stormwater can lead to costly infrastructure damage, road closures and disruption to daily life.
Finally, where stormwater runs rapidly it can erode stream banks and washes away topsoil, further degrading water quality and habitats. So, beyond pollution control and flood prevention, treating stormwater supports sustainable urban development too.
Consequently, municipalities have integrated new nature-based solutions like wetlands. These solutions allow water to infiltrate the ground while simultaneously filtering out pollutants and recharging groundwater supplies.
 

Can you explain some current stormwater management solutions?

Stormwater is treated through a combination of physical, chemical and biological processes, each designed to remove pollutants before water is discharged into natural water. The choice of treatment method depends on land use, pollutant concentration and site-specific conditions like the urban catchment. Therefore, it is initially important to decide which treatment solutions you want to use.
In urban environments, where there are impervious surfaces that prevent natural infiltration, stormwater systems must be designed to capture and filter large volumes efficiently, so the most common stormwater technique methods involve sedimentation, filtration, and biological treatment.
 

Can you tell us more about biological stormwater treatment?

Biological treatment processes such as constructive wetlands, rain gardens or bioswales rely on plants and microbes to break down organic pollutants and absorb excess nutrients. These systems mimic natural processes and are particularly effective in treating stormwater in areas where land is available for that kind of green infrastructure. If you have enough space to implement things like huge wetlands or bogs, they are perfectly designed to treat the stormwater in biological way. However, they require careful design and maintenance to remain functional over time.
Additionally, physical treatments like filter chambers or vaults can be used before the polluted stormwater reaches the biological treatment area. You can use filter chambers and stormwater vaults separately or alone. But you can also integrate them before and after, that is upstream or downstream of, other treatment solutions.

How do stormwater vaults and filtration units help?

By capturing stormwater underground, stormwater vaults help prevent excessive runoff from entering local waterways like ponds, reducing flood risk and sediment pollution to the ponds and contributing to the effectiveness of your main stormwater solutions. They include sedimentation units and filtration units.
Filtration units can be used for a variety of stormwater challenges, but their storage volume capacity is less. With vaults you can extend your storage capacity and store more water, which prevents more floods, but the most important thing is you are giving the inflow more time to release its sediments before reaching the filtration units.
Of course, all solutions depend on the kind of problem you are managing. Basically, if your catchment area has a consistent behaviour, and it is relatively small, you can choose filter chambers as a solution. Additionally, if you are using less space in an urban environment, and you are sure you can use the filtration unit in the correct way, then it can be an efficient solution. Larger urban catchments with more uncertainty may require a stormwater vault.
 

How important is the good maintenance of stormwater solutions?

Maintenance is extremely important for treatment efficiency. The systems are not miracles. You must take care of them and maintain them regularly, and you have to know what is going on inside them. If there is no regular or periodical maintenance plan, that means your sedimentation units reach their capacity quickly, and after a high flow, all sedimentation you have collected in your sedimentation unit can be released or become outflow from your vault.
 

How often should we maintain these units?

The frequency of maintenance depends on how much sediment you are collecting over a certain time. For example, the size of your catchment will directly affect your sedimentation performance. I cannot say you have to maintain your sedimentation unit every six months, because if your catchment is relatively small your maintenance period can be longer. However, I can say that it should be completely cleaned at least once a year.

What future stormwater management innovations can we expect to see?

Basically, integrating more nature-based solutions into urban environments, whether it's constructing wetlands, rain gardens, etc.
Stormwater management is evolving to address new challenges, including that of urbanization or emerging concerns such as PFAS. While traditional methods are effective for removing sediments or nutrients, they are sometimes insufficient for persistent pollutants, which require advanced treatment technologies. Case studies demonstrate that hybrid systems combining filtration biological processes, like ponds, and filtration units create more effective solutions.
In general, blue-green infrastructure is offering multiple environmental and social benefits while improving stormwater retention and quality at the same time. In my opinion, stormwater treatment must adapt to function among stricter regulations and evolving pollution concerns. So, maybe future development should focus on more scalable, more cost-effective, and more resilient technologies to ensure cleaner water and urban environments.