How to treat polluted stormwater?

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. This post is based on the interview with Rickard Granath – Stormwater Solutions expert at Uponor Infra.

Have you seen any change in the customer habits regarding planning or renewing stormwater systems?

- The awareness regarding the changing situation is there, and for a long time we have been working with floods and the flow of water. But now we are extra aware of the pollution in the stormwater and it’s a topic that we are all discussing and looking for solutions to.

So, there are a lot of hidden things that enter the stormwater system?

- You have car fumes that cause pollution, but also the materials that the cars are built of. When a car brakes, the rubber from the tyres is corroded and that all goes down the drain. The same goes for buildings. Roofs are made of copper and zinc that eventually corrodes and that too enters the stormwater.

What about these particles? How are they treated?

Do you also need to control how the water enters the tank?

- There is a certain volume, metres per second, that enters and then by having the correct volume leaving you can control the water retention time of the system. The water never stops moving, but it is slowed to a speed that gives the particles time to fall and be retained.

What about lighter components that float rather than sink?

- It’s actually a similar process, but now the particles are lighter than the water, so they float to the surface and then before the outlet, there will be some sort of screen that prevents the surface water from leaving. In conclusion, you could say that we take the middle layer of water at the end of the process.

And there is an oil separator. How does that help?

- Oil separators, and similar treatment techniques also separate the particles, but also filter the heavy metal ions that are in the water.  So, the range of tanks and filters available cater for a broad range of pollution.

Is there any other way to treat water?

- In the filters there is a chemical process. For example, an ion exchange where the filters are overloaded with calcium which is not considered a pollutant. When the heavy metals come into the filter, the filter materials will catch them and release calcium instead. They work like small magnets. Therefore, we have a few processes that are going on at the same time.

What about with potable water sources / lakes?

- Potable water sources, such as lakes, are also exposed to the same pollution. Any such lake is a really sensitive situation and you must take extra care with it. Everything that is built near or around the lake must be very carefully planned to reduce the risk of pollution.

Is it typical that the water comes from higher to lower lands, or does it also pass through other urban areas?

How do you manage smaller spaces, like parking lots?

- If you have a situation where the status of a downstream recipient is low, and the water quality cannot be made worse, it might be the case that you cannot even connect the drain to the city system. Then you would have to take care of it onsite, perhaps combining it with green areas and soil filters. Or you could retain the water, keep it there in a tank and then remove it later or reuse it to water the plants.

How does this kind of filter system work?

- You have the filter material, and the plants themselves that help open up channels which help the water infiltrate down into the soil. This is the natural part of the process, but there is also a tank that can retain water and give you the retention time we mentioned earlier. Here the particles will have the time to separate from the treated water. We could say it’s a ‘built’ natural solution. You could even take this collected water to use, for example when flushing the toilet. This way, we save on the clean water that would normally be used to flush with.

How confident are we in the effectiveness of these systems ?

- This area has been researched now for twenty to twenty five years. There are many universities and academies providing results which we can then use to predict a new, let’s say, solution woodwork. We have a lot of data to model pollution from an industry area, or a residential area, but you also have a lot of data to model any solution that you are building. This means you can predict. And when you can predict, you can also design and dimension the systems and also the solutions and be quite confident that it works. The treatment results are often up to 90% particles removed.

How much maintenance do the systems need?

What would you recommend to any urban planner thinking about a stormwater project?

- Visit our homepage www.uponor.com , where we have a lot of information both for the early stages of a project, i.e., what kind of solutions you could use, as well as a lot of documentation, online tools, and contact details of  our experts who can also help. We try to help from early in the project. And there is not a solution that fits all projects. It really depends on what you are doing, what kind of activity you have, etc. We are really happy to help with the early stages of a project, so that is a good way for you to start.