Why do we need to separate oil from the stormwater?

Remember when, back in school, our physics teachers gave us an experiment to mix oil and water and then we watched them separate?
Well what if, instead of a small glass full, we had a huge amount of water mixed with oil? And instead of a beaker of water, we had huge flows of water in constant movement, just like the conditions in a stormwater solution? How would we separate that water from the oil mixed with it and how do we clean it?
That is the question we put to our guest today on the Urbanista, Rickard Granath - Solution Manager, Stormwater at Uponor Infra.

How do we define an oil separator, and why do we need them?

In this case we are talking about oil separation from stormwater, but it could also be in the context of a sewer. The separator is a technical device that allows us to separate the oil from the water.

And where does this oil come from?

Well, if we talk about stormwater, it could be a parking area or a road where there is transport. And you could have oil coming from the dripping hydraulics. Then stormwater can flush the oil from the asphalt down to the drain. A lot of traffic might be one source, but it could be in the context of industry. If you are in the process of doing mechanics, and you need oil for that, that will mean that oil will enter the sewer system.

Why is it important that we separate the oil from these waters?

These drains, eventually, end up in a recipient such as a river or a lake. In an older combined system, the stormwater might even end up in the sewer. In either case, you don’t want oil because in the lakes it’s a pollutant and in the treatment system the oil will not be treated so it will basically pass through the treatment system.
It’s also easier to separate the oil where it has a higher concentration, this means upstream or near the parking lot where the contamination is occurring.

Are there different types of oil separators for different oils?

Not really for different oils, but there are different types of oil separators. Mainly there are two designs. Class I and class II. The class I is a better oil separator, but it is also more costly. You can choose which is suitable for your situation, but I would say normally the class I is used.

Is there a legal framework which tells which class of oil separator you should use?

Basically, yes. There is legislation that classifies the situations when and where you need a class I and a class II. Parking lots, for example. Or a car wash where there is a lot of oil and you are using detergents and other chemicals. It’s actually harder to separate in that case, because you get a mix of oil and water, which is also harder to separate. In this case you would need a class I to get the oil out of the water again.

What would be the main consideration when choosing the type of oil separator?

The type of site that you have. However, it varies in size, so if you have a big parking area you couldn’t use the smallest separator. You need a certain retention time that varies according to the flow. Because of that you need to consider the size.
The water needs a certain time to stand still and allow the oil and water to separate which also creates a sludge which needs to be cleaned.

How do we know when the separators need cleaning or maintenance?

If you have chosen a separator suitable for the size of the site, then maybe you will need to maintain it yearly. We have an alarm in the bottom saying that the sludge value is full, and one in the top indicating the oil storage is full, and finally there is an alarm to tell if the level of the water goes up. It could tell you different things, for example if a valve is closed or if there is a blockage. Basically, there are three sensors. The newer ones will tell you if it is 20% or 30% full, for example, but the more traditional models simply indicate when they are full.

Does someone need to come and check regularly?

Normally it will work with a control box where you will see an indication that something is happening. More modern versions can also be connected to a cloud, and you can get mobile phone notifications. If you are a company with many filters to clean, this will help you plan which filters to visit and in which order.

What about the retention time. How do we know how much retention we will need?

It could be useful to use a retention method with an oil separator. Another possibility is to use a bypass. When there is a really big flow, part of the water can bypass the oil separator. Which might seem questionable, but during the year 90% of the water will pass through the oil separator. And the water that bypasses the system will usually be the cleaner, most diluted water. And this will happen maybe once or twice a year during extreme events. It’s a balance between performance and investment. We help the customer find the optimal size.

If we think about the context of a parking lot. Traffic, cars and trucks, might be passing over the filter system. Is this something we need to consider?

Just as there are different types of oil separators, there are also different materials used to construct them. Ours is made for parking areas and you can have trucks passing over them because they are designed for that. This gives flexibility because when planning we don’t always have the perfect green space to install the separator. For maintenance purposes, it is also better to have the flexibility to choose where to place it.

What is it made from?

The lid is cast iron and the tank is polyethylene. Our Weholite system has been on the market for forty years, and we have produced chambers and separators from that system. It’s really robust. Also, these oil separators last forever.

And how long do the filters last?

At some point they could clog and it is possible to change them, but it’s not like a filter that collects oil. It doesn’t work like that, so the filter need changing maybe only every five or ten years. However, the Weholite system will be there for 100 years or so.

In the end, the result is cleaner water?

It’s a good point. After all the technicalities and specifications, it’s the recipients, the rivers and the lakes that we are saving. We are helping prevent pollution. That’s the most important thing to remember.