Why Wastewater matters

STOCKHOLM, Sweden. The road Henriksdalsringen meanders on a rocky hill in the east of Stockholm. At the roadside there are brown multi-storey buildings, it is a quiet residential area. Standing there, it is hard to imagine the roar and the smell under one’s feet. Because that is where the sewage from more than 1 million people comes together: Taking a shower, flushing the toilet or brushing our teeth is just as much a part of our everyday lives, as going to the supermarket or driving. Yet we rarely think about the infrastructure these acts require.

Operated by Stockholm Vatten, the Henriksdal wastewater treatment plant is one of the world’s biggest underground sewage plants. It covers a total area of 300,000m³ with 18km of associated tunnels and is buried under a rocky mound¹. At the foot of the hill an entrance and office building can be seen, but this is only a tiny part of the actual treatment plant. When entering and going downstairs, the first thing you see in the basement are parked bikes. “This way we can move faster through the sewage plant,” Tobias Salmonsson, a process engineer at the treatment plant, laughingly explains, “otherwise it would take us too long to get from A to B.”

The toilet as a dustbin

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Salmonsson picked his job at the plant because it is a mixture of all the subjects he dealt with during his studies: “It uses mechanical, biological and chemical filtration processes to clean the sewage.” He studied environmental and water engineering in Uppsala, the largest university city in Sweden. And while other people never think about their sewage again, once they have flushed the toilet, that is what Salmonsson’s job is all about. Every pollution of the wastewater by humans concerns him, as the aim of the plant is to return clean water to the Baltic Sea.

“Our biggest problem is what people throw into the toilets,” Salmonsson admits. Wet wipes, cotton swabs, tampons, condoms and bandages, do not dissolve in water as toilet paper does. That is why they already cause problems before they arrive at the treatment plant, clogging canals and pumps, that must be maintained or replaced then. The wastewater treatment plant itself removes all waste pieces larger than a few millimetres in the first cleaning step.

The biggest problem, however, is the substances that are invisible to the eye. Nitrogen and phosphorus for example, would cause eutrophication, if they got into the sea. Both substances are nutrients and lead to an excessive growth of algae. These algae then limit the sunlight and the oxygen for other inhabitants in the water. The biodiversity is declining. The limnologist Fred Erlandsson from Stockholm Vatten even believes that the Baltic sea is one of the most threatened waters in the world, due to eutrophication. “The plant is basically designed to get rid of nitrogen and phosphorus,” says Salmonsson. For example, a chemical is added that binds to phosphorus, making it larger and heavier. This makes it more likely to settle in the sediment tanks or get stuck in the sand filters. In another cleaning stage nitrogen gets removed by microorganisms.


Other micropollutants have not been filtered out at all in the Henriksdal treatment plant – until now. According to the Baltic Sea Centre, approximately 130 tons of microparticles from cosmetics and personal care products are flushed out with household waste every year. And of those, up to 40 tons passes through the sewage treatment plants and continues into the Baltic Sea². Once it reaches the sea it is difficult to remove them ever again and marine animals often eat the particles. Another issue are pharmaceutical residues, hormones, pesticides and multi-resistant germs. If they get into the Baltic Sea, there will be serious consequences for the aquatic ecosystem: Even small amounts of the active ingredient in the contraceptive pill can affect the reproduction of fish, antibiotics inhibit the growth of algae and underwater plants, and the painkiller diclofenac damages internal organs such as the liver and kidneys of the sea dwellers.

New challenges

Against this backdrop, Henriksdal is meeting two major challenges at the moment: First, Stockholm is one of the five fastest growing cities in Europe³. “And when the population grows, the Infrastructure must grow as well. Wastewater treatment is just as necessary as transportation or housing,” Fredrik Åkesson from SUEZ finds. In addition, a new EU water directive and the so-called Baltic Sea Action Plan require even stricter limits for phosphorus and nitrogen than before. Something must change. But that is easier said than done.

“The difficulty is to optimize the capacity of the plant although the volume in the rock is limited,” Åkesson explains. Because the plant was built underground in 1941, an indefinite expansion is not possible as would be the case in an open space. “The only way to meet the requirements is a membrane-based solution,” Åkesson says. He is an expert on the technology of the membrane bioreactor (LEAPmbr), which is soon going to be installed in Henriksdal.

This solution reuses already existing tanks, by adding membranes to them. The first two tanks are currently being retooled; they will be ready in one year. Until all the sewage treatment plant tanks are equipped with the membrane bioreactor it will still take 10 years. Once the upgrade is complete, the Henriksdal plant will be able to process up to 864 million litres of wastewater per day, which is double the plant’s current capacity.

“Simplified one could say that the membranes work like a coffee filter with much smaller pores,” kesson says. Solids that are larger than 0.04 μm do not fit through the pores of the membrane and are therefore separated from the clean water. This is more efficient than waiting for particles to settle, as is currently being done. “The key driver is to meet the limits for phosphorus and nitrogen,” Akesson admits, “but the pores are so small that they will also filter out most micro-plastics. The water will be almost free of particles afterwards.”

The membranes are so efficient hat later cleaning steps such as the sand filters could be omitted in the future, Tobias Salmonsson speculates. The newly gained space could then be used for a cleaning stage that completely removes or destroys microplastics and pharmaceutical residues. But since every cleaning step costs energy and money, it is highly unlikely a treatment plant will ever do more than required by laws. “We are a very big plant,” Salmonsson adds, “it is better for smaller plants to try out new technologies.”

He is convinced, that the government has to work “upstream”, to solve environmental problems.  “We can only work with what we get here. Of course, we will keep trying to remove all harming substances like microplastics out of the aquatic environment, but we can’t make them disappear, it will still arrive here and maybe then land on fields.” That is why it is important to him that legislators and private households change their minds and fight problems at the source.

A look on the bright side

But something is moving. Since January 1, 2019, the sale of cosmetic products containing plastic particles for cleaning, scrubbing and polishing purposes has been forbidden by the government in Stockholm. This concerns toothpastes, facial and body scrubs, shower gels and shampoos. In general, the interest in environmental subjects is quite high in Sweden. It is in Stockholm, that the activist Greta Thunberg started the so-called “Fridays for Future”, a protest in which students skip school to fight for climate protection. Sweden is also the country, that invented the word “Flygskam”, the shame of flying, because it is detrimental to the climate.

Tobias Salmonsson thinks that people in Stockholm are aware of Henriksdal. “That is one of the advantages of having an underground plant,” he says, “it is much closer to the city centre, which is why people maybe drive past on their way to work and wonder what happens inside.” Also, the Swedish curriculum requires that each class visits a sewage treatment plant once. The treatment plant itself launches campaigns to inform people from time to time. After all, all costs are paid by the municipality and therefore by taxes.

Salmonsson always wanted to give something back to the society with his job. “I am not saving the planet!”, he laughs, “But I do what I can.” He is convinced, that his job is contributing to Stockholm as a green city. Alexandra Fleetwood from Stockholm Vatten, the municipal water company of Stockholm, says, “the Henriksdal wastewater treatment plant has a major impact on water quality in Stockholm and on the reduction of nutrients and toxic substances.” In addition, a lot is won back in the plant.

For example, the sludge that accumulates in the primary settlers is pumped out. But instead of going to the garbage, it goes to the so-called digestion towers. It ferments there for 14 days. Just as bacteria convert sugar into alcohol during beer brewing, organic substances are converted into biogas here. This biogas is combustible and can therefore be used to generate electrical energy or to operate vehicles. All buses in Stockholm are powered using biogas from different plants and landfills.

After fermenting in the digestion towers, the sludge gets dehydrated and is distributed on fields in the north of Sweden. This way farmers don’t have to buy mineral fertilisers. The sewage sludge can therefore be used to produce both a fuel for vehicles and a soil conditioner.

Finally, heat is recovered. The clarified water, which after treatment is 10 to 20 degrees Celsius, is directed to Hammarbyverket, where its temperature can be transferred to a heating system. The water, which is sent into the sea, has only one degree left. All in all, the biogas and the heat together produce around 14 times more energy for the city than the wastewater plant consumes. “You should not look at it as waste, you should look at it as resource,” Tobias Salmonsson postulates.

The treatment steps