Frequently asked questions

We want better air quality in cities! To achieve this, we rely on the natural abilities of mosses and combine them with smart technology. This is complex and not always easy to understand. In the following, we answer the questions we have been asked most frequently in recent years and hope to make our approach easier to understand. If you have any further questions, please feel free to contact us at We also welcome your feedback.

The founders Liang Wu and Peter Sänger both have personal experience of air quality. Liang comes from Shanghai, where he experienced the negative impact of polluted air first hand: his health deteriorated visibly and only improved after he returned to Germany. Peter, too, encountered the consequences of smog on the quality of life in cities while travelling. When Liang and Peter met at university, they shared the desire to develop a sustainable solution to this problem. With a young team of experts from the fields of horticulture/biology, computer science, architecture and mechanical engineering, Green City Solutions was launched in 2014.

Air pollution is one of the biggest environmental problems worldwide and is responsible for every 7th death. With its products, Green City Solutions wants to help people in particularly polluted urban locations to enjoy cleaner air and thus better health and quality of life. Especially in the current Corona pandemic, the urgency has become more than clear. Poor air quality promotes not only cardiac arrhythmia and dementia, but also lung and respiratory diseases. These are the very exposures that are now turning families, friends and neighbours into risk groups. According to recent surveys, up to 20 percent of Covid-19 deaths are due to air pollution.

Our core product is a moss filter. It is based on the idea of combining nature with digital technology to improve air quality. There is more to the natural superhero moss than meets the eye! Mosses bind fine dust, some are antiseptic, antiviral and fungicidal; real all-rounders for air pollution control and health. With the interaction of sensors, intelligent ventilation, irrigation and software, this natural cleaning performance is made usable and measurable. The results are the CityTree & the CityBreeze: a combination of street furniture, respectively outdoor communication and biofilter with complex technology inside that controls the irrigation and ventilation of the moss modules and collects all kinds of environmental and air quality data.

CityTree & CityBreeze are thus the first biotech filters to demonstrably improve air quality.

Mosses have a huge surface, comparable to the human lung. On this surface, fine dust particles from the air sucked in are electrostatically attracted and stick, so to speak, similar to a microfibre cloth. The difference to a microfibre cloth is that the moss metabolises the fine dust particles, i.e. “eats” the fine dust. Extensive measurements have shown that the moss modules can remove up to 82% of the fine dust from the air. However, the distance up to which this effect is measurable depends on numerous factors such as wind direction and strength. Under good conditions, the effect is still scientifically detectable at 5-10 metres. Each CityTree can clean, cool and humidify about 3500, each CityBreeze 1200 cubic metres of air per hour, which corresponds to the breathing volume of 7,000 and 3,000 people respectively. Both filters are also highly efficient cooling systems that only need the power comparable to about 5 light bulbs (approx. 125W) for cooling. The advantage of these bio-filters over conventional air filters is also that they never become “full”, as the fine dust is converted into biomass by the moss. In other words, a filter that cleans itself, so to speak. A filtration without filter waste!

There are about 20,000 species of moss all over the world, from the polar regions to the desert. The moss used by Green City Solutions is of course not taken from nature, where it serves an important purpose. Green City Solutions has built the world’s first vertical moss farm for the purpose of moss cultivation in Bestensee near Berlin. Here, mosses grow on a good 1,200 square metres in the form of vertical moss mats. The moss researchers at Green City Solutions are constantly striving to find better and better moss species and mixtures for the purpose of air purification and freshening. In nature, mosses need about 4 years to grow together into a dense, lush green moss surface. Through special cultivation and ideal supply conditions, the time in the moss farm could be shortened to less than 12 weeks. Once inserted into the CityTree or CityBreeze, the specially developed bio-algorithm ensures that the mosses are optimally cared for. This means that they can theoretically remain in the biofilters for an unlimited period of time. We are working on perfecting the algorithm so that exactly this is realised. Currently, however, we replace the moss mats every 6 months to ensure ideal performance at all times. They can then regenerate on the moss farm under ideal conditions and are then reused for urban use.

Since its launch, Green City Solutions has already worked with numerous cities and companies across Europe. The first version of the CityTree was used in over 50 projects for several years. The novelty of the technology, the complexity of linking hardware and software and also the demands of the living creature moss brought numerous challenges in the first years. Technical problems, special site requirements, complications with the maintenance service and changing water qualities were some of the factors that affected the vitality and performance of the organism. At the end of 2018, based on a critical analysis, it was decided to develop a completely new product generation that would take into account all the findings of the past.
In a development project that lasted two years and was co-financed with almost two million euros from the European funding programme Horizon 2020, significant improvements were made to the CityTree. The funding was instrumental in using the findings from the previous projects to make the moss module smarter and further improve the performance of the CityTree.

It is now a good year since the two founders Peter Sänger and Liang Wu unveiled the new generation of the CityTree in front of the press and many other spectators near the Gedächtnis-Kirche in the centre of Berlin. In the two years prior to the launch, the CityTree underwent significant further development, which was co-financed and thus made possible by the European funding programme Horizon 2020. The seven-figure funding was instrumental in using the knowledge gained from the previous projects to make the moss module smarter and further improve the performance of the CityTree. These are the main four milestones at a glance:

1. The bio-algorithm: best conditions even far away from the natural habitat.

Mosses are capable of amazing things and are also highly adaptable. But in the middle of the city, on a busy square or next to a busy road, our little superheroes need our support to stay vital – that’s the only way they can perform. A specially developed bio-algorithm ensures that the mosses are supplied according to their needs, depending on weather conditions and other factors such as local air quality. With the help of more than 30 built-in sensors and a linked analysis database, factors such as aeration and irrigation are controlled in such a way that the mosses are optimally supplied at all times and can concentrate fully on filtering and refreshing the city air.

2. The moss farm: learning to understand mosses

Mosses have a huge surface area, basically like human lungs. On this surface of moss leaves, stems and hairs, fine dust particles from the air sucked in are held and thus filtered. The particles are attracted electrostatically and absorbed into the interior of the cell with the addition of water. There, the fine dust particles are decomposed and metabolised. For this we need to understand which of the approximately 20,000 known mosses is best suited for the purpose of air purification and freshening and how we can support this super moss in doing so, regardless of location and weather. That is why we have built the world’s first moss farm in Bestensee near Berlin, where the mosses are grown vertically (join our next virtual moss farm tour). Here we explore the green depths of our mosses and are able to cultivate finished mats full of dense, green and hungry moss in just 12 weeks.

3. AirCare: Air quality becomes visible

Nobody would drink a glass of dirty water. But what about breathing dirty air? In addition to air purification and cooling, an essential part of our work is to make air quality visible. For this purpose, we have developed our AirCare dashboard, in which the data measured by the sensors are evaluated and visualised. This allows us to analyse the air quality and the impact of our products on the microclimate in real time and present it in an easy-to-understand way, both on the product itself or in the web interface from anywhere.

4. Scientific validations & CE certification

The verifiability of the performance of our biofilters in outdoor applications is extremely demanding and could only be realised in cooperation with renowned institutes and long-term measurement campaigns. In 2020, the time had finally come. Supported by the European Commission, the Leibnitz Institute for Tropospheric Research Leipzig and the Institute for Air-Conditioning and Refrigeration Dresden, the CityTrees were set up at various locations and accompanied by measurements. These tests were carried out in the laboratory, in a hall and outdoors, and the results were constantly validated. Depending on the particle size, up to 82% of the fine dust can be filtered through the mosses. Since we irrigate the mosses and create a moist surface, there are also significant cooling and moistening effects on the ambient air.

With the CE Lab in Ilmenau, we have also tested the CityTree for safety, quality and compatibility and thus awarded the CE label. In this way, we guarantee that every biofilter complies with the applicable standards and can deliver the promised performance in series production.

[The detailed final report of the European Union on our project can be found here].

This question is actually somewhat unfortunate, as both organisms provide different environmental services. We see our products as a complement to trees and not as their replacement. CityTree & CityBreeze can also be used where trees have a hard time. Trees convert CO2 into oxygen, provide shade and are important for the climate and biodiversity. However, they can do little to combat air pollution caused by fine dust; in fact, they suffer massively from it. This is where our products come into play, because mosses can filter and “digest” fine dust in a natural way. In addition to this natural air purification, the moss filters also produce a noticeable cooling effect, exactly at the height of the people. Unlike trees, these environmental services can be monitored, measured and controlled through the use of sensors.

The normal city tree and CityTree and CityBreeze are therefore not in competition with each other, but rather complement each other as a solution to different problems. As a measure, a tree tends to have a long-term effect, while the moss filters have a short- to medium-term effect. So together they form a perfect team!

We don’t want to & can’t clean the air of a whole city. That is not our approach, because it is not needed. There are so many places in the city where the air is fine or where people are not staying. Our products are useful in places where there is a high level of particulate matter and where people are present or even have to be present at the same time. Think of a bus stop on a busy street, a street café, a train platform, an inner-city schoolyard, even a company campus. In these places, CityTree and CityBreeze can efficiently clean the air with a good use of resources and create a fresh air zone with a high quality of stay for people.

With the CityBreeze, we also take up existing infrastructures and supplement them with the green, purifying mosses. Especially in connection with outdoor communication, the shoe is on the other foot. For one thing, the out-of-home segment is undergoing an amazing transformation right now. Climate-friendly and sustainable content and media dominate trade articles and trade fairs. The CityBreeze enables advertising with added value and offers the ideal presentation space for sustainable initiatives. On the other hand, advertising spaces are fixed points in cities, placed where as many people as possible can see them. This fits in perfectly with Green City Solutions’ approach of establishing fresh air zones where as many people as possible can benefit. In this way, we combine ecology and economy and generate an immediate and sustainable improvement in air quality in cities.

The vision of Green City Solutions is to bring fresh and clean air to everyone. After a test phase, the aim is to roll out the production-ready CityTree and CityBreeze products in many European cities. We are currently preparing this. The next phase will involve setting up Clean Air Networks with other moss-based air filters that are also suitable for end consumers, for example, or that function as vertical facade greening systems. In the long term, our moss filters will be offered nearly free of charge and will be financed through the analysis and visualization of high-quality data. In this way, we want to provide clean and fresh air for 500+ million people by 2030.

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