In a couple of months, participants of a circular cultivation project can start harvesting their own lettuce, tomatoes, and herbs in a basement beneath an apartment building in Stockholm, as well as catch their own fish. The project is part of the EU project Gröna Solberga, in which multiple sustainable solutions are being tested in the south of Stockholm.
In the cultivation room, they use an organic cultivation system that works with bacteria. The excrement of the fish provide nourishment for the crops and the crops provide a cleaning function for the water so that the fish can thrive.
Tilapias are swimming in two open containers. They grow fast and thrive in a small water surface, so the basement farm can yield around 200 kilograms of fish each year. There are also three different hydroponic cultivation beds where basil, tomatoes, sweet peas, fennel, and chili are being grown. At the moment basil is the main crop with around 1500 basil plants grown each year. The biggest challenge for this cultivation method is to find a balance and to maintain that balance in the biological system. Supervision and maintenance of the cultivation cellars are up to the occupants themselves.
“Among other things, it is important to achieve the right level of bacteria needed to convert the ammonium in the fish excrement into nitrite and then into nitrate. The pH value is important as well, just like the alkalinity,” says Torbjörn Frisö, founder of Kretsloppsbolaget, who realized the construction of the cultivation cellar together with colleague Dag-Tore Johannessen.
Because of the shifting climate, the interest in city cultivation is increasing. That is why Frisö hopes that in the future, more cultivation cellars can be found in Stockholm. Supervision and maintenance of these basements are then also up to a group of occupants.