Urban farming in Berlin

Tucked away behind a furniture store, a hardware store and a post office, you'll find what could be the future of urban farming: What once was a brewery in the Schöneberg district of Berlin has been turned into a creative hub for artists, startups - and a city farm.

The farm has set up shop producing organic fish and vegetables for eco-conscious Berliners. The farmers combine conventional aquaculture - raising fish in tanks - with hydroponics, which involves cultivating plants with special nutrients on rock wool. This symbiotic system is called aquaponics.

Sustainable city farming

It is estimated that by 2050, two-thirds of the world's population will live in cities. And cities are responsible for 75 percent of the world's energy consumption, as well as for 70 percent of all carbon dioxide emissions. Against this backdrop, aquaponics could be a tool to make the megacities of tomorrow more sustainable.

Three years ago, Christian Echternacht and Nicolas Leschke founded their startup Ecofriendly Farmsystems and collected some 1.5 million euros ($1.6 million) to set up a farm in Berlin. The effort could revolutionize food production in cities, following a "just-in-time" model that produces food when it is needed.

Interconnected system

Dagh Sommerfeld is responsible for the fish farm. In a warm hall with 13 huge tanks, Sommerfeld stands on a ladder and checks the fish.

"The fish need a temperature of about 28, 29 degrees Celsius [82 to 84 degrees Fahrenheit]. That's why the temperature here is so high," he said.

Just a few days ago, he put 2,000 tilapia in the tanks. "Tilapia is quite a sturdy breed, which really works well in terms of aquaponics," Sommerfeld said. "It has a low food conversion ratio - that means it gains as much mass as you feed it," he explained.

The young fish are about as small as a fingertip. Once they've matured, in about eight months, they'll weigh around 750 grams (1.7 pounds).

Sommerfeld points to several pipes running along the ceiling that allow filtered rainwater collected from cisterns to flow into the fish tanks.

Just behind the fish tanks stands the centerpiece of the city farm: A bioreactor that converts fish excrement to fertilizer - thus making joint fish and vegetable farming possible.

No harmful chemicals

It's quiet and peaceful in the greenhouse, as plants creep their way up the wires. The floor is covered in long channels filled with rock wool. Gardener Robert Dietrich and his colleagues have planted about 1,000 seedlings.

With the help of the fertilizer, tomatoes and cucumbers will grow here. Greens and herbs are growing next door in another greenhouse. This farm needs about 90 percent less water than conventional farms.

Pesticides are verboten here, says Dietrich. "We use the natural enemies of harmful insects, which eat aphids, spider mites - anything that's not good for a plant," he said. The fish farm also thrives without antibiotics.

The carbon footprint for the tomatoes and fish grown here is quite small. After all, the produce doesn't get shipped halfway across the globe, and there's no need for a special cooling system.

But compared to big agricultural companies, the farm in Berlin operates on a much smaller scale: Per week, they can deliver up to 300 boxes of organic produce to customers in a city of around 3.5 million people. Per year, up to 40,000 tilapia fish could come onto plates locally.

Scaling up

Leschke and his team are working on a low-cost, lightweight system version of the system for developing countries. "Maybe a greenhouse with foil instead of glass; simpler tanks, and less technical equipment," he said. "Ideally, we wouldn't need electricity at all."

But regardless of the high costs associated with setting up such a project, there is still great interest. Grocery store owners have been inquiring as to whether the company could install such aquaponics systems on their rooftops.

Leschke and his team have already built an aquaponics system for a vegetable wholesale endeavor in Switzerland.

But it would be better to put up such farms in the countryside, as you could set up bigger systems, Echternacht said. "The bigger you get, the cheaper the price per square meter to build such a farm," he said. "The computer systems that regulate the farm can operate a 1,000 hectare farm nearly as easily as a 10 hectare farm."

For now, Leschke and Echternacht are only offering lifestyle products for city-dwellers. But aquaponics systems - especially the bigger and cheaper versions - could not only help put an end to overfishing and excessive use of pesticides: They could contribute to food security globally.