Update on Green Spark and the Greenhouse Challenge

WhatsApp Image 2018-07-25 at 16.31.56.jpeg

We just pulled an all-nighter.

After over half a year of refining our ideas, our team’s final dossier for Wageningen University’s Greenhouse Challenge was finally handed in, a rendering of which you see above.

The Greenhouse Challenge is a competition in which teams of students get to come up with a design for a food-producing tower in the former prison of the Bijlmerbajes in Amsterdam. I was part of the technical group of our team, Green Spark.

We have been working on this for a good six months. Here are some of the ideas in our final design. There were plenty of interesting ideas from the perspectives of business, architecture, and social activities – but I will mainly focus on the technical parts. Just remember:

If our small minds, for some convenience, divide this glass of wine, this universe, into parts — physics, biology, geology, astronomy, psychology, and so on — remember that nature does not know it!
– Richard Feynman

Left to right: Miguel, Emilio, Nick, and Max – the last members of the group in Wageningen soldiering on!

The AMI system

Naturally, I pushed to have an AMI (aquaponics-mushrooms-insects) system in our design to breathe life back into waste. This time we took it a few steps further. Recently, UrbanFarmers in The Hague went bankrupt. We think this is down to the fact that they were selling run-of-the-mill crops that could be found in greenhouses all around the area. Likewise, they were growing tilapia, a cheap and relatively undesirable fish (to be fair, easy to grow though).

Instead of using tilapia, we decided to grow shrimp, thanks to contact with Francisco from inLoops Tech, a startup in Wageningen. Currently, most shrimp is imported from Asia and South America. Shrimp aquaculture is also devastating mangrove forests, compromising the livelihoods of thousands of people. Shrimp also sells for a much higher price than tilapia.

The inLoops system combines shrimp production with the production of spirulina. Spirulina is highly nutritious. It also grows in saline water and takes up more sodium than plants do.

This presented an opportunity. In soilless growing systems, the concentration of sodium builds up, since plants hardly take up any. This can get to a level that compromises yield. To keep the sodium concentration in check, growers leach some of the water. With that, precious nutrients are lost. This can also lead to eutrophication. Since spirulina grows in saline water and takes up sodium, we decided to direct leach water to the spirulina.

The insect in our AMI system was of course the miraculous black soldier fly. However, in combination with shrimp, there was a catch. Shrimp are bottom feeders. This meant that the larvae had to sink to the bottom before they could be eaten. When I spoke to Darren Ho from Citizen Farm and the great Seppe Salari, they recommended we use a sinking agent (as is used by fishermen) to solve this problem, though it would increase costs.

Wastewater management system

Around the AMI system was a wastewater management system. This would extract the value in black-, grey-, and yellow water. Wastewater management is not essential to running the AMI system, but would augment the circularity of the whole building.

Yellow water – urine – is a good source of nitrogen. We decided to feed it to a struvite reactor. A struvite reactor uses magnesium to precipitate a mineral called struvite. Struvite is a slow-release fertiliser. A friend of mine, Radu Giurgiu, is using struvite reactors with SEMiLLA Sanitation Hubs and their ‘Waste to Taste’ project:

One of SEMiLLA’s Waste to Taste containers, which have been installed at festivals. They generally grow herbs like basil and mint.

Black water has the most potential. It is full of energy and nutrients. First, it would go to the biodigester, to produce methane gas. Sludge is generated as a by-product. Sludge is full of nutrients, but it’s unpredictable – it tends to contain drugs, hormones, and other unwanted chemicals.

This is why the next step was to feed the sludge to a pyrolyser. Sean from our team had just done his dissertation on pyrolysis. In pyrolysis, biomass is heated to above 400 ºC under anaerobic conditions, producing something similar to charcoal called biochar and destroying unwanted organic pollutants. To find out what happens to the biochar, read on…


Biochar is used as a soil amendment and has similar properties to perlite. This makes it a candidate for being a good substrate. There are a two issues with biochar in soilless growing systems:

  • it has a very high pH
  • it hogs nutrients when used in some systems (Bright Agrotech advise against using biochar with hydroponics)

These problems could be solved by flushing the biochar with acid, for example. Studies have also shown that biochar mixed with sawdust works well as a substrate. We decided to go with this, eventually settling on 30% biochar, 30% sawdust, and 40% soil. To find the best mix, we would have to conduct real-life tests.

Sawdust is packed with lignocellulose, which mushrooms can feed on. Therefore, once the substrate had been used, it could be fed to oyster and shiitake mushrooms, closing the circle enhancing the spiral.


Another idea our team had was to grow seedlings. Urban farms struggle to make money, and often this is due to lack of differentiation (like UrbanFarmers’ standard crops, as mentioned before). It is quite tricky for urban farms to source specialty seedlings. This is why we decided to export some of the seedlings grown in the tower to surrounding urban farms.

The total flowchart of our system, with the system boundaries drawn around the tower. Draw.io is a blessing.

Closing remarks

The Greenhouse Challenge has been a great experience. There were many overlaps with the AVF’s Brusselaer Circulair white paper. However, this was not only a feasibility study but a design. There was a seemingly infinite number of possibilities and decisions to make.

It’s not over yet. In late August, the teams will be doing their final pitches to the jury in Wageningen, before finding out the final results.

If you think Green Spark deserves to win the Greenhouse Challenge, please consider voting for us here.


What are your thoughts?

Leave a Reply

Your email address will not be published. Required fields are marked *