Ondernemen, Rond BlueCity

From pressure cooker to scale; how modular growing on coffee grounds can result in far-reaching system change


Interview with Siemen Cox, co-owner of circular oyster mushroom farm: rotterzwam

On the site of De Kroon, a breeding ground for entrepreneurs in Rotterdam West, eight converted sea containers offer a remarkable sight. Here you’ll find rotterzwam, an oyster mushroom farm that runs on an important residual stream known to every city: coffee grounds. The men behind rotterzwam are also co-initiators of BlueCity; five years ago they started growing mushrooms in the cellars of the former swiming pool BlueCity now calls its home. In this interview with Siemen Cox, we talk about his dream for the future, how rotterzwam has developed over the past five years and how rotterzwam aims for far-reaching system change.

First, let’s take a quick look back: “BlueCity is a pressure cooker and an inspiring environment where innovations originate and spring, but those new ideas need to be scaled up elsewhere in the city.” said Cox in an interview in 2016. 

Four years later he’s slightly revised this opinion: “It depends on what kind of entrepreneur you are. If you’re developing an app against food waste, for example, you don’t need a big production space. But even if you wanted to do that, the new FoodHub in BlueCity provides an opportunity to do that too.” But what Cox is trying to say is that it depends very much on what kind of business you’re trying to run: “For us it wasn’t a choice. We simply needed a larger production location and so we relocated from BlueCity to De Kroon.”

From beauty salon to reefer containers

Rotterzwam believes in a society that closes loops, optimizes the use of raw materials and consumes more plant based than animal proteins. They implement this vision by working together locally, regionally and internationally to stimulate and facilitate the production of mushrooms on local, residual flows. The result is a wide range of products and services related to mushroom production and awareness. The rotterzwam bitterballs, based on vegetable proteins, are a well-known afternoon snack in the city. 

Rotterzwam is also one of the initiators of BlueCity and the first circular entrepreneur to settle in the former swimming pool Tropicana. However, after just a few years, BlueCity became too small: in 2019, rotterzwam opened its circular farm on the site of De Kroon, a breeding ground for entrepreneurs a few kilometers down the river.

“We started in the former beauty center in the cellars of Tropicana.” Cox explains: “That was a nice, but also quite difficult space where we could produce 200 to 250 kilos of mushrooms per month. This means we regularly had to say no to potential customers.”  By way of comparison: in the eight reefer containers in use at the new location, rotterzwam converts 6,000 to 7,000 kilos of coffee grounds into 1,200 to 1,400 kilos of oyster mushrooms every month.

Crisis as accelerator for growth

The fire in 2017 (read more about the fire here, ed.) made the choice to relocate to a bigger location easier, Cox says: “In a single instant, everything was gone. So we made the decision: we’re going to scale up elsewhere.” Crisis as an accelerator for growth – it’s a hard lesson drawn from the Blue Economy, a vision Cox and Slegers share with Gunter Pauli, the founder of the Blue Economy.

The search for a new, larger location took quite some effort; after a few detours Cox and his business partner Mark Slegers ended up at De Kroon. “Truly a beautiful place and, just like BlueCity, also an ecosystem of entrepreneurs, but more focused on creativity than on sustainability.” 


Despite this focus on creativity instead of sustainability, there is synergy and cooperation at the new location as well. An example: “Mark and I had designed a climate wall. The guys from Culture Campsite are super handy, and came up with the idea of making a climate wall that you can use in a container.” That’s a great idea that would never have arisen if they had been alone in their bubble.

Almost grown out of their jacket again

Originally rotterzwam thought it needed 750 m2 for the farm – but two years later they were already running out of space. “We continue to evolve,” Cox tells us: “We would like to compost onsite with the substrate for example.” Substrate is the soil on which oyster mushrooms grow and which remains after harvesting. For rotterzwam, this is one of the few waste streams they still have. “Unfortunately, we don’t have enough room for a composting unit here, so we’re already secretly looking for a new, even bigger location.”

Modular growing and structural knowledge sharing

To be completely clear: the aim is not to become the largest mushroom farm in the Netherlands, the aim is to make an impact. Rotterzwam has therefore developed a modular farm, made out of insulated reefer containers (refrigerated containers, red). Perfect for an oyster mushroom farm. “We have a For Sale sign attached to the containers by default,” Cox says, “We strongly believe in open source. We like to share our knowledge a lot – sometimes for a fee and sometimes for free.”

With over seven years of experience in mushroom cultivation, rotterzwam also offers a mushroom master program in which they teach entrepreneurs the basics of growing mushrooms – on straw or any other type of substrate. But most important: “We give them access to the business case and to the earning models.” Cox explains. In addition, rotterzwam offers an ingrowth plan; everything you need as an entrepreneur to start a local farm.

“You can also google and do your own research of course,” he continues, “but then you’re reinventing the wheel all over again. We’ve turned seven years of experience into four days of training. After that, all you have to do is make an investment – and we can help with that too.”

Circular dream scenarios

And just like that, Cox en Slegers are building on their dream for the future: every city will have its own 100% circular farm, growing oyster mushrooms on local coffee waste, self-sufficient and with all loops closed. But that is a big challenge. Partly because of that reason, Cox and Slegers maintain an office in BlueCity. “This gives us the opportunity to distance ourselves from the farm, and to focus on scaling up the concept. And to devise and develop new concepts too – there is still a lot to be done.”

System change

The closing of all waste streams in the farm is an important aspect. One of the ways in which rotterzwam wants to realize this, is investigated in ‘Back to the Soil’, a three-year project that rotterzwam executes in collaboration with the Louis Bolk Institute, the University of Wageningen and a soil-bound farmer on Voorne-Putten. 

Back to the Soil

In ‘Back to the Soil’, three materials that are currently considered as waste, are applied to farmland: coffee grounds, oyster mushroom substrate from rotterzwam and substrate from traditional cultivation, on straw. “The bizarre thing is that these three streams are considered as waste and may therefore not be applied to land. And this while it’s a 100% natural material. There’s nothing artificial or chemical added to it.”

‘Back to the Soil’ is currently in its final phase; a lot of data is being gathered. Cox: “After the baseline measurement there was an in-between measurement in which the effect of these three materials on the crops and soil life was examined.” First wheat was planted on the soil, then onions and next year wheat again. After that harvest, the final results can be measured and a report will be submitted to a committee. The goal? “To see if we can change the status of these three materials from waste to fertilizer. If we can do that, then we really have changed the system.”

From waste to fertilizer

As long as the project ‘Back to the Soil’ is running, and the finished substrate is still classified as waste, the residual flow goes to bio-fermentation. Low-grade, but still acceptable, according to Cox and Slegers. “With a composting unit we can convert our own residual material to compost. Once we’ve sorted that out, and the legislation changes, we’ve really closed the loop”. 

And what if legislative reform does not happen? “Then we can still compost and spread coffee grounds across the country ourselves. It doesn’t matter whether others consider it as waste or not. It’s going to happen one way or another.”

Big impact, big challenge

Oyster mushrooms are, similar to meat, a good source for protein. However, they are a lot more environmentally friendly. Processing oyster mushrooms into bitterballs (those snacks we’ve mentioned before), for example, saves 50 to 90 liters of water compared to one made with meat. 

Yet it is a huge challenge to make an impact on a large scale. “Not only do you need an oyster mushroom farm in every city, but you also need a market for it. That is restaurants that want to sell them and consumers that want to eat them.” Cox says. And that’s not particularly easy with a niche product like theirs. 

Therefore, Cox is very happy with regular customers like Old Scuola and Jack Bean. And yet: “The challenge is to find more companies that are willing to pay just a little more. After all, it is a vegetarian product, grown on local waste, by local people. And it’s delivered by electric car, by local couriers.”

The inevitable additional cost that this entails is an obstacle for many hospitality entrepreneurs. “But actually the problem is that regular meat bitterballs are far too cheap. The impact that one meat bitterball has on the environment is huge, but remains invisible for the consumer.” Cox explains, “As long as the polluter doesn’t have to pay, that will not change.”

Coffee as a service

Oyster mushroom farms like rotterzwam all across the country, from Maastricht to Groningen and beyond – that’s rotterzwam’s dream for the future. To realize that, they are currently rolling out their ‘coffee as a service’ concept. “The idea is simple: the roaster remains the owner of the coffee bean. You as the user leases it: you can grind it, pour hot water over it and enjoy the coffee, but then it is collected and processed within the city, because valuable things can still be done with it.”

If the coffee drinker is not the owner of the bean, then he or she is also not the owner of the waste problem. “The waste disposal is included in the price,” Cox explains. “In a society where you pay for the waste you generate, leasing a coffee bean is a financially interesting idea.” 

Rotterzwam is currently testing their ‘coffee as a service’ concept with Hesselink, a large coffee roasting company for the hospitality industry. Hesselink now has a small number of customers who purchase coffee as a service. “It’s still small-scale, but nonetheless very interesting because it allows us to test where the challenges and successes lie.”

Do you want to read more about rotterzwam? Check out their website.

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