How we developed a misting system for Fryslân Fungies that keeps a grow cabinet humid without limescale, without legionella, and without monthly maintenance.
When Wander Oele from Fryslân Fungies called us, his question was about mushrooms. His problem was about water.
Fryslân Fungies builds grow cabinets out of discarded refrigerators. A FungiFarm, as they call it: a cabinet you place in a restaurant or an office where oyster mushrooms, shiitake, and lion's mane grow while everyone watches. To make those mushrooms grow, the air in such a cabinet must be between 85 and 95 percent relative humidity. You achieve that range by misting, and you mist with water. That is where it started.
Tap water is full of dissolved minerals. Misted water evaporates; the minerals do not. They remain on the glass, on the walls, on the sensors, and eventually on the mushrooms themselves. A white haze over your product is not a great start if you want to put those mushrooms on a plate. On top of that, the misters themselves would clog up and produce noticeably less mist after a few months.
That is exactly our area of expertise. We build water filtration and osmosis systems, and we immediately recognized the challenge at hand: how to get water so clean that it leaves nothing behind, in a setup that is small enough to fit in a refrigerator and that must run for twelve months without maintenance.
First the requirements, then the technology
We started by writing down exactly what the system needed to be able to do. This resulted in thirteen hard requirements, determined together with Fryslân Fungies. The most important ones: misting with low-mineral water, adding no heat because the cabinet must stay below 24 degrees, droplets so fine that there is no wet precipitation on the product, twelve months of operation without maintenance, and no legionella or algae in the reservoir.
Those last two requirements conflict with each other. I will come back to that shortly.
With those requirements in hand, we screened seven misting techniques and weighed three of them seriously against each other: ultrasonic misting, high-pressure misting, and air-assisted misting. In terms of droplet size, ultrasonic wins by a wide margin. An ultrasonic vibrating plate creates droplets of one to five micrometers, so fine that they hang in the air instead of condensing against the cabinet wall. High-pressure is at five to fifteen micrometers, air-assisted at ten to twenty.
But when we looked at service life, the picture reversed. Ultrasonic vibrating plates last for about three thousand operating hours. After that, the mist output declines and cleaning no longer helps. Twelve months of continuous operation is 8,760 hours. Ultrasonic only meets that requirement of being maintenance-free for a year if the system is on less than about a third of the time. High-pressure misting with osmosis water, on the other hand, is known for being low-maintenance and is easier to scale up to multiple cabinets.
We deliberately did not force a final decision on this, but turned it into a two-stage choice. We used ultrasonic for the prototype because it provides the finest mist and is the fastest route to a working proof of concept in the cabinet. For the version that will eventually stand in customer locations, we will stick with high pressure as the robust route. Because we designed the misting module to be interchangeable, that choice can be made later without having to rebuild the rest of the system.
How much mist do you actually need?
In practice, we received the following answer to that question: about two liters per day. A fine estimate based on feeling, but you cannot dimension a system based on that.
So we calculated it. With a cabinet volume of 0.4 cubic meters, four air exchanges per hour, a target of 90 percent humidity, and a cabinet temperature of 22 degrees, the net evaporation demand comes out to about 0.3 liters per day, or fourteen milliliters per hour. With a generous overcapacity factor of three for door openings and growth peaks, the recommended module capacity comes to one liter per day.
The sensitivity of that calculation lies almost entirely in the number of air exchanges per hour, and that is immediately the most important value you want to measure in practice before choosing the final capacity. We have therefore published the calculation model as an interactive model, so that the outcome can be recalculated with actual values.
Why you must not work with heat here
Back to those two requirements that conflict with each other. Legionella grows between 20 and 45 degrees, with an optimum from 25 degrees, especially in standing water where biofilm forms. The standard solution in water technology is thermal disinfection: you heat the water.
That is not possible here. The cabinet must remain below 24 degrees, otherwise the mushrooms will spoil. Every degree you add works against the product.
Hygiene therefore had to be built in mechanically instead of thermally. The buffer tank drains automatically when idle, it includes a rinse and purge cycle, there is low-level protection against dry running, there is an overflow to the drain, and there are no dead pipe sections in the system. The real risk turned out to be not the temperature, but the stagnation. As long as the water stays in motion and there is no place where it can sit still for days, biofilm does not get a chance. In the tests that followed, no legionella was found.
The system in short
What is finally in place runs from tap to mist in eight steps: tap water, a pre-filter with sediment and activated carbon, a reverse osmosis system, a demineralization polisher for the final minerals, a solenoid valve as a water lock, a buffer tank with level control, the interchangeable misting module, and finally the cabinet itself with humidity and temperature measurement.
The latter sensor completes the control loop. We deliberately do not work with one setpoint but with a bandwidth: below 88 percent the misting turns on, above 93 percent it turns off. That prevents the system from switching nervously all day. On top of that is a temperature lockout: if the cabinet threatens to head toward 24 degrees, the humidification stops. Being too cold is no problem for cultivation; being too warm is.
There is also a version without a permanent water connection, using inline filters. Conversations with catering entrepreneurs revealed that there are more locations without water supply and drainage than you might think, and we did not want to exclude them.
The unexpected outcome: the cooling unit could be removed
The most beautiful result of this project was not on the agenda beforehand.
One of our requirements was the option to take over the cooling function via adiabatic cooling. Misting costs energy, that energy comes from the air, and therefore the air cools down. That is physics, not a trick.
In the tests, that effect proved large enough to allow the entire mechanical cooling system of the old refrigerator to be eliminated. That cooling system was already a problem anyway: it consists of a radiator with fins that is not designed to be cleaned, while mushroom cultivation generates many spores in the air. To reach it, you have to disassemble half the cabinet, and you cannot ask a user to do that.
By leaving temperature control entirely to the misting water, the cabinet stays at an average of about twenty degrees, a maintenance problem disappears, and energy consumption drops. A water issue that began with limescale ended with the cooling machine becoming redundant.
What we gained from it
The combination at play here—misting mineral-poor water in a small space with strict hygiene requirements and without heat input—is found in more places than you might think. Fresh products in stores, climate chambers, vertical farming. The knowledge we have built up here regarding service life, water quality, and mechanical hygiene control is directly applicable there.
And we learned something about dimensioning. The difference between approximately two liters per day from practice and 0.3 liters per day from the calculation is not just someone's miscalculation. It is the difference between a system that works and a system that is three times too big and therefore wears out three times as fast.
The prototype is now in the workshop of Fryslân Fungies, where the final iterations are being carried out. It has operated in a real-world environment and has produced mushrooms that were actually served to guests. What remains now is not proving the principle, but making it product-ready.
About this project
Smart FungiFarming is a collaborative project between Fryslân Fungies from Leeuwarden and PureAqua B.V. from Hallum, carried out between February 2025 and August 2026. Fryslân Fungies developed the grow cabinet, the substrate blocks, and the control software. PureAqua was responsible for the misting and water filtration system.
The full technical foundation, with the requirements package, the comparison of misting techniques, the capacity calculation, and the hygiene analysis, is published at smart-fungifarming.netlify.app.
Co-funded by the European Union
This project was made possible in part by the European Regional Development Fund (ERDF), via the "Versneller Innovatieve Ambities Drenthe en Fryslân 2025-2026" scheme of the Samenwerkingsverband Noord-Nederland (SNN).
