How to Make a Greenhouse Reverse Osmosis Water Supply

Supplying a medium to large size collection with high quality water can be a challenging prospect. We struggled for a long time with various systems and setups before finally figuring out a configuration that gives us zero or near zero Total Dissolved Solids (TDS) water in a convenient, easily usable way.

Water quality is very important for our purposes. If you haven't tested yours, you should. We use this inexpensive device to make sure our RO system is working properly.

Initially we used a typical low-cost, counter-top RO system such as this one:

Unfortunately, with a setup based on a system like this, you are limited to manually filling your reservoir or utilizing a float valve. Manually filling bottles with a setup like this works very well for a small collection and is definitely the best option for many growers. However, for a larger-scale operation, this becomes tedious, time consuming work, especially considering how long it would take for a system like this to produce 50+ gallons of RO water. Additionally, low-pressure systems such as this one typically operate with worse efficiency compared to high-pressure systems that utilize a dedicated boost pump. With a setup like this (the low-pressure one shown above), an "auto-shutoff valve" should theoretically be able to be installed to shut off flow to the RO system when the float valve shuts off flow. Without an auto-shutoff valve, the float valve will shut off flow into the reservoir, but water will continue to flow through the RO system and out of the wastewater line, continually and unnecessarily wasting water and ruining your RO membrane much faster. We tried several auto-shutoff valves to solve this problem, and in every case the results were poor. This could be due to too low of water pressure from the water supply, resulting in the auto-shutoff valve not consistently closing properly. The auto-shutoff valve would often cycle on and off in quick succession, shutting off momentarily, at which point the float valve would gradually leak water into the reservoir, therefore dropping the pressure in the line, causing the auto-shutoff valve to open. This resulted in a perpetual, rapid cycling of off and on. In any case, this wasn't working for us.

After consulting other growers and many hours of searching online, we found a device called a "float switch". Float switches differ from float valves in that they operate by turning a circuit on or off based on the orientation of the switch. There are two main types of float switches: "Normally open" and "normally closed". "Normally open" switches turn off the circuit when the sensor hangs straight down. "Normally closed" switches turn the circuit on when the sensor hangs straight down. Either type can be used for this application (we use a normally closed switch), but must be mounted differently based on the type of switch you use. Some listings don't specify what type a float switch is. This is one we found that specifies it is the "Normally closed" type.

This raises the question, how does a float switch control the operation of an RO system that doesn't utilize electricity? The answer: you need a RO system with a solenoid valve for the float switch to control. The solenoid valve, when switched off by the float switch, stops all flow through the RO system, preventing water from being unnecessarily wasted. This is the RO system we went with:

This system is capable of 500 gallons per day, which is WAY more than we will ever use in a day. The upside of having such a capable system is that the filters and RO membrane will last a very long time, and the replacement filters are very affordable. We have two of these, one in each greenhouse for the past year and they are both still producing water at 0 TDS from our ~120 TDS tap water.

For our reservoir we use a 55 gallon plastic barrel. 

Inside the barrel we have a 1/2 horsepower pond pump which has a PVC pipe attached to the output port. Here's the exact pump we're using. We attached the float switch to the vertical PVC pipe. When the water level falls to the point where the float is hanging straight down, the float switch turns on the RO system, which fills the reservoir until the float switch reaches an approximately 90 degree angle (as shown) at which point it turns off the RO system.

The PVC pipe from the pump is connected through a hole we drilled in the barrel to a water hose spigot. For convenience, we control the pump with a switch like this.

The 1/2 horsepower pond pump provides pressure to our water hose, enabling us to use the water from our reservoir with a water hose and nozzle/sprayer, making watering plants very fast and efficient!

We have used this setup for the past year and have been very happy with the results. If you have any questions about how to setup a system like this for yourself, please contact us ( We want you to be successful and water quality is a big part of that!



  • Thanks so much for publishing this information. A lot of growers seem to treat anything related to greenhouse setup as if it were the secret formula for coca-cola. Sharing growing tips will be vital to long term collecting and breeding programs that can hopefully end the demand for wild collected plants.

  • Hey! Just wanna day thanks for sharing all the info on your RO machine. I’m quickly growing out of my water distillers and in need of something like what you have so I appreciate the shared knowledge! Thanks again! Stephen Davis

    Stephen Davis

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