Reverse osmosis systems can be a great investment for farmers. Reverse osmosis (RO) filters separate water molecules from other dissolved solids in the water (like carbonates). Since carbonates act as a buffer, they can make pH management difficult.
The reverse osmosis system in this video is a large system for a fairly large greenhouse, but the principles used are the same as smaller filters.
Basically, the reverse osmosis filter is a membrane through which water is pumped, filtering water at a molecular level. Most dissolved solids can’t get through the membrane. This means that the water coming out of the filter is much closer to pure water.
When to use a reverse osmosis system
Reverse osmosis filters are the most reliable solution to correct bad source water quality. The primary culprits of low source water quality for hydroponics or aquaponics are carbonates.
Carbonates act as a buffer. This means that they dissolve into and precipitate out of a solution within a pH range of about 7.5–10.
Unfortunately, carbonates are also basic; when they dissolve (or “precipitate”), they raise the pH of the solution. This makes lowering pH more difficult for growers.
Having a high ppm of salts (they will mostly be carbonates) in your source water to start also interferes with the nutrients you are able to add to the solution.
Source water quality becomes a problem at 200 or 250 ppm. This means that if your source water has a ppm value over 200, you’ll want to think about a reverse osmosis system. If the ppm value is over 250, it’s definitely worth getting one. (This value can be measured with an EC/TDS meter. Just be sure to convert the units!)
A commercial-sized filter might be overkill for a small farm
The system in this video is very large. For smaller farmers, there are many smaller RO units that will work fine for you.
How a reverse osmosis filter works—molecular filtration
This commercial-scale Siemens’ system has multiple levels of filtration to increase efficiency. Water travels through a series of filters before the pure water is finally sent to a holding tank:
1) Source water enters the system and goes through a pre-filter and a sand filter. This sorts out the large solids so that they don’t block the finer filters.
2) After being filtered for large particles, the water goes through a third filter before being sent to the RO pump.
3) The RO pump runs the water to the actual osmosis filters. The water reaches the osmosis membranes in the filter and is filtered on a molecular level.
4) Each filter then sends the water into a large holding tank. The system runs constantly and tops off the holding tank, which is used according to demand.
3 operating tasks for reverse osmosis systems
The main tasks associated with a reverse osmosis system are:
1. Backflushing filters: Most filters in this system have a backflushing function. Backflushing removes the “gunk” that builds up on the filter, increasing flow and filter lifetime. Here’s how you might backflush a canister or Y-filter, for example.
2. Replacing the RO membrane: Membranes can get worn out and efficiency can drop. Check with the filter manufacturer to see their recommended replacement intervals.
3. Scheduled checks and records: Keep a maintenance sheet to track the efficiency of the pump. For instance, one maintenance entry might have the incoming source water at 433 ppm, and the filtered water at 5.8 ppm.
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