Purified Water for Hydroponics

Although reverse osmosis (RO) membranes, which constitute the most important part of any RO system, are able to remove the majority of contaminants found in tap water, they cannot remove them all. Consequently, typical domestic RO systems actually incorporate multiple methods of purification in addition to the RO component. These other filtration methods usually come in the form of 'pre-filters' that process the tap water before it reaches the RO membrane. These pre-filters are designed to remove certain contaminants and solids so that the RO membrane does not have to do all the work, but they also act to protect the sensitive RO membrane from chemicals such as chlorine which can damage it.

'Generic' RO systems use pre-filter cartridges that come in a few specific sizes. These are (dimensions are length of the filter first followed by its diameter; measurements are in inches):

One point to note is that there is usually an allowable variance in the exact dimensions within the specific size categories above. For example, many 10″ x 2.5″ pre-filters can actually measure 9.75″ x 2.75″, and in other cases they may measure 10.25″ x 2.875”. In some cases, even a pre-filter from the same manufacturer will have slightly different dimensions. However, although it is always good practice to verify that a particular brand of filter will properly fit your RO system, most generic pre-filter systems are built to accommodate these minor differences. In general, the majority of domestic pre-filters are of the 10" category, providing filtered water at a single point of use like the kitchen sink, while whole house system pre-filters are usually 20” in length and are usually part of a more complex and larger water filtration system.

RO membranes usually come in a small set of standard sizes as there are a limited number of manufacturers who actually make the membranes but a large number of distributers that bring the generic membranes under their own brand. The most common size for domestic systems is 11.75" x 1.75",  but there are larger RO membranes available which are able to filter larger volumes of water.

The size or capacity of an RO system really comes down to the rate at which the RO membrane can process water - this is ultimately the limiting factor in any RO system. RO membranes are usually classified by their rate of filtration, usually in US gallons per day (gpd). For UK and European readers who are more accustomed to dealing with litres, a list of typical RO membrane capacities and their equivalent values in litres per day (as well as in litres per hour) has been calculated below:

As you can see, most reverse osmosis membranes don't produce purified water very quickly, and most cannot even produce a decent stream of water on their own, but at least based on these RO membrane flow rates, it should be a simple matter of just choosing a system big enough for our needs, right? Unfortunately, there are a few other variables that need to be taken into account before we can make a decision on which RO system to get. In general, purification flow rates (gpd) are typically determined using water with the following characteristics:

As the functioning of the RO membrane is affected by temperature, pressure, and contaminant levels, the actual real-world purified water generation from an RO membrane has to be adjusted for these different parameters to get a more realistic estimate of the capability of any RO system.

Capacity depends on water temperature:  In the UK, water temperatures rarely, if ever, reach the standard testing temperature of 25℃! So even assuming standard values for water pressure and TDS levels, respectively, water flow rates will be significantly lower than membrane ratings. How much lower? Well, in the winter when the water temperature is around 10℃, you can expect your RO unit to make approximately half the amount of water per hour, so a 36 gpd membrane (5.7L/hr) will actually have a 18 gpd (2.7L/hr) output. In the middle of summer, water temperatures in the UK can reach 20℃, but only for a short period, so although the purified water output will be higher in the summer, the membrane output will still fall a little short of its rated value.

Capacity depends on water pressure: A low incoming water pressure also affects RO membrane output. Domestic water pressures in the UK can typically be as low as 40 psi (and sometimes lower), and at this pressure, one can expect a decrease in production rate of purified water from the RO membrane of approximately a third. Fortunately, there is an easy way to get around low water pressure in an RO system and that is to buy one that has a pump integrated into its design. This will raise the water pressure for the RO membrane to a more reasonable level. To find out whether you need a pump, you first have to determine your water pressure using a water pressure gauge - this doesn't have to be expensive - mine was bought off amazon for a tenner (see Amazon link on the right). Once you know your water pressure, you can then at least narrow down your search for RO purification systems around whether you do or do not need a pump.

Capacity depends on total dissolved solids (TDS): Contaminants level in water can be quantified as total dissolved solids or TDS using a TDS meter and are measured in parts per million (ppm). TDS levels in tap water can vary widely from location to location and can even vary in the same location during different seasons of the year. RO membrane ratings are typically carried out on water that contains 500 ppm TDS. In the UK, having tap water much higher than 500 ppm is unusual, but if you do need to filter higher ppm water, then one should be aware that with each 100 ppm above the rating level of 500 ppm is equivalent to decreasing the water pressure by 14 psi, and so output will be reduced accordingly.

With all these environmental variables negatively affecting water purification, domestic reverse osmosis (RO) systems today are typically engineered to accommodate the relative low pure-water production produced by most RO membranes. As a result, most domestic reverse osmosis systems come with a storage tank included as part of their design which gets filled when there is no demand from users and then supplies most of the purified water that comes out of the tap when it is needed.  Typically, a 50gpd reverse osmosis membrane with a size-matched storage tank is enough to provide ultra-pure water for drinking, cooking, etc for a small family with the tank having enough time to be refilled in the intervals when there is no demand at the tap. However, a typical hydroponics reservoir will be larger than the capacity of a typical reverse osmosis storage tank, so even if the storage tank is full, it will still take several hours to fill a hydroponics system reservoir as once all the water in the reverse osmosis storage tank has been used up, the remaining water needed will have to come directly from the RO membrane.  How long it takes will depend on the capacity of the RO membrane and on how big your hydroponics reservoir is. One option might be to upgrade an RO system to increase the size of the storage tank** so that a greater volume of purified water can be held at the ready. This can provide the greater volume of pure water when needed (assuming that it has enough time between uses to fill it) but if ever you need more than the capacity of the storage tank, you will be back waiting again for the RO membrane to catch up. So the best course of action is to try to match the amount of water the reverse osmosis system's membrane can produce to a decent wait time for the biggest size of hydroponics containers you intend to fill.

An example is worth a ...

So ultimately, how big a purification system do you need? Well, an example is probably the best way to help you work out how big it should be, as everybody's circumstances are different.

Typically, we use a 10L bucket reservoir to carry out a Kratky-method hydroponic grow. If the water coming out the kitchen tap is at 10℃ , has TDS of 500 ppm and is at a pressure of 60 psi, then from a 50 gpd RO membrane system with a 15L storage tank (assuming the system has had enough time to fill the tank):

so let's say about 45-50 mins to fill a SINGLE relatively small 10L bucket! Not fast as you can see!

One last thing before you place the order for your new water purification system. One has to remember that not all the 'contaminants' in tap water are undesirable all of the time. The minerals, like calcium, magnesium and other trace elements are used by our bodies to maintain us in good health. Reverse osmosis purification systems remove these good components along with the undesirable ones just by the nature of how reverse osmosis works. Now, in the case of drinking water, it should not be such a big deal for people in the developed world as we have access to a diet that is rich in all types of elements so we should be getting these minerals from our food anyway. However, there are a couple of concerns here. Not all of us eat a diverse diet and it is possible that we may encounter a nutritional deficiency by purifying our drinking water, so that's something to watch out for. Fortunately, if you are growing your own food in the first place, then it is more likely than not that your diet is diverse enough and rich enough in all the minerals that your body might need. But that brings us to the second problem. There is some evidence in the scientific literature arguing that getting one's minerals from the water supply is better for us than if they are encountered in food. So what is the solution if you are worried about this? A re-mineralisation filter attached to the end of the water purification process can go some way towards allaying this problem as the filter will be putting back at least most of the natural good minerals taken away by the purification process. Other than that, it comes down to a choice, do you prefer VOCs, heavy metals, oestrogen-like compounds, etc and trace elements (that may be good for you but are probably coming from your food anyway) with your tea or do you prefer it without these contaminants?

Now remember, we don't want this re-mineralised water entering our hydroponics setup as the minerals added back can upset the delicate balance of minerals that our advanced hydroponic nutrients create. So if you do decide to go the re-mineralisation route, then you will have to adapt your system to be able to generate both purified de-mineralised reverse osmosis (RO) water for hydroponics and re-mineralised RO water for drinking. To our knowledge, purchasing such a multi-functional system is not currently possible and so a little plumbing DIY is needed to ensure both types of water are easily available.

**Reverse osmosis storage tanks are designed with an internal air bladder to push the purified water out on demand, so one thing to remember is that they realistically only hold half their stated volume, so a 15L tank will actually only hold about 7-8L of water when full.