In considering water types for use in hydroponics, I will simplify the discussion by dividing them into 2 main groups: groundwater and purified water.
Groundwater includes water pumped out of an underground well, as well as most municipal tap water, which is often derived from surface reservoirs or underground aquifers. Due to contact with the ground, this water can have a high levels of dissolved minerals, such as calcium and magnesium. The total levels of dissolved minerals is referred to as Total Dissolved Solids (TDS, measured in parts per million, ppm). Many resources discourage using water with high TDS levels (> 100 ppm), but I have found that even though my tap water TDS is 400 ppm, my plants still grow well. If I wanted to be thorough, I would reduce the amount of calcium and magnesium in my fertilizer blend to compensate for the amount of these minerals in my tap water, but I have found this level of detail is not necessary for the plants I grow.
Also consider that groundwater can contain dissolved carbonate. The level of calcium carbonate in your water is often referred to as “hardness”. Carbonate provides the useful feature of buffering the pH of the hydroponic solution, allowing the solution to resist changes in pH. For example, when I use tap water to make my hydroponic solution, the pH stays stable throughout the grow period. However, when I use water purified by reverse osmosis (RO), the pH is not stable and drops to low levels during the growing period. Since I practice a hands-off form of Bucket Hydroponics where minimal upkeep is preferred, I would need to initially add some sort of buffer to the purified water to keep the pH stable.
If your groundwater is “hard”, you may have installed a water softening system. These systems work by replacing the calcium and magnesium ions in your water with sodium. The sodium level in this water is too high for use in hydroponics, so it is better to find an alternative water source.
Many municipal water supplies are disinfected using chlorine or chloramine. Chlorine rapidly dissipates from water, so if your tap water contains chlorine, fill a bucket and just let it sit out for a day before using. Chloramine, however, is a stable compound which does not easily dissipate from tap water. To remove chloramine, you would need to employ a filtration or neutralization method. My tap water contains chloramine, but I don’t even bother trying to get rid of it. The plants grow just fine. Perhaps someday I will experiment with removing the chloramine to see if there is any positive effect on growth.
There are several kinds of purified water: rainwater, distilled, and reverse osmosis (RO)-purified. Rainwater (collected in a clean vessel) and distilled water are the purist forms of water. In both cases, water is evaporated and then condensed out of air, resulting in pure water with a TDS around 0 ppm. In reverse osmosis, water is collected through a semi-permeable membrane. RO-purified water is still quite pure, with a TDS of < 40 ppm.
In most places in the developed world, a cheap and plentiful supply of groundwater exists. So why use purified water? The simple reason: more control over EXACTLY what is in the hydroponic fluid. With purified water, there is no question of the exact levels of calcium, magnesium, etc, since the water has none of these to start with. There also won’t be any harmful chemicals, pharmaceuticals, or chloramine.
What are the downsides to purified water? Cost obviously. If you can collect clean rainwater, this provides a great option for cheap, pure water. Distilled water gets more expensive due to the energy cost of boiling water and the cost of equipment. Reverse osmosis equipment is also expensive, and the process is inefficient since only about 5-10% of the incoming water is retained as purified, while the rest is discharged.
Another potential downside to using purified water is the lack of pH buffering capacity. Buffering capacity refers to the ability of a solution to resist changes in pH. As the plants grow, water and nutrients are removed from the hydroponic solution, which can result in changes to the solution’s pH. If the pH gets too far outside of the ideal range (5.5 – 6.5), certain nutrients will become unavailable to the plants (known as nutrient lock-out), and the plants will suffer. As described above, my carbonate-laden tap water does a much better job of holding a stable pH than RO-purified water. If you would like to provide your purified water with a pH buffering system, you could add carbonate or MES. I have no experience with this, but it seems beneficial to provide some type of buffering system, since ideally we will not be performing frequent monitoring of the hydroponic solution.