Reverse osmosis (RO) forces water under pressure through a semi-permeable membrane that rejects most dissolved ions and many organic compounds. Typical rejection is around 90–99% of dissolved solids, depending on membrane condition and pressure. Systems usually include sediment and carbon pre-filters; the carbon stage protects the membrane from chlorine, and catalytic carbon is needed for chloramine.
In practice
RO gives a clean, predictable base, typically below about 0.02–0.05 mS/cm (20–50 µS/cm). That is ideal for hydro and for growers with hard, high-sodium or high-alkalinity tap water.
Trade-offs: no calcium or magnesium, so the nutrient (or a Cal-Mag supplement) must supply all of it; almost no alkalinity, so pH drifts more and pH Down overshoots easily; and a wastewater stream, often 1–4 litres per litre produced. Some growers blend RO with tap water to keep a little buffering. Replace pre-filters on schedule and watch output EC: a rising reading means a worn membrane.
Don't confuse with
- Distilled water: purified by boiling and condensing; similar result.
- Filtered tap water: carbon or sediment filters do not remove dissolved salts.
Myth check
RO water is a genuine reason a grow needs Cal-Mag. LED lighting is not: the LED claim is a Myth. See Cal-Mag.
TDS (total dissolved solids) is the mass of dissolved material in water; grow meters estimate it from EC with a conversion factor and display it as ppm.Tap water is municipal or well water used for irrigation; its EC, alkalinity, hardness, sodium and disinfectant vary widely, so it should be tested before use.Cal-Mag is a supplement supplying calcium and magnesium, usually as nitrates; need depends on cultivar, water source (RO vs tap) and medium (coco), not on LED lighting.Alkalinity is water's capacity to neutralise acid, mainly from dissolved bicarbonate and carbonate, reported in mg/L as CaCO3; it decides how much pH Down you need.pH measures how acidic or alkaline a solution is on a 0–14 scale; in growing it controls nutrient uptake, with soil best near 6.0–7.0 and hydro 5.5–6.5.pH Down is any acid used to lower solution pH, commonly phosphoric, nitric or sulfuric acid; each adds a nutrient, and all need gloves and eye protection.Runoff EC is the conductivity of solution draining from the pot; compared with input EC, it shows whether salts are building up or being used up in the root zone.An LED (light-emitting diode) grow light uses solid-state diodes to produce light; modern fixtures are the most efficient grow lights, at roughly 2.5–3 µmol/J.General hardness (GH) is the calcium and magnesium dissolved in water, expressed as mg/L CaCO3 or German degrees (dGH); hard water supplies some Ca and Mg.Buffering capacity is how strongly a water, solution or medium resists pH change when acid or base is added; high capacity means stable but hard-to-move pH.µS/cm (microsiemens per centimetre) is a unit of EC one thousandth of mS/cm, used mostly for low-conductivity water such as RO and rainwater.
