Reverse Osmosis (RO) is a water purification method that uses pressure to force water molecules through a semi-permeable membrane. This membrane rejects nearly all dissolved inorganic solids and impurities, resulting in highly purified drinking water. The process generates a concentrated byproduct stream known as reject water or brine. This wastewater contains the concentrated version of all substances removed from the source water. RO reject water is generally not suitable for plants in its undiluted state.
Understanding RO Reject Water Composition
The primary characteristic of RO reject water is its elevated concentration of Total Dissolved Solids (TDS). The RO membrane separates the incoming water into a clean stream and a waste stream, concentrating minerals, salts, and other impurities into the reject portion. A typical household RO system often produces three to five parts of reject water for every one part of purified water. This concentration factor means that if the source water has a TDS level of 300 parts per million (ppm), the reject water may range from 900 ppm to over 1,500 ppm. The concentrated substances include hard minerals like calcium and magnesium, alongside increased levels of sodium and chloride salts, which pose a risk to plant health.
How High Salt Concentrations Affect Plants
The elevated salt concentration in RO reject water causes two types of harm to plant life: osmotic stress and specific ion toxicity. When high-TDS water is applied to the soil, the high concentration of dissolved salts lowers the soil’s water potential. This creates an osmotic gradient where the water outside the root is less pure than the water inside the root cells. Instead of drawing water from the soil, osmotic pressure can cause water to move out of the plant roots, dehydrating the plant even when the soil appears moist. This water deficit mimics drought conditions, leading to wilting and reduced growth.
The second issue is the long-term accumulation of specific ions, particularly sodium (Na+) and chloride (Cl-), which are toxic to most plants. Sodium accumulation interferes with the plant’s ability to take up potassium (K+), an essential nutrient, because the two ions compete for the same uptake sites in the roots. This disruption impairs cellular processes and overall plant nutrition. Chloride ions can also accumulate in the leaves, leading to visible damage such as leaf burn, yellowing, and tissue death.
Dilution and Application Guidelines for Reusing the Water
Reusing RO reject water requires careful management to mitigate the risks posed by its high salt content. The most effective strategy is dilution with a source of low-TDS water, such as tap water or rainwater. A common guideline is to mix one part of reject water with at least two or three parts of fresh water; a handheld TDS meter can confirm the concentration is well below 1,200 ppm. Even diluted, the water should only be used on highly salt-tolerant species like mature trees, shrubs, or succulents. Avoid using reject water on sensitive plants or vegetable crops, and alternate its use with pure, low-TDS water to prevent long-term salt accumulation.