Reverse Osmosis uses a membrane that is semi-permeable, allowing water to pass through it, while rejecting the contaminants that are too large to pass through the tiny pores in the membrane. Reverse Osmosis, or RO is the opposite of the natural process by which moisture is taken up by living cell. Root cells of plants, for example, have special cell walls the allow water to pass through them. In Reverse Osmosis, water is forced against a membrane that is semi-permeable, allowing water to pass through it, while rejecting the contaminants that are too large to pass through the tiny pores in the membrane. Impurities are left behind and are rinsed away to a waste drain.
Water which begins its natural cycle as rain, comes into contact with many sources of contamination before eventually finding its way to your tap. The various minerals (including heavy metal ions) and salts that have been dissolved by the water during this cycle are called Total Dissolved Solids (TDS). TDS meter measures the PPM (parts per million) value of inorganic dissolved substances in the water.