5.2 Reverse Osmosis (RO) & Deionization (DI) Systems

Reverse Osmosis (RO) & Deionization (DI) Systems

Quick Answer: Reverse Osmosis (RO) removes up to 99% of dissolved inorganic salts, organic compounds, bacteria, and endotoxins from feed water. RO performance is tracked via Percent Rejection (minimum 90%), while auxiliary DI systems, UV irradiators, and ultrafilters provide secondary chemical polishing and microbial protection.

After passing through the pre-treatment components, the water has been conditioned: it is at the correct temperature, free of large particulates, softened to remove calcium and magnesium, and stripped of toxic chlorine and chloramines. However, the water still contains high levels of dissolved inorganic salts, organic compounds, bacteria, and endotoxins. The primary purification phase occurs in the Reverse Osmosis (RO) system, which is the heart of the dialysis water treatment room. The RO system removes up to 99% of these remaining contaminants, producing water that meets the stringent safety standards required for hemodialysis.

Reverse Osmosis Physics & Membrane Operation

Osmosis is a natural process where water moves across a semi-permeable membrane from an area of lower solute concentration to an area of higher solute concentration, attempting to equalize the concentration on both sides. In Reverse Osmosis, we want to achieve the exact opposite: we want to force water out of a highly concentrated solution (the feed water) into a clean, low-concentration solution (the product water). To overcome the natural osmotic pressure, the RO system utilizes a high-pressure pump. This pump forces the feed water against a semi-permeable membrane at high pressure. The pores of the RO membrane are microscopic—small enough that water molecules can squeeze through, but dissolved salts, metals, bacteria, and endotoxins are too large to pass.

Product vs Reject Streams & Percent Rejection Monitoring

As water is forced against the membrane, it splits into two streams. The water that successfully passes through the membrane is highly purified and is called "product water." The water that cannot pass through, which now contains a highly concentrated load of rejected contaminants, flows down the drain and is called "reject water" or "concentrate." The performance of the RO system is constantly monitored using conductivity meters. Conductivity measures the ability of a solution to conduct an electrical current, which directly correlates to the amount of dissolved ions (salts) in the water. By comparing the conductivity of the incoming feed water to the conductivity of the purified product water, we can determine the "Percent Rejection." The Percent Rejection formula is: ((Feed Conductivity - Product Conductivity) / Feed Conductivity) x 100. For example, if the feed conductivity is 500 and the product conductivity is 25, the percent rejection is ((500 - 25) / 500) x 100 = 95%. Regulatory standards dictate that the RO system must maintain a percent rejection of at least 90%, though most modern systems achieve 95% to 99%. If the percent rejection falls below the acceptable threshold, it indicates that the RO membrane is failing or compromised, and action must be taken immediately.

Deionization (DI) Resin Polishers

In some facilities, a Deionization (DI) system is used either as a primary purification method (if an RO system fails) or as a supplemental polisher following the RO system. DI tanks contain a mixture of cation and anion exchange resins. Unlike the water softener which exchanges calcium for sodium, DI resins exchange all positively charged ions (cations, such as sodium, calcium, aluminum) for hydrogen ions (H+), and all negatively charged ions (anions, such as chloride, fluoride, sulfate) for hydroxyl ions (OH-). When the H+ and OH- ions combine, they form pure water (H2O). As a result, DI produces water of extremely high chemical purity.

DI System Limitations, Bacterial Growth & Ion Dumping

However, DI systems have significant limitations in hemodialysis. While they excel at removing dissolved inorganic salts, DI resins do not remove bacteria, endotoxins, or non-ionic organic compounds. In fact, the porous resin beads inside a DI tank can actually serve as a breeding ground for bacterial growth. If a DI system is used, it must be followed by an ultrafilter to catch any bacteria or endotoxins shed by the resin. Furthermore, when DI resins become exhausted, they do not simply stop working; they can suddenly release massive, toxic loads of previously captured ions into the water stream—a phenomenon known as "dumping." For example, if a DI tank exhausts and dumps aluminum or fluoride into the product water, it can cause immediate, fatal toxicity to patients on the dialysis floor. For this reason, water exiting a DI system must be continuously monitored by a highly sensitive resistivity monitor equipped with visual and audible alarms. If the resistivity drops below the minimum safe level (typically 1 megohm-cm), the system must automatically divert the water to the drain and prevent it from reaching the patients.

Auxiliary Sterilization: UV Irradiation & Ultrafiltration

To further safeguard against microbial contamination, ultraviolet (UV) irradiators are sometimes installed in the product water distribution loop. A UV irradiator utilizes ultraviolet light at a specific wavelength (254 nanometers) to alter the DNA of bacteria, rendering them unable to reproduce. It is important to note that UV light does not physically remove the bacteria from the water; it merely sterilizes them. Because the dead bacterial cell walls can degrade and release endotoxins, a UV irradiator is typically followed by an ultrafilter. An ultrafilter is an extremely fine mechanical filter designed specifically to trap bacteria and endotoxins before the water reaches the dialysis machines.

Ultimately, the combination of the high-pressure RO system, potential DI polishing, UV irradiation, and ultrafiltration ensures that the water supplied to the dialysis machines is chemically pure and microbiologically safe. Technicians must understand both the mechanical operation and the critical monitoring parameters (like percent rejection and resistivity) to maintain this lifeline for the patients.