Reverse Osmosis Water Purification Systems

RO water is the term for water that has been purified with Reverse Osmosis (RO) technology.  Pure water flows through a semipermeable membrane while 95-99% of impurities such as bacteria, particles, organics and salts are rejected and drained.   RO water is not defined by specific quality parameters but rather the feed water and rejection rate of the RO module. Therefore, it can differ between modules, regions and seasons.

Reverse osmosis uses pressure and a semipermeable membrane to remove inorganics, organics, bacteria and particulates efficiently and cost-effectively from feed water. Pressure in the module overcomes the natural osmosis pressure, allowing water molecules to be pushed through the pores in the membrane. Most of the impurities (95-98%) are retained and then drained with the reject water. The rejection rate is adjustable with some systems optimizing the ratio of rejection water and product water automatically based on feed water quality. The standard ratio is 40 % product water and 60 % rejection water or vice versa, depending on water quality. If the reject water percentage is higher, the conductivity is lower, with a lower water flow rate. Reverse osmosis is used for producing RO water to be used as such or as a pretreatment step for producing ultrapure water. RO modules are also common as an integrated step in pure water systems.

You can usually find  feed water specifications in your water purification system’s technical information, such as the datasheet. If the feed water has a high value of organics, salts, calcium, etc., a pretreatment unit might be needed. Pretreatments are used to avoid clogging and scaling (calcification) of the membrane, which would shorten its life span. Depending on the initial state of the feed water, a pretreatment step might be needed to improve the RO water quality to adhere to application specs. Common pretreatment steps are activated carbon and depth filter, used individually or as a combination.

The quality of RO product water depends on the feed water from where it originates and the rejection rates of the RO module.  To determine levels of specific impurities after the RO module, you will need the incoming level and rejection rate of the RO module for that specific impurity. With this, an approximation can be calculated.

As water is passed through the RO module, calcium carbonate that is removed from the water will start to scale (calcify) the membrane and diminish its effectiveness. Regular cleaning will keep this from happening. It is best to use cleaning solutions at certain intervals and to minimize calcification by backflushing the membrane. 

RO water is often used for general laboratory tasks such as rinsing, washing or feeding laboratory instruments such as dishwashers, autoclaves and ASTM Type 1 (ultrapure) water purification systems. ASTM Type 2 (pure) water purification systems often use an integrated reverse osmosis step as pretreatment. RO water quality depends on the feed water and differs depending on the modules, region and season. Therefore, it should not be used when water of a consistent quality is called for.

RO and deionization modules are two different purification technologies. RO modules use the physical phenomenon of reverse osmosis, using pressure and a semipermeable membrane to reject impurities. Deionization modules use artificially produced resin to remove molecules based on positive or negative charge. The advantage of the RO module is that it removes a wide variety of impurities (ions, particles, organics, bacteria, salts, etc.) while the deionization module only removes ions. If ion removal is the only goal, deionization may be the better solution as it ensures constant ion removal on a higher base than a RO system. However, deionization is used to achieve Type 2 (or Type 1) water values, which typically also requires the removal of organics and other impurities. Therefore, a combination of both techniques is usually required.

You will need a system with a RO (reverse osmosis) module. Activated carbon and depth filters are generally added as a pretreatment step, to be used individually or in tandem. Finally, as the flow rate is relatively low for RO water purification systems, you will typically need a storage tank to house the RO water in your laboratory. From the storage tank, the RO water can be directed as feed water to instruments or to a dispensing unit.

Water purification technology that uses membrane filtration lowers flow rate drastically. To dispense water at a sufficient flow rate and pressure, a storage tank is generally required in laboratory environments. As reverse osmosis uses a semipermeable membrane, and therefore has a low flow rate, an internal or external storage tank is highly recommended in combination with a RO water purification system. If the RO module is big enough, a storage tank might not be needed, but as laboratory environments usually have limited space, the module cannot be big enough to achieve high enough flow rate or pressure without a storage tank.