What is Reverse Osmosis?
Of all the water purification technologies, Reverse Osmosis, commonly referred to as RO, is the most advanced and unique in nature. The technology uses a semi-permeable membrane which eliminates ions, molecules, and larger substances from water and makes it safe and purest for drinking as well as cooking. In the RO technique, applied pressure is used, which surmounts osmotic pressure. It is a colligative property, driven by potential differences of the chemical solvent. Through RO system, any kind of dissolved and suspended particles including bacteria gets removed from water. As a result, the solute retains on the pressurized side of membrane and the pure solvent passes on the other side of the membrane. The water purification process also involves diffusion, which makes it dependent on rate of water flow, pressure and other stimulating factors.
The RO purifiers are best to be used at homes. There are different industries like water and wastewater purification, food production, production of maple syrups, hydrogen production, reef aquariums and window cleaning which uses RO technology filters. These water purifiers are known for delivering healthy and tasty water, in a cost-efficient manner.
What are the basic parts of RO system?
Most of the Reverse Osmosis purifiers have similar look and functionality. The only difference lies in the quality of its components. Typically, an RO system comprises of the following parts:
- Line Valve for Cold Water: The cold water line valve is the one that fits into the cold water supply line. The tube f the valve is connected to the inlet side of the RO pre-filter. This valve is the source of water for RO systems.
- Pre-Filter: The cold water supply line supplies water which first enters the RO pre-filter. In most systems, there are two pre-filters: one for sediment and the other for carbon. The main function of the two filters are to protect the Reverse Osmosis membranes by eliminating sand, dirt slit and other minute sediments and particles which clogs the system. Carbon filters have additional utility- they remove chlorine, which have the potential to damage the RO membranes.
- RO membrane: Now comes the heart of the water purification system. This semi-permeable membrane has been specially designed to remove various aesthetic and health-related pollutants. Water passes through this membrane and goes into the pressurized storage tank. As it passes, it is treated and ultimately stored in the tank.
- Storage Tank: A standard storage tank is capable of holding 2-4 gallons of water. Inside the tank, there is a bladder which keeps water pressurized, when the tank is full. A RO tank is 12 inches in diameter and 15 inches tall.
- Post-Filter: Treated water enters the post-filter after leaving the RO storage tank and before entering the RO faucet. Post filter is a carbon filter where remaining bad taste or odor is removed from the product water by post-filtration polishing filter.
- Automatic Shut Off Valve (SOV): RO systems are well installed with automatic SOV, for conserving water. Once the storage tank is full, this valve closes, blocks water flow to the drain and prevents water from entering the membrane. After water is drawn from the RO faucet, pressure in the tank lowers down. SOV then opens and sends drinking water through the membrane and diverts the contaminated wastewater down the drain.
- Check Valve: This valve is located at the end of outlet of RO membrane housing. Its primary function is to restrict backward flow of treated water from the RO storage tank since backward water flow cracks the RO membrane.
- Flow Restrictor: Located in the RO drain line tubing, the flow restrictor regulates the water that flows through the RO membrane. Different RO systems have varied flow control ways; however the main purpose is to maintain the required water flow rate for obtaining highest quality of drinking water. Besides this, the restrictor also helps in maintaining the pressure on the membrane inlet. If this additional pressure is missing from the flow control, very little drinkable water will be produced since there will be least resistance in the incoming water and all water will only flow down the drainage.
- Faucet: Each RO unit has its own faucet which is installed on the sink of the kitchen. Non-air gap faucet models are commonly in use; however some systems also adhere to plumbing regulations that demand non-air gap faucet.
- Drain Line: The drain runs from the RO membrane housing outlet end into the drain. It main use is to dispose the wastewater containing all sorts of contaminants, impurities and pollutants which have been filtered out by the RO membrane.
How RO technology works?
Reverse Osmosis process uses high pressure pump to enhance pressure on the salt side of the system. It forces the water across the semi-permeable membrane and leaves behind 95% to 99% of dissolved salts in the reject stream, which carries the concentrated pollutants that did not pass through the RO membrane.
The amount of pressure required for streaming out salt deposits varies according to the concentration of salt of the feed water. More concentrated is the feed water, more amount of pressure is essential for surmounting osmotic pressure.
Water that is desalinated or de-ionized is known as permeate or product water. While the feed water enters the RO membrane, having sufficient pressure to overcome osmotic pressure; the water molecules and ions pass through the semi-permeable membrane. Salts and other contaminants are not allowed to pass and are discharged through the brine stream, also known as concentrate stream or reject stream. This stream either goes to the drain or recycles feed water through the RO system and save water. Water which passes the RO membrane is the product water, with around 99% of impurities removed.
Across all RO systems, cross-filtration is more prevalent than standard filtration. Here, all harmful particles are collected in the filter media. By means of cross filtration, the final water passes through the filter or crosses it, with two outlets: one for the purified filtered water and the other for contaminated water. Cross filtration process enables water to sweep away pollutants and avoids contaminant deposits. It also allows adequate instability for maintaining a clean membrane surface.
Stages of RO process
RO being a modern and innovative water purification technology is capable of eliminating about 99% of 65 different types of contaminants including chlorine, fluoride, dissolved salts and other pollutants. Typically, there are four steps in the RO process:
- Sediment Filter: Also known as the pre-filter stage, the sediment filter stage is the first step in RO technology. At this stage, silt, sediments and dirt drains out. The sediment filter is important since it protects dirt from penetrating into soft RO membrane, and prevents damage to the semi-permeable membrane.
- Carbon Filter: The primary purpose of the carbon filter is to remove contaminants and chlorine which can further affect the life and functionality of the RO membrane. This filter also improves taste and smell of water.
- Reverse Osmosis Membrane: The brain and heart of RO process, the semi-permeable membrane filters out all kinds of contaminants and drains out purest quality of water.
- Polishing Filter: This is the final stage in which a final carbon filter polishes off the water and eliminates remaining odor and taste from purified water. The filter also makes certain that outstanding drinking water is there.
Factors affecting the performance of RO systems
While ROs give healthy and tasty drinking water, there are some factors which might intervene in proper functioning of the system:
- First of all, RO systems don’t work efficiently in case of lower water pressure. Most municipality and city taps have 45-80 psi; however if the incoming water pressure is too low, the system won’t operate.
- Next, the temperature of water also determines the performance of RO systems. Cold water takes longer time to filter, as compared to normal or warm water.
- The number and types of Total Dissolved Salts (TDS) in the tap water also affects overall performance of RO systems.
- Finally, the quality of filters and RO membranes used in the system influences RO’s functionality. Obviously, filters and membranes from good brands run longer and therefore provide good support to the RO purifiers.
With this, RO water purifiers are one of a kind technology, which results in purest quality and form of water.