Reverse Osmosis is the process of Osmosis in reverse. Whereas Osmosis occurs naturally without energy required, to reverse the process of osmosis, energy needs to be applied to the more saline solution. A reverse osmosis membrane is a semi-permeable membrane that allows the passage of water molecules but not the majority of dissolved salts, organics, bacteria, particles and pyrogens. However, water needs to be `pushed´ through the reverse osmosis membrane by applying pressure that is greater than the naturally occurring osmotic pressure in order to desalinate (demineralize or deionize) water in the process, allowing pure water through while holding back a majority of contaminants.
How does Reverse Osmosis work?
Reverse Osmosis works by using a high pressure pump to increase the pressure on the salt side of the RO and force the water across the semi-permeable RO membrane, leaving almost all (around 95% to 99%) of dissolved salts behind in the reject stream. The amount of pressure required depends on the salt concentration of the feed water. The more concentrated the feed water, the more pressure is required to overcome the osmotic pressure. The desalinated water that is demineralized or deionized, is called permeate (or product) water. The water stream that carries the concentrated contaminants that did not pass through the RO membrane is called the reject (or concentrate) stream.
As the feed water enters the RO membrane under pressure the water molecules pass through the semi-permeable membrane and the salts and other contaminants are not allowed to pass and are discharged through the reject stream (also known as the concentrate stream), which goes to drain or can be fed back into the feed water supply in some circumstances to be recycled through the RO system to save water. The water that makes it through the RO membrane is called permeate or product water and usually has around 95% to 99% of the dissolved salts removed from it as compared to feedwater.
To avoid build-up of contaminants, cross flow filtration allows water to sweep away contaminant build up and also allow enough turbulence to keep the membrane surface clean. This automatic cleaning can happen if the membrane has a strong cross flow over its surface. This is achieved by having a good power pump behind the membrane. The permeate flow depends on feedwater pressure exerted by pump and the temperature of water. Each degree increase in temperature of water will increase the permeate flow by 3%. Thus if a RO membrane is “tuned” to generate 12 ltr/ hour flow at 15oC, if the feedwater temperature is 25oC (which generally is the case in any part of India, irrespective of seasons) then the permeate flow will increase by around 30%, means a standard 12ltr/hr RO will give out approximately 15.6ltr/hr. Now if a system has EDI cell rated at 12ltr/hr, after the RO it surely can’t handle such increase in flow. Even if it gives out bigger flow physically, the quality will be drastically reduced due to the small amount of resin contained in the EDI cell. To prevent the deterioration of water quality after EDI particularly, there is only one way. The design can be done such that the flow from RO permeate is reduced. To reduce the permeate flow the pump reduces the amount of water pumped on the membrane by reducing the voltage to the pump. Thus the pump rotates at a smaller speed and reduces the pressure on membrane.
However this continuous adjustment is possible till water temperature remains below 30oC. If temperature exceeds 30oC such temperature feedback mechanism to the pump does not work as then pump will fully stop as it won’t get the bare minimum voltage to run. Hence this is a big disadvantage in tropical or Indian conditions. Further due to this continuous adjustment in the rotation speed of pump motor, the motor becomes weak and eventually fails.
Now let’s look at a differently. What happens if temperature of water reduces? It goes to 4oC which is very much possible in European countries. The flow reduces from RO and inturn EDI, the quality of water from EDI may improve but quantity is less than the rated flow rate. The temperature feedback mechanism works here and it increases the voltage to keep the flow rate constant. In European countries the tap water temperature very very rarely exceeds 30oC and thus the temperature feedback mechanism is perfect for European countries, not for India or tropical countries.
Now there are distinct disadvantages of this temperature feedback mechanism:
1. It is designed for European countries to increase flow and it works there.
2. For Indian conditions after 30oC it won’t work and increased flow from EDI cell will still provide bigger quantity but worse water quality.
3. With this continuous varying voltage, the pump gets frequent shocks of voltage increase and decrease. This causes failure of pumps and incurs huge running costs.Which is experienced by majority users.
4. Water system is a backbone of lab. A failure of water system brings the lab to standstill and causes loss of analysis time.
5. Upgradation of such systems with EDI is highly expensive and sometimes difficult than buying an entirely new system.
What is the mechanism to prevent above?
TKA has a special design by which the control of flow is done semi-automatically. The tuning of all TKA water systems can be done at 25oC instead of typical Eurpoean design of 15oC. And this can be done even on site by the engineer who installs the system!! Thus a small increase in temperature from 25oC to 30oC will cause a 15% increase in flow. The TKA ion exchanger is capable to tolerate that increase. In fact, the TKA ion exchanger cartridge is capable to tolerate the flow from 6 ltr/hr to 40 ltr/hr. Thus TKA systems can handle feedwater irregularities efficiently and still maintain the quality of water for a longer time, consistently. Therefore it is technically safe to upgrade the system flow rate from 6 upto 40 ltr/hr with same hardware such as pump and consumable ion exchange but by simply replacing the RO membrane of a bigger size.
Advantages with the TKA design:
1. Upgradation is easy and economical. Just replace the smaller RO cartridge with bigger. Replacement of RO cartridge itself can be done even by user. Engineer is not required.
2. Save on capital cost on procurement of new system.