All RO plants are not the same. While the basic technology is the same, there are different levels of filtrations and designs. It is important to understand these parameters since these have a direct impact on the operational efficiency of the RO plant. Regular evaluation of the plant is critical for maintenance and to pinpoint faults before these became unmanageable.
There are certain calculations used by reverse osmosis plants manufacturers use to evaluate the performance of an RO system.It also helps them in perfectly design flaws. A typical system will typically display certain key parameters like pressure, flow, temperature and more. Some of the essential parameters that are needed to judge performance are feed pressure, concentrate pressure, permeate pressure, feed conductivity, permeate conductivity, permeate flow, feed flow and temperature.
Some methods applied in evaluating an RO plant are:
Salt rejection
This is given as a percentage of feed water contaminants rejected. It tells us the effective of the RO membrane. It is evaluated by subtracting the conductivity of permeate water from the conductivity of feed water. It is then calculated as percentage of the conductivity of the feed water. In this it gives us an estimate of the efficiency of all the membranes taken together. However, it does not tell us about the performance of each individual membrane. We are looking for a high percentage of salt rejection. A low percentage means that the membranes have to be cleaned.
Salt passage
Here we do the reverse of the salt rejection. Instead of checking the percentage of the salt rejected, we look for the amount of salt that passed. Like salt rejection, it is also expressed in percentage. However, an ideal condition is when we have a low salt passage. High salt passage indicates a malfunctioning RO plant.
Flux
Flux is the volume of water that permeates a membrane in a given period. It is typically expressed as volume per area per unit of time, usually gallons/square foot/day or litre/m/hour. The flux of membrane is directly proportional to pressure and temperature. The flux tells us about the efficiency of the membrane. Reverse osmosis plants manufacturers include this in their design specifications. It should ideally remain consistent throughout the membrane. Flux numbers are checked regularly to ensure that the membrane are working to their optimal capacity.
Recovery
The RO technology cleans out the feed water, leaving us with permeate water that is safe to use. On the other hand is the concentrated water that is left to drain. Checking the amount of cleared or recovery water tells us that we are draining less water and more contaminants are being taken out. The recovery percentage can also tell us about the state of the membrane.
Reverse osmosis plants manufacturers will have an ideal recovery percentage. This is the amount that the plant design allows to pass through. It is decided while keeping in mind factors like the contaminants in the feed water and the pre-treatment. While a lower than specified recovery percentage is obviously a sign of worry, a higher percentage is also not a welcome sign. If the recovery percentage higher than the design will allow, the membranes are susceptible to increased fouling and scaling.
Concentration factor
This is the flip side of the recovery percentage. The more the recovery percentage, the more is the concentration factor or the contaminants collected on the other side of the membrane. As in the recovery percentage, here too we are looking for unusually high numbers to avoid dangers of scaling or fouling. Recovery percentage and concentration factor tell us that the membranes are functioning as per the design. It also lets us know when the water is too contaminated, creating dangers of fouling and scaling.
Mass Balance
The key to checking the performance of your membrane is through reading its instrumentation. So, if your instrumentation is not working as it should, your readings will all be faulty. Mass balance is a calculation that is performed with various parameters like feed flow, concentrate flow and feed conductivity.
An RO plant that is not working at its optimum level indicates failing membranes, a design flaw or simple wear and tear. This is why reverse osmosis plants manufacturers have design parameters that help us to keep a check on its regular functioning.
