What Makes RO Membrane Life Short?
Reverse Osmosis (RO) membranes are used for puri?cation of water, for use in industrial processes and for wastewater treatment to eliminate impurities, salts and other related compounds in water. These membranes are important in as much that they help to achieve the right degree of purity and quality that is wanted. Although essential in such processes, RO membranes are short-lived, and their endurance can be shortened by the following factors.
We will discuss the main factors that may be attributed to shorten the life expectancy of the RO membrane and how to avoid such factors to get the most of it.
1. Fouling
Membrane fouling is defined as the deposition of a layer of particles sized from microorganism to colloids and other organic and inorganic materials on the RO membrane. One of the most critical parameters that define both the efficiency and longevity of RO membranes. Membrane fouling is characterised by the decrease in permeate flux, increase in operating pressure and a corresponding decrease in salt rejection, which results in poor membrane performance or shortened useful life.
Fouling can be caused by various factors, including:
· Silt Density Index (SDI): High values of SDI for feed water mean the presence of various particles and colloid matter, which can form fouling layer on the membrane surface. Thus, the necessity to maintain and regulate SDI, so as to avoid significant increase in fouling.
· Organic fouling: Substances such as humic acids, proteins, and algal extracellular polymeric substances may foul the membrane surface and decrease its performance. Coagulation, sedimentation or filtration before the membrane can also assist to eliminate the problem of organic fouling.
· Inorganic fouling: Deposition of such materials as calcium carbonate, calcium sulfate, silica etc lead to scaling which negatively impacts on the performance of the membrane. The major category of unwanted deposits is inorganic foulants can be prevented through water chemistry management such as controlling the pH level of water used in the operation and the use of antiscalants.
· Microbial fouling: Microorganisms increase their rate of attachment to the membrane surface and contribute to biofouling, which averts high fouling trends and low permeate flux. It means that by improving the pre-treatment, proper and consistent cleaning and ensuring right operating conditions can limit the microbial fouling.
2. Concentration Polarization
Concentration polarization refers to a situation where the concentration of the solutes rises in the layer of the boundary close to the membrane surface and consequently the osmotic pressure difference is reduced across the membrane. This phenomenon results into the decrease of permeate flux and the salt rejection, leading to the shortening of the operational life of the membrane.
In order to mitigate concentration polarization, the values of operational parameters such as crossflow velocity, feed pressure and temperature must therefore be properly fine tuned. The spacer grids used in the design of the membrane channel can also help to reduce the extent of concentration polarization through the improvement of the convection mass transfer in the channel.
Read: Commercial RO Plant Manufacturer
3. Membrane Stress
Another factor that affects the use of RO membranes is the mechanical stress resulting from the feed pressure, temperature changes, as well as thermal stress during operation. If such stresses are sustained for long enough, then the membrane will become compacted, decrease the membrane surface area and can fail.
In order to reduce membrane stress, steady operating pressure should be managed, temperature level should be regulated and there should be no drastic change in feed water properties. Daily observation of several system parts like pumps, valves, and pressure vessels will also go a long way towards avoiding associatively possible problems likely to put pressure on the membrane.
4. Chemical Degradation
RO membranes are vulnerable to chemical attack by chemical agents available in the feed water including chlorine, chloramines, ozone, and hydrogen peroxide. These chemicals may precipitate and/or cause hardening of the membrane polymer structure and therefore reduced performance and lifeline.
Because chemical degradation is inevitable, there is need to pretreat the feed water to remove these oxidants and ensure they are present in concentration that will not harm the membrane. The dissolved gases can also be removed by installing degasifier or using special type of membrane which has high chlorine tolerance technique.
5. Oxidative Degradation
Oxidative degradation takes place because the selected membrane reacts with other species, including hydroxyl radicals, peroxides, and ozone. These species can induce chain scission and cross-linking of the membrane polymer reducing its performance and the useful life of the membrane.
The best method of preventing oxidative degradation is to reduce or avoid the chance of the membrane coming in contact with these reactive species. Pre-treatment treatments such as carbon filtration or catalytic oxidation, can also be applied to minimize the concentration of oxidants in the feed water. Cleaning can also prevent or reduce instances of oxidative degradation as well as it also include the inspection of any signs of problems that may result to oxidative degradation.
6. Membrane Age
These characteristics are inherited by the membrane material and its performance decreases as well as its life cycle is shorten. Over time, the membrane material is prone to fouling, concentration polarization and even chemical degradation. Suppliers often give expected service life of the membranes according to the predicted conditions under which they are going to be used.
It is possible and advisable to change the membrane before it completely deteriorates so as to avoid repeating the unsatisfactory results and achieve maximum water treatment effectiveness. Usually observation of the membrane’s parameters such as flux and level of salt rejection reveals when it is time to replace the membrane, therefore such factors cannot be ignored.
Conclusion
The normal lifespan of RO membranes can be reduced through different ways such as fouling, concentration polarization, membrane stresses, chemical and oxidative degradation and membrane’s age. Knowing your feed water quality and some of the previously mentioned factors, along with following general pre-treatment, operational and maintenance guidelines can help you get the most out of your RO membrane.
If these problems are dealt with at their source, the longevity of the RO membrane will be realized along with minimizing overall operating expenses and disruption. Understanding and avoiding the main factors contributing to shortening membrane life will enable a more effective, durable, and possibly cheaper water treatment system.
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