Dear friends in today’s post i am going to provide a very important piece of information about water purification system which utilises RO technology or Membrane technology
To understand the basics here are some basics of the osmosis and reverse osmosis membrane technology
What is Osmosis: Osmosis is a process of diffusion of a solvent (water) through a semi-permeable membrane, from a solution of low solute concentration (hypotonic solution) to a solution of high solute concentration (hypertonic solution.) Here solvent moves physically and passively ,without applying any input of energy, across the semi-permeable membrane
(a membrane which allows solvent to diffuse through it , but do not allow solute to pass through) separating the two solutions of different concentrations.
The flow of solvent continues till the concentrations of the two solutions reach an equilibrium.
we of movement of solvent can be countered by increasing the pressure of the hypertonic solution, with respect to the hypotonic solution.
Osmotic pressure: This pressure is required to maintain an equilibrium, with no net movement of solvent across the semi-permeable membrane.
Reverse osmosis: This process is exactly reverse process as that of osmosis, in which a solvent passes through a semi permeable membrane in a direction opposite to that for natural osmosis when we apply a hydrostatic pressure greater than the osmotic pressure.
Now consider a semi permeable membrane separating two solutions of different composition (same temperature and column height) pure solvent (a hypotonic solution) and a solution of a solute in the same solvent ( a hypertonic solution).
Here solute cannot pass through the semi permeable membrane but solvent can pass. The rate of migration of solvent across the membrane is not equal. The solute molecules causes obstruction to the solvent molecules across the membrane. The rate of migration of the solvent is more from the hypotonic solution to the hypertonic solution than from the hypertonic solution to the hypotonic solution. The net effects of this migration are that the concentration of the hypertonic solution goes on decreasing and the level of the hypertonic solution goes on increasing till an equilibrium is attained. Here hydrostatic pressure in the columns of the solutions is equal to the osmotic pressure of the hypertonic solution. This pressure now prevents further migration of the solvent across the membrane in to the hypertonic solution.
Now if we apply a pressure on the solution to overcome the osmotic pressure (Pressure caused by hypotonic solution for migration of solvent) the solvent will start flowing across the membrane in a reverse direction , from the hypertonic solution side to the hypertonic solution . This phenomenon is called reverse osmosis (RO) and is used to get the pure water from a source of raw water which contains higher particulate ,contaminant.
The quality of water produced by the reverse osmosis (RO) system depends on the quality of raw water , physical properties of the membrane, and pressure applied across the membrane.
Physical properties of the reverse osmosis membrane which are important for further designing :-
1. Hydrophilicity : This is degree of affinity of membrane material towards water molecules. higher affinity means better output. higher affinity too, leads to other operational and design issues, and hence, a balance is required.
2. In face : Capacity of membrane to release water . It is important to have affinity towards water molecule, also membrane should release this water .
3. Morphology: porosity of membrane, Average porosity . more the porosity, more the output. However, higher average porosity also leads to leakage of other higher molecular weight molecules which are actually contaminants for water. On the contrary very low porosity will require very high pressures to be applied for migration of water through the membrane which increases operating costs, hence, a balance needs to be established.
The reverse osmosis RO membranes are classified into three types, based on their structure, material of construction , shape, and their structure further categorised as—isotropic/symmetric/thin, and anisotropic/asymmetric.
Based on their material of construction:-Reverse osmosis membranes are categorised as
1.Cellulose acetate:- Cellulose acetates membranes fall under anisotropic structure membranes. Because of the thick semi permeable membrane, pressure drop required across these membranes is high and hence requires higher feed water pressure, causes higher energy input. Cellulose acetates membranes can withstand chlorine up to one part per million (ppm) levels.
2.Aromatic polyamide:- Aromatic polyamides membranes fall under isotropic structure membranes. Because of thin semi permeable membrane, pressure drop required across these membranes is low and hence requires lower feed water pressure, saving energy input. However, Aromatic polyamides membranes can’t tolerate chlorine and hence extra care is required for removal of oxidizing agents prior to RO.
Depending up on their shapes RO membranes are categorised as flat sheet, tubular, hollow fiber, and spiral wound.
What is Feed: Raw water Input to RO system
What is Permeate: Output from membrane having lower concentration of contaminant
What is Concentrate (reject):This is output from Ro system having higher concentration of contaminant. In case of membranes used for water purification, what is Recovery: It is a ratio of permeate flow to feed flow.
What is salt passage: it is a ratio of permeate (Pure water TDS) contamination to feed contamination (TDS)
What is Percent salt passage: it is a ratio of permeate (Pure water TDS) contamination to feed contamination (TDS) multiplied by 100
What is Percent salt rejection: is reversal of Percent salt passage ie 100 – Percent salt passage
What is Concentration factor: Ratio of reject concentration to feed concentration.
What is Flux: This is flow of water through membrane measured in gallons per day per unit area of membrane.
When Raw feed water flows through membrane, water molecules pass through the membrane and the dissolved contaminants in water get attracted towards membrane surface, but do not flow through the membrane, they contaminants get diffused back to subsequent incoming flow of water and get rejected out with reject water. At this point concentration of contaminants (ions) near membrane becomes very high, this phenomena is called as ‘concentration polarisation’.
If the concentration of ions near membrane reaches equal to its super saturation level, they get precipitated as solids and form a layer on membrane. This is phenomena called scaling.
So above phenomena makes it clear that any changes in flows of feed, permeate, reject can cause problems to membrane. Also it is very important that quality of feed water should be contain low TDS, otherwise it may cause scaling.
How to avoid damaging effects of concentration polarisation
1. Keep lower recovery.
2. Go for higher membrane surface area for lower flux . This increases the filtration area, prevents concentration reaching to super saturation level.a minimum flow per unit area of membrane at reject side is required to flush out the dissolved high concentration.
3. Using anti scalant chemicals. anti scalant chemicals increases the saturation level of individual ions. Do not over or under dose these chemicals
Scaling: is referred to concentration polarisation. The solidification of dissolved contaminant/s, which forms scale and blocks membrane surface available for filtration
Fouling: Colloidal particles and suspended solids in feed water increases which , get deposited on membrane surface, blocking the membrane surface, reducing available filtration area, is called fouling. If contaminant is biological it is referred as ‘bio fouling’.
Damage: is referred to physical damage to membrane. Because of improper handling, mechanical damage, bio-film attack .
Hydrolysis of membranes: Because of continuous water flow through membranes, it sows some decay .This is called as hydrolysis of membranes. This is dependant on pH of water. Cellulose acetate membranes are more prone to pH. In case of thin film membranes, it is less prone to PH and allows to operate on wider range of pH values.
What care should be taken to minimise these failures:
1.Scaling:
Keep lower recovery,
Use lower flux rates
Monitor feed water quality
Do not use excessive chemical flocculants
Use good and proven Anti scalant in correct concentration.
2. Fouling :
A good pre treatment to raw water
Keep silt density index (SDI) less than 4
use micron prefilter prior to membrane
3. Hydrolysis :
Operate within pH range specified by system designer/membrane manufacturer.
Keep silt density index (SDI) less than 4
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