RO membrane technology for water purification

RO membrane technology for water purification

Kishor D Datar

Water and air management are two very important (and very difficult to design and maintain) utilities for any segment in the pharmaceutical industry. This article focuses on water management.

Due to the technological advancement in the last few years, today very advanced techniques are available to produce high purity water. One of the most common components in all these systems is the reverse osmosis (RO) module.

The article will focus on the basics of RO membrane because the basic understanding of the RO membranes functioning will help pharmaceutical engineers to maintain their existing systems, troubleshoot and correct the problem scientifically and more efficiently, and design new systems optimally.

Some of the terms are defined as below:

Osmosis: The process of diffusion of a solvent (frequently water) through a semi-permeable membrane, from a solution of low solute concentration (hypotonic solution) to a solution of high solute concentration (hypertonic solution. It is a physical process in which the solvent moves passively ie without any input of energy, across the semi-permeable membrane (permeable to the solvent, but not to the solute) separating the two solutions of different concentrations.

The flow of solvent continues till the concentrations of the two solutions reach an equilibrium.

This effect can be countered by increasing the pressure of the hypertonic solution, with respect to the hypotonic solution.

Osmotic pressure: The pressure required to maintain an equilibrium, with no net movement of solvent across the semi-permeable membrane.

Reverse osmosis: The process in which a solvent passes through a semi permeable membrane in a direction opposite to that for natural osmosis when it is subjected to a hydrostatic pressure greater than the osmotic pressure.

Some common questions that arise in every one’s mind are—what is meant by membrane technology, what principle it uses to purify the water, what kind of technology is used in these membranes to purify the water?

Consider that a semi permeable membrane separates two solutions of identical composition (and concentration, temperature and column height). (Figure 1)

The solute cannot pass through the semi permeable membrane but the solvent can. The rate of migration across the membrane is equal in either direction and hence there will be no change in any of the above mentioned parameters ie a state of equilibrium shall be maintained.

Now consider that the semi permeable membrane separates two solutions of different composition (and temperature and column height) (Figure 2) eg pure solvent (hypotonic solution) and a solution of a solute in the same solvent (hypertonic solution).

The solute cannot pass through the semi permeable membrane but the solvent can. The rate of migration across the membrane is not equal. The solute molecules cause obstruction to the solvent molecules across the membrane. Consequently, the rate of migration of the solute 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 reached. (Figure 3)

The hydrostatic pressure of the difference in the columns of the solutions is equal to the osmotic pressure of the hypertonic solution. This pressure now prevents further net migration of the solvent across the membrane in to the hypertonic solution. (Figure 4)

If we now apply deliberate pressure on the solution side to overcome the osmotic pressure the solvent will start flowing across the membrane in reverse direction ie from the hypertonic solution side to the hypertonic solution side. This phenomenon is called reverse osmosis (RO) and is used to get the pure water from contaminated water.

From the above explanation it is clear that the quality of water produced by the RO system will depend on the quality of feed water, physical properties of the membrane, and pressure applied across the membrane.

Consider that a semi permeable membrane separates two solutions of identical composition (and concentration, temperature and column height)

The basic physical properties of the membrane which have impact on further designing are as follows:

1. Hydrophilicity : Is the property of membrane material of affinity towards water molecules. Generally higher affinity means better output. However, higher affinity also leads to other operational and design issues, and hence, a balance needs to be established.

2. In face : Water releasing capacity of membrane. It is not important only to have affinity towards water molecule, but membrane should release this water and should not hold back.

3. Morphology: Average porosity of membrane. Higher the porosity, higher the output. However, higher average porosity also leads to leakage of other higher molecular weight molecules which are actually contaminants for water. On the other hand, very low porosity will require very high pressures to be applied for transferring water molecules through the membrane thus increasing operating costs, and hence, a balance needs to be established.

The RO membranes are broadly categorised into three types—based on their structure, material of construction (MOC), final form of use (shape), and their structure further categorised as—isotropic/symmetric/thin, and anisotropic/asymmetric.

Based on their MOC the membranes are categorised as cellulose acetate and aromatic polyamide. Cellulose acetates are membranes that fall under anisotropic structure membranes. Due to the thick semi permeable membrane, pressure drop across these membranes is high and hence requires higher feed water pressure, causing higher energy input. However, these membranes can withstand chlorine up to one part per million (ppm) levels. Aromatic polyamides are membranes that fall under isotropic structure membranes. Due to thin semi permeable membrane, pressure drop across these membranes is low and hence requires lower feed water pressure, saving energy input. However, these membranes can’t tolerate chlorine and hence extra care is required for removal of such oxidizing agents during operation.

Based on their shape RO membranes are categorised as flat sheet, tubular, hollow fiber, and spiral wound.

Feed Flow x Feed Concentration = (Permeate Flow x Permeate Concentration) + (Reject Flow x Reject Concentration)

Some of the terminologies commonly used with respect to membranes by membrane manufacturers and system designers are as follows (refer to Schematic 1):

  • Feed

    : Input to membrane

  • Permeate: Output from membrane having lower concentration of contaminant

  • Concentrate (reject): Output from membrane having higher concentration of contaminant. In case of membranes used for water purification, the concentrate is normally called as ‘reject’ and same terminology will be used in this article.

  • Recovery: Ratio of permeate flow to feed flow. Reference to above schematic, it is Y/X.

  • Salt passage: Ratio of permeate contamination (in case of water, TDS) to feed contamination (TDS). Refer to schematic it is b/a.

  • Percent salt passage: Salt passage multiplied by 100

  • Percent salt rejection: is reversal of Percent salt passage ie 100 – Percent salt passage

  • Concentration factor: Ratio of reject concentration to feed concentration. Refer to schematic it is c/a.

  • Flux: Flow through membrane measured in gallons per day per unit area of membrane. This term resembles to filtration area normally used for any physical filtering system.

  • One very important equation: Please ensure to read carefully following equation, since this equation is very important and will be helpful during monitoring of system and altering the system parameters.

The equation below is applicable for total contamination as well as individual contaminant also.

It is important to know some thing about concentration polarisation. Refer above equation, in case of water purification required for pharma industry; let us assume that feed flow is 1000 L/Hr, with TDS of 200 ppm. The permeate flow is at 600 L/Hr, with TDS of 5 ppm. The reject flow becomes 400 L/Hr. Now if we substitute these values in above equation, the TDS of reject comes to 492.5 ppm.

Although the recovery is 60 percent, the reject TDS concentration has gone up by approximately 2.5 times of feed TDS. This makes it very clear that near reject surface of membrane the contaminant’s concentration reaches very high.

Refer to Schematic 2

When feed water flows on membrane, water molecules travel across the membrane and the dissolved contaminants in water get attracted towards membrane surface, but can’t flow through the membrane. These contaminants get diffused back to subsequent flow of water and get rejected out along with reject water. During this process, concentration of contaminants (ions) near membrane becomes very high, this phenomena is known as ‘concentration polarisation’.

If concentration of any ion near membrane reaches equal to its saturation level, they get precipitated out forming undissolved solids and form a layer on membrane. This is called scaling.

With the above phenomena it is clear that making changes in flows of feed, permeate, reject can cause problems to membrane. Also it is evident that the fluctuation in quality of feed water should be monitored closely to avoid concentration of any individual ions going high in feed water, causing scaling.

To avoid danger due to concentration polarisation, ensure the following at the time of design and monitor it closely:

1. Keep lower recovery. This may impact on initial higher capital cost, but pays back within years span. Thumb rule says to keep concentration factor less than five

2. Ask for higher membrane surface area for a given flow ie lower flux rate. This increases the filtration area, reducing concentration reaching to saturation level. However, care has to be taken, since some minimum flow per unit area of membrane at reject side is must to flush out the dissolved high concentration

3. Using anti-scalant chemicals. These chemical increases the saturation level of individual ions. However, please consult your system designer as well as chemical manufacturer, since these chemicals need to be selected based on the feed water quality and system parameters. Do not over or under dose these chemicals. It is advisable to keep sending the feed water analysis report top both system designer and chemical manufacturer, to understand changes required in dosing concentration, due to seasonal variations in water quality.

It is important to know types of failures normally faced when using membranes for water purification. The following terminologies are commonly used by system designers very frequently:

1. Scaling: Referring to concentration polarisation. The solidification of dissolved contaminant/s, forming scale, blocking of membrane surface available for filtration is called scaling

2. Fouling: Higher level of colloidal particles/suspended solids in water, getting deposited on membrane surface, blocking the membrane surface, reducing available filtration area, is called fouling. In case such fouling is due to biological contaminates it is normally referred as ‘bio fouling’.

3. Damage: Physical damage to membrane. Some of the common causes are improper handling, mechanical damage, and bio-film attack (live organisms tend to use membrane material as their food, damaging the membrane surface).

4. Hydrolysis of membranes: Due to continuous water flow through membranes, membranes decay and their life reduces. This phenomena is called hydrolysis of membranes. This phenomena is dependant of pH value of water. Cellulose acetate membranes are more prone to these phenomena and need to be operated within narrower range of pH. In case of thin film membranes, this phenomena is less prone and has wider range of permissible pH values.

In either case this phenomena is logarithmic function, and hence important to ensure operation within correct pH value. The pH value going out of permissible range by 1 may reduce the life considerably.

Following care should be taken to minimise these failures:

1.Scaling:

  • Keep lower recovery, balancing economical design. Do not play with recoveries without knowing the impact, feed water analysis etc

  • Use lower flux rates

  • Monitor feed water quality and keep consulting your system designer/chemical manufacturer to change operating parameters etc

  • Do not allow excessive chemical flocculants used in pre treatment to reach the membrane. This will cause secondary precipitation due to concentration polarisation

  • Use correct Anti scalant in correct concentration.

2. Fouling :

  • Design appropriate pre treatment based on incoming water analysis

  • Ensure that silt density index (SDI) It is kept less than 4

  • Ensure using micron filter prior to membrane

  • Keep values of turbidity, oil, grease etc less than membrane/system designers advise.

3. Hydrolysis :

  • Strictly operate within pH range specified by system designer/membrane manufacturer.

Silt Density index (SDI) is a commonly used terminology by system designers as well as membrane manufacturers. Basically this index provides comparative analysis of degree of colloidal/suspended particles present in water. Higher concentration of these particles caused fouling.

Procedure/method to find out SDI of water is very simple and does not require any high skill or high end instrumentation. Although, today online SDI instruments are available, using a procedure explained below, giving same accuracy.

Referring to the above figure above:

1. Start the water maintaining 30 psig pressure

2. Collect X volume of water (normally not less than 100 ml).

3. Measure time required to collect X volume of water. Say ‘T0’.

4. Allow water to flow through filter for 15 minutes, count these 15 minutes from the beginning ie includes the time ‘T0’.

5. After 15 minutes, collect same X volume of water and measure time taken for collecting this water. Say ‘T15’Calculate SDI as follows:

SDI=((T15-T0)/T15) x 100 —————————–

15

If you cannot draw X volume of water after 15 minutes, it is very clear that SDI of water is very high and is not recommended for use as is for membranes.

Through this article the user will clear their basic fundamentals about RO membrane which will help them in selecting the systems, verifying the design and to monitor the installed systems.

(The author is Managing Director, Technolutions Projects)