Selecting the Right Separation Process for the Water
Membrane technology isn't new, experimental or mysterious. It is a well-established family of separation processes used throughout multiple sectors requiring water treatment to remove suspended solids, colloids, microorganisms, organic molecules and dissolved salts.
The important part is selecting the right membrane for the application.
Microfiltration, ultrafiltration, nanofiltration and reverse osmosis may all use a membrane barrier, but they perform very different jobs.
At Hydra-Clear, we design membrane treatment systems around the incoming water chemistry, required treated-water quality and operating conditions rather than selecting a technology simply because it produces "purer water".
Sometimes the answer is reverse osmosis. Sometimes it isn't.
How Membrane Separation Works
A membrane provides a selective barrier between the feedwater and the treated water.
Water passes through the membrane while selected contaminants are retained.
As the required level of separation increases, the membrane becomes progressively more selective.
Broadly, membrane processes can be considered as:
Microfiltration → Ultrafiltration → Nanofiltration → Reverse Osmosis
Moving from left to right increases the ability to remove smaller contaminants and dissolved material, but also changes the required operating pressure, membrane design, pretreatment requirements and energy consumption.
Selecting the correct process therefore requires more than simply choosing the "finest" membrane available.
Microfiltration – MF
Microfiltration is primarily used to remove suspended material and microorganisms from water and process liquids.
Typical MF membranes operate in the approximate range of 0.1 to 10 micrometres, although membrane specifications vary considerably between manufacturers and applications.
Microfiltration can be effective for removing:
* Suspended solids
* Fine particulate
* Sediment
* Many bacteria
* Oil droplets
* Yeast and similar microorganisms
It does not significantly remove dissolved salts or hardness.
Typical Applications
Microfiltration can be used for:
* Process-water clarification
* Food and beverage clarification
* Removal of suspended solids
* Oil/water separation
* Wastewater treatment
* Protection of downstream membrane systems
* Reduction of particulate loading before finer separation stages
Where the primary problem is suspended material rather than dissolved contamination, MF can provide a significantly more appropriate solution than reverse osmosis.
Ultrafiltration – UF
Ultrafiltration provides finer separation than microfiltration and is particularly effective at removing suspended solids, colloidal material, bacteria and high molecular weight organic compounds.
UF membrane selection is commonly specified by either nominal pore size or molecular weight cut-off – MWCO.
Typical UF membranes operate approximately within the 0.01 to 0.1 micrometre range, although the exact characteristics depend on the membrane chemistry and manufacturer.
UF can provide extremely low-turbidity treated water and is widely used where consistent particulate and microbiological control is required.
Ultrafiltration Can Remove
* Suspended solids
* Colloidal material
* Bacteria
* Many viruses, depending on membrane specification
* High molecular weight organic material
* Fine particulate
Like microfiltration, UF does not normally remove significant quantities of dissolved salts.
Typical Applications
* Reverse osmosis pretreatment
* Surface-water treatment
* Process-water production
* Food and beverage applications
* Industrial wastewater treatment
* Water recovery and reuse
* Removal of colloidal material
* Pharmaceutical and hygienic process applications
One of the major advantages of UF upstream of reverse osmosis is the consistency of the RO feedwater.
Where incoming turbidity or colloidal loading varies considerably, UF can significantly reduce the fouling burden placed on the downstream RO membranes.
Nanofiltration – NF
Nanofiltration occupies the separation range between ultrafiltration and reverse osmosis.
Unlike MF and UF, nanofiltration is capable of removing a significant proportion of dissolved ions.
However, NF is deliberately more selective than RO.
It generally provides high rejection of multivalent ions such as calcium and magnesium while allowing a greater proportion of monovalent ions such as sodium and chloride to pass.
This makes NF particularly useful where complete demineralisation isn't required.
Nanofiltration Can Be Used To Reduce
* Calcium
* Magnesium
* Hardness
* Sulphate
* Larger organic molecules
* Colour
* Selected dissolved contaminants
The exact rejection characteristics depend heavily on the membrane selected, feedwater chemistry, pH, temperature, pressure and ionic composition.
Typical Applications
* Membrane softening
* Partial demineralisation
* Sulphate reduction
* Organic matter reduction
* Colour removal
* Food and beverage processing
* Process stream concentration
* Water reuse applications
NF can sometimes deliver the required treatment quality at a lower operating pressure than reverse osmosis, making it an attractive option where selective ion removal is more important than maximum salt rejection.
Reverse Osmosis – RO
Reverse osmosis is the most selective of the pressure-driven membrane processes commonly used in industrial water treatment.
Rather than treating RO as a simple mechanical sieve, it is more accurate to consider it a molecular separation process.
Pressure greater than the natural osmotic pressure of the feedwater is applied across a semi-permeable membrane.
Water permeates through the membrane while the majority of dissolved ionic material is rejected and concentrated into a separate reject stream.
RO can typically achieve very high rejection of dissolved salts and is therefore widely used for producing low-conductivity process water.
Reverse Osmosis Can Reduce
* Dissolved salts
* Calcium and magnesium
* Sodium and chloride
* Sulphate
* Nitrate
* Silica
* Many dissolved metals
* Selected organic contaminants
* Microbiological loading
The actual rejection achieved depends on the individual ion, membrane selection, feedwater chemistry and system operating conditions.
This is why an RO system should be designed using a water analysis and membrane projection, rather than simply selecting a pump, pressure vessel and nominal flow rate.
Where We Use Reverse Osmosis
Hydra-Clear designs RO systems for applications including:
* Industrial process water
* Boiler make-up water
* Food and beverage production
* Pharmaceutical water systems
* Manufacturing processes
* Laboratory and high-purity applications
* Cooling and utility systems
* Water recovery and reuse
* Borehole treatment
* Brackish-water treatment
* Pre-treatment for further polishing processes
Where extremely low conductivity is required, RO can also form the first stage of a wider treatment process incorporating technologies such as electrodeionisation, ion exchange or polishing systems.
It's Not Just About the Membrane
The membrane element is only one part of a successful membrane system.
An RO plant with the wrong pretreatment, recovery, flux rate or control philosophy can perform poorly regardless of the quality of the membrane installed.
Hydra-Clear considers the complete process.
This includes:
* Feedwater analysis
* Membrane selection
* System recovery
* Membrane flux
* Crossflow velocity
* Scaling potential
* Fouling potential
* Concentrate chemistry
* Pretreatment requirements
* Antiscalant selection
* Operating pressure
* Temperature correction
* Cleaning philosophy
* Instrumentation
* Controls
* Redundancy
* Future membrane replacement and maintenance
For RO and NF systems, we use membrane projection software to model system performance against the actual feedwater analysis wherever appropriate.
This allows predicted permeate quality, recovery, pressure, concentrate chemistry and membrane performance to be assessed before the system is manufactured.
Pretreatment Matters
Membrane systems rarely operate successfully in isolation.
The treatment installed upstream of the membrane can have a substantial effect on membrane life, cleaning frequency and long-term operating cost.
Depending on the incoming water, pretreatment may include:
* Multimedia or depth filtration
* Activated carbon
* Water softening
* Iron and manganese removal
* Microfiltration
* Ultrafiltration
* Cartridge filtration
* Chemical dosing
* pH correction
* Antiscalant dosing
* Dechlorination
There is no universal pretreatment arrangement.
A mains-water RO feeding a boiler has very different requirements from an RO treating borehole water containing iron, manganese and high silica.
The process needs to reflect the water.
Recovery Isn't Everything
It can be tempting to operate a membrane system at the highest possible recovery because this reduces reject-water volume.
That isn't always good engineering.
Increasing recovery also increases the concentration of dissolved material within the membrane concentrate.
This can increase the risk of:
* Calcium carbonate scaling
* Calcium sulphate scaling
* Barium and strontium sulphate deposition
* Silica scaling
* Increased osmotic pressure
* Reduced membrane performance
* More frequent chemical cleaning
The optimum recovery is therefore a balance between water efficiency, membrane performance, feedwater chemistry and operating cost.
We would rather design a membrane plant that operates reliably than advertise an impressive recovery percentage that the water chemistry cannot support.
Membrane Fouling and Cleaning
All membrane systems experience some degree of fouling over their operating life.
The objective is to minimise it and recognise when membrane performance is beginning to deteriorate.
Potential foulants include:
* Suspended solids
* Colloidal material
* Biological growth
* Organic contamination
* Iron
* Manganese
* Aluminium
* Mineral scale
* Silica
A properly instrumented membrane system allows changes in performance to be identified through parameters such as:
* Normalised permeate flow
* Differential pressure
* Salt passage
* Conductivity
* Operating pressure
* Temperature
* Recovery
Cleaning should then be undertaken based on actual membrane condition and the probable foulant rather than simply circulating a generic chemical through the plant.
Hydra-Clear provides membrane cleaning, performance investigation and optimisation as part of our service capability.
Standard Systems. Bespoke Engineering.
Hydra-Clear manufactures a range of standard reverse osmosis systems, but standardisation stops where the application requires something different.
Our membrane plants can be engineered with:
* 316 stainless steel process pipework
* Hygienic components
* Duty/standby feed and transfer pumps
* Duplex RO arrangements
* Two-pass RO
* CIP systems
* Permeate flushing
* Automatic diversion
* Conductivity control
* UV treatment
* Chemical dosing
* Treated-water storage
* Siemens PLC and HMI control
* Remote monitoring
* Customer BMS or SCADA integration
For regulated and high-purity applications, systems can also be manufactured with the appropriate materials, documentation, testing and validation requirements incorporated into the project.
Choosing the Right Membrane Technology
The correct question isn't:
"Which membrane produces the purest water?"
It is:
"What needs to be removed, and what water quality does the process actually require?"
If suspended solids are the problem, microfiltration may be appropriate.
If colloids and microorganisms need to be controlled, ultrafiltration may be the better solution.
If selective hardness or multivalent ion removal is required, nanofiltration may provide the right balance.
If the objective is substantial dissolved-salt removal and low-conductivity water, reverse osmosis is usually the appropriate technology.
And sometimes the correct treatment process uses several of them together.
Talk to a Water Treatment Engineer
If you have a membrane application, send Hydra-Clear the incoming water analysis and the required treated-water specification.
Our engineers can review the chemistry, determine the appropriate separation process and design the system around the actual operating requirement.
Understand the water first. Then select the membrane.
Call 0800 999 6770 or contact Hydra-Clear to discuss your membrane treatment application.