Careful selection needed

Now a multi-million dollar business, membranes form the backbone of many modern water supply systems. Recent advancements have seen materials such as ceramics adopted and even graphene considered. As part of WWi's ongoing technology series, we ask several manufacturers: Is polymeric membrane integrity suitable for modern day water production demands, or should new materials be considered?

Manwinder Singh, senior VP, technology, Koch Membrane Systems

Historically, polymeric membranes have been the backbone of microfiltration (MF) and ultrafiltration (UF) for water treatment due to the low cost, wide variety of pore sizes, configurations, and manufactures.

Alternatively, other materials (ceramic membranes) have been used when conditions are too aggressive (high temperatures and aggressive chemical conditions) for polymeric membranes. Polymeric membranes do have advantages over ceramic, including less cost.

Further improvements to polymeric membranes are also improving robustness to handle aggressive aeration and cleaning conditions. Improvements in polymeric membrane formulation and membrane spinning techniques are resulting in more chemically stable formulations. These improvements have resulted in membranes that can withstand chemical exposure with wide range of chemicals without any deterioration in chemical properties of membranes.

Supported, braided polymeric membranes where the membrane is casted and penetrates the support layer have proven to increase membrane life and practically eliminate fiber breaks. These membranes were traditionally used in wastewater applications but with further advancements in polymer coating technologies, they are being applied in water treatment applications.

Specific polymeric membrane selection can prove to have excellent membrane life for particular applications.

Where Polyvinylidene Fluoride (PVDF) membranes are ideal for applications where regular chlorine cleaning is required to remove organic foulants from Natural Organic Matter (NOM), Polyether Sulfone (PES) membranes can handle high pH cleans.

Overall, polymeric membranes are currently the most cost effective for MF or UF applications but careful selection must be implemented to provide the end user the most reliable product.

Dr Jeff Koehler, principal scientist, LG NanoH2O

Historically, all commercial membranes for water purification have been based solely on polymers synthesised from similar monomer classes.

This stagnation in developing new chemistries has yielded similar membranes across the industry. These membranes obviously have their performance benefits but they suffer from robustness issues upon repeated cleanings, chlorination events, etc.

A great deal of interesting research has delved into new chemistries and molecules (i.e., aquaporins, graphene, etc.), which show very interesting and improved performance. Yet these tend to be very costly processes for full commercialisation.

While developing new chemistries for membranes, we must always keep in mind that membranes for the water industry are commodities, and as such, cannot be made of materials that significantly raise the cost of the end membrane module. Polymeric compounds are still the most cost effective route to obtain excellent quality water, but small amounts of nanoparticles and other additives can greatly enhance the performance of the membrane.

Over the next few years we will be testing a variety of new chemistries that will be based on a hybrid polymeric architecture. We cannot limit ourselves to the same polymer chemistry that has been used in the past, but we will continue to develop chemistries (polymer, hybrid and potentially inorganic) that are ready for commercialisation which will improve the performance, longevity and chemical resistance of the finished product.

Loet Rosenthal, director of drinking water, PWN

Polymeric membranes have been used for water production demands for a very long time. In 1999, PWN was the first utility in the world that implemented a fully integrated polymeric membrane plant at its Heemskerk facility. During that time polymeric membranes were the best solution available on the market for our challenges and to date our Heemskerk facility operates well, especially in terms of water quality, although we have had some issues with membrane fouling and integrity.

For our water treatment plant in Andijk, that needed to be upgraded because of its age, capacity and water quality, we compared existing technologies with the new innovations developed and tested by our R&D team. The results clearly indicated that a combination of suspended ion exchange to remove dissolved organic matter, nitrate and sulphate, and ceramic membrane filtration led to less waste, a smaller footprint, less energy consumption and a better water quality.

There were multiple reasons why we chose ceramic membranes. In the first place ceramic membranes have a very long lifetime and high durability. Also, their high permeability and high backwash rates allow for much higher gross fluxes (three to four times) at a stable operation leading to less membrane area and therefore a smaller footprint.

The only limiting factor for the application of ceramic membranes in the past has been the high capital costs because of the huge amount of steel and valves associated with the production of a ceramic membrane plant. One soultion - CeraMac - developed by our R&D team, combines up to 192 ceramic membrane elements into one vessel, making it a highly economical and compact solution.

Life cycle costs are especially important to consider when weighing the different technologies. For ceramic membranes, the benefits need to be looked at over the long-term.

Public utilities like our own have an obligation to develop and introduce new technologies to make our processes more reliable, sustainable and cost efficient. We believe that ceramic membrane filtration is becoming the new water treatment standard.

Dave Holland, senior application engineer, Aqua Aerobic Systems

The main advantage of a low-pressure polymeric membrane over one made of an alternate material - such as zirconium oxide -- remains its lower capital cost. However, the price gap between the two is narrowing as manufacturing processes and costs improve.

When one looks at the total life-cycle cost of the plant, some systems with non-polymeric membranes are already less costly to operate and maintain over the long run. And adding to the overall cost of many polymeric systems are expenses related to the lack of membrane integrity, which several studies have attempted to quantify and found that this is not an insignificant value.

While non-polymeric membranes have been gaining traction in the US even for drinking water treatment, there are still some reasons for not giving up on polymeric membranes. For one, the American engineering community has been slow to convert. Even though many alternate membrane materials have been shown to achieve much higher fluxes, fear of the unknown (not altogether unjustified) causes many consultants to derate the new materials, resulting in artificially-high life cycle costs that either penalise the client or prevent the sale in the first place.

Second, there have been some recent improvements to polymeric membranes that have improved their overall integrity. Most manufacturers are now using the more-flexible polyvinylidene fluoride (PVDF). Many now also produce the membrane using a thermally-induced phase separation process (TIPS). Lastly, polymeric membranes continue to make improvements in operating costs. To improve flux, coagulants are sometimes injected into the feed; this can be done continuously, but injecting it periodically to "pre-coat" the membrane surface saves chemicals.

For low-solids applications, an inside-out membrane can be used, which doesn't require air scouring. And many membranes now use small bursts of backwash water and/or air to clean the membranes, requiring less power.

More hydrophilic materials -- such as polyethersulfone (PES) or various hydrophilic additives -- resist fouling and require less backwash water/air and cleaning chemicals.

In summary, the answer is "yes" - recent advances in polymeric membrane technology give the material more integrity at a lower life-cycle cost, yet non-polymeric materials with even better integrity continue to add value to the market. Time will tell if the contest will end with a clear winner or in a tie.

Willy Yeo, regional marketing leader -- process systems, Asia, Pall

Polymers have been the go-to-material for a significant amount of time in water solutions, and have been regarded as the backbone of some of the most important membrane technologies that have solved our pressing resource needs for a long time.

Even in the light of new materials being developed, from ceramic membranes to nanotechnology, the polymeric membrane remains highly important in meeting today's water production demands. What is equally, if not more important than membrane material, is the design of the overall product and solution around the polymer, that will allow us to see yet more significant developments in today's water-starved world.

Specific to the area of zero-liquid-discharge (ZLD) and brine minimization, both of which are key to ensuring our water needs today are met, polymeric membranes can be designed as disc-tube reverse osmosis (RO) modules to ensure the molecular and ionic separation of the whole spectrum of pollutants in all aqueous environments: from suspended matter to the smallest ions, including colloids, bacteria, viruses and organic matter.

Such effects can be achieved when the polymeric membrane module is integrated with spacing discs separating the membrane cushions, thereby offering even better filtration and hence water production solutions than traditional spiral or tubular RO modules.

This is because there will be greater turbulence in the feed stream, lowered clogging or crystallising risks, even distribution and self-cleaning hydraulic circulation and hence more effective cleaning, longer operating life and lowered operating expenses.

Moreover, the design of the module can be further enhanced by using an open channel technology, leaving the module even less vulnerable to fouling and scaling. If the module is designed so that it can also be opened easily, the membranes can be easily investigated, tested for the optimum cleaning procedure and easily subject to laboratory investigations.

On top of the membrane material as well as module design, one key aspect of ensuring the product's suitability for meeting water production needs today is its ability to work under extreme conditions, for maximum recovery.

Today, there are modules that can operate up to 160 bars of pressure, thereby ensuring the meeting of ZLD standards, if not significantly lower brine minimisation results than most other technologies. This is the case despite the use of polymers, and in the absence of any new materials. Such is a prime example of how innovation in the design around the polymer can nevertheless propel the industry forward in meeting today's water challenges.

Claudia Staudt, principal scientist, advanced materials & systems research, BASF

Despite a broad availability of various polymers today, ultrafiltration (UF) membranes are mainly produced from polysulfone (PSU), polyethersulfone (PES) or polyvinylidene difluoride (PVDF).

The hydrophobic nature of these polymers, however, causes a fouling phenomenon, which demands a frequent chemical cleaning procedure.

Membranes made from Ultrason polymers (PSU, PES) exhibit excellent chemical resistance to caustic soda (sodium hydroxide) and high resistance to sodium hypochlorite.

Equipping these membranes with anti-adhesive surface will reduce the chemical cleaning requirement and thus improve the operational efficiency and sustainability. The development of new polymeric materials needs to consider the fact that the production of porous membranes using a non-solvent or temperature induced phase inversion process is well established.

Often, with new bulk polymers, many production parameters require some adjustment to obtain porous structures with desired pore size and distribution as well as high porosity.

In addition, new membrane materials with specific functionalities usually require complex synthesis routes, such as block-copolymers with precise block lengths.

Therefore, the current trend today is to use polymeric additives applicable in the conventional membrane production in order to achieve higher hydrophilicity and/or surface functionality.

Furthermore, different ´post treatment´ techniques have been developed, e.g. hydrogel and coating approaches, grafting and cross-linking in order to functionalise the membrane and pore surface.

Major traits of these approaches are that the parameter adjustment in production process is minimal and that only small amount of additive chemicals is needed for the functionalisation. The membrane pore could also be adjusted to smaller size via coating approaches.

The aforementioned concepts could lead to a new generation of high performance membranes. Surface properties can be tuned deliberately between purely anti-adhesives to cross-linked, enabling the uncovering of new application fields, such as heavy metal removal/recovery and produced water treatment applications, by using only one type of polymer as the basis.