Combating Fouling in Membrane Separation Systems
Use of membrane separation such as reverse osmosis and nanofiltration systems has increased in recent years.
Previous studies seeking to overcome the "Achilles heel" of membrane separation – biofouling – have focused on the quality of sea water as a variable in the equation. Mark Sewell, Catsy Lam and Stephen Morrison present findings from a new discovery into the problem: elastomeric seals.
Use of membrane separation such as reverse osmosis and nanofiltration systems has increased in recent years. Increasingly stringent quality standards for drinking water, the rise of seawater desalination and use of ever more sensitive industrial processes have driven these systems forward. Membrane technology is efficient and reliable in producing water that meets these high quality standards, but the process is prone to fouling that can have adverse effects on the filtration system.
Fouling in membrane separation systems
Fouling has been causing headaches for membrane separation system operators since the filtration method was invented. Reverse osmosis and other membrane filtration systems provide optimum conditions for microorganism colonisation. The surface colonisation develops into a biofilm if propagation of microorganisms is supported by organic nutrients present in the system.
Biofouling is the process in which biofilms clog membranes, degrade membrane performance and prevent the system from operating at peak efficiency. When biofouling occurs, more pressure and, in turn, more energy is needed to push the water through the membranes, resulting in higher operating costs. The higher pressure exerted on system components can damage the equipment. More frequent cleaning or changing of filtration media may also be required, leading to further costs and increased plant downtime.
Research into the causes of biofouling has been ongoing for decades. Among the factors that have been determined to have some role in the severity of biofouling are the presence of organic and inorganic nutrients, temperature, pH, dissolved oxygen concentrations, type of membranes and chemicals used in the water. In the desalination industry, there have been various analyses on the effect of sea water quality on membrane fouling. However, little attention is paid to other possible sources of organic nutrients in the systems that will affect water quality and membrane performance.
Piping components such as mechanical couplings had not previously been suspected as a cause of biofouling in the water filtration industry. Water chemists and microbiologists have been aware that elastomeric materials can produce a direct effect on the microbiological quality of water by leaching organic substances into the water and supporting the growth of microorganisms. Research conducted at Victaulic, a manufacturer of grooved mechanical piping systems, further supported the fact that nutrients present in elastomeric materials, such as gaskets, seals and O–rings, can feed the microorganisms and support the growth of biofilms.
|Biomass before and after: Two biofilm samples collected in test tubes before and after centrifuge|
As reported by Hans–Curt Flemming of the University of Duisburg, biofouling is the "Achilles heel" of the membrane separation process because "all other fouling components, such as organic and inorganic dissolved substances and particles, can mostly be removed by efficient pretreatment; however, microorganisms are particles which can multiply". He went onto say: "Thus, if they are removed to 99.99%, there are still enough cells left which will grow at the expense of biodegradable substances in the water."
The objective of the pretreatment process is to remove suspended solids and to degrade organic substances. However, organic nutrients cannot be completely removed, especially in poor quality feed water in which dissolved organic material is present in large quantities. Moreover, there are always gaskets and O–rings present in the filtration process where chlorine cannot be added to avoid deterioration of the membrane.
Gaskets and microbial growth
Some European and Asian countries such as the United Kingdom, Germany and China have introduced compulsory tests for microbiological resistance of all elastomeric materials used in contact with drinking water. In countries without such regulations, there are still many commercially available rubber compounds—from which gaskets, seals and O–rings are made—used in potable water applications that support microbiological growth.
These compounds are also used in reverse osmosis and other membrane separation systems. Elastomeric materials are prone to microbiological growth in the optimum environment of membrane separation systems because gasket seal compounds contain organic ingredients such as plasticisers and processing aids. These ingredients promote propagation of microorganisms on contact with either chlorinated or non–chlorinated water.
|A typical biomass sample scraped from an elastomeric plate during testing|
With this knowledge in mind, Victaulic commenced a research project designed to evaluate microbiological resistance of both commercially available and internally developed gasket compounds, identify ingredients that do not support microbial growth, and develop a "clean" compound to prevent leaching of organic substances into water. The test methods used are widely accepted and were developed by drinking water agencies in Germany, the UK and China to evaluate microbiological resistance of elastomeric materials: biomass volume, mean dissolved oxygen difference, and chemical oxygen demand.
The research determined that commercially available compounds are the least resistant to microbiological growth. Anti–microbial agents incorporated in the gasket compound do not provide significant improvement in reduction of microbiological growth. As a result, attention was turned to the selection of compounding ingredients and processing conditions.
Different trial compounds were tested to identify specific ingredients as supportive or non–supportive of microbiological growth. Very few ingredients were identified with little or no biomass formation, so a compound was optimised using the ingredients that displayed resistance. This optimised compound performed 75 times better than commercially available compounds using the aforementioned test protocols.
These results show that an important step in reducing biofouling in all clean water filtration systems is the careful selection of wetted system components including elastomeric materials. Such gaskets, seals and O–rings should be chosen to eliminate all "dirty" rubber ingredients that support biomass in contact with water.
Dealing with a biofouling problem
There are steps that a plant manager can take in dealing with the problem. Changing out old coupling gaskets to new ones made of clean rubber compounds during a maintenance shutdown essentially removes the gasket as a variable from the biofouling equation. This is certainly an avenue that a plant with persistent biofouling problems should explore. Although membranes can be cleaned, and indeed, this is the first step in remediating a problem caused by fouling, the process is labor–intensive and results in a shutdown of that section of the plant. Furthermore, the more frequent the cleanings, the shorter the lifespan of the membrane will be, because every cleaning degrades the membrane. As such, it is advisable to reduce biofouling in the first place.
There are several things a plant manager can control to reduce the threat of biofouling. In fact, feed water aside, biofouling in many cases can be attributed to human and mechanical error. To ensure equipment and employees aren't contributing to the problem, make sure the pumps in the pretreatment process are in good working order, and that any chemicals are being dosed properly and continuously. If the plant is shut down for any reason, it needs to be laid up properly, which means flushing the system with clean water and adding a preservative to the membranes if it's going to be left out of operation for any length of time. Wet, dark areas are a perfect place for bacteria to grow, so proper maintenance practices are critical. Although there are aspects of biofouling that a plant manager can't control, there are steps that can be taken to reduce the severity. These steps can result in long–term time and cost savings, so they are certainly worth pursuing.
Authors' note:Mark Sewell is manager of materials technology, Catsy Lam is a product engineer, and Stephen Morrison is global water systems technology market manager with Victaulic. For more information, please visit www.victaulic.com.