By Greg Reyneke, MWS
While almost three quarters of the earth’s surface is covered in water, only about one percent of that water can be used to meet the needs of humanity without significant processing or purification. Earth’s population is expected to exceed 10 billion people by 2050 and water scarcity is a term being heard more often now than ever before. Our old habit of harvesting, distributing, using and discarding water has put us in a position where many communities around the world now lack sufficient water to live normal lives. Water rationing, lawn watering bans, intermittent municipal water availability schedules as well as an uptick in boil-water and other water quality failure notifications are now becoming commonplace.
In addition to the reduction of traditionally available water, we are now painfully aware of so many more contaminants lurking in the water than ever before. More than seven million recognized chemical compounds are known to be in existence and approximately 100,000 of them are believed to be in use worldwide. Regulatory agencies have no idea exactly which specific compounds are used in consumer goods or what precursors are used to produce them. There are also a host of secondary compounds that can form when chemicals are exposed to fire, sunlight, oxidizers, solvents and other reagents. In the 21st century, it is safe to say that all water everywhere is contaminated with some kind of man-made chemical at some level.
Towards the end of the last century, evidence began to accumulate that certain pesticides, surfactants (used in detergents) and synthetic birth control drugs were causing skewed sex ratios, reproductive disorders and population declines in frogs, alligators and fish. Some researchers have further suggested that antibiotics and antimicrobials can pose a serious threat to human health by enhancing the antibiotic resistance of disease-causing microorganisms due to their (over)use in products such as soaps, mouthwash, toothpaste and of course, the ubiquitous hand sanitizer. As we now have started to more fully understand the presence of per- and polyfluoroalkyl substances (PFAS) in our environment, we are beginning to realize the gravity of our contaminated situation.
The need for robust water purification technologies is more important than ever before. Membrane separation technologies are powerful tools in the fight against emerging contaminants and in alleviating water scarcity. Membrane separation technology is an invaluable resource for residential, commercial and industrial water treatment applications in purification, desalination and reuse.
When speaking about membrane separations, many people immediately think about RO purifiers, but in the water quality improvement industry, membranes are simply defined as physical barriers that separate solutions and allow passage of contaminants within a certain range of size, molecular mass, or even charge polarity and strength. When driving pressure is applied, contaminants will be selectively rejected or concentrated by the membrane (depending on the membrane material[s], pore size and electrical charge) while water and unrejected contaminants will pass through as a permeate stream.
Membranes can be operated in dead-end or crossflow configuration. In dead-end configuration, the rejected contaminants concentrate into the influent stream and eventually accumulate against the surface and pores of the membrane. Concentrated contaminants will inevitably clog the membrane pores entirely, so this process is reserved for applications where the cost/inconvenience of replacing fouled membrane sheets is less important than losing any of the raw fluid or, where the engineered process flow design specifically calls for it. A modified dead-end configuration is backwashable hollow fiber lumens, which are robust and less susceptible to fouling than spiral-wound membrane sheets. Many ultrafiltration manufacturers are successfully using this technology to provide safe drinking water to millions of people worldwide.
In crossflow filtration, the membrane geometry is designed for contaminants to be scrubbed away from the membrane surface when the concentrated discharge stream is passed to drain or a secondary process. Leveraging the principles of Fick’s laws of diffusion, designers can manipulate macromolecule concentration molecules at the membrane surface as a function of the velocity of fluid that is flowing parallel to it.
Membrane materials and element construction
Crossflow technology is cost-effective and practical due to material and manufacturing improvements over the last 20 years. Those of you who have been around the industry for a few years will remember cellulose triacetate (CTA) membranes that were once ubiquitous. The CTS membrane popularized RO and allowed it to become a household product. While revolutionary in its time, CTA membranes were plagued by permeate production inefficiency and susceptibility to bacterial colonization. A game-changer popularized in the mid-1990s was thin film composite (TFC) membrane technology, which lowered total cost of ownership and increased flux at lower driving pressures. Composite membranes can be made from a number of materials, such as polyethersulfone (PES), polysulfone (PSU), polyphenylsulfone (PPSU), polytetrafluoroethylene (PTFE), polyvinyldene fluoride (PVDF) and even polypropylene (PP). The vast majority of installations these days utilize composite membranes while other materials such as ceramics made from silica, aluminum, titanium and other materials are used where pH, temperature, abrasiveness, cleaning chemistry, or other operational parameters prohibit the use of polymers.
Polymeric membranes can be manufactured symmetrically or asymmetrically, depending on the intended use and operating conditions. Contrary to popular belief, there are many ways to build a crossflow membrane, including type of polymer, length of membrane leaves, membrane support configuration and membrane density. These configuration options are significant in mission-critical operations and also important when selecting regular drinking water membranes on which you stake your reputation.
Today’s mainstream membrane separation technologies can be separated into four broad categories of separation by relative contaminant exclusion size (see Figure 2):
Reverse Osmosis (RO). Sometimes called hyperfiltration, RO is the finest form of filtration used today. The membrane pores are small enough to enable the reversal of osmotic pressure through ionic diffusion when sufficient external energy (pumping pressure) is applied. This reversal of osmotic pressure (see Figure 1) drives pure water away from molecular contaminants and enables processes like seawater desalination where sodium ions are physically removed from water greening the desert and bringing clean, safe drinking water to places where it was previously impractical. Reverse osmosis is also used industrially in many innovative applications such as concentrating fruit juice, concentrating whey protein and of course, wastewater sludge dewatering.
Nanofiltration (NF). Developed as an extension of RO, NF functions according to the same principles of ionic diffusion as RO, but with a pore-size configuration and surface charge that allows passage of all contaminants except divalent and larger ions. Monovalent ions such as sodium and potassium pass right through an NF membrane while it rejects calcium and magnesium compounds. This allows it to be used as a highly effective, salt-free softening technology where RO might be considered overkill. NF is also highly effective at addressing semi-volatile organics such as certain pesticides and also in removing color and colloids from water.
Ultrafiltration (UF). Ultrafiltration is a true physical exclusion process and doesn’t rely on osmotic principles. UF membranes are categorized by their molecular weight cut-off rating (MWCO). The typical range of MWCO’s for UF is from 1,000 to 1,000,000 Dalton, which correlates to approximately 0.005 – 0.1 micron (µm). UF is extremely effective in removing suspended solids, colloids, bacteria, virus, cysts and high molecular weight organics like tannins. UF membranes are operated in dead-end configuration, occasional flush (forward flush and/or backflush) or crossflow configuration. Membrane configuration can vary between manufacturers, but the hollow fiber type is the most commonly used. Membranes in the hollow fiber type are cast into small diameter tubes or straws referred to as lumens. Thousands of these lumens are bundled together and the ends are carefully bonded/potted into an epoxy bulkhead. The bundles are then sealed into a housing, which is usually PVC, fiberglass, or stainless steel. The sealed potting creates a separate space that isolates access to the inside of the fibers from the outside. This membrane and housing combination is called a module. A number of UF membrane assemblies on the market are certified for reduction of pathogens in drinking water such as bacteria and viruses, enabling the provision of safe drinking water more cost effectively and efficiently than ever before.
Microfiltration (MF). Microfiltration technology has been successfully employed in both crossfiltration spiral-wound, intermittent-flush hollow fiber (forward flush and/or backflush) and dead-end plate and frame configurations, depending on the nature of the application and space available. This membrane technology typically has an exclusion size of 0.2µm – 1 µm and is very well suited for the removal of particulates, turbidity, suspended solids, as well as certain pathogens such as Cryptosporidium and Giardia. MF has an established industrial track-record for sterile clarification of wine and beer, whey concentration and fruit juice sterilization. In the wastewater treatment field, microfiltration is invaluable for dewatering flocculant sludge and economically lowering BOD and COD in discharge streams. Microfiltration is also extremely effective in protecting other downstream membrane separators in mission-critical applications. Membrane separation technology paired with intelligent oxidation and effective absorption/adsorption can be highly effective in addressing even low molecular weight organo-synthetic molecules in water.
Pretreatment and maintenance
It is important to select the appropriate pretreatment for the membrane separation process being used. Composite polymeric membranes are particularly sensitive to oxidative damage, so special care should be taken to ensure that chlorine, chloramine, ozone and other oxidative disinfectants are removed from the raw water before entering the membrane. Careful consideration should also be given to macro particles and organic/inorganic contaminants in the water stream that could affect proper membrane function. As a good general rule, the smaller the pore size, the greater amount of physical pretreatment required to ensure long run times and economic operation.
Regardless of the membrane pore size, operational fouling is almost inevitable, even with adequate physical pretreatment. The types and amounts of fouling are dependent on many different factors, such as feed water chemistry, membrane type, membrane materials and operational process. The most common types of membrane fouling are scale precipitation and biological growth. Fouling causes a decrease in flux (passage of purified water), which in turn requires greater pressure against the membrane to produce a satisfactory permeate flowrate. As fouling worsens, the increased pressure (energy) requirement will cause the operating cost to increase and possibly even blind the membrane completely, leading to significant damage and operational failure.
Many innovative membrane antiscalant, metal sequestering, microflocculating and CIP (clean-in-place) compounds are available to help keep your membrane(s) clean and operating as energy-efficiently as possible. Some good questions to ask your supplier before using a pretreatment or antiscalant chemical:
• Is it chemically compatible with the membrane(s) in the application and pumps/piping?
• Can it react adversely with any other contaminants in the feed water like iron, heavy metals and other inhibitory chemicals?
• What are the recommended dosing rates and maximum dosing rates?
• What are the projected limits of solubility for individual scaling and fouling components?
• What material handling, transportation, and personal protective equipment (PPE) measures are required/recommended?
• Are there any special discharge/reclamation regulations or concerns?
• Do you offer technical support services like water testing, chemical analyses, and membrane autopsies?
Water waste and environmental protection
Some well-meaning but misinformed people accuse membrane separation systems of being wasteful since water is used to clean the membrane(s) during operation. I disagree with the negative and simplistic description of drain concentrate water as wasted water, since it really is not. Saying that a membrane separator wastes water is akin to saying that a tree dropping its unpicked fruit is wasteful. The fallen fruit returns nutrients to the earth and feeds the tree, which then grows more fruit. Discharged concentrate water from a membrane separation system is not necessarily lost forever, it will return through the building’s drainage system to a municipal plant, or back to the earth in an off-grid application, or can even be repurposed onsite.
We obviously can’t ignore the opportunity cost of the water though, since it must be cleaned, stored, treated, pressurized and distributed before entering the membrane separator. Since the discharge from a typical drinking water membrane separator is also sanitary potable water (this is obviously not considered wastewater, since it is never in contact with soils, dirt, or biological contaminants; it is merely concentrated clean water), much of this opportunity cost can be recovered through innovative reuse techniques such as greywater recovery, blending with harvested rainwater, repurposing in secondary process or used as landscape irrigation. There are certain applications where the concentrate from the membrane separation process can’t be easily recovered due to extreme levels of specific contaminants, but through coagulation, flocculation and dewatering, much of the raw water can be eventually recovered and repurposed. Recent innovations in concentrate energy recovery and storage are also helpful in demonstrating how truly efficient membrane separation technology can be.
Membrane separations are highly effective in reducing chloride discharge and cleaning chemical requirements in locations where traditional ion exchange softening is not appropriate. Many dealers are using nanofiltration and blended RO systems as salt-free softeners – providing their clients with water that has hardness levels below 17.1 mg/L (1 gpg). New innovative materials like synthetic ceramics and advanced polymers, along with enhanced spacer design and other techniques allow for progressive improvements in membrane longevity, reduction in driving energy requirements and minimization of fouling, while continuing to lower the initial acquisition-cost, total cost of ownership and energy footprint.
Education is the key to success
Properly designed and maintained membrane separation systems are a sustainable method for improving water quality and an invaluable tool for us as water quality improvement professionals. It is critically important that you either secure the education you need to ensure proper system selection, design and deployment, or work with vendors who you can rely on to help before getting yourself into trouble by making uninformed decisions. The WQA’s Modular Education Program (MEP) offers a great starting point for learning more about RO and other membrane separation technologies to help you do a better job. WQA’s Certified Water Specialist (CWS) and Certified Treatment Designer (CTD) credentials are recognized worldwide as proof of one’s commitment to best-practices and technical proficiency.
Sustainability and efficiency goals can be met through the judicious and practical application of membrane separation technologies. Advanced technologies, sensible legislation and smart operators are essential to keeping the clean water flowing on our planet that continues to be ravaged by pollution, rampant population growth and climate change.
Dalton – The unified atomic mass unit (symbol: u) or Dalton (symbol: Da) is the standard unit that is used for indicating mass on an atomic or molecular scale (atomic mass). One unified atomic mass unit is approximately the mass of one nucleon (either a single proton or neutron) and is numerically equivalent to 1 g/mol. It is defined as one-twelfth of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, and has a value of 1.660539040(20) × 10−27kg.
Fick’s laws of diffusion were derived by Adolf Fick in 1855. Expressed as follows: They can be used to solve for the diffusion coefficient, “D”. Fick’s first law can be used to derive his second law which in turn is identical to the diffusion equation. Fick’s first law relates the diffusive flux to the concentration under the assumption of steady state. It postulates the concept that a solute will move from a region of high concentration to a region of low concentration across a concentration gradient. Fick’s second law (non-steady state diffusion) predicts how diffusion causes the concentration to change with time. While Fick holds that flux is directly proportional to the concentration gradient, the diffusion coefficient “D” is affected by temperature and pressure.
Flux is the rate of mass transfer across unit surface area of barrier and mathematically expressed as:
J ≡ atoms/area/time.
Micron (µm) – One millionth of a meter. One inch is equal to 25,400 micron.
Plate and frame – Also known as a membrane filter plate. This type of filter press typically consists of alternating plates and frames supported by rails. A pressure pump is used to deliver liquid to be filtered (slurry) into each of the separating chambers. For each of the individual separating chambers, there is one hollow filter frame separated from two filter plates by filter cloths. The slurry flows through a port in each individual frame and the filter cakes are accumulated in the hollow frame. As the filter cake thickens, the filter resistance increases as well. The filtration process is halted once the optimum pressure difference is reached. The filtrate that passes through filter cloth is collected through collection pipes and stored in the filter tank. Filter cake (suspended solid) accumulation occurs at the hollow plate frame, then being separated at the filter plates by pulling the plate and frame filter press apart. The cakes then fall off from those plates and are discharged to the final collection point.
About the author
Greg Reyneke has almost three decades of ongoing experience in the management and growth of water treatment dealerships. His expertise spans the full gamut of residential, commercial and industrial water quality management applications. A recipient of the Ray Cross and Regents Awards, Reyneke has been active in the WQA since 2004 and has served on numerous committees and task forces. He is past-President of the Pacific Water Quality Association and serves on the WQA Board of Directors and Board of Governors. Reyneke writes prolifically and travels worldwide, helping to improve human life through better water quality. You can follow him on his blog at www.gregknowswater.com.