Membranes: the solution to a good many problems

Seit dem Anfang der 90er Jahre steigt der Stellenwert der Membranen im Bereich der Wasseraufbereitung. Je nachdem wie man sie nun einsetzt, können sie klären, konzentrieren, reinigen oder scheiden. Heutzutage ist diese Technologie aus den Kinderschuhen herausgewachsen. Entsprechend sinken die Preise. Man muss sie nur anwenden… ]

Since the early 90s, membranes have occupied an ever-growing place in the water cycle. Depending on where they are positioned, they can be used to clarify, concentrate, purify or separate. Today, the technology is tuned up, and prices are coming down. And the applications are legion…

“As long as you have the necessary experience of water and variations in turbidity and can control the pretreatment process, membranes are the ideal physical barriers,” says Michel Faivre, Chief Engineer in charge of the membrane process at Anjou Recherche. Membranes have been used in manufacturing processes in the food industry for over 20 years, and now are gradually penetrating the water processing sector.

Membrane processes are based upon semi-permeable thin films which separate incoming water into two elements: the permeate and the concentrate. Separation is achieved by applying an impelling force, which can be pressure, an electrical field, a temperature gradient or a concentration difference. Most of the technologies used in water processing are based on pressure gradients. A physical screening process then allows or prevents the transfer of certain components from one medium to another from which it is separated by the membrane.

This separation takes place without a change of phase and at ambient tempera-

Table: Main characteristics of pressure-gradient membrane processing

Document: Anjou Recherche

It can sometimes allow the concentration of products that are amenable to beneficiation or are recyclable without change of phase, while producing good quality water.

They act like a sieve

Micro- and ultrafiltration processes are based on the same principle as the sieve, in other words the passage through calibrated holes in a membrane. The membrane acts as a barrier to any particles whose size exceeds a value inherent in the technology. This value corresponds to the separation threshold. Whether or not a given species is held back therefore depends entirely on the size of the pores in the membrane and the size of the species.

In ultrafiltration, the membrane acts as an impenetrable barrier to material in suspension larger than 0.01 µm. Without the addition of coagulant and flocculant, and by simple physical filtration, ultrafiltration enables drinking water to be produced from natural water. The raw water is filtered through the membranes, which trap impurities. The water thus produced is free from pathogenic germs, fecal contamination and viruses. Only very light chlorination is needed, simply to ensure persistence during distribution.

The levels of treatment attained by micro- and ultrafiltration can be improved by converting certain dissolved species into larger particles falling within the separation capacity of the membranes. The conversion is achieved by adsorption on activated carbon, coagulation or even by biological or chemical oxidation or reduction.

For example, “when raw water contains significant quantities of organic material that interfere with the performance of the membrane this effect must be neutralised before filtering”, explains Basile Jarmak, Major Systems Manager at Aquasource, a subsidiary of Degrémont, “by adding powdered activated carbon (PAC) to the raw water”.

The combination of PAC and ultrafiltration is at the heart of the Cristal process, patented by Lyonnaise des Eaux. In addition to the reduction of organic matter, it performs well in eliminating pesticides and enhances the rate of filtration.

Nanofiltration and reverse osmosis function in a different manner since they can even filter out dissolved substances.

Filtering out dissolved substances

Nanofiltration can, most notably, filter out divalent ions and small organic molecules, such as pesticides and molecules of dissolved organic matter. This technology is situated between ultrafiltration and reverse osmosis.

It allows the separation of solutes with a molecular weight of 300 to 800 Daltons, corresponding to dimensions in the order of nanometres.

The development of nanofiltration membranes is a fairly recent phenomenon. Originally used for the partial desalination of brackish water, they have in recent years aroused considerable interest for the treatment of industrial waters and are beginning to penetrate the drinking water production market.

In the case of reverse osmosis, the majority of dissolved substances are retained. The degree of retention (or rejection rate) of organic and inorganic substances depends on the size of the molecules in question and also on their solubility and diffusibility within the membrane material itself. “In osmosis, the membrane is semi-permeable”, explains Abdel Khadir, commercial engineer at Permo, a firm specialising in the use and implementation of membrane techniques. The technique is based on the principle of applying a considerably higher pressure than the osmotic pressure of the concentrated liquid. Dissolved salts, iron, manganese and organic matter are stopped and concentrated on the surface of the porous medium. Only water molecules can cross the membrane.

However, “the use of osmosis requires a micro- or ultrafiltration type of pretreatment to eliminate anything that might clog the membrane”, notes Abdel Khadir. For Robert Niay of USF Memcor: “the optimal conditions for reverse osmosis are well known. It is important to heat the water, prefilter all the material in suspension, including colloids and microorganisms, reduce the silt density index, stabilise the quality of the water supply and, where necessary, adjust the pH and add scale preventing agents.” Under these conditions, the performance of osmosis membranes exceeds their nominal performance. Cleaning operations are not required so frequently and membranes have a service life of more than 5 years.

Cleaning the membranes

Only water molecules can cross the membrane, which acts as a barrier, stopping material in suspension, bacteria, viruses, spores, etc. Evidently, these particulates will then accumulate on the surface of the filter, preventing the water from crossing the membrane. Permeability is reduced and the membrane becomes clogged. To counter this drop in flow, the pressure gradient across the membrane can be increased. However, this will only lead to further clogging until a point is reached where production is no longer possible.

The most common way of eliminating deposits on the membrane is to direct pure water (or air) in a countercurrent direction, in other words from the permeate to the raw water, under a higher pressure than that of the porous medium.

filtration. This operation to detach the particulates is known as backwashing. To reduce clogging, Aquasource creates a movement of water within the fibre. “The cycle alternation is a single shot process depending on the level of turbidity.” Memcor, on the other hand, uses an air-based defouling system. A countercurrent flow of compressed air is automatically triggered. The system consists of a jet of compressed air being suddenly released through the fibres, against the flow. The material which has been forced back into suspension by the jet of air can then be ejected by a current of raw water, avoiding the need to inject oxidants, disinfectants or adjuvants.

To complete the defouling process and avoid the formation of biofilm in the pores, chemical cleaning has to be carried out as a preventive measure. This is done by circulating a chemical solution at least once a year to clean the membrane. This operation often places a heavy strain on the membrane and is costly, given that the filtration module is temporarily out of service. To reduce negative effects on production, membrane manufacturers have tried to find ways of increasing as far as possible the time between cleaning operations.

Membrane manufacturers are all working along the same lines. Membranes have become easier to clean and their service life is longer. Several producers announce a service life of over five years and have apparently verified that the membrane remains water-tight during that period.

A parameter that reflects the characteristics of a membrane is the fouling index of the water that it has filtered. This is based on the variation in the volume of water passing through a calibrated membrane within a given period. Aquasource currently produces membranes with an index close to 1: “in over 50% of cases we only clean once a year”.

A leak-free membrane

A pierced membrane no longer fulfils its role as a sieve and must therefore be detected as quickly as possible. To carry out this operation, Memcor uses an integrity test which can verify the absence of faults in the fibres within the space of four minutes. Every time the membrane is unclogged the entry and exit pressure profiles for the block of modules is controlled. Any departure from the reference profiles is immediately detected, allowing a rapid diagnosis of the state of the membranes.

To check the integrity of its membranes, Cirsee has up to now used the air test. The isolated module is inflated through its outlet to a pressure of 2 bar. Once this pressure has been reached, the module is observed for 5 minutes to see whether there is a fall in pressure. However this method is time-consuming and the module has to be taken out of service. To test large installations, the Research Centre at Lyonnaise des Eaux uses a particle counter. “Thirty modules can be tested overnight. If there is a problem, we then carry out an air test to identify it more precisely”, explains Jean-Michel Lainé, Head of the Drinking Water Processing Department at Cirsee. “The most critical problem is to monitor large-scale installations, with more than 1,000 modules.” For this type of installation Lyonnaise des Eaux Research Centre is currently developing a method based on acoustic monitoring. A microphone is used to detect leaks. This patented technique allows the integrity of the system to be continuously guaranteed with a 100 % success rate. It is undergoing industrialisation and is due to be incorporated into an ultrafiltration unit being installed in Lausanne.

The right material for each membrane

Numerous polymers are used in the manufacture of membranes. In France, materials for use in the production of drinking water must have been appraised and approved by the French Public Hygiene Council in application of Article L21 of the Public Health Code.

As a general rule, manufacturers do not reveal the chemical composition of their membrane components and prefer simply to indicate the main properties in terms of:

• mechanical resistance, to determine service life and the integrity of the membrane,
• hydrophilicity, in other words its resistance to clogging,
• chemical stability, or resistance to cleaning agents.

Polypropylene is used in the manufacture of microfiltration membranes. It is an elastic material with good mechanical resistance to frequent backwashing. Its hydrophobic properties make it somewhat prone to clogging. Chemical stability is good over a wide range of pH but it can be destroyed by chlorine, which must therefore be avoided.

Polysulfones for the manufacture of ultrafiltration membranes are either used as they are or as a support for a fine separation layer to form composite membranes for nanofiltration or reverse osmosis. They have excellent mechanical properties and chemical resistance. They can be used with a wide range of pH and can withstand continuous exposure to chlorine. The hydrophobic nature of the material does however make it prone to clogging by adsorption of organic molecules.

Cellulose derivatives are used in the manufacture of asymmetric membranes for ultrafiltration, nanofiltration and reverse osmosis. Under high pressure they have a tendency to become compacted, leading to an irreversible reduction in permeability. This phenomenon is not observed in nanofiltration, where the pressure remains low. These materials are highly hydrophilic, reducing the risk of clogging. Their chemical stability is reduced. Operating pH values must be kept to within a range of 4 to 6.5 and the temperature must not exceed 40 °C so as to avoid hydrolysis of the material. These membranes can withstand continuous exposure to low concentrations of chlorine, which prevents complete deterioration by micro-organisms.

Orelis, formerly Tech-Sep, markets four families of membranes. Organic membranes made of PES (polyether sulfone), PVDF (polyvinylidene fluoride) and an acrylonitrile copolymer in the firm’s Iris range are intended for micro- and ultrafiltration.

Their removal capacity extends from 3·10-3 to 50·10-3 Dalton. They are designed for highly viscous or concentrated media. Manufactured in PES and TFC polyamide, the Persep range encompasses the whole range of processes from osmosis to microfiltration. Designed mostly for industrial applications, the Persep range is suitable for low viscosity liquids. Ultrafiltration mineral membranes of the Carbosep product family are made of ZrO₂-TiO₂ on a carbon support. They can withstand extreme pH levels and are solvent proof. They can be used at high temperatures. Kerasep membranes are monolithic and, made of Al₂O₃-TiO₂, they can withstand very high filtration flows, are solvent-proof and are not chemically sensitive.

USF Memcor markets the MF fibre. This membrane is made of an extruded polypropylene hollow fibre forming a filtration barrier with a molecular weight cut-off of 0.2 µm. Bundled together to form a nominal surface of 15 m², it is used in the assembly of filters in a rigid case, referred to as a module.

Module assembled membranes

Membranes are either flat, tubular (> 5 mm), capillary (0.5 mm < Ø < 5 mm), or made of hollow fibre (Ø < 0.5 mm). The separation layer is on the inner side in the case of tubular membranes, on the inner or outer side for capillary membranes and on the outer side for hollow fibre membranes. “Membranes are bundled together and their extremities are embedded into a glue plug to isolate the permeate from the processed water”, explains Hervé Buisson. For practical purposes, the processed water is directly in contact with the separation layer. Depending on whether it is placed on the inner or outer side of the tube, filtration takes place from the inside towards the outside of the tube or vice versa. In a filtration case configuration, the incoming water is under pressure. When the membranes are submerged without a case, the permeate is sucked up through the end of the fibres using a low depression of < 0.3 bar.

Flat membranes are assembled in cartridges or spiral-wound membrane modules. These assemblies are always made of different elements: two circulation spaces, one for the water to be processed (the inflow spacer) and one for the permeate (permeate spacer). They surround the membrane. The two flow spacers consist of plastic grids.

To achieve much more compact package solutions, manufacturers develop membrane techniques. These devices include all the necessary processing components. They include: BRM from Degrémont, Pleiade from Orelis, M10C from USF Memcor and Biosep from CGE. Ultrasource, for example, is a self-contained clarification and disinfecting unit marketed by Aquasource. The process is based on membrane ultrafiltration. The standard machine can be fitted with 2 to 4 separate drawers, each containing 3 or 4 ultrafiltration modules. It can produce 80 to …

200 m³/hour of drinking water. This unit is equipped with a microprocessor board, sensors, and a pump, in fact everything needed to optimise its operation and cleaning procedures.

With Biosep, CGE has gone one step further, since, by sinking the fibre bundles into the water to be processed, the company has been able to eliminate the module concept. The fibres sunk into the medium to be processed are depressurised. Clean water is drawn in. To reduce clogging, an air scour is created at the bottom of the tank, to maintain a permanent disturbance of membrane fibres. Implemented in France at the Ocana station, this process is one of a number of membrane-based solutions used in drinking water production.

A solution for drinking water…

For the past two years, the trend has been to build ever-bigger drinking water processing plants. Surface water is processed with the main objective being to eliminate chlorine-resistant pathogenic germs. “In the United States this trend can be seen in cities with a population of 100,000 inhabitants, such as Marquette and Kenosha, but it is also present elsewhere, for example in Tauranga, a regional capital in New Zealand which is supplied with river water”, explains Robert Niay. Up to now, these towns simply chlorinated the water supply. Nowadays, either due to the risks this may pose or as a result of epidemics, towns opt for 100 % microfiltration of drinking water. Similar measures have been taken in the United Kingdom, where the risk of cryptosporidiosis is taken very seriously. Infrastructure programmes have consequently been implemented.

The advantages of membrane techniques in drinking water production are now well known. They offer numerous advantages over the classical techniques of separation by coagulation/sedimentation, sand filtration or filtration through granular activated carbon. They allow a wide range of pollutants to be eliminated in a single step and ensure disinfection by the physical retention of microorganisms and pathogens. They produce water of consistent quality while at the same time reducing the use of chemical reagents, and thus of treatment by-products. Moreover, the quality of the water produced exceeds that obtained with conventional techniques. The quality remains stable. And in addition, membrane techniques allow the treatment of chronically polluted waters with problems such as turbidity after heavy rainfall and organic or bacteriological contamination.

“Today, changes in the regulatory framework and the deterioration of usable resources are playing a large part in the development and spread of these techniques for the production of drinking water”, explains Hervé Buisson of Anjou Recherche. After years of research, these processes are now available for large-scale applications. One only has to look at some of the treatment units currently being installed:

• - The extension at Méry sur Oise (France), 140,000 m³/day, inaugurated in 1998. This unit, built by OTV, is equipped with 83 ha of nanofiltration membranes to eliminate broad-spectrum micropollutants. The spiral wound membrane is a case of technology transfer from seawater desalination.
• - Ocana (France): adaptation by OTV of the Biosep process (microfiltration) to replace sand filters. As a result, filtration capacity has been increased and disinfection improved.
• - Lausanne (Switzerland), 76,000 m³/day, has adopted the Cristal technology of Lyonnaise des Eaux. The production unit built by Degrémont is expected to have its ultrafiltration unit placed under acoustic control. It is due to come into service in two stages, in 2000 and 2002, respectively.
• - Del Rio (United States of America), 106,000 m³/day, built by Degrémont. It is due to come into service at the end of 1999.

While underground waters are generally of better quality than surface waters, their processing is now compulsory to eliminate the risk of occasional substandard sanitary quality. This is particularly relevant for karst water with no major pollution problems, but which may be subject to variations in quality (deterioration in terms of turbidity and microbiology). The random and unpredictable nature of such phenomena makes it necessary to apply safety treatment on a continuous basis to ensure that the sanitary standards are always respected. The development of a membrane-based filtration system is an integral part of this approach.

At L’Apié in the south of France, the water to be treated is taken from a reservoir. There are wide variations in turbidity associated with stormwater and a considerable increase in algae in summer. To improve water quality, Degrémont has installed the Cristal process, which is preceded by prefiltration at 200 µm to avoid membrane clogging. The use of powdered activated carbon has resolved the problem of tastes and odours caused by algae.

The Biosep process can also be used in the treatment of karst waters. “Aerating water is a step in the right direction since it starts biological nitrification”, explains Michel Faivre, “and we therefore do not need to use so much chlorine”.

Low-pressure osmosis can be useful for drinking water processing. For Jean-Michel Lainé: “This technique eliminates micropollutants and pesticides. However, the problem is that water becomes unbalanced. It must then be rebalanced before it is distributed to avoid degradation and corrosion problems during transport.

In the future, it should be possible to process all the water by ultrafiltration and part of it by reverse osmosis. After initial processing, separation stabilises at lower pressures, which allows energy savings. This approach then becomes cost-effective”. The European Joule project is currently testing.

using membrane combinations on industrial prototypes. It brings together Cirsee-Lyonnaise des Eaux, Mekorot Water Company (Israel) and Agbar (Spain).

It should lead to the development of processes for contaminated surface waters (France), brackish waters (Israel) and sea water desalinisation (Spain).

and for sewage

USF Memcor receives numerous requests for small capacity tertiary sewage treatment units to enable the final effluents of urban or industrial wastewaters to be recycled. The aim is to protect small rivers. Generally, retrofitting consists of installing a bioreactor immediately after the biological process. A call for tenders has been issued by the city of Lille (France) for the treatment plant at Comines. Whatever the biological wastewater treatment already used, even a naturally aerated lagoon system, effluent can be clarified and disinfected by microfiltration to a sufficient level of hygiene to be able to irrigate, spray or discharge a perfectly clear water even in an environmentally sensitive area.

In some parts of the world, the lack of resources or deterioration of quality means that there is no alternative to processing effluents. “One of the most elegant ways is to recycle them in the same way as industrial process waters” says Robert Niay, “this can be achieved with reverse osmosis as the final step. Potential drinking water resources can thus be conserved and effluents minimised.” This type of solution has been adopted by the Los Angeles west basin water board for Carson, where six units of 90 M10C modules process 625 m³ of municipal effluent per hour before reverse osmosis, and before recycling for use by the Mobil oil refinery. This plant became operational in 1998. In Australia, Eraring has used a similar approach since 1995, enabling Pacific Power to recycle up to 146 m³/h of municipal effluent in an electricity generating station.

In Scottsdale, Arizona, and El Segundo, California, municipal effluent is used for phreatic groundwater accretion. The capacity is enormous. In California, for example, the two processing plants Water Factory 21 and El Segundo carry out tertiary treatment at the rate of 112 m³/h and 470 m³/h with an M10C module from USF Memcor before polishing by reverse osmosis and accretion into the ground water aquifer.

Membrane-based treatment can also be applied to sludge. Engineers from Anjou Recherche working on residual water processing with Biosep technology, have replaced the final settling tank with a membrane. The study was financed by the Seine Normandie area water board: “This approach goes further than biomass separation”, explains Hervé Buisson, researcher at the Maisons-Laffitte Research Centre. “We have developed conditions to increase the filterability of the biomass”. In fact, sludge produced with this system is ten times more compact than sludge produced by conventional methods. Concentrations reach 15 to 20 g/l compared to 2 to 3 g/l obtained by the activated sludge method. Extended aeration, induced by the Biosep system, allows a 50 to 70% reduction in sludge production after a processing period of 30 to 50 days. Furthermore, the membrane approach is easier to operate because process management is based upon simple flow and pressure metering. Every day, 1/50th of the reactor content is drawn off to maintain a 50-day processing period. “This development is in keeping with the new nitrogen/phosphorus standards and filters out most of the micro-organisms.” While the application of this procedure to the treatment of municipal wastewater has yet to be developed, it is operational at a number of industrial sites, such as Smart, located at Hambach (France), Cartonneries et Papeteries du Rhin and Pierre Fabre Médicaments SW.

Processing of industrial effluents

Membrane processes have a strong presence in the treatment of industrial effluents.

For the past four years, l’Oréal's Sicos plant in the north of France has been processing its effluents with a BRM ultrafiltration membrane reactor from Degrémont. During this period, the same process has been installed at two Sanofi sites, in Sisteron and Aramon, respectively, a major advantage being that 2/3 of the effluents treated are now recycled. The quality of a car body paint depends on the efficiency of the water rinse. This process requires large quantities of deionised water to remove surplus paint before baking. Recycling this water by ultrafiltration allows the paint thus removed to be reprocessed. A first implementation has been achieved in Germany by Orelis. This installation processes 4.5 m³/h.

Reverse osmosis can also be used. This technology, which industrial managers are already familiar with, allows a clean separation of water from suspended or dissolved particulate matter, reusable or otherwise, without the need for chemical products. “It is a rapidly expanding market”, emphasises Abdel Khadir. “The use of such technologies in industry can lead to substantial savings.”

For instance, in France, a distillery in the Chalons area has installed a Permo reverse osmosis system. This equipment processes the 50 m³ of water which used to be pumped every hour into an aeration lagoon. The plant now recycles 80% of this water for use in its own process.

The advantages for industrial operators go beyond simply recovering water. Pollutants present in effluents often include salvageable material, which can be upgraded if a means of collecting it can be found. Membrane technologies have numerous advantages: they are non-destructive separation techniques and do not require the addition of reagents.

Runoffs and effluents containing identified pollutants.

In this context, membranes can be used for concentration or purification/separation. The farther upstream the processing takes place, the more these criteria will be respected: immediately after the production line for example. Many examples can be found in a wide range of activities, such as the agro-food industry, the automobile industry, papermaking and the pharmaceutical industry.

Treating water close to the industrial process

The tightening of standards on discharges is a strong incentive to develop membrane technologies. An increasing number of parameters have to be taken into account, and greater accuracy is required for each substance. Regulations on concentration levels are becoming increasingly stringent and difficult to comply with. For a number of years, some industry sectors have had restrictions placed on the amount of water they can use, hence the importance of developing upstream recycling techniques.

Membrane techniques, which stand out due to their capacity for clarification, concentration and non-stop separation, have potential applications for industrial wastewater processing where the objective is recycling. Among membrane techniques, nanofiltration allows the separation of solutes. For over a year now, Orelis, in conjunction with the La Rochette group and the French, Swedish and Finnish CTPs (technical research centres for the paper industry), has been developing a technique to process the effluents from the bleaching of paper pulp. The aim is to work at the processing temperature of 70 °C and at a pH of 10.5 to 11. The technical process tested in the La Rochette plant is the Cerasep technology developed by Orelis. The aim is to recycle water and chemicals so as to solve the effluent problem.

The electronics industry is also very interested in the problem of recycling. This industry needs considerable quantities of very pure water, and is investigating ways of recycling its effluents. The manufacture of printed circuits generates a rinsing water with a small metallic ion and organic additive load.

While metal elimination techniques are now fully operational, this is not yet the case for the destruction of organic products. The reuse of this water requires an efficient elimination process for organic compounds, non-impacting on the quality of the treatment baths. Such is the aim of the Brite Euram III Recycat project, conducted by Anjou Recherche, with the participation of Bull Electronics (France), Philips CFT (Netherlands), OTV Industries, Enirisorse (Italy), INCM (Italy) and ESIP (France). Based on a combination of ultrafiltration and reverse osmosis, this process needs to be finalised by ozone oxidation and PAC absorption steps, in order to maintain a suitable quality of recycled water.

To study the potential applications of nanofiltration technologies in industrial wastewater processing, an inter-technical group study is being conducted in France, with the participation of the IFTS (Institut de Filtration et des Techniques Séparatives), EDF and the Seine-Normandie water agency. The study is on effluents in the paper, leather, textile and mechanical industries.

The programme is aimed at testing the clean-up capacity of nanofiltration to reduce toxic components and its ability to produce a permeate of a suitable quality for recycling in an industrial process.