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Desalination

Desalination with Reverse Osmosis (R.O.)

Desalination Installation

Desalination Installation Design System

Desalination Pre-treatment

Membrane Technology

Scaling and Antiscalant

Softening Water and Filtering Water

Membrane Cleaning

Biofilm Removal

Some Considerations Before Buying a Water Treatment Solution

What does a typical R.O. take out of water?

 

 

Desalination

Desalination is the removal of salts from sea water or brackish water to produce drinking water.

Normal sea water can contain up to a few percents of salt, while drinking water contains only more than 10 ppm of salt.

The most commonly used technique to desalinate is the use of reversed osmosis. This technique is able to remove the salts in one step.

Due to the high osmotic pressure of sea water the working pressure can be very high (up to 100 bar) for reversed osmosis desalination systems.

Alternative desalination techniques are distillation, electrodialysis, ion exchange and carbonate removal.

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Desalination with Reverse Osmosis (R.O.)

Osmosis is a natural phenomenon, without which life would be made impossible. Osmotic processes enable plants to absorb nutrients from the ground. Our kidneys purify the blood in our bodies by means of osmosis.

Although osmosis has been discovered and studied as early as 1850, it has taken us until 1960 to be able to apply the process for water desalination.

Membrane filtration has been approached as a futuristic, expensive and complex process for a very long time. However, in the past fifteen years this process has developed to a mature and reliable technique, which is very usable for (drinking) water purification.

When we are dealing with a system, consisting of two fluids that are separated by a semi permeable membrane (permeable, just not for salts) and we add salt on one side of the system, pure water will start flowing through the membrane. This flow will continue until the pressure is equal on both sides of the membrane. Afterwards the water level will be higher where the salt was added. The difference in water level, caused by the addition of a specific amount of salt, is called osmotic pressure. The osmotic pressure of seawater is around 26 bar.

We can explain the term “reverse osmosis” as follows:

To desalinate water, we must create a flow through a membrane, causing the water to leave the salty side of the membrane, flowing into the unsalted side. To achieve this, pressure must be created upon the water column on the salt side of the membrane; firstly, to remove the natural osmotic pressure and secondly, to create extra pressure on the water column, in order to push the water through the membrane. For the desalination of seawater, the pressure must be about 50-60 bars.

There are several different techniques that can be applied for water desalination. Examples are reverse osmosis, electro dialysis, distillation and ion exchange (see image).

Reverse osmosis is the most economic process for the desalination of brackish water and seawater. When we compare this process to the traditional thermic process of distillation, the capital investments and the energy use are much lower.

Reverse Osmosis modules

There are four different types of reverse osmosis modules, which are used for reverse osmosis processes, mainly desalination processes. These are the tubular, plate & frame, spiral, and hollow fiber modules.

System costs:

Tubular, plate & frame >> hollow fiber, spiral

Design flexibility:

Spiral > hollow fiber > plate & frame > tubular

Required system space:

Tubular >> plate & frame > spiral > hollow fiber

Susceptibility to fouling:

Hollow fiber >> spiral > plate & frame > tubular

Energy use:

Tubular > plate & frame > hollow fiber > spiral

The system costs for the treatment of mineral water are the same for spiral modules and hollow fiber modules. Pre-treatment costs for the purification of surface water are higher when hollow fiber membranes are used. These membranes require a more specific pre-treatment, because they are more susceptible to fouling.

The use of tubular modules and plate & frame modules may be more expensive than that of hollow fiber modules or spiral modules. The costs of the use of tubular modules and plate & frame modules are approximately equal.

The required system space for the modules is very high when hollow fiber modules or spiral modules are used. Tubular modules take up much less space.

It has been said that for reverse osmosis systems for the desalination of seawater, spiral membranes are used most often.

  • Reverse Osmosis desalination installation (seawater(

  • System design

  • Factors that influence prestations

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Desalination Installation

A desalination installation consists of a pre-treatment system, a desalination unit and a subsequential treatment. A high-pressure pump increases the feed water pressure to the required workload. The workload is considerably higher than the osmotic pressure, in order to bring about a satisfactory flow through the membrane. A higher workload will result in a higher amount of treated water. A desalination installation
A high-pressure switch regulates the pressure within the membranes. The installation also contains flowmeters and pressure meters for process regulation. Furthermore, there are a number of security measures such as a low-pressure switch and a conductivitymeter with a security barrier.

Desalination istallation

Membrane element with pressure vessel

The desalination unit in a desalination installation consists of a number of Reverse Osmosis membranes in several pressure vessels, which are placed in a certain order. The feed water that is pumped into the module will be separated into water with a low salt content and water with a high salt content.
The water with a low salt content is called ‘permeate’ and the water with a high salt content is called ‘concentrate’. The concentrate flow is regulated by a pressure vessel, which is placed within the concentrate. The pressure vessel regulates the percentage of feed water that leaves the module through the concentrate flow.

In a spiral module that consists of multiple membranes situated in a pressure vessel, the feed water flows within the pressure vessel, through the spiral channels of the element.
The spiral membranes in the pressure vessel that are interconnected will make sure that the feed water becomes more concentrated. When the concentrate passes through the last membrane, it will reach a pressure lid, which releases pressure.
The permeate of each membrane element is collected in a tube that is place centrally in each spiral membrane. Outside the pressure vessel a drain catches the permeate off the spiral membranes.

A specific benefit of spiral membrane units is the fact that they can be arranged serially within any pressure vessel. This means that the feed water flows through a membrane with one tubular system multiple times.
Hollow fiber units do not have this benefit, which means that they need separate drains for the feed water, the permeate and the concentrate. Hollow fiber installations that are built on a grand scale will have very high system costs, mainly for the built-up of the drainage system.  

* graphic images come from FILMTEC membranes

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Desalination Installation Design System

System design

The desalination installation is a complete system, with feed water input and separate discharge pipes for concentrate and permeate. The data of the input and output pipes should always be compared with water analysis, feed water pressure and salt retention.
The designer of a Reverse Osmosis system will aim at the lowest possible membrane pressure and installation costs and maximum recovery and salt retention.

The recovery of a desalination installation for brackish water is around 85%. It depends on the solubility of suspended solids that are present in the feed water. During seawater desalination a recovery of forty to fifty percent is desirable. The recovery of seawater desalination depends on the osmotic pressure of the feed water and the applied membrane types during the desalination process.

Continuous process

A membrane filtration system is usually designed to attend a continuous process. The choice for a continuous process can be made, because the process conditions, such as feed water flow and permeate flow, are continuous.



Schematic representation of a continuous process

The same goes for Reverse Osmosis systems. These are also designed to attend a continuous process with a continual permeate flow and a system recovery that is steady.
Variations in water temperature and fouling degree of the feed water are compensated by adjustments of the feed pressure.

Multi-stage system

Systems that consist of more than one stage are called multi-stage systems. These systems can reach higher system recoveries, without exceeding the single element recovery limits. To gain a recovery of up to 70%, two stages must be implemented in the feed water treatment system.

 

Schematic representation of a two-stage system

For higher recoveries, three stages must be used. These values are based on the assumption that standard pressure vessels with six elements are used. For shorter vessels the number of stages must be doubled.
When the system recovery is higher, more membrane elements have to be connected in series. A typical two-stage system uses a stage ratio of 2:1 for the desalination of seawater with high dissolved solids contents.

Plug-flow and concentrate recirculation

The plug-flow concept is the standard Reverse Osmosis system design for water desalination applications.
The feed water is passed through a plug-flow system only once. A fraction of the feed water passes a membrane to produce permeate. The rest of the feed water is not derived of salts and will become more and more concentrated.

When the number of membrane elements in a plug-flow system is too low to achieve a high enough system recovery, concentrate recirculation may be implicated. During recirculation part of the concentrate is directed back to the feed water side of the module. The recycled concentrate mixes with the feed water and will be treated once more.

 

Schematic representation of an installation with concentrate recirculation options

 

The number of elements in each pressure vessel

Reverse Osmosis systems are usually designed for a specific permeate flow. To achieve this flow, a number of membrane elements is required. The number of membrane elements that is placed within the installation depends upon the designed flux.
For the desalination of seawater the limiting factor is the maximum feed pressure; this may not exceeds 69 bars.

Based on the designed flux, the production per unit membrane can be determined:

Production per element = flux * element surface

Number of elements = permeate flow / production per element

Number of pressure vessels = number of elements / number of elements per vessel


By means of the permeate flow and the required recovery, the feed water flow is calculated:

Feed water flow = permeate flow / recovery

 

Feed water pressure

A certain feed pressure is required, depending on the system design. The flux, the energy loss in the system and the osmotic pressure determine the feed pressure that the system requires. The required feed pressure will increase when the membrane elements are becoming contaminated through the years. A feed pump that enables a higher flow than the flow that is theoretically required will than be implicated to keep the feed pressure continual. A feed pump that increases the feed pressure by 25% will be satisfactory in practice.
When the system is started up, the initial situation is recorded. All relevant parameters should be registered and noted in a log. Based on this data the performance of the installation can be examined and regulated after the system has been put into action.

 

Monitoring

During monitoring of the system, measurements will take place of the flow, pressure and conductivity of the water. To check the hydraulic affectivity of the system the feed pressure per stage and the permeate flow need to be measured. The feed pressure depends upon the temperature of the feed water. When feed water temperatures are low, more pressure is required to achieve the same recovery that would be reached when feed water temperatures are high. When water temperatures fluctuate, one needs to normalize the permeate flow, to enable comparison with the starting situation.

When the installations function correctly the conductivity of the permeate is low, because of the removal of univalent and bivalent ions. When a leak is situated in the membrane element, the conductivity will increase. That is why conductivity measurements are applied.
These measurements take place in the permeate collection drain. Measurements can be performed for each individual stack or for all present stacks.
A sufficient monitoring of the system will enable the user to know when the system needs cleaning.

Material protection

Within desalination installations, there is a course environment when it comes to corrosion of separate parts of the system. Because of this, the material needs to possess a certain resistance to corrosion. This goes for external parts, which are exposed to a salty atmosphere (spillage, leaks), as well as for internal parts. Corrosion of external system parts can usually be prevented by providing them with a surface layer (painting, galvanizing) and by periodic maintenance of the system and closing of leaks.
Despite the fact that materials are protected against potential corrosion, they also need to be able to be resistant to pressure, vibrations and changing temperatures.
To prevent corrosion and chemical reactions in the part of the system where pressures are low (<10 bar), such as in membrane elements and pressure vessels, non-metals such as PVC and fibreglass are often applied. For high-pressure parts (10-70 bar), such as pumps, drains and lids, one needs to use metals to provide them with the same kind of protection.
PVC and some metals cannot sufficiently resist corrosion. When they start corroding they can contaminate membranes. When corrosion protection is in order, we need to keep this in mind.
The main material that is used for high-pressure parts is stainless steel. The benefit of stainless steel is that it is resistant to corrosion and erosion corrosion. Stainless steel is rarely stricken by galvanic corrosion.

Pipes and parts of the installation should follow up demands of the company where the installation is located. Pipes and components of the installation are usually built using the following materials:

Candle filters and pressure vessel: polypropylene filter in PVC or stainless steel vessel
Pumps: stainless steel
Low-pressure pipes: PVC
High-pressure pipes: stainless steel
Cleansing system: PVC or other chemical-resistant synthetic material

Energy saving

In a desalination system the concentrate is released under high pressure, that is why it is important to win back energy from the concentrate flow. This can be done by application of a pressure exchanger. The concentrate flow from the membranes is directed through the pressure exchanger, where it directly transfers energy to part of the incoming feed water with maximum affectivity.
The feed water flow now has the same volume as the concentrate flow. It will be directed to a small booster pump that corrects the hydraulic losses of the flow.
The upgraded feed water flow will join the feed water flow from the high-pressure pump.
In an installation that uses a pressure exchanger, the high-pressure pump will give off 41% of the energy, the booster pump will give off 2% of the energy and the pressure exchanger will give off the remaining 57%. The pressure exchanger does not use any external energy, so the total energy savings will be 57%.

By practising pressure directly to incoming seawater in a membrane system, a reduction of 60% in size of the high-pressure pump can be achieved. This does not only save energy; it also saves purchase costs of high-pressure pumps.

* Graphic images come from FILMTEC membranes

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Desalination Pre-treatment

For the preservation of the affectivity and life span of a Reverse Osmosis (RO) installation, a sufficient pre-treatment is required. A proper selection of pre-treatment methods for feed water will improve affectivity and extend the life span of the system by preventing or minimizing biofouling, scaling and membrane plugging.

To perform an uninterrupted and reliable pre-treatment of the feed water a special approach is used. A pre-treatment that is not geared to the installation may cause a system overload. When this occurs the system parts need cleaning much more often to restore productivity and salt retention. Cleaning costs, system performance and standstill time are very significant in that situation.

The kind of pre-treatment system that is used greatly depends on feed water quality. Consequentially, sufficient feed water pre-treatment is dependent on:

  • The source of the feed water

  • The composition of the feed water

  • The function of the feed water

When the source of the feed water that needs treatment is specified, a complete and exact water analysis is performed. This action is an important step for the design of a pre-treatment system and the entire Reverse Osmosis system, because this often determines the type and size of the pre-treatment.

Feed water analysis

Most water types that are treated in a Reverse Osmosis system are either:

  • Brackish water with a low salt content and a total dissolved solids content of up to 5,000 ppm, or

  • Brackish water with a high salt content and a total dissolved solids content of between 5,000 and 15,000 ppm, or

  • Seawater with a total dissolved solids content of around 35,000 ppm

Seawater with a total dissolved solids content of around 35,000 ppm is regarded standard seawater, because of its global abundance. Globally, the differences in total dissolved solids content of seawater can be very sufficient. The Baltic Sea for example, has a total dissolved solids content of about 7,000 mg/L, whereas the total dissolved solids content of the Red Sea is 45,000 mg/L.

Standard values for the concentrations of dissolved solids in seawater have been determined. During seawater analysis, one should always keep in mind that land influences are of great significance for the composition of seawater. A sample that is taken on open sea can have a very different composition than a sample that is taken near the shore.

The limiting factor for seawater treatment with a Reverse Osmosis system is the osmotic pressure, caused by a high total dissolved solids content. The limiting factor for the treatment of brackish water with a Reverse Osmosis system is mainly its chemical nature. This means precipitation and scaling (caused by calcium carbonate or sulphates). The chemical composition of brackish waters varies greatly and is very location specific.

To create a satisfactory process design, a very accurate water analysis should be carried out. On a water analysis form the concentrations of inorganic salts and other data of the feed water are collected. The water analysis should be fully balanced. When this is not the case sodium or chlorine ions should be added, in order to neutralize the water balance.

Scaling prevention and control

Scaling is the accumulation of (partially) insoluble salts in a membrane. When a Reverse Osmosis installation has a recovery of 50%, the concentration of salts in the concentrate flow is double the concentration of salts in the feed water flow. When recovery increases, the chances of scaling increase, as well. Because of this it is of great importance that the saturation limits of the (partially) insoluble salts are not exceeded.

Partially insoluble salts that cause problems in a Reverse Osmosis system are mainly calcium carbonate (CaCO3), calcium sulphate (CaSO4) and silica, but also calcium fluoride (CaF2), barium sulphate (BaSO4) and strontium sulphate (SrSO4).

Calcium carbonate scaling can be prevented by the addition of acids. Adding anti-scalents can prevent the precipitation of barium and strontium salts, silicates and iron. One must always keep in mind that anti-scalents can add to biofouling (microrganisms fouling). This can be prevented by tightening the demands of applied chemicals and by using chemicals with a different composition.

The use of anti-scalents cannot always be avoided. Decreasing recovery by 25% will reduce the chances of over-saturation of precipitating salts (scaling).

The membrane elements will need replacement within one or two years, because that causes the use of anti-scalents to be limited. This method is applied for relatively small installations, like installations that produce drinking water from seawater for domestic use.

Another cleaning method is opening the concentrate lid to a forward flush under low pressure.

Prevention of fouling by colloids

In Reverse Osmosis elements colloidal pollution can seriously diminish the performance by decreasing productivity. An early sign of this pollutant is usually an increasing pressure gradient. The sources of this pollution in feed water can vary greatly. They are usually bacteria, clay, and iron corrosion products.

Chemical products that are used during pre-treatment may also cause fouling of membranes. The best available technique for the determination of the fouling potential of feed water by colloids is the MFI (Modified Fouling Index) measurement. This is an important type of measurement that takes place prior to the design of a pre-treatment system. This measurement must be done regularly when the Reverse Osmosis system is put to use.

In order to maintain a MFI of (less than) five, there are a number of pre-treatment methods to decrease the MFI:

  • Sand filtration and candle filtration

  • Ultra filtration and micro filtration

  • Coagulation and flocculation

Biofouling prevention

All surface water contains microrganisms, algae, fungi, viruses and higher organisms. Microrganisms are colloidal pollutants, because they are never larger than 1 to 3 nm. Because of this, bacteria can be removed with techniques that remove colloids.

The consequences of biological growth within a membrane system negatively influence the system, just like other contaminants. The symptoms of fouling can vary from a higher pressure at the supplier side of the membrane, to telescoping (shoving apart of the membrane) and membrane damage. Sometimes bio film can develop in the permeate; causing it to become polluted again.  

The number of microrganisms in the surface water, in the feed water and in the concentrate can provide us with valuable information on the degree of contamination of the water. The types and amounts of nutrients present in the feed water are factors that determine bio film growth. Despite the fact that there are several investigators that determine the growth of bio films, it has not been fully comprehended yet.

The best way to discover biofouling during its development phase is to check a testing surface in the feed water flow. The ‘Robin Sampler’ is a simple device, which exposes small testing surfaces to water. These surface check-ups can be performed regularly and can be tested for the growth and attachment of bio film.

A careful periodic inspection of candle filters and pipelines can also be useful. The presence of mucous or strange smells can be an indication of the occurrence of bio film.

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Membrane Technology

Membrane technology has become a dignified separation technology over the past decennia. The main force of membrane technology is the fact that it works without the addition of chemicals, with a relatively low energy use and easy and well-arranged process conductions.

Membrane technology is a generic term for a number of different, very characteristic separation processes. These processes are of the same kind, because in each of them a membrane is used. Membranes are used more and more often for the creation of process water from groundwater, surface water or wastewater. Membranes are now competitive for conventional techniques. The membrane separation process is based on the presence of semi permeable membranes.

The principle is quite simple: the membrane acts as a very specific filter that will let water flow through, while it catches suspended solids and other substances.

There are various methods to enable substances to penetrate a membrane. Examples of these methods are the applications of high pressure, the maintenance of a concentration gradient on both sides of the membrane and the introduction of an electric potential.

Membranes occupy through a selective separation wall. Certain substances can pass through the membrane, while other substances are caught.

Membrane filtration can be used as an alternative for flocculation, sediment purification techniques, adsorption (sand filters and active carbon filters, ion exchangers), extraction and distillation.

There are two factors that determine the affectivity of a membrane filtration process; selectivity and productivity. Selectivity is expressed as a parameter called retention or separation factor (expressed by the unit l/m2·h). Productivity is expressed as a parameter called flux (expressed by the unit l/m2·h). Selectivity and productivity are membrane-dependent.

Membrane filtration can be divided up between micro and ultra filtration on the one hand and nano filtration and Reverse Osmosis (RO or hyper filtration) on the other hand.

When membrane filtration is used for the removal of larger particles, micro filtration and ultra filtration are applied. Because of the open character of the membranes the productivity is high while the pressure differences are low.

When salts need to be removed from water, nano filtration and Reverse Osmosis are applied. Nano filtration and RO membranes do not work according to the principle of pores; separation takes place by diffusion through the membrane. The pressure that is required to perform nano filtration and Reverse Osmosis is much higher than the pressure required for micro and ultra filtration, while productivity is much lower.  

Membrane filtration has a number of benefits over the existing water purification techniques:

  • It is a process that can take place while temperatures are low. This is mainly important because it enables the treatment of heat-sensitive matter. That is why these applications are widely used for food production.

  • It is a process with low energy cost. Most of the energy that is required is used to pump liquids through the membrane. The total amount of energy that is used is minor, compared to alternative techniques, such as evaporation.

  • The process can easily be expanded.

Process management of membrane filtration systems

Membrane filtration systems can be managed in either dead-end flow or cross-flow. The purpose of the optimization of the membrane techniques is the achievement of the highest possible production for a long period of time, with acceptable pollution levels.

Membrane systems

The choice for a certain kind of membrane system is determined by a great number of aspects, such as costs, risks of plugging of the membranes, packing density and cleaning opportunities. Membranes are never applied as one flat plate, because this large surface often results in high investing costs. That is why systems are built densely to enable a large membrane surface to be put in the smallest possible volume. Membranes are implemented in several types of modules. There are two main types, called the tubular membrane system and the plate & frame membrane system. Tubular membrane systems are divided up in tubular, capillary and hollow fiber membranes. Plate & frame membranes are divided up in spiral membranes and pillow-shaped membranes.

Membrane fouling

During membrane filtration processes membrane fouling is inevitable, even with a sufficient pre-treatment. The types and amounts of fouling are dependent on many different factors, such as feed water quality, membrane type, membrane materials and process design and control.

Particles, biofouling and scaling are the three main types of fouling on a membrane. These contaminants cause that a higher workload is required, to be able to guarantee a continuous capacity of the membranes. At a certain point the pressure will rise so much that it is no longer economically and technically accountable.

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Scaling and Antiscalant

Scaling means the deposition of particles on a membrane, causing it to plug. Without some means of scale inhibition, reverse osmosis (RO) membranes and flow passages within membrane elements will scale due to precipitation of sparingly soluble gas, such as calcium carbonate, calcium sulfate, barium sulfate and strontium sulfate. Most natural waters contain relatively high concentrations of calcium, sulfate and bicarbonate ions.

In membrane desalination operations at high recovery ratios, the solubility limits of gypsum and calcite exceed saturation levels leading to crystallization on membrane surfaces. The surface blockage of the scale results in permeate flux decline, reducing the efficiency of the process and increasing of operation costs.

The effects of scale on the permeation rate of RO systems are illustrated in the following figure. Following an induction period, plant flow decrease rapidly. The length of this period varies with the type of scale and the degree of super saturation of the sparingly soluble salt.

As it is evident from the graph, the induction period for calcium carbonate is much shorter than that for sulfate scales, such as calcium sulfate. It is economically preferable to prevent scaling formation, even if there are effective cleaners for scale. Scale often plugs RO element feed passages, making cleaning difficult and very time consuming. There is also the risk that scaling will damage membrane surface.

There are three methods of scale control commonly employed:

  • Acidification

  • ion exchange softening

  • Antiscalant addiction

Acidification: acid addiction destroys carbonate ions, removing one of the reactants necessary for calcium carbonate precipitation. This is very effective in preventing the precipitation of calcium carbonate, but ineffective in preventing other types of scale. Additional disadvantages include the corrosively of the acid, the cost of tanks and monitoring equipment and the fact that acid lowers the pH of the RO permeate.

Ion exchange softening: this method utilizes the sodium which is exchanged for magnesium and calcium ions that are concentrated in the RO feed water, following the chemical equations:

Ca2+ + 2NaZ => 2Na+ + CaZ2

Mg2+ + 2NaZ => 2Na+ + MgZ2

)NaZ represents the sodium exchange resin(

When all the sodium ions have been replaces by calcium and magnesium, the resin must be regenerated with a brine solution. Ion exchange softening eliminates the need for continuous feed of either acid or antiscalant.

Antiscalants: they are surface active materials that interfere with precipitation reactions in three primary ways:

  • Threshold inhibition: it is the ability of an antisclant to keep supersaturated solutions of springly soluble salts.

  • Crystal modification: it is the property of an antiscalants to distort crystal shapes, resulting in soft non adherent scale. As a crystal begin to form at the submicroscopic level, negative groups located on the antiscalant molecule attack the positive charges on scale nuclei interrupting the electronic balance necessary to propagate the crystal growth. When treated with crystal modifiers, scale crystals appear distorted, generally more oval in shape, and less compact.

  • Dispersion: dispersancy is the ability of some antiscalants to adsorb on crystals or colloidal particles and impart a high anionic charge, which tends to keep the crystals separated. The high anionic charge also separates particles from fixed anionic charges present on the membrane surface.

Threshold Mechanism

Dispersancy

During the past two decades new generations of antiscalants have emerged commercially, in which the active ingredients are mostly proprietary mixtures of various molecular weight polycarboxylates and polyacrylates.

Calculation procedures exist for predicting the likelihood of scale formation. Use of these predictors depends upon an up-to-date water analysis and a knowledge of system design parameters. The ions contained in the feed water concentrate though the RO system, the point of maximum scale potential is the concentrate stream. Antiscalant type and dosage is therefore based upon the mineral analysis at this point.

It is important to find the optimization of antiscalant treatment with respect to type and dosage, identifying the proper antiscalant to use and the dosage-induction type relationship for the extended level of super saturation.

Lenntech can help you in the selection of the best antiscalant for your particular application.

Economical analysis

Acid addition is not very cost effective because of the cost of acid, tanks and monitoring equipment. Unless removed by degasification, excess of carbon dioxide contained in the permeate of acid-fed systems increases the cost of ion exchange regeneration.

Antiscalants are relatively cheap products and have no additional costs.

When compared to either acid or antiscalant addition, the main disadvantage to softening is cost factoring in equipment costs. Through a present worth analysis there is no level of hardness in which softening competes economically with antiscalants addition.

The following table gives a cost comparison between softening and antiscalant treatment options for different levels of hardness, on a basis of a RO system designed to produce 75 gpm (17 m3/h) of permeate at 75% recovery.

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Membrane Cleaning

There are a number of cleaning techniques for the removal of membrane fouling. These techniques are forward flushing, backward flushing, air flushing and chemical cleaning, and any combination of the methods.

There are several different membrane cleaning methods, such as forward flush, backward flush and air flush.

  • When forward flush is applied, membranes are flushed with feed water or permeate forward. The feed water or permeate flows through the system more rapidly than during the production phase. Because of the more rapid flow and the resulting turbulence, particles that are absorbed to the membrane are released and discharged. The particles that are absorbed to membrane pores are not released. These particles can only be removed through backward flushing.
     

  • Backward flush is a reversed filtration process. Permeate is flushed through the feed water side of the system under pressure, applying twice the flux that is used during filtration. When the flux has not restored itself sufficiently after back flushing, a chemical cleaning process can be applied.
     

  • During a chemical cleaning process, membranes are soaked with a solution of chlorine bleach, hydrochloric acid or hydrogen peroxide. First the solution soaks into the membranes for a number of minutes and after that a forward flush or backward flush is applied, causing the contaminants to be rinsed out.
     

  • A newer cleaning method is the so-called air flush or air/ water flush. This is a forward flush during which air is injected in the supplier pipe. Because air is used (while the water speed remains the same), a much more turbulent cleaning system is created.

Forward flush

When forward flush is applied in a membrane, the barrier that is responsible for dead-end management is opened. At the same time the membrane is temporarily performing cross-flow filtration, without the production of permeate.
The purpose of a forward flush is the removal of a constructed layer of contaminants on the membrane through the creation of turbulence. A high hydraulic pressure gradient is in order during forward flush.

Backward flush

When backward flush is applied the pores of a membrane are flushed inside out. The pressure on the permeate side of the membrane is higher than the pressure within the membranes, causing the pores to be cleaned. A backward flush is executed under a pressure that is a bout 2.5 times greater than the production pressure.
Permeate is always used for a backward flush, because the permeate chamber must always be free of contagion. A consequence of backward flush is a decrease in recovery of the process. Because of this, a backward flush must take up the smallest possible amount of time. However, the flush must be maintained long enough to fully flush the volume of a module at least once.

Air flush or air/ water flush

Fouling on the membrane surface needs to be removed as effectively as possible during backward flush. The so-called air flush, a concept developed by Nuon in cooperation with DHV and X-flow, has proved to be very useful to perform this process. Using air flush means flushing the inside of membranes with an air/ water mixture.
During an air flush air is added to the forward flush, causing air bubbles to form, which cause a higher turbulence. Because of this turbulence the fouling is removed from the membrane surface.
The benefit of the air flush over the forward flush is that it uses a smaller pumping capacity during the cleaning process. 

Chemical cleaning

When the above-mentioned cleaning methods are not effective enough to reduce the flux to an acceptable level, it is necessary to clean the membranes chemically.
During chemical cleaning chemicals, such as hydrogen chloride (HCl) and nitric acid (HNO3), or disinfection agents, such as hydrogen peroxide (H2O2) are added to the permeate during backward flush. As soon as the entire module is filled up with permeate, the chemicals need to soak in. After the cleaning chemicals have fully soaked in, the module is flushed and, finally, put back into production.

Cleaning methods are often combined. For example, one can use a backward flush for the removal of pore fouling, followed by a forward flush or air flush.
The cleaning method or strategy that is used is dependent on many factors. In practise, the most suitable methods is determined by trial and error (practise tests).

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Biofilm Removal

A biofilm is a layer of microorganisms contained in a matrix (slime layer), which forms on surfaces in contact with water. Incorporation of pathogens in biofilms can protect the pathogens from concentrations of biocides that would otherwise kill or inhibit those organisms freely suspended in water.

Biofilms provide a safe haven for organisms like Listeria, E. coli and legionella where they can reproduce to levels where contamination of products passing through that water becomes inevitable.

It has been proven beyond doubt that chlorine dioxide removes biofilm from water systems and prevents it from forming when dosed at a continuous low level. Hypochlorite on the other hand has been proven to have little effect on biofilms.

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Softening Water and Filtering Water

Filtering water involves separating mineral particles, like manganese, iron, hydrogen sulfide or other organic matter, from pure H2O. By passing water through a "filter bed," or "media bed," these granular particles are trapped - and clean, purified water passes through the bed.

Softening water involves something called "ion exchange" to remove dissolved minerals - like calcium, magnesium, iron and manganese - that can't be trapped in a filter bed. Softeners use fresh resin beads with sodium attached to the resin. As water enters the tank, dissolved calcium and magnesium are attracted to the resin. The resin passes up the sodium in exchange for the dissolved chemicals and the water is then rid of these impurities.

Free of contaminants and minerals, water treated with softeners or filtration systems will make an impression on you and your household, and at the work place.

Whether you use a filter or a softener depends on whether the contaminates in your water are particles or dissolved minerals.

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Some Considerations before buying a water treatment solution

Here are some helpful considerations you may want to take into account before purchasing a water treatment system.

How hard is your water?

Having your water tested will help you determine your exact needs, even though you may already know you have one or more water problems.

In determining your needs, a water expert will look at a number of things. For example, the hardness level of the water and the size or type of equipment necessary for water softening. Additional problems may require additional equipment. The same principles hold true for drinking water systems.

Your water usage and pressure.

The amount of water used as well as water pressure is factors to consider when fitting with a water quality improvement system.

Why you should avoid a "quick fix"

Fixing your existing water problem is your primary goal, but don't be eager to settle for the least expensive solution. A higher-priced unit may serve your needs better by being more efficient, and reducing operating costs and maintenance time. Be sure, however, that you're getting your money's worth. Before you buy, get a detailed estimate of equipment, installation, and operating costs.

Buy from a reputable dealer.

A reputable water quality improvement equipment dealer is an excellent resource in helping you determine your water conditioning needs.  ASSK is the most trusted name in water treatment solutions.

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What does a typical R.O. take out of water?

CHEMICALS

REDUCES BY

 

OTHER CONTAMINENTS

REDUCES BY

THMs (chloroform)

95%

 

barium

97%

benzene

83%

 

bicarbonate

94%

carbon tetrachloride

87%

 

cadmium

97%

p-dichlorobenzene

93%

 

calcium

97%

TCE (trichloroethylene)

98%

 

chromate

92%

1, 1-dichloroethylene

86%

 

copper

97%

1, 1, 1-trichloroethane

93%

 

detergents

97%

1, 2-dichloropropene

95%

 

fluoride

90%

cis-1,3-dichloropropene

95%

 

lead

97%

chlorobenzene

95%

 

magnesium

97%

ethylbenzene

95%

 

nickel

97%

hexachlorobutadiene

95%

 

nitrates

80%

ortho-xylene

95%

 

total dissolved solids

95%

PCE (tetrachloroethylene

95%

 

potassium

92%

toluene

95%

 

radium

97%

trans-1, 2-dichloroethene

95%

 

selenium

97%

1, 1, 2, 2-tetrachloroethane

95%

 

silicate

96%

1, 2-dichlorobenzene

95%

 

silver

85%

1, 2-dichloropropane

95%

 

sodium

92%

1, 1-dichloroethane

95%

 

strontium

97%

chlorine

99%

 

sulfate

97%

EDB

99%

 

PCB's

97%

DBCP

99%

 

insecticides

97%

Atrazine

97%

 

herbicides

97%

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