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Demineralization

Introduction

Demineralization is the removal of minerals and nitrate from the water. The three that we will be discussing in the lesson are ion exchange, reverse osmosis and electrodialysis. These methods are widely used for water and wastewater treatment. Ion exchange is primarily used for the removal of hardness ions like magnesium and calcium and for water demineralization. Reverse osmosis and electrodialysis, which are both membrane processes, remove dissolved solids from water using membranes.

How each Process Works

Ion Exchange

The ion exchange units are used to remove any charged substance from the water but are mainly used to remove hardness and nitrate from groundwater.

The water is pretreated to reduce the suspended solids and total dissolved solids load to ion-exchange unit.

The methods used for pretreatment include:

Cold lime without soda ash

Hot lime with or without soda ash

Coagulation and filtration

Filtration

Evaporation or distillation

Reverse osmosis

Ultrafiltration

In this type of process, 90% of barium, arsenic, cadmium, chromium, silver, radium, nitrites, selenium and nitrates can be effectively removed from water. Ion exchange is one of the choices that is better for small systems that need to remove radionuclides.

Advantages of Ion Exchange

Ion exchange can be used with fluctuating flow rates.

Makes effluent contamination impossible

Resins are available in large varieties from suppliers and each resin is effective in removing specific contaminants.

Limitations of Ion Exchange

Pretreatment is required for most surface waters

Waste is highly concentrated and requires careful disposal

Unacceptable high levels of contamination in effluent

Units are sensitive to the other ions present.

Ion Exchange Process

Inorganic removal is accomplished through the adsorption of contaminant ions onto a resin exchange medium. One ion is substituted for another on the charged surface of the medium that is usually a plastic resin. This type of surface is designed as either cationic or anionic-negatively charged. The medium is saturated with exchangeable ions before the treatment operations.

The contaminant ions during ion exchange, replace the regenerant ions because they are preferred by the exchange medium. When no ions are left to take the place of the contaminant ions, the medium is regenerated with a suitable solution that saturates the medium with the appropriate ions. Since there is a required down time, the regeneration cycles are done only once per day.

For resin exchange, capacity is expressed in terms of weight per unit volume of the resin used. Calculation of the breakthrough time for an ion exchange unit requires knowing the resin exchange capacity, influent contaminant concentration and the desired effluent quality.

Ion Exchange Equipment

The ion exchange unit contains prefiltration, disinfection, ion exchange, storage and distribution elements.

Chemicals Used

To regenerate the exchange medium in ion exchangers, the chemical, sodium chloride is often used because of its low cost. When using this chemical one must remember that high sodium residual will result in the finished water which may cause it to be unacceptable for individuals with a salt restricted diet. To avoid this problem, other regenerant materials can be used such as potassium chloride.

Reverse Osmosis

In a reverse osmosis system, the water is put under pressure and forced through a membrane that filters out the minerals and nitrate. These systems are compact and easy to operate and require minimal labor, which make them suitable for small systems and for systems where there is a high degree of seasonal fluctuation in water demand.

Reverse osmosis effectively removes nearly all the inorganic contaminants from water. This process removes over 70% of the following:

• Arsenic-3

• Arsenic-4

• Barium

• Cadmium

• Chromium-3

• Chromium-6

• Fluoride

• Lead

• Mercury

• Nitrite

• Selenium-4 and selenium-6

• Silver

When the units are operated properly, ninety-six percent removal rates will be attained. Reverse osmosis also affectively removes natural organic substances, pesticides, radium and microbiological contaminants. To work effectively, reverse osmosis should be used in series. Near 0 effluent contaminant concentrations can be achieved by water passing through multiple units.

Advantages of Reverse Osmosis

Nearly all contaminant ions and most dissolved non-ions are removed

Suitable for small systems with a high degree of seasonal fluctuation in water demand

Insensitive to flow and TDA levels

Operates immediately without any minimum break-in period

Possible low effluent concentrations

Removes bacteria and particles

Simplicity and automation operation allows for less operator attention which makes them suitable for small system applications.

Limitation of RO

High operating costs and capital

Potential problem with managing the wastewater brine solution

Pretreatment at high levels

Fouling of membranes

RO Process

In this process, contaminants are removed from water by using a semi-permeable membrane that permits only water, not the dissolved ions, to pass through its pores. The contaminated water is subjected to high pressure that forces the pure water through the membrane, leaving the contaminants behind in a brine solution. The membranes come in a variety of pore sizes and characteristics.

RO Equipment

RO units include the following:

Raw water pumps

Pretreatment

Membranes

Disinfection

Storage

Distribution elements

Units are able to process any desired quantity or quality of water by configuring units sequentially to reprocess waste brine from earlier stages of the process. Principle design for reverse osmosis units are:

Membrane type and pore size

Operating pressure

Required pretreatment

Product conversion rate

Electrodialysis

Electrodialysis is effective in removing fluoride and nitrate from water. This process also uses membranes but direct electrical currents are used to attract ions to one side of the treatment chamber. This system includes a source of pressurized water, direct current power supply and a pair of selective membranes.

Advantages of Electrodialysis

All the contaminant ions and many of the dissolved non-ions are removed

Insensitive to flow and TDS levels

Possible low effluent concentrations

Limitations of Electrodialysis

Operating costs and capital are high

Level of pretreatment required is high

Twenty to ninety percent of feed flow is rejected stream

Replacement of electrodes

Electrodialysis Process

In this process, the membranes adjacent to the influent steam are charged either positively or negatively and this charge attracts counter-ions toward the membrane. These membranes are designed to allow the positive or the negative charged ions to pass through the membrane, where the ions move from the product water stream through a membrane to the two reject water streams.

Electrodialysis Equipment

The electrodialysis system has three essential elements:

Source of pressurized water

Direct current power supply

A pair of selective membranes

The average ion removal rate varies from one-fourth to two-thirds percent per stage. Using multiple stage units can increase the efficiency of removal. The membrane pairs are stacked in the treatment vessel.

Chemicals Used

The amount of water treated may be limited by the fouling of the membranes. Fouling is caused if the membranes pores become clogged by salt precipitation or by the obstruction of suspended particulates. The particulates that are suspended in the water can be removed in pretreatment but the salts that exceed the solubility product at the membrane surface has to be controlled chemically by pH reduction or chelation of the metal ions by using phosphate. Reversal of the charge on the membranes, electrodialysis reversal (EDR) helps flush the attached ions from the membrane surface which helps extend the time between cleanings.

Deionized water also known as demineralized water is water that has had its mineral ions removed. Mineral ions such as cations of sodium, calcium, iron, copper, etc and anions such as chloride, sulphate, nitrate, etc are common ions present in water. Deionization is a physical process which uses specially-manufactured ion exchange resins which provides ion exchange site for the replacement of the mineral salts in water with water forming H+ and OH- ions. Because the majority of water impurities are dissolved salts, deionization produces a high purity water that is generally similar to distilled water, and this process is quick and without scale buildup.

Electrodeionization (EDI) is a water treatment process that removes ionizable species from liquids using electrically active media and an electrical potential to effect ion transport. It differs from other water purification technologies such as conventional ion exchange in that it does not require the use of chemicals such as acid and caustic for reactivation. EDI is commonly used as a polishing process to further remove trace ionic salts of the Reverse Osmosis (RO) permeate to high purity water of multi-megohm-cm quality.

The continuous electrodeionization (EDI) process, is distinguished from other electrochemical collection/discharge processes such as electrochemical ion exchange (EIX) or capacitive deionization (CapDI), in that EDI performance is determined by the ionic transport properties of the active media, not the ionic capacity of the media. EDI devices typically contain semi-permeable ion-exchange membranes and permanently charged media such as ion-exchange resin. The EDI process is essentially a hybrid of two well-known separation processes - ion exchange deionization and electrodialysis, and is sometimes referred to as filled-cell electrodialysis.

How it works

The electrically active media in EDI devices may function to alternately collect and discharge ionizable species, or to facilitate the transport of ions continuously by ionic or electronic substitution mechanisms. EDI devices may comprise media of permanent or temporary charge, and may be operated batchwise, intermittently, or continuously.

There are two distinct operating regimes for EDI devices: enhanced transfer and electroregeneration (Ganzi, 1988). In the enhanced transfer regime, the resins within the device remain in the salt forms. In low conductivity solutions the ion exchange resin is orders of magnitude more conductive than the solution, and act as a medium for transport of ions across the compartments to the surface of the ion exchange membranes. This mode of ion removal is only applicable in devices that allow simultaneous removal of both anions and cations, in order to maintain electroneutrality.

The second operating regime for EDI devices is known as the electroregeneration regime. This regime is characterised by the continuous regeneration of resins by electrically produced hydrogen and hydroxide ions. The dissociation of water preferentially occurs at bipolar interfaces in the ion-depleting compartment where localized conditions of low solute concentrations are most likely to occur (Simons). The two primary types of interfaces in EDI devices are resin/resin and resin/membrane. The optimum location for water splitting depends on the configuration of the resin filler. For mixed-bed devices water splitting at both types of interface can result in effective resin regeneration, while in layered bed devices water is dissociated primarily at the resin/membrane interface.

"Regenerating" the resins to their H+ and OH- forms allows EDI devices to remove weakly ionized compounds such as carbonic and silicic acids, and to remove weakly ionized organic compounds. This mode of ion removal occurs in all EDI devices that produce ultrapure water.

Under Direct Current (DC) electrical potential, Water (H2O) behaves as follows:

H2O -> H+ + OH-

Technology Overview

A typical EDI device contains alternating semipermeable anion and cation ion-exchange membranes. The spaces between the membranes are configured to create liquid flow compartments with inlets and outlets. A transverse DC electrical field is applied by an external power source using electrodes at the ends of the membranes and compartments.

When the compartments are subjected to an electric field, ions in the liquid are attracted to their respective counterelectrodes. The result is that the compartments bounded by the anion membrane facing the anode and the cation membrane facing the cathode become depleted of ions and are thus called purifying (or sometimes, diluting) compartments. The compartments bounded by the anion membrane facing the cathode and cation membrane facing the anode will then “trap” ions that have transferred in from the purifying compartments. Since the concentration of ions in these compartments increases relative to the feed, they are called concentrating compartments, and the water flowing through them is referred to as the concentrate stream (or sometimes, the reject stream).

In an EDI device, the space within the ion depleting compartments (and in some cases in the ion concentrating compartments) is filled with electrically active media such as ion exchange resin. The ion-exchange resin enhances the transport of ions and can also participate as a substrate for electrochemical reactions, such as splitting of water into hydrogen (H+) and hydroxyl (OH-) ions. Different media configurations are possible, such as intimately mixed anion and cation exchange resins (mixed bed or MB) or separate sections of ion-exchange resin, each section substantially comprised of resins of the same polarity: e.g., either anion or cation resin.

Comparing New and Old Technology

When compared with conventional resin-based, chemically regenerated deionization equipment, EDI systems offer a variety of benefits. Most obvious is the elimination of the regeneration process and its associated hazardous regeneration chemicals - acid and caustic. Since EDI operates through a combination of ion-transfer across the resins and membranes, as well as electrochemical regeneration of a portion of the bed, the resins and membranes are always functional as long as the DC voltage is applied. The resin in a conventional deionizer only purifies water when in its active (regenerated) form. As a result, the EDI system product water quality stays constant over time, whereas in regenerable deionization, product water quality degrades as the resins approach exhaustion. For those processes requiring DI water on a continuous basis, conventional systems must be duplexed so that one system can provide water while the other is regenerated. Duplexing adds cost, complexity, and size to conventional DI systems. Because EDI is continuous, and not a batch process, duplexing is not necessary. As a result of this, as well as the avoidance of regenerant chemical storage and transfer equipment, EDI system footprints are often one half of the size of their conventional counterparts.

There are significant tangible cost benefits associated with the elimination of regeneration. The costs of regeneration labor and chemicals are replaced with a small amount of electrical consumption. A typical EDI system will use approximately 1 kW-hr of electricity to deionize 1000 gallons from a feed conductivity of 50 microsiemen /cm to 0.1 µS/cm product conductivity. Since the EDI concentrate (or reject) stream contains only the feed water contaminants at 5-20 times higher concentration, it can usually be discharged without treatment, or used for another process. Thus facility costs can also be reduced since waste neutralisation equipment and ventilation for hazardous fumes are not necessary.

There are also less tangible cost reductions, which are harder to quantify, but usually favor the use of EDI systems. By eliminating hazardous chemicals wherever possible, workplace health and safety conditions can be improved. With today's increasing regulatory influence on the workplace, the storage, use, neutralisation, and disposal of hazardous chemicals result in hidden costs associated with monitoring and paperwork to conform to EPA and OSHA requirements as well as the "Right To Know" laws. In addition, the fumes, particularly from acid, often cause corrosive structural damage to facilities and equipment.

For the most part the elimination of regenerant chemicals is considered advantageous, but the chemicals do offer at least one benefit. In conventional demineralisers, acid and caustic is typically applied to the ion exchange resins at concentrations of 2-8% by weight. At these concentrations the chemicals not only regenerate the resins but clean them as well. The electrochemical regeneration that occurs in a EDI device does not provide the same level of resin cleaning. Therefore proper pretreatment is even more important with a EDI device, in order to prevent fouling or scaling. This is one of the reasons that RO pretreatment is normally required upstream of a EDI system. In general the feed water requirements for EDI systems are stricter than for a chemically regenerated demineraliser.

Demineralization ( DM ) Water Treatment Plants

Demineralization is the process of removing mineral salts from water by using the ion exchange process.

Demineralised water is water completely free (or almost) of dissolved minerals as a result of one of the following processes :

• Distillation

• Deionization

• Membrane filtration (reverse osmosis or nanofiltration)

• Electrodyalisis

• Or other technologies.

resins which provides ion exchange site for the replacement of the

salts in

mineral water with water forming H+ and OH- ions.

Because the majority of water impurities are dissolved salts, deionization produces a high purity water that is generally similar to distilled water, and this process is quick and without scale buildup.

De-mineralization technology is the proven process for treatment of water.

A DM Water System produces mineral free water by operating on the principles of ion exchange, Degasification, and polishing. Demineralized Water System finds wide application in the field of steam, power, process, and cooling.

Principle

Raw water is passed via two small polystyrene bead filled (ion exchange resins) beds. While the cations get exchanged with hydrogen ions in first bed, the anions are exchanged with hydroxyl ions, in the second one.

Process :

In the context of water purification, ion-exchange is a rapid and reversible process in which impurity ions present in the water are replaced by ions released by an ion-exchange resin.

The impurity ions are taken up by the resin, which must be periodically regenerated to restore it to the original ionic form.

(An ion is an atom or group of atoms with an electric charge. Positively-charged ions are called cations and are usually metals; negatively-charged ions are called anions and are usually non-metals).

The following ions are widely found in raw waters :

Cations	Anions
Calcium (Ca2+)	Chloride ( Cl-)
Magnesium (Mg2+)	Bicarbonate (HCO3-)
Sodium (Na+)	Nitrate (NO3-)
Potassium (K+)	Carbonate (CO32-)

Ion Exchange Resins :

There are two basic types of resin - cation-exchange and anion-exchange resins.

Cation exchange resins will release Hydrogen (H+) ions or other positively charged ions in exchange for impurity cations present in the water.

Anion exchange resins will release hydroxyl (OH-) ions or other negatively charged ions in exchange for impurity anions present in the water.

The application of ion-exchange to water treatment and purification. There are three ways in which ion-exchange technology can be used in water treatment and purification :

first, cation-exchange resins alone can be employed to soften water by base exchange; secondly, anion-exchange resins alone can be used for organic scavenging or nitrate removal; and thirdly, combinations of cation-exchange and anion-exchange resins can be used to remove virtually all the ionic impurities present in the feedwater, a process known as deionization.

Water deionizers purification process results in water of exceptionally high quality

Deionization :

For many laboratory and industrial applications, high-purity water which is essentially free from ionic contaminants is required.

Water of this quality can be produced by deionization.

The two most common types of deionization are :

• Two-bed deionization

• Mixed-bed deionization

Two-bed deionization :

The two-bed deionizer consists of two vessels - one containing a cation-exchange resin in the hydrogen (H+) form and the other containing an anion resin in the hydroxyl (OH-) form.

Water flows through the cation column, whereupon all the cations are exchanged for hydrogen ions.

To keep the water electrically balanced, for every monovalent cation, e.g. Na+, one hydrogen ion is exchanged and for every divalent cation, e.g. Ca2+, or Mg2+, two hydrogen ions are exchanged.

The same principle applies when considering anion-exchange. The decationised water then flows through the anion column.

This time, all the negatively charged ions are exchanged for hydroxide ions which then combine with the hydrogen ions to form water (H2O).

Mixed-bed deionization :

In mixed-bed deionizers the cation-exchange and anion-exchange resins are intimately mixed and contained in a single pressure vessel.

The thorough mixture of cation-exchangers and anion-exchangers in a single column makes a mixed-bed deionizer equivalent to a lengthy series of two-bed plants.

As a result, the water quality obtained from a mixed-bed deionizer is appreciably higher than that produced by a two-bed plant.

Although more efficient in purifying the incoming feedwater, mixed-bed plants are more sensitive to impurities in the water supply and involve a more complicated regeneration process.

Mixed-bed deionizers are normally used to ‘polish' the water to higher levels of purity after it has been initially treated by either a two-bed deionizer or a reverse osmosis unit.

Electrodeionization EDI :

Electrodeionization Systems remove ions from aqueous streams, typically in conjunction with reverse osmosis (RO) and other purification devices.

Our high-quality deionization modules continually produce ultrapure water up to 18.2MW/cm. EDI may be run continuously or intermittently

Advantages :

• Variety of cost effective standard models.

• Improved aesthetics and rugged design.

• User friendly, low maintenance and easy to install.

• Simpler distribution and collection systems.

• Quick availability.

• Pre dispatch assembly check.

• The multiport valves are top mounted as well as side mounted with the necessary high pressure rating PVC piping.

• Single valve operation as compared to the six valves in conventional filters

• Each operating step is clearly marked on the valve, thereby eliminating chances of error in the operating sequence.

• Single valve assembly, with its simplified frontal Piping, simpler distribution collecting systems is Very easy to install.

• Rust free

• Less power consumption

• Durable

• Economical

• High shelf life

Major Applications :

• Boilers feed water, Textiles, Pharmaceuticals, Chemicals, Breweries, Swimming pools, Potable Water, Hospitals, Automobile, and

Battery, Fertilizers.

• Ion Exchange Plants

- Softener

- Industrial DM Plant

- Two Stage & Multi Stage DM Plants

- Mix Bed Demineraliser

- De-Gasifiers

- Cation Polisher

- Manual/Automatic Plants

- Pharmaceutical Industry

- Power Plant

- Oil & Gas sector

- Chemical Industries

- Textile Industries

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