Applications of electrodialysis

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Conventional electrodialysis is today commercially by far the most relevant ion-exchange membrane separation process. Electrodialysis was first developed for the desalination of brackish water to produce potable water. In this application, electrodialysis is replaced today to some extent by other membrane processes such as reverse osmosis and nanofiltration. Nevertheless, water desalination is still the most important large-scale application of electrodialysis. But other applications of electrodialysis in the food, the pharmaceutical and chemical industry as well as in wastewater treatment are gaining increasing importance. Another large-scale application of electro-dialysis is the preconcentration of seawater for the production of table salt. Some of the more important large-scale industrial applications of conventional electrodialysis and the stack and process design used in this application as well as the major limitations are listed in Table 1.

5.1.1 Brackish water desalination by electrodialysis

The production of potable water from brackish water is presently the largest single application of electrodialysis. In brackish water desalination, electro-dialysis competes directly with reverse osmosis. However, in a certain range of feedwater salt composition, electrodialysis has a clear economic advantage over other desalination processes. Electrodialysis is mainly used in small- to medium-sized plants with capacities of less than a few 100 m3 per day to more than 20,000 m3 per day with a brackish water salinity of 10005000 mgL-1 total dissolved solids. Since in electrodialysis both the energy consumption and the required membrane area increase with increasing feedwater concentration, reverse osmosis is considered to have an economic advantage for the desalination of water with total dissolved salts in excess of 10,000 mgL-1. The advantages of electrodialysis compared to reverse osmosis are the following: high water recovery rates, long useful life of membranes, operation at elevated temperatures up to 50 °C, and less membrane fouling or scaling due to process reversal. The disadvantage of electrodialysis compared to reverse osmosis is that neutral toxic components such as viruses or bacteria are not removed from a feed stream. Thus, the

Table 1 Industrial applications of conventional electrodialysis

Industrial applications

Stack and process design

Status of application

Limitations

Key problems

Brackish water desalination

Sheet flow, tortuous path stack, reverse polarity

Commercial

Concentration of feed and costs

Scaling, costs

Boiler feedwater production

Sheet flow, tortuous path stack, reverse polarity

Commercial

Product water quality and costs

Costs

Waste and process water treatment

Sheet flow stack, unidirectional

Commercial

Membrane properties

Membrane fouling

Demineralization of food products

Sheet flow or tortuous path stack, unidirectional

Commercial pilot phase

Membrane selectivity and costs

Membrane fouling, product loss

Table salt production

Sheet flow stack, unidirectional

Commercial

Costs

Membrane fouling

Concentration of reverse osmosis brine

Sheet flow stack, unidirectional

Pilot phase

Costs

Waste disposal

Lactic Acid Electrodialysis
Figure 19 Industrial-size electrodialysis reversal desalination plant. Photograph, courtesy of Ionics Incorporated.

product water may require a posttreatment procedure when used as potable water. In spite of the fact that electrodialysis reversal is significantly less sensitive to membrane fouling than reverse osmosis, some pretreatment of the feedwater is required. Especially, the iron and manganese ion concentrations must be kept below 0.3 and 0.05 mgL-1. A typical large-scale electrodialysis water desalination plant is shown in the photograph of Fig. 19. The plant is built by Ionics Inc., Watertown, MA, USA, using the Aquamite EDR system.

5.1.2 Production of industrial water by electrodialysis

Depending on its application, industrial process water must meet certain quality standards in terms of total dissolved solids and colloidal material. Traditionally, precipitation, filtration, and ion-exchange are used in the production of industrial water. Today, these processes are replaced or increasingly used in combination with microfiltration, reverse osmosis, and electrodialysis. Major applications of electrodialysis in industrial water processing include re-demineralization of boiler feed and desalination of contaminated industrial water for reuse.

5.1.3 Electrodialysis wastewater treatment

During recent years, there has been a substantial increase in water costs. Also, the discharge of certain industrial wastewaters is often problematic and costly because of high salt concentrations or toxic water constituents. This has resulted in a trend to reuse industrial wastewater. A typical application of industrial water reuse is the recycling of cooling tower blow-down water. Electrodialysis is particularly suited for this purpose since high recovery rates up to 95% and high brine concentrations up to 100,000 mg can be achieved, which results not only in lower feedwater costs but also in a reduction in wastewater discharge. Furthermore, ion-exchange membranes can be operated at temperatures up to 50 °C, which is in the range of most cooling systems.

The recovery of water is not always the main objective in the treatment of industrial effluents. Very often toxic or valuable components such as heavy metal ions are removed to avoid pollution of the environment and save disposal chemical costs by recycling valuable materials. A large number of interesting applications for electrodialysis are in the galvanic industry and in metal surface treatment processes. A typical example is the recovery and concentration of nickel from the still rinse water and the recovery of acids such as HCl, HNO3, HF, etc. from spent pickling solutions. There are many more interesting applications of electrodialysis in wastewater treatment. Some of these applications require only relatively small electrodialysis units, as, for example, the treatment of small amounts of effluents from chemical processes containing highly toxic compounds. In other applications, rather large quantities of water must be treated. This is the case in the paper and pulp industry. Here electrodialysis has been successfully used to remove NaCl selectively from the chemical recovery cycle of Kraft Pulp production.

5.1.4 Food processing by electrodialysis

In the food and beverage industry but also in biotechnology and the pharmaceutical industry, electrodialysis has found a number of applications. Some of these applications can be considered as state-of-the-art processes such as the deionization of whey. Other applications such as the removal of salts from protein and sugar solutions or salts and organic acids such as lactic acid and certain amino acids from fermentation have been studied on a pilot plant scale. However, only a few applications have been commercialized so far. Many applications of conventional ion-exchange resins in the food industry can be replaced by electrodialysis providing better economics, simpler operation, and less waste material, which must be deposited.

5.1.5 Electrodialysis in hybrid processes and the preconcentration of salts

The use ofelectrodialysis in desalination is most economic in a certain range of concentration as far as the feed solution, the diluate, and the concentrate is concerned. To utilize the most effective range of operation of electro-dialysis in desalination, a combination with other separation processes has proved to be very effective. Of very special interest, however, is the concentration of NaCl from seawater prior to evaporation for the production of table salt in Japan, which has no native salt deposit. Thus, for the production of table salt and as raw material for the chlorine alkaline production, the use of electrodialysis as a preconcentration step prior to evaporation of seawater results in substantial energy savings. Large plants with a capacity of20,000 to more than 200,000 tons of salt per year are now in operation in Japan. Preconcentration of diluate solutions by electro-dialysis is also of interest for the production of certain salts such as KBr or KI from raw water sources and for the treatment of certain industrial effluents to recover valuable or toxic wastewater constituents. In the production of ultrapure water, the combination of electrodialysis with ion-exchange is very effective and has finally resulted in the development of the continuous electrodeionization process and to the continuous regeneration of ionexchange resins. Another application, which is presently studied on a laboratory scale, is the integration of ion-exchange membranes in the so-called membrane reactors. In many chemical and biochemical reactions, the reaction products or the reaction by-products inhibit the reaction when a certain concentration is exceeded. This often limits the achievable product concentration and requires additional separation and concentration steps. A continuous removal of the reaction inhibiting components often makes a continuous more economic production possible.

A substantial effort has also been concentrated on reversing electro-dialysis to recover energy from mixing a concentrated salt solution such as seawater with river or surface water in an electrodialysis stack. Although the process is technically feasible, it is presently not economic.

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Responses

  • amalda
    What is electrodialysis used for?
    2 years ago
  • roosa himanen
    What is use of electro process in effluent ro?
    1 year ago
  • oliwier
    What is the process of electrodialysis and reverse osmosis?
    1 year ago
  • vito
    What is the processes of electrodialysis and reverse osmosis?
    1 year ago
  • JOHN
    What ions are left over post reverse osmosis processing?
    2 months ago
  • Katelyn
    What are the applications of electro dialysis?
    19 days ago

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