EDI/ EDR

Gemwater designs both EDI and EDR systems for removal of Ionic contaminants in water.   
 
EDI  
Electrodeionization (EDI) is a continuous, self-regenerating process for producing high purity water. EDI is the combination of two ion removal technologies, electrodialysis (ED) and ion exchange resins, to provide highly efficient ion removal on a continuous basis. The essential components of an EDI system are:
ion exchange membranes;
ion exchange resin;
electricity.

Gemwater generally uses EDI  when high quality product water is required, and feed water is available which has already been treated to remove suspended and dissolved solids.

EDI is an advanced water treatment process based on electrodialysis (ED). Electrodialysis works based on the principles governing the behavior of an ionic solution when it is subject to direct current potential (DC).  

Utilizing concentrate recycle, typical recoveries are 85-95%.
An EDI system is created from an ED system by addition of mixed bed ion exchange resins into the feed (demineralizing/product) compartments. The transfer of ions from the feed water thus becomes a two-stage process, ions are first captured on the resin, and then migrate through the resin and the membranes to the concentrating channel.

Periodic cleaning of the EDI system is usually required to remove scale, and sometimes to remove organic materials or other foulants. This cleaning is achieved by circulating chemical solutions through the stack(s) using the Clean In Place (CIP) system.

EDI BENEFITS
The EDI system is the state of art of demineralization. The process is based on membrane technology and introduces a significant number of benefits versus the conventional resins plant.

Chemicals
The absence of chemicals  reduces operational costs and reduces the problems related to the handling and personnel safety.   

Area and civil works
With respect to conventional resin system the area required for the plant is reduced. At the same time the absence of wastes to be handled permits to save the area related to the neutralization basin.

Flexibility
The EDI is continuously regenerated by means of electrical current, this solution allows an higher flexibility of the plant that can run on the on/off basis without dead time needed to reach the correct production requirement; the plant needs only few seconds to reach the quality of the water.

Spares
The EDI system has a small amount of automatic valves and rotating equipment, that means a significant increasing of reliability of the plant and a reduced number of spares.

Improvement
The EDI is made on the basis of modular construction, this choice allows to increase the production in a fast and easy way with a reduced low price impact.

Operational costs
According to the previous item,   the operational costs of EDI are extremely reduced comparing with the conventional resins system; for this reason the increased investment will be recovered in few years.

High Product Water Quality
EDI has high efficiency in the weak ions removal as carbon dioxide, silica, organic acid salts, boron, achieving ultrapure water quality.

Process guarantees
The design of the plant will ensure:
proper and safe operation;
easiness of maintenance;
safe operation during normal and emergency conditions;
proper monitoring of the status of the plant during normal and emergency conditions.


EDR (ELECTRODIALYSIS REVERSAL)
ED process applies an electric potential which moves dissolved salt ions (e.g. (+) ions are attracted to the (-) electrode) through an electrodialysis stack consisting of alternating layers of cationic and anionic ion exchange flat sheet membranes, creating alternate channels of desalted product water and concentrated reject water.
A modification of the ED process, named EDR (electrodialysis reversal), periodically reverses the polarity of the applied electrical potential on the stack, to minimize the effects of inorganic scaling and fouling by converting product channels into concentrate
channels. EDR is most attractive for the desalination of brackish water and in cases where TOC removal and microbial control are not important. Below, a working diagram of ED process.

Electrodialysis induces contaminant ions to migrate through a membrane, removing them from the water. In an electrodialysis unit, contaminated water is pumped into narrow compartments separated by alternating cation-exchange and anion-exchange membranes, selectively permeable to positive and negative ions.  
It is important to note that RO/NF energy cost is based on the volume of water treated; whereas for ED/EDR processes, it is proportional to the salts removed. Therefore, these processes are usually only suitable for brackish feed waters with a salinity of up to 12,000 mg/L TDS. With higher salinities the ED/EDR process becomes more costly than other desalination processes. As a rule of thumb, approximately 1 kWh is required to extract 1kg additional salt using ED/EDR. Bacteria, non-ionic substances and residual turbidity are not affected by this process and can therefore remain in the product water and require further treatment before certain water quality standards are met.



PRETREATMENT FOR RO AND EDR
The choice of a particular pretreatment process is based on a number of factors such as feed water quality characteristics, space availability, and RO/NF/ED membrane requirements. Typically, pH adjustments (between 5 and 6) and/or antiscalant additions are made to prevent scale formations on the membrane surface. As mentioned before, feedwater recovery is another key parameter that can be optimized to limit the scale formation. To control and minimize colloidal, organic, and biofouling either conventional pretreatment (e.g. coagulation/ flocculation/ sedimentation/ filtration), or modern pretreatment methods (membrane filtration using MF/UF) are employed prior to the desalination unit.