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Wastewater Treatment by Ion Exchange Method: a Review of Past and Recent Researches

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Wastewater Treatment by Ion Exchange Method: a Review of Past and Recent Researches id1152921 pdfMachine by Broadgun Software - a great PDF writer! - a great PDF creator! - http://www.pdfmachine.com http://www.broadgun.com EEnnvviirroonnISmmSN : 0ee97n4n - 74tt51aall SSccVioilueemen n12 Iccssuee 4 An Indian Journal Critical Review ESAIJ, 12(4), 2016 [143-150] Wastewater treatment by ion exchange method: a review of past and recent researches Nirali Kansara*, Lavleen Bhati, Mansi Narang, R.Vaishnavi Department of Chemical Engineering, BITS Pilani, (INDIA) E-mail: [email protected] ABSTRACT KEYWORDS Ion exchange is an exchange of ions between two electrolytes or between Ion exchange; an electrolyte solution and a complex. Ion exchange can be a process of Electrolyte; purification, separation and decontamination of aqueous or ion contain- Resin; ing solutions. Ion exchange resins are used in wastewater treatment pro- Water treatment. cess plants to interchange one ion with another to fulfill the purpose of demineralization. In this assignment, we look at the working principle, equipment design, advantages and disadvantages, and industrial applica- tions of ion exchange as a water pollution remediation technique. 2016 Trade Science Inc. - INDIA INTRODUCTION that have to be removed. The resin is then regener- ated by backwashing the resin to remove the accu- Ion exchange is an exchange of ions between two mulated solids, flushing removed ions from the resin electrolytes or between an electrolyte solution and with a concentrated solution of replacement resin. a complex. Ion exchange can be a process of purifi- Hence the production of backwash limits the use of cation, separation and decontamination of aqueous ion exchange in wastewater treatment. or ion containing solutions. They are usually ion ex- In this assignment, we look at the working prin- change resins, zeolites, clay or soil humus. They are ciple, equipment design, advantages and disadvan- primarily either cation exchangers that exchange tages, and industrial applications of ion exchange as positively charged ions or anion exchangers that ex- a water pollution remediation technique. change negatively charged ions. Along with absorp- tion and adsorption, ion exchange is a form of sorp- PRINCIPLE tion. Ion exchange systems generally contain ion ex- Ion exchange resins are used in wastewater treat- change resins which are operated on a cyclic basis. ment process plants to interchange one ion with an- Water is passed through the basis till it is saturated, other to fulfill the purpose of demineralization. There in which condition, the water coming out of the resin are 2 types of ion exchange resins, one is the system contains more than desired concentration of the ions of cation resins and the other one is the anion ex- 144 Wastewater treatment by ion exchange met.hod: a review of past and recent researches ESAIJ, 12(4) 2016 Critical Review Figure 1 : An ion exchanger change resins. Its materials can be also broken down be separated into various types depending on its to different groups and systems but it depends on solubility. Obviously for it to be operational, the whether it is weak acid anion, or a strong base cat- constituents must be insoluble in normal circum- ion or it can also be a weak base anion. The type of stances and for that to occur, usually high molecular exchangers is not that important as resins are very weight composite has to be chosen. Bead size is also sensitive of the phenomena of fouling which is another measure that decides the type of resin suit- caused by active presence of the organic matter in able for general use. Essentially a certain granular water which is not treated. Therefore, it is impor- size is wanted so that the molecules that form the tant that a separate section should be used to re- entire structure does not clasp on together too com- move any type of suspended solids, if present. It pressed and there should be a certain null and void should be done before the influent undertakes any volume to avoid massive fluid head loss. The physi- treatment process. Also, soluble organics should also cal characteristics on how the beads are organized be removed if possible so as to put fewer loads on will also disturb the ion exchange bed physical ro- the ion exchange unit. bustness whether it is good enough to endure expan- The working of the whole process can be ex- sion and contraction due to change in temperature plained by a simple example. In a demineralizer, without danger of bursting or failing taking place. water of the influent passing through cation exchange One more thing to take note is that conditional to the resin will lose all cations. These metallic cations procedures used for backwashing and particularly become acids and the ion loss is replaced by same in cases whereby there is an unrestrained high flow corresponding number of Hydrogen ions. These gen- rate caused by mismanagement; this can lead to blow erated acids are then removed via a regenerated an- off of resins coming out from the vessel and causing ion exchange resin which is alkaline. But this time, in capacity loss of the beds. Consequently, typical the anions are replaced by corresponding amount of attrition losses can be predictable to be within 5 to hydroxides in wastewater. The bed capacity is lim- 20% dependent on which type of ion exchange resin ited to a certain fixed amount, so the resin becomes present in the systems. exhausted and thus undergoes the regeneration pro- Ion exchange softening, similarly known as So- cess. Anion resin is regenerated via sodium hydrox- dium Zeolite Softening is a characteristic example ide whereas cation exchange resin is regenerated of how the ion exchange process works to decon- using either sulphuric or hydrochloric acid. taminate the water by eliminating the hardness level Distinct ion exchange bed, other than clustered instigated by existence of calcium and magnesium. together conferring to its functional groups can also This is generally used in steam boilers and indus- Environmental Science An Indian Journal ESAIJ, 12(4) 2016 Nirali Kansara et al. 145 Critical Review trial water treatment presentations. When the bed monest cause of softener failure. gets totally saturated with Ca and Mg ions, it is re- Adsorption of organic matter newed and regenerated mainly by increasing the to- tal of sodium ions. This is done by first doing Presence of organic matter in water supplies is backwashing to minimalize compactness of the bed a very common problem. Untreated water from lakes and to discharge surrounded air pockets. Afterwards, and rivers usually contains dissolved organic mate- it will be monitored by the real regeneration pro- rial derived from decaying vegetation which imparts cess by means of brine solution. When this is ac- a yellow or brown colour to it. These substances complished, rinsing will be voted for to eliminate can become irreversibly adsorbed within the anion beads, reducing their exchange capacity and leading whatever extra brine left in the exchange bed. In cur- to a reduction in treated water quality. The afore- rent designs, all these processes are done using au- mentioned organic matter is removed, prior to tomated programs which can sense the level of satu- demineralisation, by flocculation with alum or fer- ration and tell the operators to act as a result. Gen- ric salts followed by filtration which removes the erally, depending on the influent characteristics, this metal hydroxide floc and the co precipitated organic will define the frequency desired to carry out the compounds. This treatment also removes any fine full regeneration cycle. silt which represents another source of resin foul- ing. Both organic and iron fouled units can be chemi- ADVANTAGES AND DISADVANTAGES cally cleaned on site but complete removal of impu- rities is rare and resin performance usually suffers The advantage of ion exchange as a water after fouling. remediation technique is that it is very cost effec- tive. Very little amount of energy is required and Organic contamination from the resin regeneration of resins is very economical. If resins The resins themselves can be a source of non- are maintained efficiently, they last for many years ionized organic contamination. New commercial before they have to be replaced. However, there is grade resin often contains organics remaining after a long list of limitations attached to this particular manufacture, unlike very old resins, which shed or- technique, which makes it one of the lesser used tech- ganic fragments as the polymer structure opens up nique. very slowly. Although such contamination can be Calcium sulphate fouling ignored for many uses, it is necessary that they are “ removed, by passing the demineralised water through Sulphuric acid is one of the cheapest cation resin an ultra filtration membrane. regenerant and is widely used. Some water supplies contain a high proportion of calcium which on reac- Bacterial contamination tion with sulphuric acid forms calcium sulphate. This Resin beds do not act as filters for the removal fouls the resin and blocks drain pipes with a buildup of bacteria or other micro-organisms. Persistent ac- of scale. Under such circumstances, hydrochloric cumulations of organic matter, if not flushed, serve ” acid is used as a substitute . as a growing bed for bacteria, since they are a source Iron fouling for nutrients. Hence, when sterile water is required it can be obtained by treating the demineralised wa- Bores yielding anaerobic water from under- ter by non-chemical means such as heat, ultraviolet ground supplies contains iron in Fe2+ state in abun- irradiation or very fine filtration. Resins beds can dance. Small amounts are readily removed by so- be decontaminated with disinfectants such as form- dium cycle softeners but care must be taken to pre- aldehyde, but heat or oxidizing disinfectants such as vent contact with air prior to treatment.
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