1. Lime – Soda Softeners
2. Hot Lime –Hot Phosphate
3. Sodium Zeolite Softening
4. Hydrogen Zeolite Softening ( Hydrogen Cation Exchange)
5. Silica
6. Demineralization
7. Regeneration of Demineralization Exchangers
8. Mixed Bed Demineralizers
9. Condensate Polishing
10. Electrodialysis
11. Electrodeionization (EDI)
12. Reverse Osmosis (RO)
1. Lime
– Soda Softeners:
What is hard water?
“Hardness” in water is primarily the result of concentrations of calcium and magnesium.
Thus, some water utilities remove calcium and magnesium to soften the water and improve its quality for domestic use. Other ions that produce hardness include iron, manganese, strontium, barium, zinc, and aluminum, but these ions are generally not present in signifi-cant quantities. Therefore, total hardness is usually defined as the sum of magnesium and calcium hardness in milligrams per liter (mg/L),as calcium carbonate
(CaCO3). Total hardness can also be differentiated into carbonate and non carbonate hardness. Carbonate hardness is the portion of total hardness present in the form of bicarbonate salts [Ca(HCO3)2 and Mg(HCO3)2] and carbonate compounds
(CaCO3and MgCO3).
Non carbonate hardness is the portion of calcium and magnesium present as non carbonate salts, such as calcium sulfate (CaSO4), calcium chloride (CaCl2), magnesium sulfate (MgSO4), and magnesium chloride (MgCl). The sum of carbonate and noncarbonate hardness equals total hardness.

Hardness
Soft
Moderate
Hard
Very hard mg/L as CaCO3
0 to 75
75 to 150
150 to 300
Above 300
For most applications, total hardness of 120mg/L or less and magnesium hardness of
40mg/L or less appear to be acceptable design criteria for softening facilities.
Lime-soda softening involves the removal of scale-forming dissolved solids such as calcium and magnesium salts from water. Calcium hydroxide (lime) and sodium carbonate (soda) cause scale-forming materials to precipitate. A coagulant aid settling out precipitated material.
Reaction of lime and soda in softening process
Calcium hydroxide (hydrated lime) reacts with soluble calcium and magnesium carbonates to form insoluble precipitates. They form a sludge that can be removed by settling and filtration. Lime, therefore, can be used to reduce hardness present in the bicarbonate form (temporary hardness) as well as decrease the amount of bicarbonate alkalinity in water. Lime reacts with magnesium sulfate and chloride and precipitates magnesium hydroxide, but in this process soluble calcium sulfate and chlorides are formed. Lime is not effective in removing calcium sulfates and chlorides (permanent hardness). Soda ash is used primarily to reduce non-bicarbonate hardness (permanent hardness). The calcium carbonate formed by the reaction precipitates as sludge and can be filtered out. The resulting sodium sulfate and chloride are highly soluble and non-scale forming.
Methods of lime-soda softening
The older method of intermittent softening consists of mixing the chemicals with the water in a tank, allowing time for reaction and forming of sludge, and filtering and drawing off the clear water. The modern method of continuous lime-soda softening involves use of compartmented tanks with provision for (a) proportioning chemicals continuously to the incoming water, (b) retention time for chemical reactions and sludge formation, and © continuous draw-off of softened water. Lime-soda softening is classified as hot or cold, depending on the temperature of the water. Hot process softeners increase the rate of chemical reactions, increase silica reduction, and produce over-all better quality water.
Coagulants used in lime-soda process

In the initial clarification process, coagulants are used to agglomerate fine suspended particles, which can then be filtered out. Likewise, in the softening process, coagulants speed up settling of sludge by 25-50%. Sodium aluminate used as a coagulant in limesoda softening being alkaline, also contributes to the softening reactions, particularly in reducing magnesium. Proper uses of coagulants help remove silica in the softening process. Silica tends to be adsorbed on the floc produced by coagulation of sludge.
Disadvantages of lime-soda softening
The main disadvantage is that while hardness is reduced it is not completely removed.
Variations in raw water composition and flow rate also make control of this method difficult since it involves adjusting the amounts of lime and soda ash being fed.
Advantages of lime-soda softening
The main advantage is that in reducing hardness, alkalinity, total dissolved solids, and silica are also reduced. Prior clarification of the water is not usually necessary with the lime-soda process. Another advantage is that with continuous hot process softening some removal of oxygen and carbon dioxide can be achieved. Fuel savings can be realized with hot process softening because of solids reduction. This reduction decreases the conductivity of the feedwater, thereby decreasing blowdown and conserving heat.
2. Hot Lime –Hot Phosphate:
Using a hot phosphate softener in conjunction with a hot lime softener, water of near zero hardness is produced. The chemicals used in the hot phosphate process are sodium hydroxide, NaOH, and trisodium phosphate Na3P04.
Calcium hardness is precipitated as tricalcium phosphate that is even more insoluble than the calcium carbonate precipitated in the lime-soda process.
Magnesium hardness is precipitated as magnesium hydroxide.
Hot process softening is usually carried out under pressure at temperatures of
227240°F (108-116°C). At the operating temperature, hot process softening reactions go essentially to completion. This treatment method involves the same reactions described above, except that raw water CO
2
is vented and does not participate in the lime reaction. The use of lime and soda ash permits hardness reduction down to 0.5 gr/gal, or about 8 ppm, as calcium carbonate.
Magnesium is reduced to 2-5 ppm because of the lower solubility of magnesium hydroxide at the elevated temperatures.

6. Demineralization:
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.
Demineralisation is the Process of removing the mineral salts from water by ionexchange. Impurities that remains dissolved in water dissociate to form positive and negative charged particles known as ions. These impurities or compounds are called electrolytes. Generally, all natural water has electrolytes in varying concentrations. An ion-exchange vessel holds ion-exchange resin of the required type through which water is allowed to pass. The selective ions in the water are exchanged with ions or radicals loosely held by the resin. In this way, the water is passed through several vessels or a mixed bed vessel so that both positive and negative ions are removed and water is demineralised.
In water treatment, demineralization refers to the removal of all mineral salts using ion exchangers. A demineralization system is an arrangement of cation and anion exchange beds. They are usually in series as in Fig. , with water passing through the cation and then the anion. Upon leaving the system, the water has had all cations replaced with hydrogen ions (H + ) and all anions replaced with hydroxyl (01-ions. The effluent water is virtually free of dissolved minerals.
There are many demineralization arrangements in use. Normally composed of multiple exchangers, they are designed according to:
• The properties of the raw water
• The desired properties of the treated water
• Equipment costs
• Regeneration costs
• Ease of operation and control

Regeneration techniques for DM plants
Parallel flow (co-current) Regeneration Technique The exchange resin is loaded under down -stream conditions only.
Fixed bed (counter-current) Counter Flow Process The Exchange resin is loaded under down-stream and regenerated under up-stream conditions with dynamic back pressure created with water or air.
Floating Bed / counter flow process The regeneration is performed under downstream flow, without hydraulic problems. The resin is loaded up-stream flow, so that the bed is kept floating.
Automatic Regeneration The regeneration procedure (back-wash, regen-eration & rinse) is automatically controlled, implying reduced operating costs. To control the regeneration cycle either of the following systems can be used: a) Relay based interlock and operation sequence logic.
b) PLC based interlock and operation sequence logic.
11. Electrodeionization (EDI)
Electrodeionization is a water treatment technology that utilizes an electrode to ionize water molecules and separate dissolved ions (impurities) from water. It differs from other water purification technologies in that it is done without the use of chemical treatments and is usually a tertiary treatment to reverse osmosis (RO). There are also
EDI units that use a small bed with ion-exchange resin to enhance the deionization further, this is often referred to as Continuous electrodeionization (CEDI) since the

electric current regenerates the resin mass continuously. CEDI technique can achieve very high purity, with conductivity being below 0.1uS/cm.
An electrode in an electrochemical cell is referred to as either an anode or a cathode, terms that were coined by Michael Faraday. The anode is defined as the electrode at which electrons leave the cell and oxidation occurs, and the cathode as the electrode at which electrons enter the cell and reduction occurs. Each electrode may become either the anode or the cathode depending on the voltage applied to the cell. A bipolar electrode is an electrode that functions as the anode of one cell and the cathode of another cell.
Each cell consists of an electrode and an electrolyte with ions that undergo either oxidation or reduction. An electrolyte is a substance containing free ions that behaves as an electrically conductive medium. Because they generally consist of ions in solution, electrolytes are also known as ionic solutions, but molten electrolytes and solid electrolytes are also possible. They are sometimes referred to in abbreviated jargon as lytes.
Water is passed between an anode (positive electrode) and a cathode (negative electrode). Ion-selective membranes allow the positive ions to separate from the water toward the negative electrode and the negative ions toward the positive electrode. High purity deionized water results.
When fed with low TDS feed (e.g., feed purified by RO), the product can reach very high purity levels (e.g., 18 Megohms/cm[1]). The ion exchange resins act to retain the ions, allowing these to be transported across the ion exchange membranes. The main usage of EDI technology such as that supplied by Ionpure and SnowPure are in electronics, pharmaceutical, and power generation applications.
One important aspect in the water treatment application is that water to the EDI needs to be free from CO2, since this in its dissolved form will put unnecessary strain on the
EDI unit and will reduce performance.

How it works:
An EDI stack has the basic structure of a deionization chamber. The chamber contains a ion exchange resin, packed between a cationic exchange membrane and a anionic exchange membrane. Only the ions can pass through the membrane, the water is blocked.
When flow enters the resin filled diluiting compartment, several processes are set in motion. Strong ions are scavenged out of the feed stream by the mixed bed resins. Under the influence of the strong direct current field applied across the stack of components, charged ions are pulled off the resin and drawn towards the respective, oppositely-charged electrodes. In this way these charged strong-ion species are continuously removed and transferred in to the adiacent concentrating compartments.
As the ions go towards the membrane, they can pass through the concentration chamber but they cannot reach the electrode. They are blocked by the contiguous membrane, that contains a resin with the same charge.
As the strong ions are removed from the process stream, the conductivity of the stream becomes quite low. The strong, applied electrical potential splits water at the surface of the resin beads, producing hydrogen and hydroxyl ions. These act as continuous regenerating agents of the ionexchange resin. These regenerated resins allow ionization of neutral or weakly-ionized aqueous species such as carbon dioxide or silica. Ionization is followed by removal through the direct current and the ion exchange membranes.
The ionization reactions occurring in the resin in hydrogen or hydroxide forms for the removal of weakly ionized compounds are listed below:
CO
2
HCO
SiO
NH
3
2
+ OH ==> HCO
3
3
-
H
3
BO
3
+ OH ==> CO
3
+ OH ==> HSiO
+ H + ==> NH
4
-
3
2-
-
+ OH ==> B(OH)
4
+
-
Advantages
As a substitute for the more traditional ion-exchange process, EDI brings advances in both energy and operating expenses to the high purity water treatment train. By eliminating the periodic regeneration requirement of ion exchange resin, environmental benefits are also realized by avoiding the handling and processing of acid and caustic chemicals brought to the site.

Some of the advantages of the EDI as opposed to the conventional systems of ionic interchange are:

Simple and continuous operation

Chemicals for regeneration completely eliminated

Cost effective operation and maintenance

Low power consumption

Non pollution, safety and reliability

It requires very few automatic valves or complex control sequences that need supervision by an operator

It requires little space

It produces high pure water in a constant flow

It provides complete removal of dissolved inorganic particles

In combination with reverse osmosis pre-treatment, it removes more than 99.9% of ions from the water
Disadvantages

EDI cannot be used for water having hardness higher than 1, since the calcium carbonate would create a scab in the camera of the concentrated one, limiting the operation

It requires purification pretreatment

Carbon Dioxide will freely pass through an RO membrane, dissociating and raising the conductivity of water. Any ionic species formed from the carbon dioxide gas will lower the outlet resistivity of the water produced by EDI. The management of CO2 in water is typically handled in one or two ways: the pH of the water can be adjusted to allow the RO membrane to rejuect the ionic species or the carbon dioxied can be removed from the water using a strip gas.
12. Reverse Osmosis (RO)
Osmosis is based upon the fundamental pursuit for balance. Two fluids containing different concentrations of dissolved solids that come in contact with each other will mix until the concentration is uniform. When these two fluids are separated by a semi permeable membrane (which lets the fluid flow through, while dissolved solids stay behind), the fluid containing the lower concentration will move through the membrane into the fluid containing the higher concentration of dissolved solids (Binnie e.a., 2002).

After a while the water level will be higher on one side of the membrane. The difference in height is called the osmotic pressure.
What is Reverse Osmosis?
By applying a pressure that exceeds the osmotic pressure, the reverse effect occurs.
Fluids are pressed back through the membrane, while dissolved solids stay behind.
To purify water by Reverse Osmosis membrane, the natural osmosis effect must be reversed. In order to force the water of the brine stream (high salt concentration) to flow towards the fresh stream (low salt concentration), the water must be pressurized at an operating pressure greater than the osmotic pressure. As a result, the brine side will get more concentrated.
The operating pressure of seawater is around 60 bar.
1. Water flows from a column with a low dissolved solids content to a column with a high dissolved solids content
2. Osmotic pressure is the pressure that is used to stop the water from flowing through the membrane, in order to create balance
3. By pursuing pressure that exceeds the osmotic pressure, the water flow will be reversed; water flows from the column with a high dissolved solids content to the column with a low dissolved solids content