Water Softening

When water is treated to remove turbidity, some of the hardness is also removed.Sodium aluminate will not only act as a coagulant, it will also add alkalinity to the water to reduce some of the non-carbonate hardness:

Na 2 Al 2 O 4 + 4H 2 O —— > 2Al(OH) 3  ̄+ 2NaOH

MgSO 4 + 2NaOH —— > Na 2 SO 4 + Mg(OH) 2  ̄

When sodium aluminate is used as a coagulant in turbidity treatment, the amount of lime required to reach the softening pH in the cold lime method of softening can be reduced by about 10%.

i. Cold Lime Method

Cold lime softening is a proven process and is more than 150 years old. In this process, calcium oxide (CaO) is added to hard water to form calcium hydroxide.

Calcium hydroxide reacts with magnesium and calcium bicarbonates and free CO 2 to form insoluble calcium carbonate and magnesium hydroxide.

CaO + H 2 O —— > Ca(OH 2 )

CO 2 + Ca(OH 2 ) —— > Ca CO 3  ̄ + H 2 O

Ca(HCO 3 ) 2 + Ca(OH 2 ) —— > 2Ca CO 3  ̄ + 2H 2 O

Mg(HCO 3 ) 2 + Ca(OH 2 ) —— > Mg(OH) 2  ̄ + 2Ca CO 3  ̄ + 2H 2 O

Magnesium hydroxide has good flocculating properties and helps to precipitate calcium carbonate particles. Excess lime is converted to calcium carbonate by adding carbon dioxide to the water as it leaves the primary settling tanks. At the same time, magnesium hydroxide will be converted to magnesium carbonate. After this carbonation step, sodium carbonate is added to react with noncarbonate salts to form calcium carbonate, which will precipitate. The other compounds are soluble and will stay in the water:

Na 2 CO 3 + Ca SO 4 —— > Na 2 SO 4 + Ca CO 3  ̄

Na 2 CO 3 + Ca Cl2 —— > 2Na Cl + Ca CO 3  ̄

After this treatment, water hardness will be about 80 mg/l, placing it in the moderately hard category. Sand and gravel filters are used to remove precipitated salts from the water.

ii. Ion Exchange

Many ion exchange processes are used to treat water. Water softening is one of the common treatments. In the system, calcium and magnesium ions in the water are exchanged with sodium ions, while the water flows through a resin bed. After a time, the sodium ions will become depleted and the resin will require regeneration, a process where calcium and magnesium ions are removed and sodium ions are replaced. This is done by pumping brine solution through the resin. This process will not reduce turbidity or remove other chemicals.

The exchange reactions may be written as follows:

And the regeneration reaction is as follows:

Where Z = zeolite radical.

The ion-exchange process can be used with cations and anions, both organic and inorganic. However, most of the applications of ion exchange involves inorganic species which often require the use of extremely highly concentrated regenerant and/or the use of organic solvents to remove organic species. Different varieties of cation and anion exchangers are available for exchange with a particular ion of different stabilities. Generally, ions with higher charge will form more stable salts with the exchanger than those with lower charge; hence, polyvalent species can more frequently be removed from a solution than monovalent ones.

Base exchange resins used in the food industry utilize an ion exchange bed of natural or synthetic zeolites that are hydrous silicate or styrene-based resins. In sodium exchange resins, sodium from the resin is exchanged for calcium and magnesium in the water. Some exchange of iron, copper, manganese, and aluminum will also occur.

Many modern resins are based on a sulfonated styrene divinylbenzene structure.There are resins for almost every kind of application. Selection is based on water analysis, operating temperature, and required outflow water quality. Resins are costly, and durability under usage conditions is one of the important selection criteria.The sulfonic groups in styrene-based resins are responsible for the cation exchange property. The resins are able to operate over a wide pH range. Water flows through a bed of spherical resin beads. After some use, the resin is regenerated in three steps.

1. The resin is backwashed to remove solids that could have accumulated on the resin bed surface. The beads are suspended in the upward flow that will loosen any packed bead clumps.

2. A saturated salt solution is introduced on top of the resin and allowed to flow downwards. The salt will remove calcium and magnesium ions and replace them with sodium ions. This leaves the resin in the sodium condition.

3. In the final step, the resin is rinsed to remove excess salt and calcium and magnesium chlorides. The softener is ready to return to service.

In plants, more than one softener are usually available so that some will be regenerated while others are operational. Regeneration can take from 35 to 70 min. In most plants, the regeneration of softeners is completely automated. This ensures a continuous supply of soft water at the lowest operating cost.