pH Correction of Cooling Water & Effluent Water: Comparison between Lime and Caustic Soda
Atul Krishna Sahoo and Rajat Palkar
AB-ChemTECH

Author, here in this article, provides a comparative account for pH correction of cooling water and effluent water by using lime and caustic soda, for industrial benefit

Generally industrial cooling water tends to have a pH of more than 7.5 to 8 due to the presence of bi-carbonates or other alkaline compounds in it. As such pH correction is not required for Cooling Water. Effluent water can have a pH less than 7.0, thus pH correction is required here for further treatment.

However, due to condensate water mixing from various sources, in specific industries (as in sugar industry) - the pH value for circulating cooling water revolves around 5.0. Lime treatment is very favored here for pH management by the industry, basically due to lower cost of lime. Therefore, some other industries also find it easy to use lime for same or similar purpose. This is with specific reference to cooling water treatment in sugar industries. Sodium hydroxide is not so favored, because of its price. However, the author here explained that how ultimately sodium hydroxide is more economical & the very suitable treatment option to increase pH, thus making the plant operation simple.

Disadvantages of Lime Treatment:
1. Contrary to general perception, Lime is not suitable for pH treatment as its solubility is very low i.e. approx 1.73 g/lt water.
2. Due to low solubility, the balance Ca(OH)2 settles as sludge in the cooling tower or effluent treatment tanks.
3. With increase in temperature, solubility decreases.
4. Due to this low solubility, the maximum pH level of saturated lime solution goes to 12.4.
5. Ca(OH)2 having higher reactivity replaces the dissolved salts into Casalts. Also, we know that Ca-salts have low solubility.
6. Removing this sludge is very tedious & often requires lot of manpower.
7. Lime further forms carbonate while coming in contact with air (CO2), and which is sparingly soluble in water. These carbonates get deposited on fills of cooling tower & effluent tanks.

Problem with Calcium Hydroxide:
1. Large quantities of dissolved Calcium Hydroxide are needed for pH treatment, if sludge formation is to be avoided.
2. Sludge formation in cooling tower can be difficult to handle or to remove.
3. Sludge settling on splash fills can break them and thus affect the splash fill area for heat transfer.
4. Lime is dosed manually, inadvertently the dosage is far in excess, resulting in sludge formation.
5. Calcium salts form scaling on heat transfer surfaces.

Advantages of Caustic Soda Treatment:
1. It has a very high solubility of 1000 g/lt.
2. All sodium salts are soluble in water, thereby forming no scales.
3. Small amount of NaOH causes a high rise in pH i.e. 0.4g of NaOH crystals in 1 litre of water causes a pH rise from 7 to 12.
4. With increase in temperature, the solubility of NaOH increases.

Results & Discussion: For Comparison, if we take a saturated solution of Lime & 20 g of Caustic Soda each in 1 litre of water to change the pH from 5 to 8 at different flow quantities.



Although Lime is cheaper than Caustic Soda, it is not appropriate for pH treatment of Cooling Water or Effluent water.

Conclusion:
1. Caustic Soda eliminates the problem of scale formation.
2. Problem of Sludge can be avoided using caustic soda for pH correction, which in turn reduces other operational cost.
3. Automatic dosing with Caustic Soda is a simple way for pH correction.