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The use of limestone for pH adjustment was once widespread. However, its use today as a treatment step must be considered carefully..

Limestone Tank

Limestone is an alkaline agent with the ability to neutralize, or partially neutralize strong acids. The neutralization process occurs when strong acids, in intimate contact with limestone chips, react with Calcium Carbonate (CaCO3, the primary constituent of limestone) to form water, carbon dioxide, and calcium salts. The following depicts the neutralization of hydrochloric acid by limestone.

CaCO3 + 2HCl → CaCl2 + CO2 + H2O

Due to the presence of other calcium products in the limestone chips typically used it is quite possible for limestone to exert alakline influenes on the wastewater stream up to and beyond a pH of 11.0, meaning that an upper pH limit of 9.0 cannot be guaranteed and can easily be violated. 

The pH neutralization process occurs as strong acids react with the calcium carbonate in the limestone through intimate contact with small limestone chips. A high surface area is important as is sufficiently long contact time. The reaction is not instantaneous and requires sufficient time. Additionally the acidic solutions must be in intimate contact with the limestone. This leads to one of the biggest problems with the use of limestone as an effective treatment process, the coating of the available limestone surface area with precipitated debris.

One of the byproducts of the neutralization process is calcium salts. Calcium salts tend to be very insoluble in water. This results in the precipitation of salts that deposit on the limestone chips forming very effective coatings. Once coated with precipitated products, the limestone is rendered useless and must be replaced. Other solids and organic materials that are suspended in the waste stream will often come out as a result of mechanical filtration thereby contributing to the coating of the limestone chips.

CONCLUSION

The chemistry stands, limestone will neutralize strong acids, at least partially. However, in practice limestone cannot be relied upon to neutralize acidic waste streams.The chips are very easily rendered useless from coating by the highly insoluble calcium salts and other products that precipitate out of the effluent stream. Additionally the design of the system cannot inherently guarantee the treatment of the effluent stream. There are no controls that regulate the addition of acidic or alkaline neutralizing agents. The system designer simply hopes that sufficient alkaline products dissolve into solution to achieve the desired treatment.

For these reasons Digital Analysis cannot endorse, encourage, or recommend the use of limestone for the purpose of treatment. Limestone certainly does not hurt the process and it can be argued that it does help. However, if the limestone contact tank is going to displace an active chemical treatment stage or system then we must strongly discourage the use of this method. We will never use, under any circumstances, limestone exclusively. In most states around the nation limestone is not an acceptable treatment method and will not be accepted by the local controlling authority. Limestone is an effective way to line the containment area of a bulk acid storage facility, which safeguards against the accidental discharge of a hazardous chemical. However, as a primary treatment agent it cannot be considered.

Disadvantages of Limestone as a neutralizing agent...

  • Does not treat alkaline (basic) waste streams
  • No controls to regulate or oversee the process
  • No guarantee that the system will treat an acidic stream
  • Limestone chips are easily coated rendering them useless
  • Periodic replacement is necessary; this is a very difficult and potentially expensive job
  • Adds to the suspended solids (TSS) and dissolved solids (TDS) loading

So what is the alternative to limestone pH Adjustment?

Active chemical treatment systems that use strong acids / strong bases (such as sulfuric acid and caustic) are the safe, proven method for treating acidand basic wastes streams. All of the pH Adjustment systems manufactured by Digital Analysis Corp. utilize active chemical treatment which guarantees thecomplete pH Adjustment of any industrial process. Our pHASE, batchTREAT and labTREAT family of pH adjustment systems are designed to handle any flow from any source at anytime.