Lime

Chemical Identity:

• Chemical Name: Calcium hydroxide
• Chemical Formula: Ca(OH)₂
• Common Names: Hydrated lime, slaked lime

Role of Lime in Effluent Treatment Plants

pH Adjustment:

• Neutralization: Lime is used to adjust the pH of acidic wastewater. Adding lime raises the pH, neutralizing acidic effluents, which is crucial for subsequent treatment processes.
• Optimal pH for Treatment: Maintaining an optimal pH range (typically between 6.5 and 8.5) is essential for effective chemical coagulation, flocculation, and biological treatment processes.

Chemical Precipitation:

• Heavy Metal Removal: Lime facilitates the precipitation of heavy metals from wastewater. It reacts with metal ions to form insoluble hydroxides (e.g., Fe(OH)₃, Al(OH)₃), which can be removed through sedimentation or filtration.
• Phosphate Removal: Lime can precipitate phosphates as calcium phosphate, reducing the phosphorus load in the effluent.

Coagulation and Flocculation:

• Coagulant Aid: Lime enhances the aggregation of suspended particles into larger flocs, which can be removed more easily through sedimentation or flotation.
• Improving Floc Strength: The addition of lime increases the strength and settleability of flocs formed during the coagulation process.

Odor Control:

• Reducing Odors: Lime helps reduce odors associated with wastewater by neutralizing acidic compounds and inhibiting the activity of odor-producing bacteria.

Application of Lime in ETP

Dosage Calculation:

• Based on Wastewater Characteristics: The dosage of lime is determined by analyzing the characteristics of the influent wastewater, including its acidity, metal concentration, and phosphate levels.
• Jar Testing: Conducting jar tests in a laboratory setting can help determine the optimal dosage for effective pH adjustment and precipitation.

Feeding Method:

• Slurry Addition: Lime is typically added to wastewater in the form of a slurry (a suspension of lime in water) to ensure even distribution and efficient reaction.
• Point of Addition: Lime can be added at various stages of the treatment process, including the equalization tank, reaction tank, or directly into the wastewater stream.

Mixing and Reaction Time:

• Thorough Mixing: Proper mixing is essential to ensure a complete reaction between lime and the constituents of the wastewater.
• Adequate Reaction Time: Sufficient reaction time must be provided to allow for the formation and settling of precipitates.

Monitoring and Control:

• Continuous Monitoring: Regularly monitor pH levels, metal concentrations, and other relevant parameters to ensure effective treatment.
• Adjust Dosage: Adjust the lime dosage based on monitoring results to maintain optimal treatment conditions.

Handling and Safety

• Storage: Store lime in a cool, dry place, protected from moisture to prevent it from forming lumps or becoming inactive.
• Handling: Use personal protective equipment (gloves, masks, goggles) to avoid irritation from dust and contact with skin or eyes. Lime dust can be hazardous when inhaled.
• Safety: While lime is not toxic, it is caustic and can cause burns upon contact with skin or eyes. Handle with care and follow safety guidelines to prevent accidents.