Lime Dosing System for Water Treatment: Design, Components and Operation

What is a Lime Dosing System?

A lime dosing system is a chemical preparation and injection package used to add lime (calcium hydroxide — Ca(OH)₂) into water or wastewater streams for pH correction, softening, coagulation, phosphate removal or heavy metal precipitation. Lime is one of the most widely used chemicals in water treatment due to its low cost and effectiveness, but its abrasive and settling nature makes it one of the most technically demanding chemicals to handle and dose accurately.

A correctly designed lime dosing system ensures consistent slurry preparation, stable dosing and minimal maintenance. A poorly designed system results in settled lime, blocked pipelines, pump failures and unreliable pH control.

Where Lime Dosing is Used

• pH correction and neutralisation: Raising pH of acidic effluents in industrial ETP plants before discharge
• Water softening: Removing hardness from raw water in lime-soda softening plants
• Phosphate removal: Precipitation of phosphate from municipal and industrial wastewater
• Heavy metal removal: Precipitation of lead, zinc, copper and chromium as insoluble hydroxides
• Coagulation assistance: pH adjustment to optimise coagulant performance in surface water treatment
• Sludge conditioning: Lime addition to sludge for stabilisation and dewatering improvement

Key Components of a Lime Dosing System

1. Lime Storage Hopper or Bulk Silo

Dry hydrated lime is stored in a hopper or silo above the slurry preparation tank. For small systems, 200–500 kg bag-filling hoppers are used. Larger plants use pneumatically filled silos with capacities of 5–50 tonnes. Lime is highly hygroscopic and must be stored in sealed, dry conditions to prevent bridging and caking.

2. Slurry Preparation Tank with Agitator

Lime powder is mixed with water in a preparation tank to form a lime slurry (typically 5–20% concentration by weight). An agitator is essential to keep lime in suspension — without continuous mixing, lime settles rapidly forming hard deposits that block pipelines and damage pumps. The agitator must be selected for the abrasive nature of lime, suitable impeller type for suspension, corrosion-resistant materials in SS316 or rubber-lined construction, and sufficient power to maintain suspension at minimum speed. Premix Technologies manufactures agitators for lime slurry service with appropriate impeller design, shaft sealing and material of construction.

3. Slurry Recirculation Pump

A recirculation pump continuously circulates lime slurry between the preparation tank and a return line to prevent settling in pipelines during periods of low demand. Without recirculation, lime settles and solidifies in suction and discharge lines.

4. Dosing Pump

The dosing pump meters the correct volume of lime slurry into the process stream. For lime service, the pump must handle abrasive solids, high specific gravity and tendency to cake in valves and diaphragms. Mechanically actuated diaphragm pumps with large-bore valves and EPDM or reinforced PTFE diaphragms are typically used. View Premix dosing pumps suitable for lime slurry service.

5. Injection Point and Static Mixer

The injection quill introduces lime slurry into the process pipe. For pH correction, the injection point should be followed by a static mixer or turbulent zone to ensure rapid mixing before pH measurement. A poorly positioned injection point results in poor distribution and inaccurate pH control.

Design Considerations

Lime slurry pipelines should be designed for a minimum velocity of 1.5 m/s to prevent settling, sloped to allow drainage, fitted with flush connections at low points, and short in length to reduce settling risk. Typical slurry concentrations of 8–15% balance pumpability against tank volume requirements.

Dosing is typically controlled by pH feedback from an online analyser downstream of the injection point. A PID controller adjusts the dosing pump output to maintain target pH.

Common Problems and How to Avoid Them

• Settled lime in tanks and pipes: Ensure continuous agitation and minimum recirculation velocity. Flush during shutdowns.
• Blocked dosing pump valves: Use large-bore ball check valves rated for slurry service. Flush pump and valves with clean water during shutdowns.
• Inconsistent pH control: Check injection point location, verify pH sensor calibration and ensure consistent slurry concentration.
• Rapid pump wear: Use abrasion-resistant diaphragm and valve materials. Avoid oversized pumps running at low stroke.

Conclusion

Premix Technologies manufactures complete lime dosing systems including slurry preparation tanks, agitators, dosing pumps, recirculation loops, injection quills, instrumentation and control panels. Each system is engineered to the specific lime type, slurry concentration, dosing rate and process conditions of the application. Contact us at sales@premixtechnologies.com or view our chemical dosing systems range.