Alum Dosing Pump Calculator
What is Alum Dosing?
Alum dosing is a critical process in water treatment where Aluminum Sulfate (Alum) is added to water to destabilize colloidal particles. This process, known as coagulation, allows small particles to clump together into larger "flocs" which can then be easily removed through sedimentation or filtration. Accurate dosing ensures water quality compliance while minimizing chemical costs.
How to Calculate Alum Dosing Rates
To determine the correct setting for your dosing pump, you must account for the flow of the water, the required concentration (dose), and the properties of the liquid alum solution being used. The calculation generally follows these steps:
The Formula
The basic formula for liquid chemical dosing is:
Feed Rate (L/hr) = (Flow Rate × Target Dose) / (Specific Gravity × % Strength × 10)
Where:
- Flow Rate: The volume of water treated in cubic meters per hour (m³/hr).
- Target Dose: The desired concentration of alum in milligrams per liter (mg/L) or parts per million (ppm).
- Specific Gravity: The density of the chemical solution (typically 1.33 for liquid alum).
- % Strength: The concentration of the commercial product (typically 48% for liquid alum).
Calculation Example
Suppose you are treating a plant with a flow rate of 200 m³/hr and the jar test indicates an optimal dose of 30 mg/L. You are using standard liquid alum with a specific gravity of 1.33 and a strength of 48%.
- Step 1: Calculate pure alum requirement: 200 × 30 = 6,000 g/hr (6 kg/hr).
- Step 2: Adjust for solution strength: 6 kg / 0.48 = 12.5 kg of liquid solution per hour.
- Step 3: Convert weight to volume: 12.5 kg / 1.33 kg/L = 9.40 L/hr.
Why is Specific Gravity Important?
Liquid alum is significantly heavier than water. If you calculated the feed rate assuming the density of water (1.0 g/cm³) instead of the actual specific gravity of alum (approx 1.33 g/cm³), you would overdrive the pump and overdose the system by roughly 33%. This leads to chemical wastage, pH issues, and increased sludge production.
Optimizing Your Pump Settings
Once you have the calculated flow rate in mL/min or L/hr, you can calibrate your metering pump. It is recommended to perform a "drawdown test" using a calibration column to verify that the pump's actual output matches the calculated requirement, as pump wear and viscosity changes can affect real-world performance.