Formula & Calculator
Coagulant Dosage Scale-Up (Jar Test to Full Scale)
Scales up an optimal coagulant dose determined by laboratory jar testing to the actual daily dosing rate needed at a full-scale treatment plant.
Interpretation
Full-Scale Dose = Jar Test Dose (mg/L) × Plant Flow (m³/day) / 1000. Scales up coagulant dose from laboratory jar tests to full‑scale plant operations. Used in water treatment design.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| Full-Scale Dose | Daily coagulant requirement | kg/day |
| Jar Test Dose | Optimal dose found in jar testing | mg/L |
| Plant Flow | Actual plant flow rate | m3/day |
What it means
Coagulant dosing is optimised using jar tests, where different doses are applied to samples to determine the optimal dose for turbidity removal. The resulting dose in mg/L is then scaled up to the full‑scale plant by multiplying by the plant flow rate and converting units. This is a standard procedure in water treatment to ensure cost‑effective use of chemicals and to maintain treated water quality. The formula assumes that the optimal dose per unit volume remains constant at full scale, though plant‑specific factors like mixing intensity may require adjustment. Example: A jar test shows that 5 mg/L of alum is needed. The plant treats 50,000 m³/day. The full‑scale daily dose is 5 × 50,000 / 1000 = 250 kg/day. This guides the chemical feed system settings and chemical procurement.
Worked example
Coagulant Dosage Scale‑Up – Two Examples
Real‑World| Parameter | Value |
|---|---|
| Jar test dose | 20 mg/L |
| Plant flow | 5,000 m³/day |
| Parameter | Value |
|---|---|
| Jar test dose | 15 mg/L |
| Plant flow | 10,000 m³/day |
Common mistakes
- Jar test dose: Typically in mg/L (ppm) – the optimal dose determined from lab experiments.
- Plant flow: The full‑scale flow rate – in m³/day.
- Conversion: Multiply jar test dose (mg/L) by flow (m³/day) and divide by 1000 to get kg/day.
- Scale‑up factors: The formula assumes linear scale‑up; however, mixing intensity, detention time, and pH may differ at full scale – verify with plant trials.
- Chemical purity: Commercial coagulants have different purity – adjust dose based on active content.
Applications
Coagulant dosage scale‑up from jar test to full‑scale uses the jar test dose (mg/L) multiplied by the plant flow (m³/day) and divided by 1000 to obtain the full‑scale dose in kg/day. Jar tests are bench‑scale simulations used to determine the optimal coagulant type and dose for removing turbidity and organic matter. This simple scaling relationship allows engineers to translate laboratory findings into practical operational parameters, ensuring that the treatment plant achieves the desired clarification efficiency. It is essential for process control, especially when water quality changes seasonally. By applying this scale‑up, plant operators can adjust coagulant feed rates quickly, reducing chemical waste and costs while maintaining effluent quality.
- Optimisation of coagulant dosing in drinking water and wastewater treatment
- Plant process control and troubleshooting during water quality events
- Estimation of chemical costs for treatment plant operations
- Design of chemical feed systems based on expected dosages
- Seasonal adjustment of coagulant type and dose for varying raw water
Frequently Asked Questions
The full‑scale dose (in kg/day) is Full-Scale Dose (kg/day) = Jar Test Dose (mg/L) × Plant Flow (m³/day) / 1000. This converts the laboratory‑determined optimum dose (in mg/L) to the daily mass of coagulant needed at the plant.
Assuming the jar test dose transfers perfectly to full scale. In practice, the full‑scale dose often needs adjustment because mixing conditions, water temperature, and other factors differ. The jar test is a starting point, not a final setting.
Jar testing simulates the coagulation and flocculation process in the laboratory. It allows operators to test different coagulant types and doses to achieve the best settling and turbidity removal. It is a practical, quick method to optimise chemical addition.
- Mixing intensity and time (different in full‑scale units).
- Water temperature and pH (may change from the time of jar test).
- Water quality variations (seasonal turbidity, algae blooms).
- Coagulant dilution and feed system inaccuracies.
Monitor the effluent turbidity and adjust the dose incrementally (e.g., ±5‑10%) until the desired turbidity is achieved. Also, check pH and alkalinity, as they affect coagulation. Sometimes a different coagulant or addition of a flocculant aid is needed.
For alum (aluminium sulfate): 10‑50 mg/L, depending on raw water quality. For ferric chloride: 5‑30 mg/L. Polymer doses are much lower (0.1‑1 mg/L). The exact dose is determined by jar testing.
First calculate the required mass of coagulant (kg/day). Then convert to volume using the solution concentration: Feed Rate (L/h) = (Dose (kg/day) / (Concentration (kg/L) × 24)). Adjust for the specific gravity of the solution.
pH affects the charge and solubility of coagulants. For alum, the optimum pH range is 5.5‑7.5. For ferric salts, it is 4‑8. Adjusting pH with lime or soda ash may be necessary to achieve optimal coagulation.
Take several beakers of raw water. Add varying coagulant doses to each, then mix rapidly (e.g., 2 min at 100 rpm) and slowly (e.g., 15 min at 30 rpm). Allow settling (e.g., 15 min) and measure the turbidity of the supernatant. Choose the dose that gives the lowest turbidity.
- Aluminium sulfate (alum).
- Ferric chloride.
- Ferric sulfate.
- Polyaluminium chloride (PAC).
- Organic polymers (cationic, anionic, non‑ionic).