Coagulant Dosing
Calculate the coagulant consumption of a water treatment plant, consumption = flow × dose ÷ 1000, multiplying the treated flow (m³/day) by the coagulant dose (mg/L) set by jar test. The result, in kg/day, sizes the storage, dilution and dosing pumps for products such as aluminium sulphate, ferric chloride or PAC, ensuring efficient coagulation of colloidal particles. Enter the treated flow and the coagulant dose.
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Coagulant dosing
In a water treatment plant (WTP), the first chemical step is coagulation: a coagulant is added — aluminium sulfate, ferric chloride or polyaluminium chloride (PAC) — which neutralizes the electric charge of the suspended colloidal particles, letting them gather into flocs that later settle out. The optimum dose (in mg/L) is determined in the laboratory by the jar test, which reproduces coagulation in a row of beakers with increasing doses. Once the dose is known, the daily consumption follows from consumption = flow × dose ÷ 1000, which turns the treated flow (m³/day) and the dose (mg/L = g/m³) into kg/day of product. That figure sizes the stock held in the warehouse, the capacity of the dilution tanks and the flow delivered by the dosing pumps. Too little coagulant leaves the water turbid (incomplete coagulation); too much wastes product, raises the operating cost and may leave residual aluminium or iron in the treated water. That is why the dose is constantly readjusted as the turbidity and the colour of the raw water change. Enter the treated flow and the coagulant dose.
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Chlorine Demand
Calculate the chlorine demand of a water, demand = applied dose − chlorine residual, subtracting the measured chlorine residual (mg/L) from the applied chlorine dose (mg/L). The result, in mg/L, is the chlorine consumed by organic matter, ammonia, iron, manganese and other reducers before free chlorine remains for disinfection. Knowing the demand is essential to dose chlorine correctly and keep an adequate residual in the network without waste or underdosing. Enter the applied dose and the measured residual.
Velocity Gradient (Mixing)
Calculate the mean velocity gradient (G) in rapid-mix and flocculation chambers, G = √(P ÷ (μ × V)), from the dissipated power (W), the water dynamic viscosity (Pa·s) and the chamber volume (m³). The result, in s⁻¹, measures mixing intensity: rapid mixing needs high G (700–1000 s⁻¹) to disperse the coagulant, while flocculation uses low G (20–70 s⁻¹) to promote floc collision and growth without breaking them. Enter power, viscosity and volume.
Corrosion Inhibitor Efficiency
Calculate the efficiency of a corrosion inhibitor, η = (CR₀ − CR_inh) ÷ CR₀ × 100%, comparing the corrosion rate without inhibitor (CR₀) with the rate in its presence (CR_inh). The result, in %, measures how much the inhibitor slowed corrosion — the standard indicator to evaluate and compare inhibitors in laboratory tests (mass loss, polarization or impedance). Effective inhibitors form protective films on the surface and reach efficiencies above 90%. It is widely used in boiler water treatment, cooling systems and well acidizing. Enter the corrosion rates without and with inhibitor.
Hydraulic Retention Time (HRT)
Calculate the hydraulic retention time (HRT) of a reactor or tank, HRT = volume ÷ flow, dividing the working volume (m³) by the influent flow (m³/h). The result, in hours, is the average time the liquid stays in the unit and is decisive in designing clarifiers, anaerobic reactors, lagoons and aeration tanks: short times prevent reactions or settling from completing, while long times raise cost and footprint. Enter the working volume and the inlet flow.
Larson-Skold Index (Water Corrosivity)
Computes the Larson-Skold index, the ratio between the aggressive and the protective anions in a water: chloride plus sulphate divided by alkalinity, all converted to milliequivalents per litre with the equivalent weights 35.45 for chloride, 48.03 for sulphate and 50.04 for alkalinity expressed as CaCO₃. The reading is direct: below 0.8 alkalinity dominates and the carbonate film protects carbon steel; between 0.8 and 1.2 corrosion stops being negligible; above 1.2 chloride and sulphate break the film and the localised corrosion rate takes off, the typical scenario of cooling tower makeup water running at many cycles of concentration. Unlike the Langelier index, this one does not say whether the water will scale — it measures only the corrosive power of the anions, which is why the two readings complement each other rather than compete. Total alkalinity was adopted as the input, instead of separate bicarbonate and carbonate, because that is what a routine laboratory reports, and converting it through the CaCO₃ equivalent returns exactly the sum of the two in milliequivalents per litre. Enter the chloride, the sulphate and the total alkalinity.
Silt Density Index (SDI)
Computes the silt density index of ASTM D4189, the test that measures the fouling potential of the feed water of a reverse osmosis membrane: the sample is filtered through a 0.45 µm membrane at 207 kPa, the time to collect 500 mL is measured at the start and again at the end of the test, and the index is the percentage of flow loss divided by the duration. The expression is SDI = (1 − initial time ÷ final time) × 100 ÷ duration, and the number tells how much the filter plugged per minute of test: membrane makers usually require under 5 for a spiral wound element and under 3 for extended warranty, and water above that forces stronger coagulation or filtration upstream. The test is only valid if the flow loss stays below 75%; if the filter plugs before that, run it again with a shorter duration. The duration was adopted as an input instead of fixing 15 minutes, the standard value, precisely because poor water demands repeating at 10 or 5 minutes — results from different durations are not comparable, which is why the value is recorded as SDI₁₅, SDI₁₀ or SDI₅. Enter the initial time, the final time and the test duration.
The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.