Three tabs for coagulation, sand filtration and chlorination. Adjust sliders to see turbidity removal curves, Carman-Kozeny headloss, and Giardia inactivation in real time.
Parameters
Raw Turbidity (NTU)
NTU
Flow Rate Q (m³/h)
m³/h
Coagulant Dose D (mg/L)
mg/L
Filtration Rate v (m/h)
m/h
Bed Depth L (m)
m
Flow Rate Q (m³/h)
m³/h
Media Constant Ce
mg/L
Flow Rate Q (m³/h)
m³/h
Contact Time T (min)
min
Target CT (mg/L·min)
mg/L·min
Results
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Headloss ΔH (m)
—
Filter Area (m²)
—
Loading Rate (m/h)
—
Cl₂ Conc. (mg/L)
—
Chemical (g/s)
—
Giardia (log)
—
Chemical (g/s)
—
Removal (%)
—
Effluent (NTU)
Coagulant Dose vs Turbidity Removal
Main
Theory & Key Formulas
Chemical flow = D × Q ÷ 3600 [g/s]
Removal from empirical turbidity-dose curve.
Sedimentation follows Stokes' law.
ΔH = Ce × v × L [m]
Filter area A = Q / v [m²]
Design range: v = 3–7 m/h
CT = C × T [mg/L·min]
Required C = CT target / T
3-log Giardia: CT ≈ 6 (25°C, free Cl₂)
What is Water Treatment Process Optimization?
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What exactly is "coagulation" in water treatment, and why do we need to calculate a dose?
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Basically, coagulation is the first chemical step where we add a substance, like aluminum sulfate, to make tiny dirt and bacteria (colloids) clump together. The dose is critical: too little, and the water stays cloudy; too much, it's wasteful and can cause other problems. In this simulator, you control the Coagulant Dose D and see how it interacts with the Raw Turbidity to predict treated water quality.
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Wait, really? So the dose isn't just a fixed number? And what's this "headloss" in the filtration part?
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Exactly! The optimal dose depends heavily on how dirty the incoming water is, which is why we have the turbidity slider. For filtration, headloss is the key concept. It's the pressure drop or energy loss as water flows through the sand filter. Try moving the Filtration Rate v and Bed Depth L sliders above. A higher rate or a deeper bed increases headloss, which tells engineers how often the filter needs backwashing.
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That makes sense. The last part is "CT" for disinfection. Is that just concentration times time? Why is that product so important?
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You got it! CT is literally Concentration × Time. But it's the fundamental metric for killing pathogens. For instance, to inactivate 99.9% of Giardia cysts, regulations require a minimum CT value. In practice, if your contact time is short, you must increase the chlorine dose (concentration) to hit the target. Play with the Contact Time T and Target CT parameters to see the trade-off. It's a balancing act between tank size and chemical use.
Physical Model & Key Equations
The chemical feed rate for coagulation is calculated from the desired dose and the plant flow rate. This determines the mass of coagulant that must be delivered per second.
Where D is the Coagulant Dose (mg/L), and Q is the Flow Rate (m³/h). The divisor 3600 converts mg·m³/(L·h) to grams per second.
The headloss through a clean granular filter bed is predicted by the Carman-Kozeny equation, which relates pressure drop to flow rate, bed depth, and media properties.
$$ \Delta H = C_e \times v \times L \quad \text{[m]} $$
Where ΔH is the headloss (m), Ce is the Media Constant (specific resistance), v is the Filtration Rate (m/h), and L is the Bed Depth (m). The required filter area is simply A = Q / v.
Real-World Applications
Drinking Water Plant Design: Engineers use these exact calculations to size chemical feed pumps, design sedimentation basins, and determine the surface area and depth of rapid sand filters. Adjusting the flow rate Q in the simulator directly impacts the required filter area and chemical consumption.
Regulatory Compliance for Disinfection: Health agencies mandate minimum CT values for different pathogens. Operators must continuously monitor chlorine concentration and contact time in clearwells to ensure they meet the target CT, guaranteeing the water is safe from viruses and bacteria.
Coagulant Optimization & Cost Savings: Water treatment plants perform frequent "jar tests" to establish the empirical curve between turbidity and optimal dose. Using a simulator like this helps operators find the minimum effective dose, saving thousands of dollars per year on chemicals.
Troubleshooting Filter Performance: A rapidly increasing headloss indicates the filter is clogging and needs backwashing. Understanding the Carman-Kozeny relationship helps diagnose if problems are due to changes in filtration rate, media condition (Ce), or poor pretreatment from coagulation.
Common Misconceptions and Points to Note
When starting to use this tool, there are several points beginners often stumble on. The first is the misconception that "a coagulation-sedimentation removal rate close to 100% is desirable." While you can achieve high removal rates by moving the sliders in the tool, in practice, chemical costs and sludge treatment expenses skyrocket. For example, if achieving 95% removal for raw water turbidity of 50 NTU requires 40 mg/L of PAC, but 20 mg/L yields 85% removal, the latter is often chosen for its cost-effectiveness.
The second is "judging a design based solely on the sand filter head loss ΔH." The ΔH calculated by the tool is merely the initial value for "clean sand." In actual operation, clogging progresses with captured impurities, and ΔH increases over time. In design, you factor in a margin for this initial ΔH and determine backwash frequency within the total allowable head loss (typically 2.5–3 m), which includes the increase due to clogging.
The third point concerns disinfection calculations: the tendency to think "if the CT value exceeds the standard, it's absolutely safe." The tool's calculation assumes ideal, complete mixing. However, in actual contact tanks, uneven flow velocity distribution and short-circuiting can occur, allowing some water to pass through quickly. Therefore, the concept of a "safety factor," aiming for a design CT value 1.5 to 2 times the standard, is essential.
Enter raw water turbidity (0–500 NTU) in the turbidity field; typical surface water ranges 5–50 NTU
Set coagulant dose (5–100 mg/L as alum or ferric sulfate) based on jar test results for your source water
Input chlorine dose (0.5–3 mg/L) to achieve target free residual (0.2–0.5 mg/L at tap)
Review calculated headloss across filter media, filter area requirement, and hydraulic loading rate in m/h
Verify disinfection log-removal for Giardia and chemical feed rates (g/s) for coagulant and oxidant systems
Worked Example
Surface water treatment plant receiving 2000 m³/h with 35 NTU turbidity: apply 40 mg/L alum dose, achieving 99.5% removal to 0.18 NTU post-filtration. Sand filter bed depth 0.8 m, sand effective size 0.5 mm yields headloss ΔH = 0.34 m at loading rate 8 m/h. Filter area required = 2000 ÷ 8 = 250 m². Chlorine dose 1.5 mg/L at full capacity = (2000 m³/h × 1.5 kg/m³) ÷ 3600 = 0.83 kg/s disinfectant feed rate. Giardia log-removal = 2.8 assuming CT = 15 mg·min/L.
Practical Notes
Jar testing is essential: bench-scale coagulation (1–3 minute rapid mix, 15–20 minute flocculation) determines optimal alum/ferric dose for your specific pH and water quality; calculator assumes well-optimized conditions
Headloss increases exponentially with filter run length; backwash when ΔH exceeds 1.5–2.0 m to maintain filtration rate and prevent mudball formation
Chlorine residual decay varies with temperature and contact time; maintain minimum 0.2 mg/L free chlorine in distribution system for regulatory compliance (EPA Surface Water Treatment Rule)
For high-turbidity events (>100 NTU), pre-sedimentation or pre-filtration reduces coagulant demand and extends filter runs significantly