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WTP Sizing Calculator — Alum Dose, Sedimentation, Rapid Sand Filter

Daily chemical consumption + unit sizing for a conventional treatment plant from plant capacity (MLD).

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Conventional surface water treatment — screening, coagulation, flocculation, sedimentation, filtration, chlorination — is the backbone of Indian urban water treatment. This calculator sizes the three most capital-intensive units: the coagulation chemical dosing rate (kg/day of alum), the sedimentation tank (area and volume from surface loading rate and retention time), and the rapid sand filter bank (total area and number of standard units).

Design parameters per CPHEEO Chapter 8 are well-established: alum 20-40 mg/L typical (jar-test confirmed), sedimentation surface loading rate 1.0-2.5 m³/m²/hr, sedimentation retention 3-4 hours, RSF hydraulic loading rate 5-7 m/hr, sand depth 600-750 mm. The calculator takes plant capacity in MLD and these parameters and produces the design sizes directly.

Based on the CPHEEO Manual on Water Supply and Treatment, published by the Central Public Health and Environmental Engineering Organisation, Ministry of Housing and Urban Affairs, Government of India.

What this calculator computes

  • Daily alum consumption (kg/day) — drives chemical storage and bulk ordering
  • Sedimentation tank surface area (m²) and volume (m³)
  • Total rapid sand filter area required (m²)
  • Number of filter units including one standby for backwash operation

Calculator

WTP Dosing & Unit Sizing (Alum / Sedimentation / RSF)

↓ Download Excel

Size coagulant dosing, sedimentation tanks, and rapid sand filters for a given WTP capacity. Based on CPHEEO Chapter 8 design parameters.

Inputs
Plant capacityMLD
Alum dosemg/L
Jar-test-determined, 20-40 mg/L typical for 50-200 NTU raw water
Sedimentation SLRm³/m²/hr
Sedimentation retentionhr
RSF hydraulic loadingm/hr
Area per filter unitm²
Standard filter unit size, 30-80 m² typical
Outputs
Alum consumption
300.0kg/day
Mass = MLD × dose(mg/L) [since 1 MLD × 1 mg/L = 1 kg/day]
Sedimentation tank area
77.2m²
A = Q / SLR = (MLD × 10⁶ / 86400) / SLR
Sedimentation tank volume
1,250m³
V = Q × retention = (MLD × 10⁶ / 86400) × t × 3600
Total RSF area required
19.3m²
A = Q / HLR
Number of filter units
2units
N = ⌈Total area / unit area⌉ + 1 standby
↪ Round up; add one for standby during backwash
CPHEEO Reference Values
Alum dose typical20 – 40 mg/L
Sedimentation SLR1.0 – 2.5 m³/m²/hr
Retention time3 – 4 hr
RSF HLR5 – 7 m/hr
Sand depth600 – 750 mm (eff. size 0.4–0.7 mm)
Download the Excel version to keep a local copy with live formulas — change inputs in the sheet and outputs recompute automatically.

How to use the inputs

  • Enter plant capacity in MLD (convert from m³/day by dividing by 1000, or from m³/hr by multiplying by 24/1000)
  • Alum dose: do a jar test for raw water, then put the optimum here. Start with 30 mg/L if jar test isn't available
  • SLR 1.5 m/hr is conservative; 2.0 is reasonable; 2.5 is aggressive and demands excellent coagulation
  • RSF HLR 5-6 m/hr is standard; 7 m/hr is upper bound
  • Standard filter unit is 50 m²; adjust to your architectural layout

Worked example

Worked example — 10 MLD WTP
10 MLD plant, alum dose 30 mg/L (from jar test, raw water turbidity 80 NTU), SLR 1.5 m/hr, retention 3 hrs, RSF HLR 6 m/hr, filter unit 50 m² standard. Alum consumption = 10 × 30 = 300 kg/day → quarterly bulk order 27 tonnes. Sedimentation area = (10 × 10⁶ / 86400) / 1.5 = 77 m² → say 8 m × 10 m tank. Sedimentation volume = 77 × 3 = 231 m³ (depth 3 m works). RSF area = (10 × 10⁶ / 86400) / 6 = 19.3 m². Units = ceil(19.3 / 50) + 1 = 1 + 1 = 2 filters (1 running, 1 standby — minimum for reliability).

Interpreting the results

The daily chemical consumption drives the chemical house sizing and bulk storage (design for 30-45 days stock). Sedimentation area determines the footprint of the largest civil structure on your plant — often 30-40% of total plant area. Number of filter units must be at least 2 (one always available when the other backwashes); for plants > 20 MLD you'll typically see 4-6 filter units.

If the SLR you're using gives a much larger sedimentation area than expected, your raw water probably has poor settling characteristics — consider PAC (polyaluminium chloride) instead of alum (30-50% less dose, larger floc) or add a lamella plate settler (3-5x the effective settling area in the same footprint).

FAQs — using this calculator

Do I need to do a jar test before designing?
Yes — always. The 20-40 mg/L alum range is a guideline; the optimum for your specific raw water can be 10 mg/L or 80 mg/L depending on turbidity, alkalinity, temperature. A jar test takes 2 hours in a lab and saves 20-30% of your chemical cost over the 30-year life of the plant. Never design from textbook values alone.
Why do I need a standby filter?
Rapid sand filters need backwashing every 24-48 hours, and backwashing takes 5-10 minutes. During that time the filter produces no water. If you have only 1 filter, plant output drops by 100% for those minutes. With 2 filters (1 running + 1 standby), output stays continuous. CPHEEO requires at least 1 standby per filter bank.
Is alum still the best coagulant?
For most Indian surface water, alum is cost-effective and well-understood. However, polyaluminium chloride (PAC) is rapidly gaining share — 30-50% less dose required, less sludge, better at cold temperatures and high turbidity. If you're designing a new plant, price PAC alongside alum. For very turbid monsoon water or very cold winter source, PAC often wins.
Can I use a slow sand filter instead of rapid sand?
Yes for smaller communities (< 50,000 population). Slow sand filter has HLR of 0.1-0.3 m/hr vs 5-7 m/hr for RSF — so it needs 20x the area. The advantage: no chemicals, no skilled operator, 99%+ bacterial removal through biological action in the schmutzdecke layer. For rural schemes in areas with surface water sources, SSF is often the right choice.
What about PAC vs alum in this calculator?
The calculator uses alum by default. If you're using PAC, reduce the dose by 30-50% (typical PAC 10-25 mg/L vs alum 20-40 mg/L). The calculator then gives PAC consumption in kg/day, which you use for cost comparison.

Related calculators & references

Other CPHEEO Calculators

Chlorine Dose Calculator
Next step in the treatment train
Water Demand & LPCD
Size the plant capacity
Service Reservoir Sizing
Clear water reservoir after WTP

Indian Codes Referenced

IS 10500 — Drinking Water Quality
IS 13405 — WTP Design

Related pages on InfraLens

CPHEEO Ch. 8 — Full treatment chapter
CPHEEO Ch. 9 — Disinfection
CPHEEO Ch. 10 — Advanced Treatment (RO, UF)