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Rispri LabsENGINEERING A CIRCULAR FUTURE

Free tool · Water and wastewater treatment design

Constructed Wetland Design Calculator

Wetland area A = Q × ln(C₀ ÷ Cₑ) ÷ (K × d × n), where the rate constant K = K₂₀ × 1.06 raised to (T − 20). A horizontal subsurface flow bed treating 50 m³/day from 150 to 30 mg/L BOD at 15 °C needs about 430 m².

Your inputs

Results update as you type. Example values are filled in.

m³/day
mg/L

After a septic tank, settler or anaerobic baffled reactor.

mg/L
°C
m

Subsurface: 0.6 m. Surface flow: 0.3 to 0.5 m.

Gravel: 0.35 to 0.40. Surface flow with plants: 0.65 to 0.75.

How to use this calculator

  1. Choose subsurface or surface flow. This sets depth, porosity and rate constant.
  2. Enter flow and the BOD in and out.
  3. Use the coldest month temperature, since that controls the area.
  4. Read the area, then shape it with the length to width ratio.

How do you calculate the area of a constructed wetland?

KT = K20 × 1.06(T − 20)

A = Q × ln(C0 ÷ Ce) ÷ (KT × d × n)

HRT = A × d × n ÷ Q

First order plug flow model with Reed et al. (1995) rate constants as adopted by US EPA. It is a classic method; the P-k-C* model (Kadlec and Wallace) gives larger, more conservative areas at low outlet concentrations.

Worked example

With wetland type: Horizontal subsurface flow, gravel bed (K₂₀ 1.104/day), average flow 50 m³/day, inlet bod 150 mg/L, target outlet bod 30 mg/L, coldest month water temperature 15 °C, water depth 0.6 m, porosity 0.38, length to width ratio 3:

Wetland bed area: 428 m²

The calculator also gives rate constant at design temperature 0.825 per day; hydraulic retention time 1.95 days; bed length 35.8 m.

Assumptions and limits

  • Primary treatment is in place so solids do not clog the bed.
  • BOD only. Nitrogen and pathogen removal need separate checks.
  • Rate constants are for typical domestic sewage.

Frequently asked questions

How much area does a constructed wetland need per person?

Horizontal subsurface flow beds typically need 1 to 5 m² per person, depending on climate and effluent target. Warm Indian climates sit at the lower end.

Which plants are used in constructed wetlands in India?

Common reed (Phragmites), cattail (Typha), Canna and Colocasia are widely used. They are hardy, root deeply and tolerate sewage.

Can a constructed wetland meet CPCB norms?

A well designed wetland after a settler can meet BOD 30 mg/L and TSS 50 mg/L. Tighter norms such as BOD 10 mg/L usually need a polishing stage or a vertical flow bed.

References

  • Reed, Crites and Middlebrooks (1995). Natural Systems for Waste Management and Treatment
  • US EPA (2000). Constructed Wetlands Treatment of Municipal Wastewaters, EPA/625/R-99/010
  • Kadlec and Wallace (2009). Treatment Wetlands, 2nd ed.

Built and maintained by the Rispri Labs engineering team. Last updated .