Water tools
Submerged aerated biofilter sizing
Submerged aerated biofilters combine biological oxidation and filtration in one compact bed. They size on volumetric organic loading, with air supplied per kilogram of BOD applied.
How it works
Bed volume from the organic load and the volumetric loading, area from the depth, upflow velocity as a check:
Process air from the aeration rate per kilogram of BOD applied, sludge from the yield:
- organic load applied, kg BOD₅/d
- flow, m³/d
- influent BOD₅, kg/m³
- bed volume, m³
- volumetric organic loading, kg BOD₅/m³·d
- bed area, m²
- media depth, m
- upflow velocity, m/d
- process air, Nm³/d
- aeration rate, Nm³ per kg BOD₅ applied
- sludge produced, kg TSS/d
- sludge yield, kg TSS per kg BOD₅ applied
For structured media the volumetric loading is the specific surface times the surface loading (about 14 g BOD₅/m²·d for a 30 mg/L effluent). Design is empirical, from pilot and full-scale data. Measured oxygen transfer in biofilters is at most about 10%, well below supplier claims. Beds backwash about weekly with air and treated effluent (3 to 8% of the treated volume); wash water and sludge can return to an upstream anaerobic reactor.
Related reading
These calculators use standard published formulas and are provided for preliminary engineering guidance. Confirm against measured data and vendor projections before design. Model your full water matrix in Nepti or post your project to compare provider proposals.