AWP Labs · Wastewater
MBBR plant estimator
Size the biological stage of a wastewater plant from flow and strength. Returns media surface area, carrier volume, reactor volume and retention time.
Your inputs
568 m³/day · 104.2 gpm average
Use the coldest month. 20 °C.
Entered at 20 °C and corrected for water temperature. Lower is more conservative. 5–10 is common for secondary treatment.
Share of the reactor volume occupied by carriers. AWP designs at 35% and never above 50%.
AWP’s standard carrier provides about 168 ft² per cubic foot (550 m²/m³).
Results
Reactor volume
25,918gallons
98 m³ of aerated tank at 35% carrier fill
Carriers in suspension · 35% fill
- Carrier volume
- 1,213ft³
- 34.3 m³
- Biofilm surface
- 203,729ft²
- 18,927 m²
- Retention time
- 4.1hours
- BOD removed
- 288lb/day
- 131 kg/day
- Nitrification
- Off
- Homes served
- ≈ 500
- at 300 gal/home/day
The media is loaded on the BOD applied, 313 lb/day. At 68 °F the design rates are 7.50 g BOD/m²·day.
This sizes the biological stage only. Screening, clarification, disinfection and solids handling are designed around it.
How this is calculated
Load. The media is loaded on the BOD coming in: BOD applied (kg/day) = Flow (m³/day) × BOD in ÷ 1,000. BOD removed = Flow × (BOD in − BOD out) ÷ 1,000 is shown as a result. Nitrification is sized on TKN, because organic nitrogen turns into ammonia in the reactor: Nitrogen load (kg/day) = Flow (m³/day) × (TKN − NH₃-N out) × 0.85 ÷ 1,000. The 0.85 allows for nitrogen taken up into new biomass.
Temperature. Biology slows in cold water. With T in °C, BOD rate = rate × 1.04^(T − 20) and Nitrification rate = rate × 1.098^(T − 15) × 0.85.
Surface. Bacteria grow on the carriers, so capacity is set by surface area: Area (m²) = Load (g/day) ÷ Corrected surface loading rate (g/m²·day).
Volume. Carrier volume = Area ÷ Carrier surface area, and Reactor volume = Carrier volume ÷ Fill fraction. Retention time is reactor volume divided by flow.
Assumptions
- AWP designs at 35% carrier fill, never more than 50%, so the carriers circulate freely.
- The BOD loading rate is entered at 20 °C and the nitrification rate at 15 °C. Both are corrected to the design water temperature, so use the coldest month.
- The nitrification rate includes a 0.85 allowance for the dissolved oxygen actually held in the tank.
- BOD removal and nitrification happen in separate stages, so their surface areas add.
- Homes served uses 300 gallons per home per day, from AWP’s 150,000 gpd plant serving about 500 homes.
- A preliminary estimate. AWP sizes every plant to its discharge permit and site.
A preliminary estimate for planning. AWP confirms every figure against your water, site and permit before quoting.
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