Design of Wastewater Treatment Project (Domestic Sewage)

Wastewater Parameters

• Daily wastewater flow rate: 800 m³/day
• Hydraulic Retention Time (HRT): 6 hours = 0.25 days
• influent COD of aerobic tank: 800 g/m³
• (hypothetical value, as typical domestic sewage COD is lower)
• Required effluent COD: 200 g/m³
• Daily COD removal requirement: (Influent COD- Effluent COD) ×Flow rate = (800- 200) × 800 = 540 kg/day

Aerobic Tank Volume Calculation

• Water volume (Vwater)= Flow rate × HRT = 800 × 0.25 = 200 m³
• Typical wastewater depth in aerobic tank: 2.5m; tank wall height: 3 m
• Total tank volume(Vtank)= 200 / 2.5 × 3 = 240 m³
• Tank surface area (Stank)= 240 / 3 = 80 m²
• If only one aerobic tank is used:
  ①Rectangular tank: 10 m × 8 m × 3 m
  ②Circular tank: Φ10.2 m × 3 m

MBBR Quantity Calculation

• Ideal MBBR filling rate: 35%-40%
• (Insufficient MBBR reduces treatment efficiency, while excessive
• MBBR increases aeration burden and may cause clogging)
• MBBRvolume (Vmbbr)= 200 × 35% = 70 m³
• Daily COD removal rate of MBBR= 5 g/m²·day
• RequiredCODremovalpercubic meterofMBBRperday=540/70 = 7.72 kg/m³
• Requiredspecific surface area of MBBR= 7.72 × 1000/5=1,544m²/m³

This specific surface area exceeds that of currently available MBBR

Recommendation

• Usetwo aerobic tanks.
• MBBRvolume= 70 × 2 = 140 m³
• Required specific surface area of MBBR= (7.72 / 2) × 1000 / 5 = 772 m²/m³
• Recommended MBBR types: MBBR37 (＞800 m²/m³) or MBBR04 (＞900 m²/m³)

Consideration of Peak Flow

• Anengineering factor k is introduced to account for peak flow variations. The value of k depends on actual daily flow fluctuations; here, we assume k = 1.1.
• Actual MBBR quantity= k × Theoretical MBBR quantity = 1.1 × 140 = 154 m³
• Actual MBBR filling rate= 154 / 2 / 200 = 38.5% (＜ 40%, within the ideal range)