Design of Return Sludge and Gas Transfer

1. SLUDGE PRODUCTION, WASTING AND RETURN

   Q_w ¾ waste sludge(SS) flowrate
   X_w ¾ waste sludge(SS) concentration, mg/l
   p_x ¾ net waste activated sludge produced, lb/day
   X_r ¾ return sludge concentration, mg/l

2. OXYGEN REQUIREMENT AND TRANSFER

      Theoretical:
     O₂^T = [Q(S_o-S)]/0.68 - 1.42P_x
      Standard oxygen requirement under field situation:
     SOR = O₂^T [(ßC^a_s-C)/C_s20]1.024^T-20 a
     C^a_s = C^T_s[P_b+ 0.21(1-O_abs)]
     C^T_s ¾ oxygen saturation conc. of tap water at field situation, mg/l
     C  ¾ maintenance oxygen conc. in wastewater, mg/l, 2 ~ 4 mg/l (Ten states standard)
     C_s20 ¾ oxygen saturation conc. of tap water at 20 °C
     a = K^w_La/K^T_La20 , 0.8 ~ 0.9
     ß = C^w_s/C^T_s , 0.7 ~ 0.98
     O_abs ¾   percentage oxygen absorbed at air bubble leaving the liquid surface, 6% ~ 10%
     P_b ¾ pressure at diffuser depth, atm

3. FINAL SETTLING TANK

      Settling phenomena ¾ discrete, flocculation, hindered and compression
     Function  ¾ clarifying and thickening sludge
     SFL×A = (Q+Q_r)X_MLSS = (1+R)QX_MLSS

     SFL ¾ Limiting Solids Flux, lb/ft²·day, SFL <50 lb/ft^2.day at peak rate
     A ¾  required area of settling tank, ft²
     Side water depth >12 ft
     SOR: 400 ~ 800 gal/ft²·day at average flow; SOR < 1,200 gal/ft²·day at peak hourly flow