Design of Return Sludge and Gas Transfer


1. SLUDGE PRODUCTION, WASTING AND RETURN

   Px = [YQ(So-S)]/(1+kdqC ) = QwXw + QeXe
   R = (1-q /qC)/(Xr/X -1)

   Qw ¾ waste sludge(SS) flowrate
   Xw ¾ waste sludge(SS) concentration, mg/l
   px ¾ net waste activated sludge produced, lb/day
   Xr ¾ return sludge concentration, mg/l

2. OXYGEN REQUIREMENT AND TRANSFER

      Theoretical:
     O2T = [Q(So-S)]/0.68 - 1.42Px
      Standard oxygen requirement under field situation:
     SOR = O2T [(ßCas-C)/Cs20]1.024T-20 a
     Cas = CTs[Pb+ 0.21(1-Oabs)]
     CTs 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)
     Cs20 ¾ oxygen saturation conc. of tap water at 20 °C
     a = KwLa/KTLa20 , 0.8 ~ 0.9
     ß = Cws/CTs , 0.7 ~ 0.98
     Oabs ¾   percentage oxygen absorbed at air bubble leaving the liquid surface, 6% ~ 10%
     Pb ¾ pressure at diffuser depth, atm

Diffused aeration system assembly, Big Blue Sewage Treatment Plant, Kansas City, Mo.

Rotary lift diffuser showing header in position for service

y13.jpg (49617 bytes)

Ceramic Grid Aeration System, Montpelier, Vermont

Aerating Devices

3. FINAL SETTLING TANK

Final Clarifier -- Plan View

      Settling phenomena ¾ discrete, flocculation, hindered and compression
     Function  ¾ clarifying and thickening sludge
     SFL×A = (Q+Qr)XMLSS = (1+R)QXMLSS

Solid Flux Curve Prepared from the Settling Data Given in Fig 13-10(Syed R. Qasim)

     SFL ¾ Limiting Solids Flux, lb/ft2·day, SFL <50 lb/ft2.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

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