Activated Sludge Process Model UCR ENVI 121, Spring 2010, Ben S.-Y. Leu, [email protected]The reactor of activated sludge process (ASP) is called completely mix activated sludge (CMAS) system, which is a completely mixed reactor followed by a clarifier as shown in Figure 1. SO,XOQ+QRQW,XWS, XQQ-QWS, XEQR,XRXW=XRVFigure 1. Schematic diagram of a completely mixed activated sludge system. (Note: Q = volumetric flow rate; V = reactor volume; S = concentration of substrate or pollutants; X = cell concentration). Assumptions: 1)Completely mixed reactor 2)No microorganisms in the waste influent (XO~0) 3)Waste stabilization by the microorganisms occurs only in the reactor unit (to be conservative, actually there may be some waste stabilization in the settling unit). 4)Zero volume assumption – volume used in calculating solid retention time (SRT) for the system includes only the reactor volume (settling tank serve as a reservoir in which solids are returned to maintain a given solids level in aeration tank). Mass balance: Accumulation = inflow – outflow + growth OWEWWDOWWWdXFor cells (X): VQX(QQ)XQX(K )XVdtdSFor substrate (S): VQS(QQ)SQ SXVdtY=−−−+−=−−−−µµ(1) (2)
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At steady state OWEWWDOWWWdXVQX(QQ)XQX(K )XVdtdSVQS(QQ)SQ SXVdtµµ=−−−+−=−−−−WWDOQXKXVXQ(SS)YV−==µµUse Equation (6) OQ(SS)FoodF/M ratioYXVMicroorganism−===µDefine: Mean Cell Retention Time (MCRT, or also called solid retention time SRT, in days) CWWmass of cells in reactorXVmass of cells wastedQX==θFrom (5) and definition DC1K−=µθSubstitute µin (6) using (8) ODC11Q(SS)KYXV−+=θCODCYQ(SS)XV(K1)−=+θθ(3) (4) (5) (6) (8) (7) (9)