Lawrence and McCarty Design Example

Lawrence and McCarty Design Example - Activated Sludge...

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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. S O ,X O Q+Q R Q W ,X W S, X Q Q-Q W S, X E Q R ,X R X W =X R V Figure 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 (X O ~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 O W E W W D O W W W dX For cells (X): V QX (Q Q )X Q X ( K )XV dt dS For substrate (S): V QS (Q Q )S Q S XV dt Y = + = µ µ (1) (2)
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At steady state O W E W W D O W W W dX V QX (Q Q )X Q X ( K )XV dt dS V QS (Q Q )S Q S XV dt µ µ = + = W W D O Q X K XV X Q (S S) Y V = = µ µ Use Equation (6) O Q(S S) Food F/M ratio Y XV Microorganism = = = µ Define: Mean Cell Retention Time (MCRT, or also called solid retention time SRT, in days) C W W mass of cells in reactor XV mass of cells wasted Q X = = θ From (5) and definition D C 1 K = µ θ Substitute µ in (6) using (8) O D C 1 1 Q(S S) K Y XV + = θ C O D C YQ (S S) X V(K 1) = + θ θ (3) (4) (5) (6) (8) (7) (9)
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