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 WW D O 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 (1) (2)
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At steady state O W E WW D O W dX V QX (Q Q )X Q X ( K )XV dt dS V QS (Q Q )S Q S XV dt µ = −− +− = D O QX K XV XQ (S S) YV −= = Use Equation (6) O Q(S S) Food F/M ratio Y XV Microorganism = = = Define: Mean Cell Retention Time (MCRT, or also called solid retention time SRT, in days) C mass of cells in reactor XV mass of cells wasted Q X = = θ From (5) and definition D C 1 K Substitute in (6) using (8) O D C 1 1 Q(S K Y XV  +=   CO DC YQ X V(K 1) = + (3) (4) (5) (6) (8) (7) (9)
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Lawrence and McCarty Model Equation (7), (8), and (9) are the basic equations used for design (at steady state). To simulate the effluent consumption of substrate (S), Lawrence and McCarty (1970) developed a steady-state relationship between X and S based upon Monod kinetics. Monod kinetics: 1 max S S KS = + µ Apply equation (10) to equation (8) max D SC S1 K −= + θ Rearrange max D K = + + max S D D CC 11 SK K S K θθ  = ++ +   max D S D S KK K  −− = +   SD C max D C 1 S 1 K + = S CD C max D K (1 K ) S ( K) 1 + =
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This note was uploaded on 02/02/2012 for the course CEE 255B taught by Professor Michaelstenstrom during the Fall '11 term at UCLA.

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Lawrence and McCarty Design Example - Activated Sludge...

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