Document Type : Research Paper
Authors
^{1}
Tehran sewage Company
^{2}
faculty member of matarials and energy research center
^{3}
Consultant for water and wastewater projects
^{4}
faculty member of materials and energy research center
Abstract
Performance Evaluation and Biokinetic Coefficients Determination of Oxidation-Ditch Process using pollutant elimination models on Tehran South Wastewater Treatment Plant
Keywords: pollutant elimination models , Monod model, Kinetic Coefficients, Performance Evaluation, Improved Operation
Introduction
Kinetic models are widely used in fundamental research of biodegradation processes to examine the hypotheses, to control and predict the operation performance in practice and to optimize the reactor design. In this work, different mathematical models including First-order substrate removal model, Grau second-order substrate removal model, Stover-Kincannon model and Monod model were conducted to investigate the reaction kinetics of the oxidation ditch reactor and kinetic coefficients were determined. The aim of this study was to evaluate different mathematical models for describing the COD removal kinetics in the oxidation ditch reactor and to compare the applicability of different models. Because of Monod model is one of the most popular models for various reactors and waste water treatments and commonly employ to describe the biodegradation kinetics, only a few studies succeeded in applying the other models to the oxidation ditch reactor. Thus, in order to determination of the biokinetic coefficients of sewage in south region of Tehran, four kinetic models were used to simulate biomass growth in the oxidation ditch reactor and were compared together, and so the best of them has been innovated to be used in design of Tehran wastewater treatment plants and then the performance of the treatment plant is evaluated and planned to improve the operation method.
Methods & materials
This cross-sectional study implemented in Tehran South wastewater treatment plant using Oxidation-Ditch(O.D) process. In this study 80 samples from the influent wastewater, reactor and effluent were collected periodically and experimented by the Standard Methods.
The wastewater quality parameters including BOD5, COD, TSS and MLSS were measured in warm and cold seasons. These systems were operated under two different MLSS concentrations in the aeration tank. then four kinetic models were applied to simulate biomass growth in the oxidation ditch reactor. the kinetic coefficient in each model obtained as below:
Removal efficiency and bio-kinetic coefficients in suitable model were also calculated.
First-order substrate removal model
Assuming the first-order substrate removal model was prevailing in the Oxidation ditch reactor, the substrate removal rate is expressed as Eq. 1.
dS/dt=-k_l S (1)
Where dS/dt is the substrate removal rate (g/L/d), k1 is first-order substrate removal rate constant (1/d), S is the substrate concentration in a reactor (g/L). In athe Oxidation ditch reactor, mass balance under pseudo-steady-state is expressed as follows Eq. 2 or 3 by introduction of the Eq. 2.
Q/V (S_0-S)=-K_l S (2)
((S_0-S))/θ_H =-K_l S (3)
Where Q is the inflow rate (L/d), V is the effective volume of the reactor (L), S0 is the substrate concentration in influent (g/L), and ƟH is HRT (d). The value of k1 is obtained from the slope of the approximate curve by plotting (S0 _ S) / ƟH against S.
Grau second-order substrate removal model
The common equation of a second-order model is given as follows:
-dS/dt=K_S X(S/S0)^2 (4)
Where kS is Grau second-order substrate removal rate constant (g substrate/g MLVSS/d), and X is the biomass concentration in a reactor (g MLVSS/L).The following Eq. 5 is obtained via integration of Eq. 4 within the boundary conditions of S=S0 and t=o to ƟH, X=constant, and linearization,
(S_0 Ɵ_H)/(S_0-S)=Ɵ_H+S_0/(K_S X) (5)
Where ƟH is HRT. As (S0 _ S) /S0 can be expressed as substrate removal efficiency and
S0/kSX is a constant, Eq. 5 is modified as follows:
θ_H/E=m+nθ_H (6)
Where m is S0/kSX and n is a constant, E is substrate removal efficiency. The values of m, n and kS are easily derived by plotting ƟH /E against ƟH.
Stover-Kincannon model
The Stover-Kincannon model considers the substrate removal rate as a function of substrate loading rate at steady state. The general equation of Stover-Kincannon model is described as follows (7);
dS/dt=(Q(S_o-S))/V (7)
On the other hand, dS/dt is defined as follows in this model.
dS/dt=(U_max ((QS_0)/V))/(K_B+QS_0/V) (8)
Therefore, Eq. 7 is converted to the following equation.
V/(Q(S_0-S))=(K_B.V)/(U_max QS_0 )+1/U_max (9)
Where is the substrate removal rate (g/L/d), Umax and KB are the maximal substrate removal rate and saturation rate constant, respectively (g/L/d). The values of Umax and KB are obtained from the slope of the approximate curve by plotting V/Q (S0 _ S) against V/QS0.
Monod model
Yield Coefficient value (Y), the decay coefficient value (Kd), the kinetic constant (K) and saturation constant value (Ks) can be obtained according to Monod model. The substrate removal rate is represented as follows:
r_su=(μ_m XS)/(Y(K_S+S)) (10)
r_g=-Yr_su-K_d X (11)
1/SRT=YU-K_d=(Y(S_0-S))/θX-K_d (12)
Where Y is the Yield Coefficient (mg COD/mgSS), and Kd is the decay coefficient value (1/d), The values of Y and Kd are obtained by plotting U against 1/SRT.
θX/(S_0-S)=1/U=K_s/K.1/S+1/K (13)
Where Ks is the saturation concentration (g/L), and K is the maximal specific substrate removal rate constant (g /d). The values of Ks and K are obtained by plotting 1/U against 1/S.
Conclusion& Discussion of Result
Plotting results are depicted in figures 1 to 4
Figure1 : First-order substrate removal model
Figure 2: Grau second-order substrate removal model
Figure 3: Stover-Kincannon model
Figure 4: Monod model
the regression line for the plotted linear equation of the model had a R2 of 0.84-0.87which was bigger than that found for three other models with R2 of 0.36-0.81. Removal efficiency and bio-kinetic coefficients in suitable model were also calculated. Monod model provided predictions having the most important relationship with factual data received from the study. In addition, Monod model turned out to be applicable to predict the biomass concentration in the oxidation ditch reactor. The solutions of kinetic studies obtained in this field will provide an invaluable tool in 'the design and process control of the oxidation ditch reactor.
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Main Subjects