Sequencing Batch Reactor Design in a Denitrifying Application - ACS

Chapter 14

Sequencing Batch Reactor Design in a Denitrifying Application 1

Basil C. Baltzis, Gordon A. Lewandowski, and Sugata Sanyal

Downloaded by UNIV OF CINCINNATI on May 29, 2016 | http://pubs.acs.org Publication Date: August 2, 1991 | doi: 10.1021/bk-1991-0468.ch014

Department of Chemical Engineering, Chemistry, and Environmental Science, New Jersey Institute of Technology, Newark, NJ 07102

A mathematical model has been developed to describe biological denitrification in a sequencing batch reactor (SBR). The model assumes noninhibitory kinetics for nitrate reduction, inhibitory kinetics for nitrite reduction, and incorporates the possible toxicity effect of nitrite on the biomass. The model has been successful in qualitatively describing experimental data from a 1200 gallon pilot unit. Extensive sensitivity analyses have been performed to indicate the kinetic parameters that have the greatest impact on reactor performance. Numerical results have also indicated ways of properly selecting the operating parameters (hydraulic residence time, time devoted to fill, fraction of reactor contents to be emptied during draw-down), in order to achieve complete denitrification and avoid high nitrate and nitrite concentrations during operation. The results of the model are currently being applied to the design of a facility to denitrify a high strength 0.42 MGD munitions waste stream.

Sequencing batch reactors (SBR) are known to offer a number of advantages over conventional continuous flow systems (9, 16), such as cycling between anoxic and aerobic periods of operation, effective process and quality control, and the fact that they do not require a separate clarifier. For cases of aerobic biodégradation, mathematical models, verified by experiments, have shown that SBRs can also have a greater volumetric efficiency than continuous flow reactors (J), achieving the same level of treatment, at an equivalent throughput, in a much smaller volume. The experience gained in this earlier work enabled us to modify the equations for a denitrifying application. Current address: Allied-Signal, Inc., Morristown, NJ 07962-1087

0097-6156/91/0468-0282$06.00/0 © 1991 American Chemical Society Tedder and Pohland; Emerging Technologies in Hazardous Waste Management II ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

14. BALTZIS ET AL

Sequencing Batch Reactor Design

283

Reported Rates for Nitrate and Nitrite Removal Rates of nitrate and nitrite removal in batch and continuous flow reactors have been reported by various researchers (2, 5-8, 12). It has been reported that (when reduction is dissimilatory), nitrite is an intermediate (11) which may accumulate in the system (4,15). This nitrite build-up is undesirable since it inhibits the overall rate of denitification (14). Nitrogen removal in SBR units has also been reported in the literature (9, 13, 16), although these studies involved nitrification/denitrification with activated sludge and municipal wastewaters.


Purpose of the Present Study The present study is for a SBR designed to denitrify a high strength industrial waste. The intent was to model the bio-denitrification process, verify the model with available experimental data, and use the model to optimally design the unit. Assumptions Made in Deriving the Model In deriving the model, the SBR operating cycle was assumed to consist of only three phases, namely, fill, "react", and draw-down, all of which operate anaerobically. TTbe volume variation during the cycle is shown in Figure 1. The inlet volumetric flow rate during the fill phase, and the outlet volumetric flow rate during the draw-down phase are constant, although not necessarily equal to one another. Reaction occurs throughout the cycle; thus, the "react" phase is simply the period during which there is neither an input to, nor an output from the reactor (pure batch mode). Settling is neglected in this model, which is based on kinetics only, and does not incorporate physical phenomena. This means that solids will be lost during draw-down that will have to be made up in growth. Therefore, the model is likely to underpredict field performance. Nitrate and nitrite are assumed to be the only substances exerting rate limitation on the process. Therefore, the carbon source (usually methanol) and phosphorous are in excess. The rate of nitrate reduction is described by Monod's (non-inhibitory) expression, while the rate of nitrite consumption is described by Andrews' (inhibitory) expression. Provision is also made in the model to incorporate the possibility of nitrite exerting a toxic effect (deactivation) on the biomass. It is also assumed that the unit operates under isothermal conditions. Mathematical Description of the Process Based on the assumptions described above, the general describing bio-denitrification in a SBR are the following:

dt

equations

f

Tedder and Pohland; Emerging Technologies in Hazardous Waste Management II ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

(1)


284

EMERGING TECHNOLOGIES IN HAZARDOUS WASTE MANAGEMENT II

V

0 U Figure

1.

Reactor contents volume variation with time.


14. BALTZIS ET AL.

285


, Q b ^ = —ï (g-.)- — n dt V Y f

(2)

1

1

^

-


dt

(P - P) + cat b

AU

Q

— = dt where,

V

(3)

Y, 2

b + (μ + u)b - kpb 1 2

/»_,_ s

A. =

A.

F

f

V

, and

(4) A* Ρ

μ = 2

2

Κ K ' + ρ + ρ /Κ. 1 ττ +ι s~ * Equation 1 represents an overall mass balance for the reactor contents, while Equations 2 through 4, represent mass balances for the nitrate, nitrite, and biomass, respectively. The specific form of the equations is different for each phase of the cycle, since some of the terms (those involving volumetric flow rates) may be equal to zero. The last term in Equation 4 represents the loss of (active) biomass due to the possible toxic effect of the nitrite. Dimensionless Form of the Model The model equations have been reduced to dimensionless form, in order to decrease the number of parameters. This was done by scaling the parameters and variables as follows : s s b Κ μ* u, = , χ = , ω = , φ = Κ Κ Υ Κ Κ μ 7

f

max

Κ —, Κ.

Υ η =

ν =

Υ 2

Q; -

σ_ =

ρ ,

ρ. ,

ν =

Κ

, ρ = αΥ ,

ν

Q' = t,3- 2t , 3

ε « kK/ι

, max

Vι =

\ύ

V , max

ύ

1

Κ

ο ρ

μm a xVmax f

1

, οc

ι

,

V max

—— y max

It can be shown that the time (in dimensionless form) devoted to each phase of the cycle is the following:


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EMERGING TECHNOLOGIES IN HAZARDOUS WASTE MANAGEMENT II

0 £ * £ ( 1 - 4 )?

Fill:

(5)

t

React : (1 - δ)σ

= (ρλ - i/AJ/?x

dx — dt>

= a

-β*λ

(11)

χ 1

1

(12)

2

+ AJ/?x - ε/?χν 1

(13)

2

where, u A =

ç>v , and A, =

1

1+u

2

2

o + v + yv

It should be mentioned that the volume variation has been incorporated wherever needed in Equations 8 through 13, and thus Equation 1 does not appear in the dimensionless formulation of the model. By making the equations dimensionless, the number of model parameters has been reduced from 17 in the original formulation to 11 in the final one, which substantially reduces the amount of numerical work needed for sensitivity studies. Special Case of the SBR Operation. When the fill and draw-down times are insignificant, the SBR becomes a conventional batch


14. BALTZISETAL


287

reactor, operated in a cyclic fashion. Although the derivation is slightly different in this case, it can be shown that the batch operation is described by Equations 11 through 13, provided that one sets β = 1 and ϋ = Χμ max

Reaching the Steady Cycle. After operation over a period of time (the extent of which depends on the start-up conditions), the unit (SBR or batch) reaches the steady cycle, in which all concentrations repeat their temporal variation. This is expressed as: e (t») = e (i>),


n+1

n

0

Sequencing Batch Reactor Design in a Denitrifying Application - ACS

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