Modeling and Control of the Activated Sludge Process - ACS

21 Modeling and Control of the Activated Sludge Process GUSTAFF OLSSON

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Department of Automatic Control, Lund Institute of Technology, Lund, Sweden

During recent years there has been an increasing better

operation

of wastewater treatment plants

rantee satisfactory interest

in

effluent

instrumentation

quality

at

in

demand

minimal cost.

and control

comes

after

The renewed a period of

huge investments in sewer networks and treatment plants. factors

have contributed

and control, better

sludge process

process for

waste. Although its

control

is

far

the

pollutants

today its in

operation

A portion

of

mass. The reactor

where the

lic

disturbances are

significant

sources

shock loads

like

are

effluent

from r a i n

Significant

and control

discussion in

is

amplitude.

Diurnal

storms or melting

Hydrau-

variations

snow may cause

disturbances also appear from internal

of linear

control methods are

are

tremendous control

sewage water

is

problems.

such,

or

that

seldom adequate.

Concentration and composition changes of the

so low, that

difficulties

primary pumps, back-washing of deep bed f i l t e r s

quasi-stationary

in the

system and a

made in (1 ) ·

sludge flow rate changes. The amplitudes

water create

flows to

separated from the

organisms in the

disturbance patterns A detailed

return

solids

of viable

summarized here. as

the

Nitri-

ratio.

are

as well

in

and water.

the concentrated sludge is recycled in order

reasonable food-to-mass

major upsets.

improved.

ammonia-nitrogen in the water to form ni-

to maintain enough mass Some t y p i c a l

can be significantly

for available

microorganisms react with orga-

mass, carbon dioxide

and more cell

a sedimentation basin,

common

of municipal and indust-

the wastewater and with oxygen dissolved

organisms oxidize

liquid.

control.

recognized as the most

dynamics is complex the potential

water to produce more cell nitrate

is

reduction

In the process heterotrophic

fying

Several

operation

from exhausted. Already with commercially

instrumentation

trite,

better

such as cheap computing power, improving sensors and

and major unit

nic

for

knowledge of process dynamics and

The activated rial

to the potential

for

order to gua-

influent

waste-

The number of components

so large and the concentrations generally

the measurement problems

seem p r o h i b i t i v e .

composition and concentrations in the reactor

can be

Microbial

significantly

0-8412-0549-3/80/47-124-367$05.00/0 © 1980 American Chemical Society

Squires and Reklaitis; Computer Applications to Chemical Engineering ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


368

COMPUTER APPLICATIONS TO CHEMICAL ENGINEERING

changed, sometimes very f a s t , sometimes g r a d u a l l y . As a r e s u l t the p o l l u t a n t e l i m i n a t i o n will be a f f e c t e d and the process may t u r n i n t o undesirable o p e r a t i o n a l c o n d i t i o n s . These p r o p e r t i e s of the process imply, that disturbance d e t e c t i o n and parameter and s t a t e e s t i m a t i o n are the major c o n t r o l problems. The process dynamics is complex. T y p i c a l response times vary from a few seconds up to weeks and months. Nonlinear phenomena are common due to l a r g e disturbance amplitudes. Measurement and t r a n s port time delays cannot be neglected. Sensor l o c a t i o n is important due to s p a t i a l d i s t r i b u t i o n s . As l i v i n g organisms are part of the system, not only parameters but the s t r u c t u r e of the system dynamics can be changed during the operation. D i s s o l v e d oxygen (DO) concentration is a key v a r i a b l e in the operation. I t a f f e c t s the economy and is also c l o s e l y r e l a t e d to the b i o l o g i c a l a c t i v i t y of the organisms in the sludge. The main emphasis in this paper is to show how the DO concentration d i s t r i b u t i o n in a non-homogeneous aerator can be used f o r the e s t i m a t i o n of the a c t i v i t y of both h e t e r o t r o p h i c and n i t r i f y i n g organisms. The paper is organized as f o l l o w s . A b r i e f review of the act i v a t e d sludge dynamics and DO c o n t r o l is made. Then the propert i e s of DO p r o f i l e s in organic removal are discussed. In the l a s t s e c t i o n it is shown, that the DO p r o f i l e concept can be favourabl y used in r e a c t o r s with combined organic and n i t r o g e n removal, and the i m p l i c a t i o n s f o r c o n t r o l are discussed. A c t i v a t e d sludge dynamics Large e f f o r t s have been spent to develop h i g h l y s t r u c t u r e d models of a c t i v a t e d sludge dynamics. The r e s u l t s in this paper are based on models derived elsewhere (1-4). The models consider the flow regime in the a e r a t o r , oxygen dynamics, c e l l growth and b a s a l metabolism. I t a l s o includes degradation of p o l l u t a n t s both organic carbon and n i t r o g e n - as w e l l as the s o l u b i l i z a t i o n of p a r t i c u l a t e degradable organic carbon. Organic waste removal. I t is assumed that the organic waste is degraded i n t o the f o l l o w i n g steps: ( i ) substrate p e n e t r a t i o n of the c e l l membrane ( i i ) metabolism of the mass stored in the f l o e phase, the "growth" phase ( i i i ) decay of the organisms to i n e r t mass T y p i c a l response times are of the order 15-30 minutes ( i ) , days ( i i ) , and s e v e r a l days ( i i i ) . The net microorganism growth r a t e is a key design parameter and i t s r e c i p r o c a l is termed s l u d ge age or mean c e l l residence time. An adequate c o n t r o l has to take i n t o c o n s i d e r a t i o n , that the growth r a t e is time-varying (5). Nitrification. In b i o l o g i c a l n i t r i f i c a t i o n ammonium n i t r o gen is o x i d i z e d i n t o n i t r i t e by means of Nitrosomonas s p e c i e s , and n i t r i t e is o x i d i z e d to n i t r a t e by the N i t r o b a c t e r s p e c i e s .


21.

OLSSON

Activated

The f i r s t

reaction

Sludge

is

ing effluent n i t r i t e final

aerator.

In the

slower than the

denitrification

if

so the

be small.

if

the

system

is

ammonia and n i t r i t e


ganisms.

concentrations

are

respectively

much more sensitive

lost.

residence

species

times

are

to

to oxygen.

and tempera-

Moreover,

the

than for heterotrophic

n i t r i f y i n g plant than plants

as well

to pH, toxic

organisms.

s i g n i f i c a n t l y smaller

Therefore a f u l l y

mean c e l l

is

so c a l l e d Monod k i n e t i c s with respect

ture changes than the heterotrophic growth rates are

the

the

well.

follow the

The growth rates

is

in

designed for

The growth rates of Nitrosomonas and Nitrobacter assumed to

result-

Nitrate

ammonium nitrogen a l c a l i n i t y

could be restored as

second one,

will

no d e n i t r i f i c a t i o n takes place

oxidation of

it

369

concentration

oxidation state

About h a l f of

Process

or-

requires much longer

designed for only

carbona-

ceous removal. Dissolved oxygen. in a reactor port

The structure of

can be divided

of DO, the production related

sumption draulic

(uptake

rate)

transport

gen transfer

to

The hyoxy-

(production). represented

due to heterotrophic (r2),

growth

considered in more d e t a i l .

by four terms, (r^),

oxygen uptake rate

oxygen consumption rate by orga-

oxygen uptake rate due to Nitrosomonas

Nitrobacter

(r^)

uptake rate

(SCOUR)

respectively.

SCOUR = and the

supply and the con-

of DO is n e g l i g i b l e in comparison with the

are

nism decay

the a i r

balance

the hydraulic trans-

due to organism growth and decay.

The oxygen uptake rate terms are Here they

the DO material

into three parts,

is

(ri

defined by + r )/cell 2

s p e c i f i c nitrogeneous

(r^)

The s p e c i f i c carbonaceous

and

oxygen

(6)

mass

oxygen uptake rate

(SNOUR) in analo-

gous way by SNOUR =

(r

+ r ^ / c e l l mass

3

In a f u l l y n i t r i f y i n g tude as le

reactor

SNOUR is of

SCOUR. Consequently, f u l l

the

the

same order of magni-

nitrification

will

almost

The DO dynamics include phenomena in widely d i f f e r e n t scales, trol has

doub-

a i r demand. from fractions

of hours to weeks.

system designed for to

gradually change

Process

the its

control

This means,

of DO as

time

that a con-

a physical variable

parameters.

i d e n t i f i c a t i o n and parameter estimation has been ap-

p l i e d in water quality and The o v e r a l l oxygen transfer

wastewater treatment coefficient

systems

(J_"9)

can be determined on-line.

The hydraulic dispersion has been i d e n t i f i e d by manipulation of the

influent

flow rate or

Reactor model. process

includes the

the

return

The structured

sludge flow rate model of the

following concentration

(9).

activated

variables:


sludge

soluble

.


370

organic s u b s t r a t e , stored mass, ammonia n i t r o g e n , n i t r i t e and n i t r a t e n i t r o g e n . Organisms are represented by v i a b l e Heterotrophs, Nitrosomonas and N i t r o b a c t e r . Inert (non-viable) n i t r i f y i n g bacte r i a are lumped together with the non-viable h e t e r o t r o p h i c orga nisms. D i s s o l v e d oxygen is i n c l u d e d , but a l c a l i n i t y , pH and gas changes are neglected, as non-covered aerators are used. The concentration dynamics is described by (5), 8c 3t

2

_ 9c

9c _,_

7~2 " * 9z" V

f

l

(

c

)

f

G

ν

F

£

-

f

2

\

( c )


dZ

where c = Ε = ν = fl= f£=

c ( z , t ) = a space and time v a r i a b l e c o n c e n t r a t i o n dispersion coefficient stream v e l o c i t y production r a t e consumption r a t e

D i s s o l v e d oxygen c o n t r o l DO dynamics and c o n t r o l is of v i t a l importance f o r the ope r a t i o n of an a c t i v a t e d sludge system. The r e l a t i o n between DO and b i o l o g i c a l a c t i v i t y will be summarized here. The choice of an adequate DO c o n c e n t r a t i o n . The d e s i r e d DO c o n c e n t r a t i o n is a compromize between economy demands and hydrau l i c and b i o l o g i c a l demands. Moreover, in a non-homogeneous reac t o r a proper s p a t i a l d i s t r i b u t i o n has to be determined. Aerobic organism growth demands a c e r t a i n amount of DO con c e n t r a t i o n . In the aerator the growth of s p e c i f i c types of orga nisms should be favoured. At low DO concentrations h e t e r o t r o p h i c organisms may be i n h i b i t e d while filamentous organisms are favour ed, r e s u l t i n g in b u l k i n g sludge. The DO demand of n i t r i f y i n g o r ganisms is higher than that of h e t e r o t r o p h i c organisms. Conse quently in a n i t r i f y i n g r e a c t o r b u l k i n g sludge seldom appears. To f i n d the proper degree of mixing is complex. I t is r e l a t e d to both DO c o n c e n t r a t i o n , f l o e s i z e , f l o e formation as w e l l as h y d r a u l i c d i s p e r s i o n and b u l k i n g sludge formation. Experiments seem to i n d i c a t e that the sludge volume index (SVI) will increase sharply f o r an i n c r e a s i n g a i r supply. The reason f o r this is un c l e a r (10). E i t h e r it is due to the a d d i t i o n of oxygen as such or by the i n c r e a s i n g mixing. The a i r supply thus has to be optimized between the growth l i m i t a t i o n and the SVI i n c r e a s e . N i t r a t e s can be used as complements or a l t e r n a t i v e s to oxy gen, both f o r energy saving and f o r the treatment of i n d u s t r i a l wastes. This is the background f o r the Kraus m o d i f i c a t i o n of the a c t i v a t e d sludge process. S i m i l a r l y an anoxic zone in the i n l e t area of the a e r a t o r can be a p p l i e d (11). DO c o n t r o l . Most of a v a i l a b l e commercial DO c o n t r o l systems today are based on an analog PI c o n t r o l l e r and one DO probe. Quite



21.

OLSSON

Activated

Sludge

Process

371

o f t e n they give u n s a t i s f a c t o r y r e s u l t due to d i f f i c u l t process and measurement n o i s e , time delays in the p l a n t and i n s u f f i c i e n t measurement information. Unsuitable actuators are common, but actuator l i m i t a t i o n s can be overcome e a s i e r with d i g i t a l c o n t r o l . A t y p i c a l r e s u l t from d i r e c t d i g i t a l c o n t r o l of the Kâppala p l a n t outside Stockholm is shown in f i g 1. The DO c o n c e n t r a t i o n target is 3 mg/1. The c o n t r o l l e d concentration (curve B) has an average value of 3.0 mg/1 with a standard d e v i a t i o n of 0.28 mg/1. Encouraging and remarkable r e s u l t s of DO c o n t r o l have been achieved r e c e n t l y (12), where two a e r a t i o n basins were compared in par a l l e l , one manually and the other a u t o m a t i c a l l y c o n t r o l l e d . With automatic c o n t r o l energy was saved and s i g n i f i c a n t improvements of the water q u a l i t y were obtained. Further p r a c t i c a l DO c o n t r o l experiences are reported in (13 - 15). The biodegradable load to the r e a c t o r can be i n d i c a t e d by the a i r flow r a t e i f a DO c o n t r o l system is a c t i v e . In f i g 2 dry mass of COD from municipal sewage is compared to such an a i r flow s i g n a l in Kâppala. D i s s o l v e d oxygen c o n c e n t r a t i o n p r o f i l e s with organic removal The s p a t i a l v a r i a t i o n of concentrations in a non-homogeneous r e a c t o r will increase the c o n t r o l complexity. However, the conc e n t r a t i o n p r o f i l e s provide e x t r a information. The immediate problem of l o c a t i n g the DO probes s u i t a b l y must be considered, which was remarked already in 1964 ( 16) : "there is no one p o s i t i o n in the tank which is r e p r e s e n t a t i v e of the whole tank a l l the time. This means that a one-point c o n t r o l system a p p l i e d to a p i s t o n flow p l a n t is l i k e l y to be, at best, a compromise...". DO p r o f i l e c h a r a c t e r i z a t i o n . In an aerator with a uniform a i r supply d i s t r i b u t i o n the DO c o n c e n t r a t i o n has a t y p i c a l p r o f i l e , f i g 3. The oxygen content is c l o s e to zero at the i n l e t but is o f t e n in excess c l o s e to the o u t l e t . As the i n f l u e n t wastewater is mixed with r e c y c l e d sludge the s o l u b l e substrate is captured q u i c k l y by the f l o e . Within 30 minutes it is s i g n i f i c a n t l y reduced. No oxygen is needed f o r this process. Stored mass is consumed as a r e s u l t of c e l l growth, and the DO uptake r a t e is p r o p o r t i o n a l to the growth r a t e . As the stored mass c o n c e n t r a t i o n decreases t o wards the o u t l e t , the growth r a t e will decrease. Consequently the DO consumption at the o u t l e t is mainly due to c e l l decay. As most organism are s a t i s f i e d with 2-3 mg/1 of DO the natur a l question a r i s e s , how to d e f i n e a s u i t a b l e DO concentration t a r g e t . Normally the a i r flow d i s t r i b u t i o n cannot be c o n t r o l l e d but only the t o t a l a i r flow. An a n a l y s i s of DO p r o f i l e s and s y s t e matic ways to d e f i n e d e s i r e d s e t p o i n t s were made in (_5, 17). F i v e features of the DO p r o f i l e are important, f i g 3: ( i ) the p o s i t i o n of the maximum slope, which is r e l a t e d to the organic o r h y d r a u l i c l o a d . I t is pushed towards the o u t l e t by an i n c r e a s i n g load, which can be compensated by an i n c r e a s i n g a i r supply.




372



21.

OLSSON

Figure 2.

Activated

Sludge

Process

DO control at Kàppala with hourly data of COD concentration, drymass flow of COD and air flow rate


373



374

( i i ) the value of the maximum slope, which is approximately pro p o r t i o n a l to the value of the maximum s p e c i f i c growth r a t e . ( i i i ) the DO c o n c e n t r a t i o n at the o u t l e t , which is r e l a t i v e l y high i f the growth is c l o s e to completion. ( i v ) the slope of the p r o f i l e at the o u t l e t , which is d i r e c t l y r e l a t e d to the completion of the c e l l growth. For incomplete reac t i o n s the slope will be s i g n i f i c a n t l y p o s i t i v e . Otherwise it is almost h o r i z o n t a l , and i t s value r e f l e c t s only the endogeneous respiration. (v) the curvature of the p r o f i l e down-stream, q u a n t i f i e d by a se cond d e r i v a t i v e . I t should be negative, and i t s s i g n and value are r e l a t e d t o the organic load, the growth r a t e and the degree of r e a c t i o n completion. The slope ( i v ) is a much more r e l e v a n t i n f o r m a t i o n f o r DO c o n t r o l than the absolute value of the DO c o n c e n t r a t i o n at the r e a c t o r end. This e x p l a i n s , why many p l a n t s demand high excess DO concentrations at tile o u t l e t . Moreover, the a i r flow demand is p r i m a r i l y r e l a t e d to the DO p r o f i l e slope than to the concentra t i o n value at the o u t l e t . The q u a l i t a t i v e f e a t u r e s of the DO p r o f i l e are the same, even i f the d i s p e r s i o n c o e f f i c i e n t Ε is i n c r e a s i n g . A l l gradients are decreasing u n t i l they approach zero f o r the extreme case, a CSTR. DO p r o f i l e dynamics. The dynamical c h a r a c t e r i s t i c s o f the DO p r o f i l e are i l l u s t r a t e d by s i m u l a t i o n of a p l a n t , c o n s i s t i n g of an a e r a t o r with four CSTR in s e r i e s , each one having organic removal but no n i t r i f i c a t i o n . The r e t u r n sludge c o n c e n t r a t i o n is assumed constant, which is a reasonable assumption as long as the r e t u r n sludge flow r a t e is constant and the s e t t l e r sludge b u f f e r is p o s i t i v e . Both the i n f l u e n t flow r a t e and the biochemical oxygen de mand (BOD) have been v a r i e d 25 % around t h e i r average values in 24 hour p e r i o d s . As they are assumed to be in phase with each other, the r e s u l t i n g organic load v a r i e s a f a c t o r of three during the p e r i o d . Without any DO c o n t r o l the DO concentrations decrease f o r an i n c r e a s i n g load and v i c e v e r s a , f i g s 4 and 5. F i g 5 shows, how the DO p r o f i l e is pushed towards the o u t l e t as a r e s u l t of in c r e a s i n g load. To parametrize the p r o f i l e , we d e f i n e two v a r i a b l e s , the " s l o p e " between the two l a s t subreactors, d

4 3

= D0(4) - D0(3)

and an estimate of the second s p a t i a l d e r i v a t i v e of the p r o f i l e of the three l a s t subreactors, d

4 2

= D0(4) - 2-DCX3) + D0(2)

where DO(i) is the DO c o n c e n t r a t i o n in subreactor i . The value of


OLSSON

Activated

Sludge

Process

375


ZNFLUEHT BOO CHQ/L5

Figure 4.

Influent BOD disturbance (in phase with the flow rate disturbance)


376

COMPUTER

APPLICATIONS TO CHEMICAL

ENGINEERING


CI42 should be negative, but tends to p o s i t i v e values f o r high loads, f i g 6, and is very s e n s i t i v e to the biodegradable l o a d . The slope d ^ is a l s o c o r r e l a t e d to the load change ( f i g s 4 and 6). As SCOUR is an a l t e r n a t i v e measure of biodegradable load, it has been i l l u s t r a t e d in f i g 7 f o r the same s i m u l a t i o n . The SCOUR p r o f i l e ( f i g 8) is c l o s e l y r e l a t e d to the DO p r o f i l e ( f i g 6 ) . The l a t t e r , however, is d i r e c t l y measurable, and the oxygen t r a n s f e r c o e f f i c i e n t does not have to be estimated. DO p r o f i l e during c o n t r o l . The s e n s i t i v i t y of the DO conc e n t r a t i o n to load changes is l a r g e s t in subreactor 3, so DO(3) has been chosen f o r c o n t r o l . With a d i s c r e t e time c o n t r o l l e r suppl i e d with a feed-forward s i g n a l from the i n f l u e n t flow rate the t o t a l a i r supply to the four subreactors was c o n t r o l l e d to keep DO(3) as constant as p o s s i b l e , f i g 9. Even i f the r e g u l a t o r is not o p t i m a l l y tuned, the improvement of the a e r a t o r performance as seen by the DO p r o f i l e is s i g n i f i c a n t . The load v a r i a t i o n s are detected by the a i r flow r a t e ( f i g 9) as w e l l as by d43 and d42, f i g 10. The slope is now l e s s p o s i t i v e and d42 is s a t i s f a c t o r y (negative) a l l the time. As the p r o f i l e parameters are more d i r e c t l y r e l a t e d to the oxygen uptake r a t e s , they are b e t t e r i n d i c a t o r s of the biodegradable load than the a i r supply, p a r t i c u l a r l y when the c o n t r o l s i g n a l is l i m i t e d . The DO c o n t r o l experiment, f i g 1, can f u r t h e r v e r i f y the r e s u l t s . The a i r flow r a t e (D) is c l e a r l y c o r r e l a t e d to the DO prof i l e slope (C). The slope is obtained by s u b t r a c t i o n of two n o i s y s i g n a l s , but the d i f f e r e n c e is l a r g e enough to give s a t i s f a c t o r y accuracy. D i s s o l v e d oxygen p r o f i l e s with combined organic removal and nitrification The general appearance of a DO p r o f i l e is s i m i l a r as before when n i t r i f i c a t i o n takes place (16). The a n a l y s i s of the p r o f i l e and i t s i n t e r p r e t a t i o n , however, is d i f f e r e n t . The d i f f e r e n t time s c a l e s of the growth r a t e s of n i t r i f y i n g and h e t e r o t r o p h i c organisms is used in the a n a l y s i s in order to separate the d i f f e r e n t phenomena. C h a r a c t e r i z a t i o n of a s a t i s f a c t o r y DO p r o f i l e . A t y p i c a l steady-state s i m u l a t i o n of DO p r o f i l e s in a plug flow r e a c t o r will i l l u s t r a t e the ideas. Ammonia-nitrogen from the i n f l u e n t flow is consumed by both n i t r i f y i n g and h e t e r o t r o p h i c organisms, f i g 11. In a completed r e a c t i o n the NH4-N approaches zero before the o u t l e t . The Nitrosomonas oxygen demand is shown in f i g 12. The c h a r a c t e r i s t i c break of the oxygen demand is caused by the disappearance of NH4-N. As the n i t r i t e is o x i d i z e d to n i t r a t e a sharp break appears in both the n i t r i t e c o n c e n t r a t i o n and the oxygen demand due to N i t r o b a c t e r growth, f i g s 11 and 12. With the long mean c e l l residence time, necessary to complete


OLSSON

Activated


2.4_l

Sludge

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UNCONTROLLED REACTOR

7—1—ÏTÏ— —a:—•—ïTi 1

Figure 6.

1

a.

The DO profile parameters d and d and the influent flow rate of uncontrolled reactor iS

SCOUR CM-1J

Figure 7.

SCOUR

h2

UNCONTROLLED REACTOR

of the four subreactors in the uncontrolled reactor


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378


21.

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Sludge Process

379


2..

ί Figure 10.

'

Ï?S

'

2Î

'

37'.S TIME OO δ*.

Same simuhtion as Figure 9. DO profile parameters and the influent flow rate

CONC MG/L \

NITRATE

\ NITRITECi»e)

4.. /

AMMONIUMV

β..

DISTANCE AL0N6 REACTOR θ

Figure 11.

8.25

'

e.'s

'

e.Vs

'

I

Concentration profiles of ammonium, nitrite, and nitrate in a plug flow reactor


COMPUTER

APPLICATIONS

T O CHEMICAL

ENGINEERING

OXYGEN UPTAKE RATES CMG/L/LENGTH) /


/ ιβ·.

NITROSOMONAS GROWTH

\

I

HETEROTROPHIC \

DECAY V

^HETEROTROPHIC

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GROWtn^^

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Steady-state spatial distribution of different oxygen uptake rates in a plug flow reactor

Figure 13.

DO profiles in a plug flow reactor with both full nitrification and with no nitrification


21.

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the n i t r i f i c a t i o n , the oxygen demand due to h e t e r o t r o p h i c growth has i t s maximum e a r l y . I f no n i t r i f i c a t i o n would occur, the c o r responding DO p r o f i l e would have i t s c h a r a c t e r i s t i c break c l o s e to the i n l e t , f i g 13.The amplitudes of SCOUR and SNOUR are s i m i l a r in the f i r s t p a r t of the r e a c t o r , but the SNOUR has a c l e a r break, which makes it p o s s i b l e to separate carbonaceous and nitrogeneous oxygen demands. Disturbances of the DO p r o f i l e . The r e s u l t of d i f f e r e n t t y pes of disturbances in o p e r a t i o n a l c o n d i t i o n s or i n f l u e n t flow c h a r a c t e r i s t i c s have been studied e x t e n s i v e l y by s i m u l a t i o n . The DO p r o f i l e behaves q u a l i t a t i v e l y as f o r carbonaceous removal (5), but the r e s u l t i n g c o n t r o l a c t i o n may be d i f f e r e n t with n i t r i f i c a t i o n . A load increase in terms of higher i n f l u e n t flow r a t e or a l a r g e r BOD or ammonia c o n c e n t r a t i o n will push the break of the p r o f i l e towards the o u t l e t . A BOD increase will favour more heter o t r o p h i c growth, thus i n c r e a s i n g SCOUR. This l i m i t s the oxygen a v a i l a b l e f o r n i t r i f i c a t i o n . An incomplete r e a c t i o n is noted as i n c r e a s i n g ammonium and n i t r i t e concentrations at the o u t l e t as w e l l as by an i n c r e a s i n g DO slope at the o u t l e t . A decrease of the sludge age or i n s u f f i c i e n t a i r flow r a t e or decrease in temper a t u r e will make the n i t r i f i c a t i o n chain of r e a c t i o n s incomplete. This will be noted in the p r o f i l e in a s i m i l a r way as a load change. Implications f o r c o n t r o l . I f the DO c o n c e n t r a t i o n has a non-homogeneous p r o f i l e at l e a s t three DO probes are needed to monitor the p r o f i l e changes. They give c l e a r i n d i c a t i o n s of v a r i a t i o n s in the b i o l o g i c a l a c t i v i t y . The c o n t r o l of the a i r flow is c r u c i a l , not only f o r the economy but f o r the e f f l u e n t water qual i t y . For o n - l i n e c o n t r o l the DO probes are s u f f i c i e n t i n d i c a t o r s of e x t e r n a l disturbances, but the measurements can be r e g u l a r l y cross-checked with a l c a l i n i t y , n i t r i t e or n i t r a t e measurements of the e f f l u e n t . By c o n t r o l l i n g the a l c a l i n i t y by n i t r i f i c a t i o n the cost f o r chemical dosage in p o s t - p r e c i p i t a t i o n p l a n t s can be s i g n i f i c a n t l y reduced. Acknowledgment Part of the research has been supported by the Swedish Board for T e c h n i c a l Development. Many of the r e s u l t s presented here have grown out of cooperative research with p r o f e s s o r John F Andrews, Univ of Houston, Texas. Literature cited 1. Olsson, G. "State of the a r t in sewage treatment p l a n t control"; AIChE Symp. S e r i e s , 1977, 72, 52-76.



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ENGINEERING

2. Stenstrom, M.K. "A dynamic model and computer compatible control strategies for wastewater treatment plants"; PhD thesis, Department of Environmental Systems Engineering, Clemson University, Clemson, S.C., 1975. 3. Sharma, B.; Ahlert, R.C. "Nitrification and nitrogen removal"; Water Res., 1977, 11, 897-925. 4. EPA "Process design manual for nitrogen control"; U.S. Environmental Protection Agency, Oct. 1975. 5. Olsson, G.; Andrews, J.F. "The dissolved oxygen profile - a valuable tool for control of the activated sludge process"; Water Res., 1978, 12, 985-1004. 6. Andrews, J.F.; Stenstrom, M.K.; Buhr, H.O. "Control systems for the reduction of effluent variability from the activated sludge process"; Prog. Water Tech., 1976, 8, 41-68. 7. Beck, M.B. "Identification and parameter estimation of biological process models"; In "System Simulation in Water Resources", Vansteenkiste, G.C., Ed.; North Holland, 1975, 19-44. 8. Olsson, G.; Hansson, O. "Modeling and identification of an activated sludge process"; Proc. IFAC Symp. on Identification and System Parameter Estimation, Tbilisi, USSR, Sep. 1976, 134-146. 9. Olsson, G.; Hansson, O. "Stochastic modeling and computer control of a full scale wastewater treatment plant"; Proc. Symp. on Systems and Models in Air and Water Pollution, The Institute of Measurement and Control, London, Sep. 1976. 10. Bosman, D.J.; Kalos, J.M. "Relation between aeration rates and settling characteristics of activated sludge"; Water Poll. Control, (S. Africa), 1978, 101-103. 11. Tomlinson, E.J.; Chambers, B. "The use of anoxic mixing zones to control the settleability of activated sludge"; Paper 23, Int. Environmental Colloquium, Liége, Belgium, May 1978. 12. Wells, C.H. "Computer control of fully nitrifying activated sludge processes"; Instrumentation Tech., April 1979, 32-36. 13. Flanagan, M.J.; Bracken, B.D. "Design procedures for dissolved oxygen control of activated sludge processes"; USEPA, 600/2-77-032, June 1977. 14. Gillblad, T.; Olsson, G. "Computer control of a medium sized activated sludge plant"; Prog. Water Tech., 1977, 9, 427-433. 15. Gillblad, T.; Olsson, G. "Implementation problems for activated sludge controllers"; Report TFRT-7137, Department of Automatic Control, Lund Institute of Technology, Sweden, 1978. 16. Jones, K.; Briggs, R.; Carr, J.G.; Potten, A.H. "Automatic control of aeration in a fully nitrifying activated-sludge plant"; Inst. Publ. Health Engr. J., 1969, LXIII, (4), 271-295. 17. Olsson, G.; Andrews, J.F. "Estimation and control of biological activity in the activated sludge process using dissolved oxygen measurements"; Proc. IFAC Symp. Environmental Systems Planning, Design and Control, Kyoto, Japan, 1977.

RECEIVED

November

5,

1979.


Modeling and Control of the Activated Sludge Process - ACS

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