Measurement of respiration of activated sludge

W a t . R e s . Vol. 22, No. 11, pp. 1405-1411, 1988 Printed in Great Britain. All rights reserved

0043-1354/88 $3.00 + 0.00 Copyright © 1988 Pergamon Press plc

M E A S U R E M E N T OF RESPIRATION OF ACTIVATED SLUDGE MILENKO Rog ~, MILAN DULARl and PETER A. FARKAS2 hBoris Kidri6 Institute of Chemistry, Hajdrihova 19, 61000 Ljubljana, Yugoslavia and 2Research Center for Water Resources Development, VITUKI H-1095 Budapest, Kvassay J. ut l., Hungary (First received June 1987; accepted in revised form May 1988)

Abstract--The use of an open respirometer (respirograph) for the determination of biochemical oxygen demand is described. Some other methods that have already been used for the determination of BOD (Warburg's, Sapromat, dilution, etc.) are time-consuming. The respirographic technique enables us to obtain the results within a few hours, the accuracy being comparable to that of the other methods just mentioned. Respirography appears to be particularly suitable for use in controlling the operation of biological treatment plants. Key words--activated sludge, oxygen uptake rate, BOD

NOMENCLATURE A = Activity of the activated sludge (rag 1-t h - i ) BOD = Biochemical oxygen demand (mg 1-1) BODe = BOD determined by the respirometric method (mg I- t) c - Dissolved oxygen concentration (mg 1 t) c~ = Solubility of oxygen under working conditions (mg 1-l) Cs = Saturated concentration of dissolved oxygen (mg l-i) COD = Chemical oxygen demand (mg 1-~) &a = Overall oxygen transfer coefficient (h- t) STBOD = Oxygen consumption (short term biological oxygen demand) mg O~ re = Oxygen uptake rate-exogenous part (rag 1-i h-i) Re = Oxygen uptake rate permit sludge mass, exogenous part (gg ~h ~) ri = Oxygen uptake rate-endogenous part (rag 1- ~h - 1) Ri = Oxygen uptake rate permit sludge mass, endogenous part (gg-i h-i) rl = Total oxygen uptake rate (mg 1-1 h -~) S = Substrate concentration (rag 1- ~) t = Time (h) X = Suspended solids (g 1-i) X, = Volatile suspended solids (g 1-1) Y = Cell yield coefficient. INTRODUCTION

Treatment parameters for the control and operation of biological treatment plants are partly chemical ones such as chemical oxygen demand (COD) total and dissolved organic carbon (TOC, D O C ) mixed liquor suspended solids and their volatile fraction (MLSS, MLVSS) etc. However, many of t h e s e - however useful they may be in practical use---do not yield much insight into the factors that control the proper biological process. One can get closer to the substrate removal (biooxidation) process using the respirometric method. Biological oxygen uptake (respiration) is closely re-

lated to substrate removal, and important parameters such as BOD, substrate removal activity and related parameters can be measured using the respirometric method, as described earlier by Chudoba (1985), Erode (1975), Fleps (1975), H u a n g (1984), Reinnarth (1983), Tebbutt et al. (1967), Therien et al. (1984), Vernimmen et al. (1969), Young and Baumann (1976a, b) and some others. One of the authors of the present paper (Farkas, 1966) proposed a respirometer that was open to the ambient air and made measurement possible in a system that was essentially a complete mix reactor continually being fed oxygen and batch fed with substrate. Blok (1974, 1976) coined the name "respirograph" for this kind of respirometer, and Farkas (1981) proposed the use of this system for treatment plant control. In our laboratories an open respirometer fitted with a microprocessor was constructed. This instrument enabled us to work out the methodology for the determination of parameters such as STBOD, oxygen uptake for the activated sludge system, the influence of toxic substances (wastewaters) on the respiration of the activated sludge, the monitoring of the adaptation of activated sludge to other types of wastewaters and some other parameters. This paper describes the manner of using the respirometer and provides a more detailed explanation of the method for the determination of BOD. Some practical examples will be given by way of illustration.

MATERIALS AND METHODS

Recorder trace obtained when using the respirograph (Fig. 1) can be divided into four phases on the basis of which certain parameters can be evaluated that are employed in

1405 WR

22,'11--E

,c/

MILENKO Ro~ et al.

1406

oerotion is discontinued

"7

/0

OD

OO

t / rain

Fig. 1. Schematic representation of the open respirometer. (1) Aquarium pump; (2) difusser stone; (3) reactor; (4) magnet; (5) magnetic stirrer; (6) oxygen probe; (7) oxygen meter; (8) recorder; (9) AID converter; (10) microprocessor; (1 I) monitor; (12) printer.

Fig. 3. Calculation of the endogenous oxygen uptake rate.

- - D O changes as a function of time are measured, ---endogenous oxygen uptake rate (Fig. 3) is calculated from the linear part of the DO vs time curve by using the following equations:

the characterization of wastewaters and in dimensioning and controlling biological treatment plants (Fig. 2).

Saturation of the suspension o f activated sludge (Phase 1) The suspension of activated sludge is transferred from the biological treatment plant to the respirograph and aerated until all the substrate in the system has been used up. The bulk of the substrate can be removed by rinsing the sludge suspension with tap water several times before aeration. This manner of rinsing is suitable if the ionic content (physiological solution) of the tap water is similar to the ionic content of the test wastewater content. On the other hand buffer solution, at a similar ionic content to the test wastewater, must be used. When the equilibrium is established (dissolved oxygen concentration is constant during constant aeration), the endogenous phase of oxygen uptake rate has been reached. Prepared in this manner, the suspension of activated sludge in the respirometer is fit for further measurements. (Actually, a true steady state can never be reached as the endogenous respiration is slowly but constantly decreasing due to the endogenous breakdown of cells.) Determination o f endogenous oxygen uptake rate (Phase H) In the suspension of activated sludge to which no additional energy (substrate) is supplied, the endogenous oxygen uptake rate has a constant value: ri = KLa (c s -- ce) = constant

(1)

If the air supply to the respirograph is discontinued after the endogenous phase has been attained (co), the endogenous oxygen uptake rate can be determined as follows:

ri=

(mgl Lh-I)

(2)

or

ri Ri = ~ ( g g - ' h

~)

(3)

Calculation o f the overall oxygen transfer coefficient, KLa (Phase 111) Our aim is now to calculate the amount of oxygen consumed by the sludge upon addition of a particular waste. To this part of the calculation, knowledge o f KLa prevailing in the respirograph becomes necessary. Overall oxygen transfer coefficient, KLa, is required for further calculations resulting from respiration measurements. It can be calculated in various ways as described by Benefield and Randal (1980), Crabtree and Wood (1976), Curi and Eckenfelder (1980), Eckenfelder (1966), Stukenberg and Wahbeh (1977) etc. Air-to-water phase oxygen transfer can be expressed by the equation: dc d~t = ct gLa (tic s -- c) (4) where -

KLa for waste water KL for tap water

fl _ cs for waste water c~ for tap water

t/rnin Fig. 2. Recorder trace obtained by using the respirometric technique.

(4a) (4b)

Respiration of activated sludge aeration is discontinued

liquor (turbulence), temperature of mixed liquor, air pressure, etc. During the measurement of KLa the following conditions must be ensured: - - a i r flow must be constant through the whole experiment; that is, for every experiment the same diffuser stone must be applied; - - f o r all measurements the reactor must be applied with known volume and shape; --stirring with a magnetic or other stirrer must be constant during the experiment time, and so must be the suspension around the oxygen probe must be adequate to oxygen probe characteristics; --temperature of mixed liquor must be constant (normally 20°C) during the experiment; - - i f air pressure is changed a lot during the experiment, KLa must be measured every 2 h and taken into consideration for further calculations.

t / rain

Fig. 4. Ln (c e - c) as a function of time.

The following equation holds for the suspension of activated sludge: dc - - = • KLa (tic s -- c) -- rt dt

(5)

The factors ~t and ti can be taken as constants, and in the following treatment we assume them as being unity. In the respirograph--where the equilibrium between oxygen input and the endogenous oxygen uptake rate has been attained (no additional energy is supplied to the syst e m ) ~ : l c / d t = 0 and equation (5) takes the form KLa (c~ -- Ce) -- ri = 0

(6)

and ri KLa --

E v a l u a t i o n o f the r e s p i r o g r a m ( P h a s e I V )

When ri and KLa have been measured in the suspension of activated sludge, the sludge is again in the endogenous phase. Once the equilibrium has been attained between oxygen input and the own oxygen consumption of the activated sludge, a known quantity of wastewater (substrate) should be added to the suspension. This addition of substrate now upsets the equilibrium of dissolved oxygen concentration due to the outset of substrate respiration (oxygen consumption for the degradation of wastewater). The recorder trace obtained is called the respirogram (Fig. 5). For the endogenous respiration it holds that: ri = KLa @5 -- G) = constant

(6a)

KLa (c~ -- G) = ri

(c, - c,)

(7)

It would be possible to calculate KLa from equation (7) under steady-state conditions, provided the values of c~ and ri are known from previous measurements. Yet this method is unreliable because it is difficult to evaluate cs and co accurately. That is why in the course of our measurements we make use of the measurement method using dynamic conditions, by deoxygenating the system followed by plotting the reaeration DO curve. Since activated sludge has its own oxygen consumption, it is not necessary either to introduce a reducing agent into the system for removing dissolved oxygen (e.g. Na2SO3) or to rinse it with N 2 gas. After the aeration has been discontinued, the concentration of dissolved oxygen begins to drop toward zero DO concentration. As soon as DO reaches 0 . 5 m g l -~, the pump is started (the system is being aerated again), and the changes in dissolved oxygen concentration are plotted as a function of time (Fig. 2, Phase III). The first part of the curve (the linear one) enables us to calculate the overall oxygen transfer coefficient from the equation:

dc dt

(C e -- C2 -

-

(10)

= KLa ( c ~ - c) -- rt

As rt has a negative sign, the left-hand-side of equation (10) is also negative: DO starts dropping. For the instant value of substrate (exogenous) respiration it holds that: dc

(11)

re = KLa (c e -- c) -- - -

dt

Figure 5 shows that at the lowest point of the respirogram equals zero; consequently, the rate of exogenous (substrate) respiration for the exactly known amount of added substrate equals

dc/dt

re = KLa (c e -- Cmin)

(12)

By integrating equation (11), where the member fo~ ddtc d t = 0,

we obtain

(c~ - cl)

II

(9)

However, when the substrate is added to the system, the equilibrium is upset by substrate oxidation. Thus:

l n - KLa --

1407

;0

(8)

12

A more reliable method is also available, namely that of taking into account a larger number of measured points (e.g. minutes, at least 6 measurements) and plotting on the basis of these points the diagram In (ce - c) vs time (Fig. 4). The absolute value of the slope of the straight line yields the value of KLa. It is assumed, however, that the (endogenous) respiration, ri prevailing can be regarded as practically constant during the time of this measurement. Besides correct preparation of the suspension of the activated sludge, KLa is one of the major parameters we need for the evaluation of respirometric parameters such as STBOD, BODr, re, Y, etc. Therefore KLa has to be measured with great attention. KLa is dependent on gas flow, bubble size, reactor dimensions, stirring of mixed

re d t = KLa

f0

(G -- c ) dt

ce

t I

t2

t3

Fig. 5. Respirogram.

tlmin

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MILENKO Rog

Figure 5 indicates that all the substrate is used up in the time from t~ to t2; therefore equation (13) now becomes

f/

re d t = K L a

(G -- C) dt.

i

(14)

i

Providing the area of the respirogram is known, we can calculate the amount of oxygen used for decomposing the added wastewater (substrate) (this amount of 0 2 consumed is called STBOD):

fj '

re

dt = KLa

F = STBOD

Aside from re and STBOD the following parameters can also be calculated from the respirogram: --time required for dissolved oxygen concentration to drop to a minimum: (16)

--reaction time for the added waste (substrate): tr = t3 - tl

(17)

--maximum decrease of dissolved oxygen concentration: ~Cma x :

C e - - Cmit~

Table 1. BOD5 values and BODs/COD ratios determined by the dilution method for fiberboard production of wastewater Dilution BODjmgl ~ BODs/COD 1:1000 5200 0.26 1:1500 5800 0,29 1:2000 7000 0.35

(15)

r

A t = t 2 -- t I

e t al.

(18)

Both the dilution method and the methods making use of apparatus (Warburg, Sapromat, etc.) for the determination of B O D are time-consuming and less applicable particularly in controlling the operation of biological treatment plants. The BOD results obtained by using different working techniques and at various dilutions differ from each other due to different kinetics of decomposition. By way of illustration, we present the case of wastewater resulting from fiberboard (hardboard pieces) production, where

--exogenous oxygen uptake rate: A c ~ ( m g l -t min i)

re = K L a

(19)

or

re

Re

= ~ (g g-~ d - ' )

(20)

--biochemical oxygen demand (STBOD) for the added waste water: 1000 ml 1-~ BOD, STBOD (21) ml of added waste water --biomass yield factor: KLa F STBOD r = 1. . . . 1- - (22) S S --activity of biological sludge: S--K

A

m

S

t

K~X S +I~

(23)

If the concentration of added substrate is high and it holds that S >>Km (which is true of most cases), then the activity is: BOD r A=- = K0X (24) t

All the parameters obtained by using respirometric measurements can be printed--with the aid of a microcomputer system--immediately after the measurements have been completed.

RESULTS AND DISCUSSION

The standard dilution method (1) is still most widely used for the determination of biochemical oxygen demand. According to this method the BOD5 values, i.e. the amount of oxygen expressed in mg 1- l that is required for biochemical degradation of organic substances in 5 days, are ascertained. This parameter allows only the evaluation of those organic substances which are easy to degrade, whereas in order to get an insight into the total process of decomposition of organic substances, we must carry out a test using an apparatus.

C O D = 20000 mg 1-~ BOD25 = 16000mgl -~ (determined by the Sapromat method at dilution of 1:5, 1:8, 1:13 and 1:27) BOD5 values obtained by using the dilution and Sapromat methods at varying dilutions, are shown in Tables 1 and 2. BOD 5 values obtained by various working techniques for wastewater resulting from fiberboard production, indicate great disparity between each other for reasons we have already mentioned. Yet another disadvantage of these methods is the considerable amount of time they require. In this paper, we present an essentially faster method that would be at the same time also accurate enough to meet the needs of practical work. Our experiments demonstrate that respirography fulfills both of these requirements. The respirographic method for the determination of B O D was tested for several systems, viz. - - a q u e o u s solutions of pure compounds (methanol, ethanol, formaldehyde, sodium acetate); - - m o d e l wastewaters (aqueous solution of peptone, aqueous solution of peptone and municipal wastewater, aqueous solution of peptone and glucose); --industrial effluents (municipal wastewater, wastewater from chemical industry, wastewater from pharmaceutical industry, wastewater from pig farms and wastewater from the pulp and paper industry).

Table 2. BOD5 values and BODs/COD ratios determined by the Sapromat method for fiberboard production of wastewater Dilution 1:5 1:8 1:13 1:27

BODs/mgl 6000 8800 11000 27000

~

BODs/COD 0.30 0.44 0.55 0.60

Respiration of activated sludge

1409

Table 3. Theoretical and practical values of C O D and BOD for an aqueous solution of methanol (1 g l -t) Method Theoretical value (Meinck et al., 1975) Standard method ( A P H A , 1981) Dilution method ( A P H A , 1981) Sapromat method Manometric method*

C O D / m g 1-~

BODs/mg I t

BODi0/mg I ~

BOD20/mg I t

1500 1469 ----

960 -930 825 1145

--1030 1150 1198

1260 -1150 1380 1395

*BSB--Messger~it, Modell 1200 ( W T W Company), was used. Table 4. Values of C O D and BOD r (respirographic BOD) for an aqueous solution of methanol (1 g l i) Experiment

COD/mg I i

BOD~/mg 1 i

BODr/COD

I 2 3 4 5 6

1329 1389 1567 1527 1504 1500

1091 954 1370 1445 1402 1334

0.82 0.69 0.87 0.95 0.93 0.89

Mean value

1469 ± 91

1266 ± 197

0.86 ± 0.09 (10.5%)

Samples were collected at various times. For each type of wastewater thus obtained (solution), a series of analyses were carried out. Likewise, all these types of wastewater were analyzed for COD, BOD5 and BOD r. For aqueous solutions of pure compounds theoretical as well as measured values of COD and BOD are given along with the respirographic results. The results are tabulated in the Tables 3, 4, 5, 6, 7 and 8. In the case of aqueous solutions of sodium acetate and peptone only COD and BODr were determined. These values are presented in Tables 9 and 10.

At least 5 experiments were performed for each type of model wastewater as well as of industrial wastewater. Individual values of BODr/COD exhibited standard deviation of < 10% in all the cases considered. The results of these measurements are given in Table 11. CONCLUSIONS

The determination of biochemical oxygen demand (BOD) is carried out mostly by using the standard dilution method (1), Warburg's method,

Table 5. Theoretical and practical values of C O D and BOD for an aqueous solution of ethanol (1 gl ~) Method

COD/mg I i

BODs/mg I t

2090 1836

1350

Theoretical value (Meinck et al., 1975) Standard method ( A P H A , 1981) Dilution method ( A P H A , 1981) Sapromat method Manometric method

1370 1840 1863

BOD,0/mg I i

BOD20/mg I-

i

1800 1710 1952 1963

2147

Table 6. Values of C O D and BOD r for an aqueous solution of ethanol (l gl L) Experiment

BODr/mgl i

BODr/COD

1 2 3 4 5 6 7 8 9

COD/mgl 1772 1850 1820 1820 1744 1706 1971 1971 1868

i

446 537 516 523 416 480 655 691 588

0.25 0.29 0.28 0.29 0.24 0.28 0.33 0.35 0.32

Mean value

1836 ± 92

539 ± 92

0.29 ± 0.04 (12.4%)

Table 7. Theoretical and practical values of C O D and BOD for an aqueous solution of formaldehyde (1 g l i) Method Theoretical value (Meinck et al., 1975) Standard method ( A P H A , 1981) Dilution method ( A P H A , 1981) Sapromat method Manometric method

COD/mg I I

BODs/mg I t

BODi0/mg I -~

BOD20/mg I I

1068 1097 ----

728 -1135 174 400

--1150 390 1015

1220 --530 1425

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MILENKO Rog et al.

Table 8, Values of COD and BOD r for an aqueous solution of formaldehyde (1 g I L) Experiment

COD/mgl i

BODjmgl i

BODr/COD

1 2 3 4 5

1t02 1022 1198 1087 1078

710 736 810 722 699

0.64 0.72 0.68 0.66 0.65

Mean value

1097 ± 63

735 ± 44

0.67 ± 0.03 (4.7%)

Table 9. Values of COD and BOD r for an aqueous solution of sodium acetate (10 g l J) Experiment

COD/mgl t

BODr/mgl i

BOD~/COD

1 2 3 4 5 6

7428 7428 7428 7428 7428 7428

1846 1852 2354 2369 1959 2173

0.25 0.25 0.32 0.32 0.26 0.29

Mean value

7428

2092 ± 240

0.28 ± 0.04 (12.7%)

Sapromat's method and some manometric methods (HACH, WTW, etc.). All of these methods are timeconsuming, requiring, for instance, 5 days for the determination of BOD 5. BOD 5 results obtained by various methods (standard dilution, Warburg, Sapromat, manometric) and at various dilutions differ from each other, but are nevertheless comparable to one another within the frame of the particular type of waste water. Likewise, BOD5 and BOD r results are comparable to one another within the given type of sewage, which gives us grounds for believing that the respirographic method for determining BOD is convenient for actual use. The most notable advantage of the respirographic method is that it enables us to

obtain results in a few hours; moreover, if the respirograph is fitted up with a microprocessor, the results can be printed immediately after the measurements in question had been completed. That is why we recommend it especially as an easy-to-handle laboratory instrument in controlling the operation of biological treatment plants, where BOD 5 obtained by the standard dilution method is of lesser value on account of the considerable amount of time it needs to supply valid data. However, the respirographic method is not applicable to the determination of BOD in less polluted wastewaters (where the values of BOD r are below 30 mgl 1), and especially for those, where polluting material is in the form of suspended particles. If the

Table 10. Values of COD and BOD r for an aqueous solution of peptone (1 gl i) (Merck, Pepton aus Casein, Art. 7213) Experiment

COD/mgl ~

BOD~/mgl i

BOD~/COD

I 2 3 4 5 6 7

1105 1109 1092 1104 1191 I108 1108

614 615 605 603 602 566 672

0.56 0.55 0.55 0.55 0.51 0.51 0.61

Mean value

1117 ± 6

611 + 31

0.55 ± 0.03 (5.6%)

Table 11. Values of COD, BODs, BOD~ and ratios between them

Wastewater Peptone Peptone and municipal wastewater Peptone and glucose Municipal wastewater Pharmaceutical industry Chemical industry Pig farms Pulp/paper industry

COD

BOD 5

BOD r

BOD 5

BODr

BOD 5

mgl ~

mgl ~

mgl ~

COD

COD

BOD r

621 329 161 146 851 595 952 387

0.69 0.64 0.68 0.68 0.49 0.34 0.70 0.21

0.52 0.67 0.60 0.62 0.61 0.35 0.66 0.14

1.34 0.96 1.13 1.10 0.78 0.97 1.07 1.48

1200 490 267 235 1396 1707 1453 2768

830 315 182 161 667 577 1014 574

Respiration of activated sludge substrate is well degradable, (e,g. acetate or septic waste) even B O D r values < 10 m g 1-~ c a n be readily detected. REFERENCES

APHA (1981) Standard Methods for the Examination of Water and Wastewater, 15th edition. American Public Health Association, Washington, D.C. Benefield L. D. and Randal C. W. (1980) Biological Process Design for Wastewater Treatment. Prentice-Hall, Englewood Cliffs, N.J. Blok J. (1974) Respirometric measurements on activated sludge. Wat. Res. 8, 11-18. Blok J. (1976) Measurements of the viable biomass concentration in activated sludge by respirometric techniques. Wat. Res. 10, 919-925. Chudoba J., Cech J. S., Farka6 J. and Grau P, (1985) Control of activated sludge filamentous bulking. War. Res. 19, 191 196. Crabtree H. E. and Wood A. J. (1979) Uprating aeration processes. Wat. Pollut. Control 78, 27-43. Curi C. and Eckenfelder W. W. Jr (1980) Theory and Practice of Biological Wastewater Treatment. Sijthoff and Noordhoff, Alpen aan den Rijn, The Netherlands. Eckenfelder W. W. Jr (1966) Industrial Water Pollution Control. McGraw Hill, New York. Eckenfelder W. W. and Ford D. L. (1970) Water Pollution Control. The Pemberton Press, Austin, Tex. Eckenfelder W. W. and O'Connor D. J. (1961) Biological Waste Treatment. Pergamon Press, Oxford.

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Emde W. (1975) Technology transfer: the development and application of research knowledge. Prog. Wat. Technol. 7, 1039-1049. Farkas P. A. (1966) Adt'. War. Pollut. Res. 19, 309-328. Farkas P. A. (1981) The use of respirography in biological treatment plant. War. Sci. Technol. 13, 125-131. Fleps W. (1975) An automatic flow respirometer, its use and description. Prog. Wat. Technol. 7, 1-12. Huang J. Y. C. and Cheng M. D. (1984) Measurement and new application of oxygen uptake rates in activated sludge processes. J. Wat. Pollut. Control Fed. 56, 259-265, Meinck F., Stoff H. and Kohlschfitter H. (1975) Scieki Przemislowe. Arkady, Warsaw. Reinnarth G. and Rfiffer H. (1983) Bestimmung der Sauerstoffverbrauchraten von Belebtschlamm. Vom Wass. 60, 223-235. Stukenberg J. R. and Wahbeh V. N. (1977) Experiences in evaluating and specifying aeration equipment. J. War. Pollut. Control Fed. 49, 66-82. Tebbutt T. H. Y., Heuken E. R. and Lamb J. C. (1967) Respirometric determination of BOD. Wat. Res. 10, 613~17. Therien N., Calv6 P. and Jones P. ((1984) A respirometric study of the influence of aliphatic alcohols on activated sludges. Wat. Res. 18, 905-910. Vernimmen A. P., Henken E. R. and Lamb J. C. (1967) A short-term biochemical oxygen demand test. J. War. Pollut. Control Fed. 39, I006-I020. Young J. C. and Baumann E. R. (1976a) The electrolytic respirometer--I. Wat. Res. 10, 1031 1040. Young J. C. and Baumann E. R. (1976b) The electrolytic respirometer--II. Wat. Res. 10, 1141-1149.