Toxic effects of cupric, chromate and chromic ions on biological oxidation

Water Research Pergamon Press 1973. Vol. 7, pp. 599-613. Printed in Great Britain

TOXIC EFFECTS OF CUPRIC, CHROMATE AND CHROMIC IONS ON BIOLOGICAL OXIDATION A. LAMB a n d E. L. TOLLEFSON Department of Chemical Engineering, University of Calgary, Calgary T2N IN4, Alberta, Canada

Abstrat:t--Using a completely mixed, continuously operated, lightly loaded, laboratory activated sludge system, the toxic effects of cupric, chromic and chromate ions under conditions of shock loading were observed. These were determined with the aid of a total carbon analyzer and simple mass balance techniques in terms of conversion of the organic nutrient fed. The distribution of the metal ion between aqueous solution and suspended solids was measured using atomic absorption spectrophotometry. Toxic effects were in the order: Cu 2+ > CrO42- > Cr 3÷ while the reductions in conversion were 90, 50 and 20 per cent, respectively, for concentrations of 5 ppm metal ion. Cupric ion toxicity was directly proportional to the weight of copper absorbed per unit mass of suspended matter within the total copper concentration range (0-5.5 ppm) studied. This toxicity decreased markedly with increased suspended solids concentration: an 80 per cent decrease in conversion at 210 ppm suspended solids was reduced to a negligible quantity (3 per cent) by increasing the suspended solids to 4000 ppm. At 210 ppm suspended solids, 34 per cent of the added copper was removed by the sludge in 7 h. The results of this work suggest that the toxic effect of metal ions on a sewage plant activated sludge system could be reduced by rapidly increasing the suspended solids concentration, possibly by theadditionofdried sludge. It is also implied that the effect on dilute systems such as lagoons would be much greater because of the low suspended solids. This work supports a mechanism involving rapid adsorption of the cupric ion by both viable and dead sludge followed by a slower rate determining step resulting in the toxic effect. The first order rate constant for substrate utilization was found to be (1.07 4- 0.6) h- t.

INTRODUCTION THE CONVENTIONAL m u n i c i p a l sewage t r e a t m e n t p l a n t consists o f a sequence o f o p e r a t i o n s designed to r e m o v e s u s p e n d e d m a t t e r a n d soluble o r g a n i c m a t e r i a l f r o m the influent, sewage s t r e a m entering the plant. Bacteria, either a e r o b i c o r a n a e r o b i c , in the sewage p l a n t influent a r e utilized to oxidize o r g a n i c m a t t e r p r e s e n t . I n the seconda r y t r e a t m e n t stage o f such a plant, a e r o b i c b a c t e r i a present in a n activated sludge o r biological floe utilize o x y g e n for the p u r p o s e o f oxidizing t h e o r g a n i c m a t t e r in the waste stream. A n a e r o b i c b a c t e r i a are utilized in the digestion o f p r i m a r y a n d s e c o n d a r y sludge. B o t h types o f b a c t e r i a are susceptible to toxic effects o f certain m e t a l ions. T h e increasing t r e n d t o w a r d s c o m b i n i n g i n d u s t r i a l a n d m u n i c i p a l wastes for t r e a t m e n t in sewage p l a n t s increases the possibility o f c o n t a m i n a t i o n o f the inttuent b y t r a n s i t i o n m e t a l ions. Such metal ions m a y originate in the waste streams, for e x a m p l e f r o m e l e c t r o p l a t i n g works, h y d r o m e t a l l u r g i c a l refining, p h o t o g r a p h i c shops, storage battery r e p a i r d e p o t s a n d t h o s e using certain soluble m e t a l ion catalysts. T h e p u r p o s e o f this r e s e a r c h was to investigate, o n a l a b o r a t o r y scale, the toxic effects o f certain m e t a l i o n s o n the b i o l o g i c a l o x i d a t i o n processes w h i c h o c c u r when a n a c t i v a t e d sludge sewage t r e a t m e n t p l a n t is " s h o c k l o a d e d " with these ions. T h o s e c h o s e n to be investigated were cupric, c h r o m i c a n d c h r o m a t e ions. C h a n g e s in substrate u t i l i z a t i o n across t h e unit were used to m e a s u r e the toxic effects. w.~. 714.---~

599

600

A. LAMBand E. L, TOLLEFSON LITERATURE

REVIEW

There have been many laboratory investigations into activated sludge systems using both batch and continuous operation. Batch systems are typified by those used by HEIDMAN et al. (1967) in which the reactor consisted of a cylinder from which one third of the mixed liquor and half of the remaining supernatant liquor were removed daily and the volume replaced with a synthetic waste. Continuous flow systems (AYRES et al., 1965; GENETELLXand HEUKELIAN, 1964) offer certain advantages over batch operations, the principal one being that they require 3 days to acclimate the sludge and reach equilibrium compared with. 3 weeks for a batch system. In addition, their use avoids the feed-starvation cycle associated with a batch system and the uneven feeding of nutrients noted by McKINNEY (1963). As a result, a more uniform biological population is produced. While many of the laboratory continuous flow units which have been employed were "open", RICiCARD and GAuI3Y (1968) and STACK and CONWAY (1959) used "closed" systems. Stack and Conway measured the rate of oxygen consumption by displacing oxygen into the system from a reservoir by the addition of water from a burette, maintaining atmospheric pressure in the system. Air, circulated through the biologically active mass, was scrubbed flee of carbon dioxide and the resulting pressure reduction compensated for by the addition of oxygen. BARTH et al. (1965) summarized the effects of heavy metals on the activated sludge process. They reported that primary treatment partially removed the toxic chemical and that the continuous addition of 1.2 ppm (parts per million) copper cyanide resulted in a significant reduction in the efficiency of the waste treatment system at the 2000 ppm solids level. These authors reported that 27 per cent of the chromate ion fed continuously at 15 ppm was removed in the excess sludge and 56 per cent discharged in the effluent. In contrast, when cupric ion was fed continuously to give a concentration of 10 ppm, 55 per cent was removed in the activated sludge and 25 per cent discharged in the effluent. According to MOORE et al. (1960) chromic ions at the 100 ppm level in conjunction with 200 ppm biological solids had little effect on a laboratory sewage plant operation, only a 3 per cent decrease in Biological Oxygen Demand (B.O.D.) being observed. McDERMOTTet al. (1963) reported that in a spiralflow, activated sludge system operating at 200 mg 1-' suspended solids, a concentration of 26 ppm cupric ion had little effect while one of 40 ppm reduced the B.O.D. removal efficiency to 50 per cent. Mo~L'roN and SHUMATE(1963) also studied the effects of cupric ion; they observed increased turbidity and increased Chemical Oxygen Demand (C.O.D.) in the effluent. Their work was extended by AYl~S et al. (1965) who concluded that the toxic effect depended on a combination of (a) the quantity of cupric ion added, (b) the concentration of suspended solids and (c) the influent sewage strength. THEORY

In the study described herein, the reaction vessel is considered to be a continuously stirred tank reactor (C.S.T.R.). The contents of the reactor are assumed to be "perfeetly mixed" so that the conditions within the liquid in the tank are the same everywhere and equal to the conditions at the outlet. A material balance for one of the components, i, can be set up which in words is expressed by:

Toxic Effectson BiologicalOxidation

601

[ accumulation ] [ massofi ] [ massofi ] [ massofi ] of mass of i in = entering the -leaving the + produced the tank tank tank by reaction Considering a system of volume, V, the rate of accumulation of component i is given by: din, = W l i s _ dt

where mi WI W2 is i, V Rl

W2i, -t- V R,

(1)

---- mass of component i in the reactor (rag) ---- rate of feed containing component i (1 h- ~) = flow rate exit the reactor (1 h-x) = concentration of component i in the feed stream (mg 1-1) = concentration of component i in the reactor (mg I- 1) = volume of the reactor (I.) = rate of production of component i (mg i- ~ h- I) and where the positive values of Rl indicate a production of component i. In the steady state, dml _ 0 dt

(21

R, = - - ( W l i ~ - - W2i,) V

(3)

Applying equation (3) to the balance of substrate around the reactor, the rate of substrate utilization Rt in the steady state is given by: Rx RI =

=

--

RL

Wlbs-

W2b,

(4)

V

where

bs = b, = Rt = and now W~ = W2 =

substrate concentration in the feed stream (mg h-t) substrate concentration in the reactor (mg h-X) rate of substrate utilisation (mg 1 -i h-~) feed rate of solution containing substrate (1 h -x) flow rate leaving the reactor.

The concentrations bI and br are measured analytically, Wt is measured and ;4"2 is calculated from the sum of the flows entering the reactor. Hence values for the rate of conversion of substrate can be calculated. Under unsteady conditions an accumulation term is included: R~ = Wlb.r - - W 2 b ,

V

(db,) dt

(5)

602

A. LAMBand E. L. TOLLEFSON

Under such conditions, the rate of change of substrate concentration in the reactor with time must be measured to determine the rate of substrate utilization at a point in time. The percentage of the substrate converted, E, is given by: E

--

R1V b• Wt

x

100.

(6)

E is also a measure of the unit efficiency in terms of substrate utilization. The energy for biological growth is obtained from the oxidation of the substrate. For the particular substrate, glucose, used in this investigation, carbon dioxide is the oxidation product. Busch and M'CRICK (1960) showed that in the utilization of glucose in an activated sludge system, one mole of oxygen is consumed for each mole of carbon dioxide produced. The proportion of substrate oxidized to carbon dioxide can therefore be obtained from measured oxygen uptake rates. MCKINNEY (1963) states the rate of substrate utilization to be first order, depending only on the substrate concentration when the substrate is limiting; under these conditions, the nutrient :ceils weight ratio is less than 2.1 : l. This holds true provided that mass transfer due to inadequate mixing is not the rate controlling step. Under the experimental conditions described herein, the nutrient : cells ratio was 0.3: 1. Data obtained at different recycle rates indicated our process was not mass transfer limiting. For such conditions the first order rate constant for substrate utilization, k, may therefore be given by equation (7) R i = kbr. (7) ANALYTICAL METHODS Oxygen uptake rates were measured directly rather than by using a B.O.D, method. Pure oxygen was metered into the air circulation system to maintain constant pressure as the carbon dioxide produced was absorbed in sodium hydroxide. Glucose, the organic nutrient, was determined using a Beckman Model 915 total organic carbon analyzer. Transition metal ions added as toxic chemicals were monitored using a Perkin-Elmer Model 303 atomic absorption spectrophotometer. Suspended bacterial solids were determined in terms of organic carbon content. Dissolved oxygen was monitored continuously using a Delta Scientific Model 88 oxygen probe. EXPERIMENTAL EQUIPMENT A glass vessel (100 cm long × 17 cm dia.) was used as the experimental reactor into which were fed nutrients plus essential minerals, caustic for p H control, and sludge recycle from the base of the inclined plane settler. The Latter consisted of a plate located within a cylindrical vessel (82 cm long x 9 cm dia.) and maintained at an angle o f approximately 57 ° to the horizontal (Fro. 1). Variable flow peristaltic pumps were used in sludge wastage, sludge recycle, nutrient feed and air circulation services. Liquid flow through the system was by gravity. Oxygen was supplied to the system by water displacement from an 8-I. container. An oxygen probe on the sludge recycle line monitored dissolved oxygen continuously and a copper-~onstantan thermocouple sensed the air temperature in the reactor. Air was distributed in the reaction mixture by means of a frit located in the base of the

foxic Effects on Biological Oxidation

1 ~ I

°s~;~N

603

OXYGEN IRESEVOIR

__ L~

~C~ RESEvo,.

LIQUID DISPLACEMENT(~

TTLER

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\\

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w%,,GE

S~U~GE

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:M~ MICROSELLOW$ PUMP ~x PERISTALTIC PUMP

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MERCURY MANOMETER FIG. I. Complete unit--flow diagram.

reactor. It then passed through caustic wash to remove carbon dioxide prior to being recycled. Before commencing a run, all vessels and interconnecting lines were steamed thoroughly to remove debris and to sterilize the equipment. The contents of the 46-1. nutrient feed tank were filter sterilized, using a biological filter (equipped with a 142 mm dia. Millipore filter, 0.22/~m pore size) and a pressure tank (Millipore Ltd.). The feed composition used is given in TABLE 1. TABLE 1. FEED COMPOSITION

Component Glucose Ammonium sulfate Magnesium sulfate (MgSO,J Manganous sulfate (MnSO4.H,O) Calcium chloride CaCI2.2H20)

I molar phosphate buffer Tap water Distilled water

Concentration (mg I- ~) 500 250 25 10 10 (ml 1- l) 5 50 945

604

A. LA~mand E. L. TOLLEFSON

The reactor was charged with 10.8 I. of feed and 1.2 l, of sewage plant primary effluent (used as a "seed"). The unit was then operated under batch conditions for 24 h. Potassium hydroxide addition was commenced after 6-10 h operation for pH control. At the end of 24 h the unit was switched to continuous operation, i.e. continuous nutrient feed and continuous sludge wastage. At 48 h the oxygen uptake rate was measured over a period of 1-2 h. Toxic chemical was then added over a 10-s period. Toxic chemical studies were commenced at 48 h because (a) operating conditions were steady and (b) minimum operating time was considered desirable in relating the toxic effects observed to the sewage from which the "seed" was taken. In all runs considered, influent feed rate, influent concentration and suspended solids were approximately constant, making such a comparison between the effects of metal ions possible. The following salts were used: chromic chloride, potassium chromate and copper sulfate; no toxic effect due to other ions present was anticipated. Based on the low mixing time in the reaction vessel (of the order of seconds), the measured peak toxic chemical concentration was equivalent to that amount added over the 10-s period. The effluent from the inclined plane settler overhead and the settler base (recycle line) were sampled at regular intervals prior to and after the addition of toxic chemical. Samples were analyzed for unconverted glucose, dissolved inorganic carbon, suspended solids and the toxic chemical ion. Mixed liquor dissolved oxygen and reactor air temperature were monitored during the experimental run. The effects of the shock loaded dosage of toxic chemical were generally followed for 6-8 h. Nutrient feed rate, being controlled by a persitaltic metering pump, was assumed constant and was determined over a 20-30 h period. Sludge wastage rate was measured twice per run. Potassium hydroxide addition, metered by a microbellows pump, was determined over a 20-30 h period. The oxygen probe was calibrated against airsaturated distilled water prior to use. RESULTS AND DISCUSSION FmugE 2 illustrates the physical form of the sludge after development of the flocs during 48 h of operation. FIGURE 3 illustrates the breakup of the floc due to a concentration of 5.5 ppm cupric ion 75 min after cupric ion addition. Prior to the addition of toxic chemical, operating conditions were steady (shown by a constant oxygen uptake rate and constant concentrations of suspended solids and soluble organics at the reactor outlet). The effect of cupric ion on the glucose content in the reactor is illustrated in FIG. 4. Glucose is measured as ppm organic carbon. The increase in glucose concentration after addition of toxic chemical is due to reduction in biological growth. The parallel plots in FIG. 4 illustrate the lack of further growth in the settler and the lack of mixing between reactor and settler. (The lower part of the settler in which there was active flow was considered to be part of the reactor.) Busch and M,clucK (1960) showed one mole of carbon dioxide is produced for every mole of oxygen consumed in an activated sludge system in which glucose is the only organic nutrient. Based on this premise, the rate of glucose oxidation to carbon dioxide was obtained from oxygen uptake data and conversions of glucose to carbon

Toxic Effects on Biological Oxidation

605

o o Recycle ~. . . . . • I=f flmln! C 0 -~

sO

o~jj/,/

of

't "~ 3C

o iii AI

o

t .......

/ ///

o

~3

-so

o

/"

I

I

I

i

50.

IO0

150

zOO

Time offer Copper Addition (minutes)

FIG. 4. Soluble organic carbon in recycle and effluent streams. d i o x i d e are given in TABLE 2. D a t a for the overall c o n v e r s i o n o r u t i l i z a t i o n o f glucose are given in TABLE 3. A d d i t i o n o f toxic metal ion depressed the o x i d a t i o n o f glucose because o f t h e i n h i b i t o r y effects o n bacterial growth. FIGURE 5 illustrates this effect a n d the slow recovery o f the system after the " s h o c k - l o a d " a d d i t i o n o f c o p p e r ion. T h e r e d u c t i o n in c o n v e r s i o n o f glucose as the c o p p e r ion c o n c e n t r a t i o n is increased as a p p a r e n t . TABLE 2. GLUCOSE OXIDATION TO CARBON DIOXIDE (~'~o) Toxic chemical Run number ppm toxic chemical

XIII

Cupric ion (Cu 2.) XVI XVII XVIII XXI

1.9

6.1

6.2

4.2

42

39

36

36

Chromic Chromate ion (CrO,J-) ion (Cr 3+) XXII XXIII XXIV XXVII XXV

5.5

8.4

16.0

15.0

4.9

11.3

52

37

49

39 39

31 31

33 33

52

37

49

13

29

35

19

33

49

5

20

29

12

33

49

5

20

26

7

33

50

23 17 14

7 7 7

22 20 18 16

50 53 53 53

Time (h) -- 1

--0.5 0 0.5

42

39

36

1

10

11

14

1.5 2 2.5 3 3.5 4 5 6 7

36 21 21

20

14 19 24 11

6 6 6

606

A. LAMBand E. L. TOLLEFSON TABLE 3. TOTAL GLUCOSECONVERSION(~,)

Toxic chemical Run number ppm toxic chemical

XIII

Cupric ion (Cu z+) XVI X-VII XVIII XXI

1.9

6.1

6.2

4.2

92

87

89

89

Chromic Chromate 1on (CrO~-'-) ion (Cr ~+) XXII XXIII XXIV XXVIII XXV XXVI

5.5

8.4

16.0

15.0

4.9

11.3

12.4

90 90

92 92

91 91

90

91

88

84

90

91

88

84

12

75

41

39

5l

63

62

3

73

28

25

46

67

60

6

76

22

21

47

65

73

19 33 33

23 22 27

44 45 46 52

81 86 87 88

77 83 83 84

Time

(h) -- 1

--0.5 0 0.5 I

92

87

89

40

8

17

89 29 22

1.5

2 2.5 3 3.5 4 5 6 7

42

23

15 15

73

54 34

11 22 24

FIGURE 6 illustrates the effect of chromate chromium on glucose conversion for various concentrations of chromate ion (4.9, 15.0, 16.0 ppm). The data for runs XXIII and XXIV (16.0 and 15.0 ppm chromate respectively) are essentially super° imposable indicating that the system gave reasonably reproducible results. The sewage 4or-

o 5.Sppm

,='°

li \

o

o

zc

\"

\\

O~

Time

(h,1

Fro. 5. Effect of cupric ion on glucose oxidation.

Toxic Effects on Biological Oxidation

15.0 ppm -" 4.9 l~pm

O

4

607

O

~-

,o'll/ eo ~

•

•

•

4o

N

30

\

\\

/

./ /

I

0

20-

T Time

(h}

Fl~. 6. Effect of chromate ion on glucose conversion.

"seeds" used in Runs XXIII and XXIV were sampled from the sewage plant on different days and kept refrigerated for different lengths of time before use. Thus the day on which the sewage seed was sampled and the length of time it was retained prior to use did not appear to have had a measurable effect on these data. In FIG. 7, the effect of 11-12 ppm chromic chromium on glucose conversion is shown. The two plots for Runs X X V and XXVI, differ in one important respect: the former was a "closed" system using recirculated air with oxygen make up; the latter

9O

O

~>,o \ \ // o \ ~/o "0 l/ ~60

5_00

o I I. 3 ppm "GIc.J~l" lymhltn 0 *,,-.----...-A 12.4ppm ~O~n's~l~l

a ' ~ ~4kf

I

I

I

!

2

4

6

8

Time

I I0

(h)

FiG. 7. Effect of 11-12 ppm chromic ion on glucose conversion.

608

A. LA.~Iaand E. L. TOLLEFSON

was an " o p e n " system. Thus there is no significant accumulation of residual gases adversely affecting the biological process while operating on closed air circulation. The lower initial conversion in Run XXVI is due to the lower retention time (8.7 h) compared with Run XXV (9.3 h). The toxicity effect can be equated to a maximum reduction in total substrate conversion or in oxygen utilization. Comparison of FIGS. 6, 7 and 8 shows that the three metal ions have differing toxic effects. For example, Fig. 7 shows 12.4 ppm chromic ion reduces the glucose conversion by 24 per cent, FXG. 6 shows 4.9 ppm chromate ion depresses the conversion by 45 per cent and FIc;. 8 shows 4.2 pprn cupric ion depresses 0~0 -

5 5 ppm -

19ppm

"N ~ 6O

g ~ o

40

2 0

N z0

0

2

4

t,

f

6

8

Time (h)

FIG. 8. Effect of cupric ion on glucose conversion. the conversion by 75 per cent. Thus, based on the reduction in conversion, the order of increasing toxicity is Cr 3 + < C r O , , 2 - < Cu 2 +. The relative toxicity of these three metal ions is clearly shown in FK;. 9 in which the maximum reduction in conversion is plotted as a function o f the peak toxic chemical ion concentration. The peak concentration is the maximum occurring in the mixed liquor and measures directly the quantity of metal added to the reaction system. The difference in metal ion toxicities can be illustrated in two ways. Firstly, for a given metal ion concentration the maximum decreases in glucose conversion are quite different. For example, for a concentration of 5 ppm metal ion, estimated decreases are 90, 50 and 20 per cent for cupric, chromate and chromic ions respectively. Secondly, the effects o f an increase in metal ion concentration are quite different. A 2 ppm metal ion increase results in a greater reduction in conversion of 20, 5, 1 per cent for cupric, chromate and chromic ions respectively. The pronounced toxic effects reported herein in contrast to those reported in current literature are attributed to the lower concentration of suspended solids (210 ppm). Since glucose feed concentration (500 mg l- ~), residence times (8-9 h), and conversions (90 per cent) were comparable with those reported in the literature, it is suggested that

Toxic Effects on Biological Oxidation IO0

/

°'~ I

~

.x

C

0

•--~-•

=,O o//oo }{

g

609

Cupric ion Chromate ~on Chromic ion

o 4()

""o

E

0

I

t

I

I

5

I0

15

20

ppm Toxic Chemicol ion FIG. 9. Relationship to the peak reactor toxic chemical concentration to the maximum reduction in conversion.

the greater proportion of dead cells present at higher suspended cell concentrations may interact with the toxic metal ions to reduce their toxicity. Thus the higher tolerable concentrations reported can be adequately explained in view of the results of this work. Data given by AYRES et al. (1965) and McDERMOTT et al. (1963) for the effects of shock loaded doses of copper can be expressed in terms of reduction in conversion caused by peak reactor concentrations of 4 and 8 ppm cupric ion. These data and those obtained by the writers are given in FIG. 10. X o o.~

I00

•

81 Tollefe~ Ayres It ol McDermott et ol

Lamb

t

"-

._~

x

\

%

~

so

\\o

E 20

18~ ~ .

8 ppm 4ppm

,

0

1000

2000

SO00

4000

5000

Suspended Solids ( p p m ) FIG. ]0. Per cent reduction in conversion as a function of suspended solids concentrations for 4 and 8 ppm cupric ion.

610

A. LAMBand E. L. TOLLEFSON

Inspection of this figure shows that the effect of a toxic chemical such as cupric ion decreases markedly with an increase in suspended solids concentration. For example, at the 4 ppm copper level, the decrease in conversion at 210 ppm suspended solids is 80 per cent compared with a negligible decrease (3 per cent) at 4000 ppm suspended solids. Important conclusions can be drawn from the relationship between toxicity effect and suspended solids illustrated in FIG. 10. Firstly, it is suggested that sewage plant activated sludge systems should be operated at the highest possible level of suspended solids consistent with efficient operation. Secondly, a toxic situation resulting from metal ion addition may possibly be alleviated by the addition of dried sludge which might complex with some of the metal ions present. Lastly, it is implied that during the operation of biological oxidation lagoons, where suspended bacterial solids could be quite low, toxicity to metal ions could be relatively high.

6 f

I::a 4 "!

31

i

clear

• Effluent

mi~ed liquor

z-.-.-~

Effluent

cleor

soPution

solution

' .

]

.

:-. o

J

,

Ol

t Recycle

•,- ....

~

g,

'~

:.

-

'

#

~#~

o

'

I

I

ioo

200

300

I 400

Time after Copper Addition (rain) F[o. I I. Cupric ion content of effluent and recycle streams (5.5 ppm added initially).

Adsorption of the cupric ion on the biological floc is apparent from FIG. l i and contrasts sharply with the behavior of chromate ion which is not adsorbed (FIG. 12). MOULTON and SHUMATE(1963), AYRES et al. (1965) and MCDERMOTT et al. (1963) apparently did not observe any initial effects because of the relatively long time interval between samples (the order of hours). Calculations show that the plots of cupric ion concentration in the reactor mixed liquor (FIG. 1I) followan exponential decay characteristic of the experimental system in which the cupric ion is added over a very short time period. Integration of these decay curves gives the quantity of cupric ion removed by the sludge. Calculations show that 34 per cent of the added copper was removed in 7 h. The difference in removal from the figures quoted by BARTHet al. (1965) (55 per cent) can be attributed to their higher level of suspended solids (2000 ppm).

Toxic Effects on Biological Oxidation

I~

~

~. -~- . . . . . a----a

~

: Recycie -- Recycle -,L Effluent Efftuenf

611

mixed liquor cloer solutlon mixed liquo¢ clear sot~t)orl

~c

g

o2

[

Time ofter Chromote Addition (rain) FxG, 12, Chromate ion content of e~uent and recycle streams (15.0 ppm added initially),

In order to investigate possible mechanisms for toxicity effects of cupric ions, the maximum depression in glucose conversion (FIG. 8) expressed as a percentage of the initial conversion is plotted as a function of (a) initial reactor peak copper concentration and (b) the weight of adsorbed copper per unit mass of suspended solids in the reactor. A linear relationship for the latter within the concentration range studied is Adsorbed Copper-g Co~)per/g Sludge x IO2 0

0.2

1.0 i

i

1.5

2.0

i

I

:>.5

Ioo

i

g 9c 8c

0

0

g =~

,c

o'~'V "~'/f//

I

O' :-4,r

I

Total Copper - P e a k

I Reactor

I Copper Concentration-ppm

F ; o 13 Relationship of initial, total and adsorbed cupric ion to the maximum reduction in conversion.

612

A. LAMBand E. L. TOLLEFSON

indicated in FIG. 13. It is anticipated that toxicity effects would be associated with the adsorption of cupric ion by a chelation mechanism, on "active sites" associated with a viable sludge. The effect of suspended solids concentration on cupric ion toxicity discussed previously suggests cupric ion may also be adsorbed on non-viable sludge. A rapid adsorption of the copper is indicated (FIG. I1). A slower rate determining step would appear necessary to explain the 1-2-h time lag in achieving minimum conversion (FIG. 8). While it was not the primary, objective of this work to investigate the kinetic aspects of biological oxidation, it is possible, using equations (5) and (7), to obtain an estimate of the rate constant for substrate utilization under conditions in which the nutrient concentration is rate limiting. Using data obtained prior to the addition of the toxic chemical, the value of the first order rate constant for substrate utilization was found to be (1.07 ~ 0.6) h - I .

CONCLUSIONS This work has shown that simple mass balance techniques can be used to follow the toxic effects of metal ions on the biological oxidation process occurring in a completely mixed activated sludge system. Toxic chemical distribution and concentration can be determined using atomic adsorption spectrophotometry. Toxicity effects were in the decreasing order Cu 2÷ > C r 0 4 2 - > Cr 3÷. Toxic effects can also be readily followed by continuous measurements of oxygen uptake rates. Rapid chromic and cupric ion adsorption followed by a decrease in substrate utilisation over a period of time supports an initial adsorption mechanism that is followed by a rate-controlling step producing the toxic effect. The toxic effect of the cupric ion depends markedly on the suspended solids concentration and method for reducing the toxic effect by increasing the solids concentration is suggested. Toxic effects on the operation of lagoons could possibly be far greater than in an activated sludge system because of the lower suspended solids concentration.

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Toxic Effects on Biological Oxidation

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RtC~.ARD M. D. and G A U D Y A. F. (1968) Effect of oxygen tension on oxygen uptake and sludge yield in completely mixed heterogeneous populations. Prec. 23rd ind. Waste Conf. Purdue Univ. Engng Ext. Ser. No. 132, 883-893. SrAct~ V. T. and CoNwAYR. A, (1959) Design data for completely mixed activated sludge treatment. J. Wat. Pollut. Control Fed. 31, 1181-1190.