Effect of nickel(ii) on activated sludge

Wat. Res. Vol. 23, No. 8, pp. 1003-1007, 1989 Printed in Great Britain.All rights reserved

0043-1354/89 $3.00+0.00 Copyright © 1989MaxwellPergamonMacmillanplc

EFFECT OF NICKEL(II) ON ACTIVATED SLUDGE ULKU YETIS and CELALF. GOKCAY* Environmental Engineering Department, Middle East Technical University, Ankara, Turkey (First received June 1988; accepted in revised form February 1989) Al~raet--There is little information in the literature to describe the effects of heavy metals on activated sludge kinetics. The purpose of this study is to evaluate the toxic effect of nickel on activated sludge. Different concentrations of Ni(lI) (5.0, 10.0 and 25.0 mg l -l) were maintained in a laboratory-scale completely mixed activated-sludge unit, without recycle, treating simulated wastewater. The feed solution contained 650 mg l-i protein (corresponding to 1300mg I-m COD) as a source of carbon. Experimental results indicated that the treatment efficiencywas not adversely affected by the presence of nickel up to a concentration of I0.0 mg l -~. However, a concentration of 25.0 mg l -~ Ni(II) caused serious upsets in the system, while 5.0 mg l-m Ni(II) in feed solution had some stimulatory effects. The maximum specific growth rate, P-m,of the culture also doubled when the nickel concentration was 5.0 mg l- i and better floc formation was noticed at this nickel concentration. Key words--nickel, inhibition, stimulation, activated-sludge, biokinetics

NOMENCLATURE b = Maintenance coefficient, h -t D =Dilution rate, h -m Dc= Critical dilution rate, h -~ k = Maximum substrate utilization rate, h-m Ks = Half substrate saturation coefficient, mg 1-m Q = Influent volumetric flow rate, m3d-~ Qw= Sludge wastage volumetric flow rate, m3d -m Sc = Exit substrate concentration, mg 1- I V,= Aeration tank volume, m3 x = Biomass concentration in the aeration tank, mg 1-m X, = Biomass concentration in the final effluent, mg 1-~ Ym= Maximum biomass yield coefficient, g g- ~ 0c ffi Biological solids retention time, BSRT (or mean cell residence time), d ]'/m Maximum specific growth rate, h -I. -----

INTRODUCTION Even moderate concentrations of heavy metals are generally regarded as toxic to microorganisms and are often thought to affect a considerable reduction in treatment efficiency. However, biological systems can adapt to moderate concentrations of heavy metals and may even remove a certain fraction, provided that a proper acclimatization period is allowed. Conversely, if the metal concentration is too high, then the introduction of this waste into a biological treatment process may cause serious upsets in the system. The toxicity of heavy metals in biological treatment depends mainly upon two factors, namely, metal species and concentration. Other factors such as pH, microorganism concentration, influent strength and type are also reported to affect the toxicity of metals, though to a lesser degree. Research with a wide range of mean cell residence time settings indicated that *To whom all correspondence should be addressed,

heavy metal toxicity decreases at longer residence times (Sujarittanonta and Shcrrard, 1981; Weber and Sherrard, 1980; Trahern et al., 1980; Bagby and Shcrrard, 1981; Poon and Bhayani, 1971; Neufeld and Hermann, 1976). Resistance of biological treatment systems to metal toxicity may be enhanced greatly by proper acclimatization. During acclimatization, either resistant organisms are selected and/or microbes arc adapted to the metal environment metabolically. In the case of metabolic adaptation, microorganisms are thought to synthesize new enzymes to substitute the metal-inactivated ones or create alternative shunt pathways (Chang et al., 1986). Barth et ai. (1965) reported that the aeration phase of a biological treatment can tolerate concentrations of up to 10mgl -~ of chromium, copper, nickel or zinc, either singly or in combination, with < 5% loss in treatment efficiency. Moreover, they reported that the continuous dose of nickel that will not give significant reduction in treatment efficiency is 1.0-2.5mgl -t. Reduced sludge bulking and better sludge settleability with heavy metals was also noted by these workers. Conversely, McDermott et aL (1965) indicated that hererotrophic organisms can tolerate a continuous dose of 1.0 mg 1-~ Ni(II) without any loss in treatment efficiency. The biokinetic constants of microbial cultures are also affected by the presence of heavy metals. For example, Sujarittanonta and Sherrard (1981)showed that the values of maintenance and biomass yield coefficients are dependent on a COD: Ni(II) ratio in the feed. These workers showed that the maximum biomass yield, Ym, and the maintenance coefficient, b, values for the reactors receiving 1.0 and 5.0 mg 1-m Ni(II) in about 400 mg 1-~ COD were greater than those without nickel. However, when the influent

1003

1004

U L K U YETIS a n d CELAL F. G O g C A Y

C O D concentration was approximately doubled in the feed containing 1.0 mg 1- ' NI(II), Ym and b values remained the same as with the control reactor which received no nickel. However, contrary to this, Weber and Sherrard (1980) found that neither of these biokinetic constants nor the C O D removal efficiency was affected by the presence of 5.15 and 9.98 mg i -~ Cd(II) concentration. Moreover, Neufeld and H e r m a n n (1976) showed that the half substrate saturation coefficient, K,, and the maximum substrate utilization rate, k, were not affected by Cd(II) up to a certain threshold concentration. Beyond this threshold level these coefficients were also affected. Chang et al. (1986) compared a biofilm system with an activated-sludge, where both were continuously receiving 1.5 or 10 mg I-1 Cu(II) or Cd(II) in the feed solution, respectively. Their results indicated that these concentrations of heavy metals do not affect the dissolved organic carbon removal efficiency, and the sludge did not settle all that well. There is also evidence that certain biological systems, notably blue-green bacteria (Henriksson and DaSilva, 1978) and a culture of Chlorella (Bertrand and DeWolf, 1967), are induced by the presence of nickel. Stimulation of growth and certain enzyme activities by nickel in some microorganisms are reported by Hutchinson (1981). F r o m the foregoing discussion it is clear that there is still a need for further research on the effects of heavy metals on biological culture systems. Perhaps, only then will it be possible to draw general conclusions on this important issue. Therefore, the present study, in order to fill this literature gap, is aimed at the effect of nickel on a once-through activatedsludge system, MATERIALS AND METHODS Laboratory apparatus

A modular Gallenkamp-500 series laboratory-scale and once-through continuously stirred tank reactor was used in the experiments. The effective reactor volume was 1.01. and the sterilized feed solution was being constantly fed into the reactor by a dual function, nutrient/harvest, peristaltic pump (Gallenkamp model No. FBL-420-010M). Aeration in the unit was provided by the aerator module (Gallenkamp model No. FBL-420-010M) and efficient mixing of the mixed liquor suspended solids was achieved by the magnetic stirrer module (Galtenkamp model No. FBL-330-010N). The temperature within the activated-sludge unit was maintained constant at 25°C by the temperature controller module (Gallenkamp model No. FBL-330-010N). Feed solution

Synthetic wastewater with known COD and nickel concentration was used in the experiments. The composition of the simulated wastewater is given in Table I. A nickel chloride solution was added to the synthetic wastewater to give 5.0, I0.0 and 25.0mgl -~ Ni(II) concentrations in the feed. Phosphate salt was introduced to the medium so as to provide both buffer action and phosphorous source to the microorganisms. The culture was carbon-limited in all the experiments and the carbon source was proteose-pepton (Oxoid).

Table 1, Composition o f the synthetic wastewater

Constituents

concentration (mg 1-1)

Proteosc-pcpton

1221.7

NaCl Na:SO, K2HPO 4 MgCI2.6H20

407.4 44.6 44.6 3.7 3.7 3.7 57 × 10 -3 31 x 10 3 8 × 10-3 46 × 10 -3 49 × 10 3 76 x 10-3

FeCI2.2H20

CaCI2"2H:O MnSO4 H2MoO, NaOH ZnSO 4 CoSO, CaSO,

Analytical procedures

Substrate concentration was determined in the centrifuged aliquots either by measuring protein according to the Folin--Ciocalteu method (Lowry et al., 1951) using Bacto ova albumin as reference, or by standard COD analysis (AHPA, 1975). The protein measurements were converted into COD units according to the ratio COD/protein = 2.0, which was determined experimentally in this study. The ratio was more or less constant within the detection limit for both effluent and feed solutions. The biomass concentration was determined gravimetrically by filtering sample aliquots through 0.45 #m pore size membrane filters and drying the filters to a constant weight at 105°C. RESULTS During initial start-up of the culture, synthetic wastewater devoid of nickel was inoculated with about 1% fresh sewage and the reactor was operated batchwise till a healthy culture was established. Continuous culture conditions were then maintained. Wall growth over the inner walls of the tank and heating coils etc. were removed daily by manual brushing. The removed slime was added back into the reactor content. Prior to taking a sample at a set dilution rate (or mean solids residence time) several samples were drawn from the mixed liquor in order to determine whether steady-state conditions were reached. Steady readings of the mixed liquor suspended solids (MLSS) concentrations were taken as an indication of steady-state conditions. The mean residence time for cells in a completely stirred tank reactor (CSTR) is given by the equation 0c = XVa/[QwX + (Q - Qw)Xe], where Q and Qw are influent and wastage stream flow rates, respectively, and 0c is the biological solids retention time (BSRT). When there is no recycle, such as in the case of once-through activated-sludge, this equation simplifies to 0c = X V , / Q w X = VJQw = I/D, where the mean cell residence time equals hydraulic detention time and its reciprocal, usually denoted by D, is termed as the dilution rate (Pirt, 1975). In this paper the dilution rate notation will be used most of the time.

Effect of nickel(II) on activated sludge 700

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Figure 1 shows the exit substrate concentrations, Se, and the suspended solids concentration, X, at different dilution rates with synthetic wastewater devoid of nickel. A typical dilute-out curve was obtained without nickel, as shown in this figure. The critical dilution rate was depicted at around 0.32 h-1, and there was very little change in the biomass concentration over a wide range of D values. The kinetic constants,/Zm (maximum specific growth rate) and K, (half saturation substrate concentration), were determined as 0.31 h - m and 85 mg l-I, respectively, from a Lineweaver-Burk plot of the data, as presented in Fig. 2. An alternative method was also adopted to e v a l u a t e / ~ by operating the continuous culture at a dilution rate greater than the critical dilution rate (De). At this operational condition the culture in the reactor vessel should start to dilute-out at a rate equal to ( P r o - D ) . A plot of the logarithm of biomass concentration vs time indeed provided a straight line, as shown in Fig. 3. From the slope of the straight line, tim was calculated from ( t i m - - D)/2.3 as 0.41 h - l by a least-square regression analysis.

Fig. 3. Evaluation of P'm for the baseline study by nonsteady-state dilution experiments using synthetic wastewater devoid of nickel.

Adding 5.0 mg l-1 Ni(H) Continuous feeding o f 5 . 0 m g l -m Ni(II)containing synthetic wastewater was started after the reactor was acclimatized to nickel for over 2 months. Surprisingly, the COD removal performance of the unit was not found to be adversely affected by nickel. Furthermore, the all-round reactor performance was found to have improved considerably with the introduction of nickel. As can be deduced frofn Fig. 4, the MLSS concentration almost doubled between the D values of 0.18 and 0.40 h -l, and the sludge was observed to settle easily due to the large floes formed. Similarly, the critical dilution rate increased to 0.58 h -~ from an early value of 0.32 h - 1. The effluent substrate concentration (Se) began to rise at D > 0.4 h - l, while at the same time the MLSS concentration was decreasing. The 0.58 h -I dilution rate was taken as the critical dilution rate for this culture and the exit substrate concentration increased to the inlet substrate concentration level at dilution rates of 0.76 h -m and above.

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Fig. 4. Dilute-out curve for synthetic wastewater conta;ning

synthetic wastewater devoid of nickel.

5 mg 1-~ Ni(II).

1006

ULKU YETIS a n d CELAL F. GOKCAY

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Fig. 7. Dilute-out curve for synthetic wastewater containing 10mgl -= Ni(]I).

5 mg 1- i Ni(II). Another interesting observation was the trend in the MLSS concentration with the changing dilution rates in Fig. 4. Up to about a 0.18h -~ dilution rate there was a gradual increase in biomass concentration with the increasing D values. The MLSS concentration was 400 mg I-~ for the lowest dilution rate studied, which increased to 950 mg 1-~ at dilution rates of 0.18 h - l and above. Contrary to this, the exit substrate concentration (S=) remained steady at about ( 1 6 0 m g l - ' COD) until a dilution rate of 0.18 h- l dilution or above, indicating that some of the substrate in the feed was utilized for purposes other than building bacterial mass, i.e. maintenance, Further treatment of the yield data will be published elsewhere, The kinetic coefficients for the wastewater containing 5.0 mg 1-~ Ni(II) were determined from the Lineweaver-Burk plot of the data, as shown in Fig. 6. The #m and Ks values were computed as 0.67h -~ and 9 7 m g l - ' , respectively, from this plot. The value calculated for the maximum specific growth rate, / ~ , from the non-steady-state dilution experiments depicted in Fig. 6 was 0.59 h-L

Adding lO.Omg1-1 Ni(H) Experiments with 10.0mgl -l Ni(II) were commenced after the usual acclimatization of the culture

to the present concentration of the heavy metal was complete. From the dilute-out curve shown in Fig. 7 it was judged that the critical dilution rate of the culture decreased to around 0.30h -~, close to the baseline value of 0.32 h-l. Both values were considerably smaller than those obtained with 5.0rag 1-1 Ni(II) (0.58 h-l). As can be seen from Fig. 7, the biomass concentration remained more or less constant at 600 mg lover a wide range of dilution rates. The biomass concentration was highest, 950mgl - ' , in the ease of 5.0 mg 1- l Ni(II) and around 480 mg 1- t in the baseline study. Moreover, there was no significant difference between the substrate removal efficiencies at 5.0 and 10.0 mg 1-~ Ni(II) cases. Apparently, the effect of maintenance disappeared in the case of 10.0 mg 1- ] Ni(II), as indicated by the plateau section of the biomass curve over the dilution rates below 0.16 h -l in Fig. 7. The maximum specific growth rate and K, values were determined from the Lineweaver-Burk plot as 0.32h -~ and 105mgl -~, respectively. The value determined from the non-steady-state dilution experiment, 0.29 h -~, matched considerably with that found from the Lineweaver-Burk plot.

Adding25.0mg1-1 Ni(H) The activated-sludge unit could not be operated at truly steady-state conditions at this nickel concen-

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Effect of nickel(II) on activated sludge Table 2. Biokineticconstants derived from different experiments Type of Dilute-out Lineweaver- N0n-steady experiment curve Burk plot dilute-out Baseline study De ffi0.32 h -~ /~m•0.31 h-' /tm= 0.41 h -~ K, ffi85 mgl-' 5.0 mgl -j Ni(lI) Dc = 0.58 h -t pm ffi0.67 h -I /~mffi0.59 h -j K,=97mgl-' 10.0mgl-' Ni(II) Dcf0.30h -I p~f0.32h -j p-m=0.29h-~ /(1 ffi 105 mg I-'

tration. However, the unit could be maintained at a constant dilution rate for over 1 m o n t h without completely washing-out. The measurements o f M L S S concentration were carried out in the second week, as shown in Fig. 8. The M L S S concentration was found to fluctuate in a completely random manner, as can be seen from the straight line obtained when biomass concentration values were plotted on a probability graph. This, in turn indicated t h a t / ~ oscillates to and fro with concomitant wash-out and no-wash-out cycles manifesting themselves as fluctuations in the biomass concentration in the reactor. DISCUSSION AND CONCLUSION The effect of nickel on a once-through activatedsludge unit performance was evaluated in this study and the biokinetic constants derived are summarized in Table 2. Different concentrations o f Ni(II) (5.0, 10.0, 25.0 mg 1-1) were dosed to the activated-sludge unit t r e a t i n g a s y n t h e t i c w a s t e w a t e r w i t h 1 3 0 0 m g l -~ C O D . Recycle was deliberately omitted for greater accuracy in determination o f the biokinetic constants. U p to 10.0mg1-1 Ni(II) did not adversely affect the activated-sludge kinetics, as judged from the kinetic constants summarized in Table 2. However, a 25.0mg1-1 Ni(II) concentration led to unpredictable oscillations in the rate of growth of microorganisms. These oscillations manifested themselves as r a n d o m fluctuations in the biomass concentration. In spite of the oscillations, the culture could be maintained for over 1 m o n t h at a pseudo-steadystate condition, without completely washing-out. The increase in biomass concentration in the tank during continuous operation indicated that microorganisms grew faster than the dilution rate at those instances. This phenomenon remained unexplained in this study. Concentrations of 5.0 mg 1-1 Ni(II) improved the performance of the unit to a great extent. The m a x i m u m specific growth and dilution rates of the culture almost doubled, while K, remained more or less constant with respect to the baseline study.

1007

However, the most drastic increase was observed in biomass concentration, where the biomass concentration doubled as compared to the baseline study. Another improvement observed with a 5.0 mg mg 1-1 Ni(II) concentration was better sludge settleability. Acknowledgements--The authors are grateful to The Middle East Technical University for financially supporting this study by AFP Grant No. AFP-85-03-11-01. Technical assistance by K. Demirtas is also deeply appreciated.

REFERENCES AHPA (1975) Standard Methods for the Examination o f Waste and Wastewater, 13th edition. American Public Health Association, Washington, D.C. Bagby M. M. and Sherrard J. H. (1981) Combined effects of Cd and Ni on the activated sludge process. J. Wat. Pollut. Control Fed. 53, 1609-1619. Barth E. F., Ettinger M. G., Salotto B. V. and McDermott G. N. (1965) Summary report on the effects of heavy metals on the biological treatment processes. J. Wat. Pollut. Control Fed. 37, 86-96. Bertrand D. and DeWolf A. (1967) C.r. hebd. seanc. Acad. Sci. Ser. 265, 2231-2235. Cited in Advances in Applied Microbiology, Vol. 29 (Edited by Laskin A. I.), p. 215. Academic Press, New York (1983). Chang S. Y., Huang J. C. and Liu Y. C. (1986) Effects of Cd(II) and Cu(II) on a biofilm system. J. envir. Engng 112, 94-104. Henriksson L. E. and DaSilva E. J. (1978) Z. allg. Microbiol. 18, 487-494. Cited in Advances in Applied Microbiology, Vol. 29 (Edited by Laskin A. I.), p. 213. Academic Press, New York (1983). Hutchinson T. C. (1981) Effects o f Heavy Metal Pollution on Plants (Edited by Lepp N. W.), pp. 171-211. Applied Sciences, London. Cited in Trace Elements in The Terrestrial Environment (Edited by Adriano D. C.), p. 371. Springer, New York (1986). Lowry O. H., Rosebrough N. J., Farr A. L. and Randall R.J. (1951) Protein measurement with the Folin phenol reagent. J. biol. Chem. 193, 265-275. McDermott G. N., Post M. A., Jackson B. N. and Ettinger M . B . (1965) Nickel in relation to activated sludge and anaerobic digestion process. J. War. Pollut. Control Fed. 37, 163-177. Neufeld R. D. and Hermann E. R. (1976) Heavy metal removal by acclimated sludge. J. War. Pollut. Control Fed. 47, 310-329. Pitt S. J. (1975) Principles o f Microbe End Cell Cultivation. Blackwell Scientific, Oxford. Poon C. P. C. and Bhayani K. H. (1971) Metal toxicity to sewage organisms. J. sanit. Engng Die. Am. Soc. cir. Engrs 97, 161-169. Sujarittanonta S. and Sherrard J. H. (1981) Activated sludge nickel toxicity studies. J. War. Pollut. Control Fed. 53, 1314-I 322. Trahern P. G., Knocke W. R. and Sherrard J. H. (1980) The effect of Ni(II) on the nitrification in the activated sludge process. A.I.Ch.E. Syrup. Ser. 77, 171-176. Weber A. S. and Sherrard J. H. (1980) Effects of cadmium on the completely mixed activated sludge process. J. Wat. Pollut. Control Fed. 52, 2378-2388.