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Reuse of biologically regenerated activated carbon for phenol removal
PII: S0043-1354(97)00337-0
Wat. Res. Vol. 32, No. 4, pp. 1085±1094, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00
REUSE OF BIOLOGICALLY REGENERATED ACTIVATED CARBON FOR PHENOL REMOVAL IVANA IVANCEV-TUMBAS*, BOZO DALMACIJA, ZAGORKA TAMAS and ELVIRA KARLOVIC Institute of Chemistry, Faculty of Sciences, 21000 Novi Sad, Trg D. Obradovica, 3, Yugoslavia (First received November 1996; accepted in revised form August 1997) AbstractÐGranular biologically activated carbon (GBAC) is in principle suited for removal of phenolic substrate from wastewater. In this study a wastewater model was used and special attention was paid to the operation of separate aerobic bioregeneration of carbon and reuse of bioregenerated carbon in a new stage of the experiment. In order to assess the eectiveness of bioregeneration, the eciency of the biosorption system both before and after its bioregeneration was monitored at the substrate concentrations ranging from 1.9 to 1053 mg lÿ1. All kinetic parameters for the biosorption system were determined for both fresh and already used GBAC. A 92±100% eciency of phenol removal was achieved irrespective of whether the fresh or bioregenerated GBAC was used. The process of bioregeneration for the once-used GBAC was faster than for the one that was used several times. It was also faster when phenol alone was used as substrate than in the case of a phenol mixture. Respirometric measurements indicated that a mixture of substituted phenols showed inhibition eects on microorganisms. Calculations based on these measurements, as well as the ®ndings of GC/MS analysis, indicated that deterioration of the adsorbent quality was taking place continuously. It was concluded that bioregeneration, carried out as a separate operation, could not be recommended for practice. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐbioregeneration, granulated biologically activated carbon, phenol, biodegradation, respirometry, biosorption
INTRODUCTION
Numerous ecotoxicological studies showed high toxicity of phenols (KuÈhn et al., 1989; Bryant and Schultz, 1994), 11 of them being included in the USEPA Priority Pollutant list (Bigley and Grob, 1985). Phenols appear in water as a result of both humi®cation process and anthropogenic activities. Use of a biosorption system with granular biologically activated carbon (GBAC) in water treatment has become a subject of intensive research in the last decades (Sutton and Mishra, 1994). Ehrhardt and Rehm (1985) proved that Pseudomonas sp. and Candida sp. immobilized onto activated carbon are more resistant to phenol than the suspended cells of these species. GBAC represents a kind of a depot of the substrate and oxygen needed for the microorganisms present on the adsorbent, and protects them from too high concentrations of the substrate or toxic substances. Microorganisms regenerate surface of activated carbon using organic substrate as a source of food and energy. The process of substrate removal includes the adsorption onto activated carbon surface, adsorption onto the bio®lm, and *Author to whom all correspondence should be addressed.
microbiological oxidation (MoÈrsen and Rehm, 1990; Xiaojian et al., 1991). Thermal regeneration of activated carbon substantially increases the costs of water treatment. On the other hand, the bioregeneration in situ extends the time of potential usage of activated carbon, and thus reduces the costs of the system's operation (Servais et al., 1991). The aim of this work was to study the possibility of separately reusing bioregenerated GBAC for continuous removal of phenolic substrate from a wastewater model, and to compare its eciency with that of fresh GBAC. The eciency of phenol removal and the kinetic parameters of the biosorption system was determined before and after the bioregeneration. Potential applicability of respirometric methods for monitoring microbiological regeneration of activated carbon was also investigated.
MATERIALS AND METHODS
Apparatus and experimental procedure All experiments were carried out on the laboratory setup schematically presented in Fig. 1. The apparatus consisted of a glass column, (i.d. 6 cm and height 150 cm), ®lled up with GBAC ``NORIT ROW 0.8 SUPRA'', a tank of model wastewater, and a pump to ensure an upward water ¯ow in the column. Air was intro-
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Ivana Ivancev-Tumbas et al. euent had become equal.Bioregeneration was performed under aerobic conditions. After the bioregeneration, the carbons from stages IIa and b were mixed and used in the third stage of the experiment. Total mass of GBAC in the column was 990 g. Phenol was used as substrate in a concentration range of 85.2±105.9 mg lÿ1. The applied water ¯ow rates were 23.2, 77.5 and 96.9 l dayÿ1. When phenol concentrations in the in¯uent and euent were equal, the water ¯ow was stopped and another aerobic bioregeneration of GBAC was performed. Analytical procedures
Fig. 1. Schematic of the setup. I, Wastewater tank; II, Pump; III, Column with GBAC; IV, Sintered glass plate; 1, 2, and 3: Taps for sampling.
duced through a sintered glass plate at the bottom part of the column with the aid of a compressor. Before beginning the experiment, GAC was colonized by the microorganisms previously adapted to phenol. The adaptation was performed in a laboratory setup by activated sludge procedure: microorganisms were cultured on special supports (Petrovic, 1984), and then put into a continuous active sludge reactor. The water ¯owing through the reactor contained phenol and the necessary nitrogen and phosphorous nutrients. The adaptation lasted 2 months. After that the contents of the reactor were decanted and the liquid phase was put into the GAC column. The experiment consisted of 3 stages. In each of them water containing phenols was passed continuously, and after each stage water ¯ow was stopped without breaking aeration which was continued for a certain period of time. In this way, separate operations of carbon bioregeneration were performed in the column. In stage I of the experiment the glass column was ®lled up with 1350 g of bioregenerated carbon, used in previous investigations to remove ekalux (Dalmacija et al., 1992) and methyl orange (Karlovic et al., 1994) from water. The model wastewater was introduced into the column at a ¯ow rate of 20 l dayÿ1. Concentrations of phenol were 1.9; 45.4; 105.5; 236.2; 455.5 mg lÿ1. For each concentration, the experiment lasted 10 days. After that the water was introduced into the column at a ¯ow rate of 41 l dayÿ1 at a substrate concentration of 1053 mg lÿ1. When the phenol concentrations in the in¯uent and euent had become equal, the water ¯ow was stopped and aeration continued. The column was aerated during a 2-month period, and after that the second stage of the experiment started. Two setups of identical construction to that in stage I were used in the second stage. In stage IIa, the column was ®lled with 980 g of bioregenerated carbon, remaining from stage I (the rest was used for respirometric measurements and determination of phenol content on carbon during previous stage). The second column was ®lled with 500 g of fresh carbon (phase IIb). The model wastewater containing a mixture of phenols (phenol 60%, 4-nitrophenol 2.8%, 2,4-diaminophenol 10%, pyrogallol 2% and 3-nitrophenol 0.2%) was introduced in both columns at the same time. Total phenol concentration was in the range of 10±40 mg lÿ1. Flow rates through the ®rst column (stage IIa) were 16.4, 24.1, 47.1 and 92.1 l dayÿ1, and through the second (stage IIb) 11.1, 27.1, 50.1, 60.3 and 96.5 l dayÿ1. Each ¯ow was applied for 15 days. The water-¯ow was stopped at the highest ¯ow rate in both setups, when the phenol concentrations in the in¯uent and
Phenol concentrations in the in¯uent, euent and on carbon were monitored by spectrophotometric method, using 4-aminoantipyrine (APHA, 1989). Activity of microorganisms present on carbon was estimated using Warburg's apparatus (model V 166, B. Braun) (Dalmacija et al., 1996). An amount of carbon (1± 3 g) was taken from the column for each applied concentration (or ¯ow rate) during water puri®cation, and several times during each stage of bioregeneration. Samples were placed in the bottle with 40 ml of dilution water for BOD5 (APHA, 1989) and 1 ml of 10% KOH solution. Oxygen consumption by the bio®lm attached onto carbon was monitored for 5 to 7 days for each carbon sample. Carbon samples (5 g) were taken from the column before the beginning of stage III, during the course of stage III, and in the last bioregeneration. They were extracted using the Soxhlet apparatus with 50 ml of hexane, methylene chloride and methanol successively, 8 h with each solvent. Extracts were evaporated to dryness under the nitrogen atmosphere and dissolved in 5 ml of the solvent. One part of each of the methanolic extracts was silylated (BSTFA: TMCS: TMSi = 3:2:3). At the end of stage III of the experiment, an euent sample (1 l) was taken for qualitative analysis by GC/MS technique. It was acidi®ed to pH < 2 with sulfuric acid and extracted 3 times with 60 ml of methylene chloride. Extracts were concentrated to 1 ml by roto-evaporation and 1 mL was injected in gas chromatograph. All GC/MS analyses were performed on a HewlettPackard 5890 Series II gas chromatograph with mass selective detector Hewlett-Packard 5971A, using Ultra 2 column (25 m 0.2 mm 0.33 mm). The injection temperature and detector temperature were 2508C and 2808C, respectively. Helium ¯ow rate was 1 ml minÿ1. The results were compared to mass spectra from the commercial WILEY library. Only those spectra with correlations above 70% were taken into account. In the course of the last bioregeneration, microbiological tests by standard cultivation methods on nutritive supports were carried out (Petrovic, 1984).
RESULTS AND DISCUSSION
The rate and eciency of phenol removal were calculated from the results presented in Table 1 and the amount of carbon ®lling for each stage, given in Experimental Procedure. Phenol concentrations presented in Table 1 are average values of individual daily measurements for each applied concentration of phenol in the ®rst stage and for each applied ¯ow rate in the second and third stages of the experiment. Coecients of speci®c rates of oxygen consumption in the bio®lm were calculated as a slope of the linear dependence (r = 0.90±0.99) of the speci®c oxygen consumption in the bio®lm (in mg O2 gÿ1
Phenol removal using GBAC Table 1. Results of phenol removal by the biosorption system Phenol concentration (mg lÿ1) Phase of system's operation I
P (l dayÿ1) 20 20 20 20 20 41.0 16.4 24.1 47.1 92.1 11.1 27.1 50.1 60.3 96.5 23.2 77.5 96.9
IIa
IIb
III
In¯uent
Euent
1.9 45.4 105.5 236.2 455.5 1053 25.8 12.9 21.7 36.2 24.4 13.5 26.8 27.6 37.6 85.2 105.9 92.5
0.14 0.19 0.17 0.50 1.46 480.3 0.08 0.04 0.63 22.3 0.11 0.25 0.83 10.7 22.3 0.002 0.35 51.4
carbon) during days 5±7 of the experiment. Table 2 shows the data obtained for the ®rst stage of the experiment. Similar results were obtained for stages II and III. Figures 2±5 illustrate how the coecient of speci®c rate of oxygen consumption in the bio®lm, (k), the rate and eciency of substrate removal and phenol content on carbon depend on phenolic load in the respective stages (I, IIa, IIb and III) of the experiment. With increasing phenolic load in stage I in the range of 0.04±31.98 mg gÿ1, the rate of substrate removal increased in the range of 0.03±17.40 mg gÿ1 dayÿ1. The eciency of substrate removal was in the range of 92.5±99.7% up to the phenolic load of 9.38 mg gÿ1 dayÿ1, and afterwards showed a decrease. The activity of microorganisms, determined on the basis of oxygen consumption, increased with increasing phenolic load, as well as the content of phenol on carbon. It can be concluded that phenolic load above 9.38 mg gÿ1 dayÿ1 was too high to achieve good eciency, despite of a high rate of substrate removal and well-developed micro¯ora.
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In stage IIa, an increase in the phenolic load from 0.43 to 3.40 mg gÿ1 dayÿ1 was accompanied by an increase in the rate of substrate removal in the range of 0.43±1.30 mg gÿ1 dayÿ1, and decrease in the removal eciency from 99.7, through 97.1 to 38.3%. It was found that the rate of removal of the mixture of phenols in stage IIa was 13 times lower compared to the removal of phenol alone. The activity of microorganisms showed ®rst an increase with increasing phenolic load, and then started to decrease in parallel with decrease in the substrate removal eciency, which can be a consequence of an inhibition process caused by substituted phenols. In stage IIb, an increase in the phenolic load in the range of 0.54±7.25 mg gÿ1 dayÿ1 caused an increase in the removal rate in the range of 0.54± 2.94 mg gÿ1 dayÿ1, whereas the process eciency dropped from 99.4 to 40.6%. During this stage, there was a certain period of microorganisms adaptation to the conditions in the biosorption system, and their activity increased when the system's eciency started to decrease (phenolic load of 3 mg gÿ1 dayÿ1), whereas the concentration of phenols on carbon showed a constant increase. This can be explained by the use of fresh carbon, on which micro¯ora introduced into the system was in the stage of biocenose formation. By comparing the operation of stages IIa and IIb it can be concluded that the system with the fresher GBAC attained a higher rate of substrate removal at the same phenolic load, which can be explained by incomplete reactivation of carbon surface in the course of its bioregeneration. In stage IIb there was no fall in the activity of microorganisms, which can be explained by higher adsorptive and protective activity of the fresh carbon. In stage III of the experiment, at increased phenolic load in the range of 0.01±9.06 mg gÿ1 dayÿ1 the rate of phenol removal increased in the range of 0.01±4.03 mg gÿ1 dayÿ1, whereas the removal eciency decreased from 100% through 99.7 to 44.5%. A longer period of adaptation of microorganisms, which is evident from Fig. 5, is probably a consequence of another change in the substrate.
Table 2. Results of respirometric measurements in stage I of the experiment Phenol concentration in the in¯uent (mg lÿ1) 45.4 time (h) 2 24 28 49 73 96 118 Ð Ð Ð Ð
105.5
mg O2 gÿ1 carbon 0.208 0.731 1.551 2.222 2.539 2.539 2.964 Ð Ð Ð Ð
236.2
time (h)
mg O2 gÿ1 carbon
1.5 5.5 10.5 25.5 30.5 48.5 73.5 93.5 Ð Ð Ð
0.357 0.998 1.606 2.709 3.195 4.055 5.109 6.082 Ð Ð Ð
455.5
time (h)
mg O2 gÿ1 carbon
4.0 10.0 21.5 31.0 46.5 69.5 101 120 Ð Ð Ð
0.931 1.467 1.467 1.915 2.31 3.828 4.993 5.513 Ð Ð Ð
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time (h)
mg O2 gÿ1 carbon
time (h)
mg O2 gÿ1 carbon
4.0 9.0 14.7 23.0 33.0 37.8 50.5 60.2 72.3 86.2 96.0
0.742 2.655 5.432 8.198 10.026 10.756 12.598 14.606 15.116 15.570 16.023
3.0 10.0 22.0 27.0 32.5 46.0 52.0 58.0 71.0 Ð Ð
0.346 0.788 2.835 5.478 8.143 10.387 11.792 12.977 14.828 Ð Ð
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Fig. 2. Dependence of the rate of substrate removal, eciency of phenol removal, phenol content on carbon, and the coecient of speci®c rate of oxygen consumption in the bio®lm on the phenolic load in stage I of the experiment.
The activity of microorganisms upon this adaptation period increased again and reached a plateau of 0.1 hÿ1. During this stage, the phenol concentration on carbon showed an increase. By comparing the processes in stages III and IIb it can be concluded that, at the same phenolic load, the system attains a higher removal rate when phenol alone is involved (stage III), regardless of bioregenerated carbon. This can be explained again by the supposition that some substituted phenols present in the mixture can exhibit an inhibitory eect (Beltrame et al., 1988). The lack of correlation between the phenol concentration on carbon and the phenolic load of the system indicates that phenol adsorption on carbon and its microbiological oxidation take place simultaneously, which is also evident from Figs 2±5.
For stages II and III, the coecients of total rate of substrate removal (ks) were calculated on the basis of the Atkinson model and the Eckenfelder equation for the bio®lter (Metcalf and Eddy, Inc., 1979): Se =S0 eÿksSa ZA=P Se=substrate concentration in the euent S0=substrate concentration in the in¯uent Sa=speci®c surface of the ®lling Z=adsorber's height ks=coecient of total rate of substrate removal P=¯ow rate of the wastewater
The logarithmic form of the equation gives a linearized dependence: ÿln Se =S0 ks V=P where V is the bio®lter volume.
Fig. 3. Dependence of the rate of substrate removal, eciency of phenol removal, phenol content on carbon, and the coecient of speci®c rate of oxygen consumption in the bio®lm on the phenolic load in stage IIa of the experiment.
Phenol removal using GBAC
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Fig. 4. Dependence of the rate of substrate removal, eciency of phenol removal, phenol content on carbon, and the coecient of speci®c rate of oxygen consumption in the bio®lm on the phenolic load in stage IIb of the experiment.
Fig. 5. Dependence of the rate of substrate removal, eciency of phenol removal, phenol content on carbon, and the coecient of speci®c rate of oxygen consumption in the bio®lm on the phenolic load in stage III of the experiment.
Graphical presentation of the latter expression rendered the data given in Table 3, with the coecient ks, being the slope of the obtained straight line. Speci®c carbon surface was not taken into account. The values for ks are approximately equal for stages IIa and IIb of the system's operation, because the same substrate (mixture of phenols)
was involved. For stage III, where the substrate was only phenol, ks increased to 124.5 dayÿ1, which supports the hypothesis on the inhibitory eect of substituted phenols. According to the Atkinson model, the intercept should have the value 0, but this happens only when organic matter is removed by sorption on the bio®lm and microbiological oxidation.
Table 3. Coecient of total rate of substrate removal Stage -ln(Se/So) V/P (day) ks (dayÿ1) Intercept R
IIa 5.7 0.13
5.8 3.5 0.09 0.05 47.2 0.46 0.91
IIb 0.5 0.02
5.4 0.10
4.0 0.04
3.5 0.02 46.9 1.1 0.82
III 1.0 0.02
0.5 0.01
10.8 0.09
5.7 0.03 124.5 ÿ0.11 0.92
0.6 0.02
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Table 4. Coecient of speci®c rate of oxygen consumption Regeneration time (day) 75 89 96
k (hÿ1) bottom
middle
top
0.015 0.029 0.098
0.006 0.078 0.075
0.008 Ð 0.025
If there is another way of organic matter removal involved (e.g. adsorption), the straight line has a positive intercept. If, however, the euent pollution takes place, then the resulting straight line has a negative intercept. For stages II and III the intercept has a positive value, which means that, apart from microbiological process, adsorption is also involved in phenol removal. The latter process was less pronounced in the case of the biologically regenerated carbon (intercept: 0.46), than in the case of a fresh carbon (intercept: 1.1). However, a negative value of ÿ0.11 obtained for the intercept for stage III of the system's operation suggests the occurrence of deterioration of the euent quality, probably caused by lysed biomass, or by desorption of the already removed material. Euent in stage III had become yellow, and both 4-nitrophenol and ekalux were identi®ed by GC/MS analysis. After stage I, and a 75-day long interruption in the system's operation, samples of carbon from the bottom, middle and top of the column were taken for respirometric analysis. It appeared that the microbiological activity on the carbon was still well developed. The results of respirometric analysis are given in Table 4. An increase in the coecient of speci®c rate of oxygen consumption in the absence
of organic load indicates the presence of a live microbiological activity. During the bioregeneration operation after stages II and III of the experiment samples of carbon were taken after phase IIa from the top, after phase IIb from the bottom, and after phase III from the middle of the column. The results of monitoring the bioregeneration process are presented in Figs 6±8 for each regeneration stage, respectively. A decreasing trend is evident for both the phenol content on carbon and microbiological activity. When the wastewater in¯ow was stopped, the microorganisms continued oxidation of phenols adsorbed on carbon surface (under aerobic conditions). In this way, the surface of the biosorption system ®lling was regenerated. The coecient of speci®c rates of oxygen consumption has gradually approached the value obtained in respirometric measurements for the unused carbon sample (blank assay) of 0.004 hÿ1. The rate of microbiological regeneration can be described by the formula: y aeÿkx where y and a can have a twofold meaning, namely y=content of phenol on carbon (solid line in Figs 6±8), or the constant of the speci®c rate of bio®lm oxygen consumption (dashed line in Figs 6±8); a=initial phenol content on carbon, or the initial constant of the speci®c rate of bio®lm oxygen consumption; and k=coecient of microbiological regeneration of activated carbon; x=time of microbiological regeneration.
The results of statistical data treatment are presented in Table 5. Coecient rates of microbiological regeneration of the once used carbon (IIb stage)
Fig. 6. Phenol content on carbon and the coecient of speci®c rate of oxygen consumption during the bioregeneration after phase IIa of the experiment.
Phenol removal using GBAC
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Fig. 7. Phenol content on carbon and the coecient of speci®c rate of oxygen consumption during the bioregeneration after phase IIb of the experiment.
Fig. 8. Phenol content on carbon and the coecient of speci®c rate of oxygen consumption during the bioregeneration after phase III of the experiment.
are 2±3 times higher than the coecient for the already regenerated carbon (IIa stage). The process of biological regeneration of carbon was slower when the substrate was a mixture of phenols than phenol alone (stage III). Also, it can be concluded that the coecient of speci®c rates of oxygen con-
sumption better describes the process of microbiological regeneration (R = 0.86±0.97) than the phenol content on carbon. This is probably related to the problem of phenol desorption, its polymerization on carbon (Grant and King, 1990, Cooney and Xi, 1994; Vidic et al., 1994), reaction with the poten-
Table 5. Rate coecient for microbiological regeneration of activated carbon case 1 Regeneration after the stage IIa IIb III
case 2
k(dayÿ1)
R
k(dayÿ1)
R
0.047 0.082 0.206
0.75 0.75 0.92
0.048 0.122 0.159
0.94 0.97 0.86
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Ivana Ivancev-Tumbas et al.
possible. On the other hand, proteolytes, saccharolytes, and lipolytes probably used the lysed biomass as substrate. Findings of qualitative GC/MS analyses carried out before the beginning of stage III, during the course of stage III, and in the last bioregeneration stages, are summarized in Table 6. Identi®cation of aminophenol and nitrophenol suggests that these compounds are hard biodegradables, as the time elapsed from their use was more than 4 months. An indication for this may also be the calculated coecient of total rate of substrate removal (ks), which for stage II was about 47 dayÿ1 and for stage III almost 3 times higher. The metabolic products of phenol, mentioned in the literature, were not identi®ed, which could be explained by complete degradation of phenol to carbon (IV)oxide and water. Phenol polymerization was evidenced by the presence of phenoxyphenol and biphenyl, fouling a portion of the adsorbent surface. The presence of ekalux and sulphur was due to previous usage of the same carbon. The origin of fatty acids and their methyl esters (probably formed by methylation in the Soxhlet apparatus) may be the lysed biomass serving as substrate for lipolytes in the system. All the above observations indicate, the deterioration of GBAC surface quality takes place not only in the course of the system's operation, but also in its bioregeneration. The cause is either the adsorption of the lysed cells, hard-biodegradable
Fig. 9. Population of aerobic heterotrophs in the last bioregeneration phase.
tially present less-biodegradable compounds, or with some metabolic products. These ®ndings show that respirometric methods can be used to monitor the process of microbiological regeneration of carbon in the biosorption system. Population of microorganisms on GBAC was monitored during the last period of regeneration. The results are presented in Fig. 9. Count of heterotrophs showed ®rstly an increase, then a decrease, and then a further increase. The count of phenol-oxidizing bacteria showed an increase, which can be explained by phenol desorption from carbon, taking place for as long as
Table 6. Compounds identi®ed on carbon. A±D denote respectively the pure unused carbon (blank), carbon bioregenerated after stage IIa, after stage IIb, and in the course of stage III Day of bioregeneration Compound [1,1'-Biphenyl]-4,4'diol 1-Hexanol, 2-ethyl2-Butenoic acid 8-Quinolinol, 2-methyl9-Hexadecenoic acid, methyl ester 9-Octadecenoic acid 9-Octadecenoic acid, methyl ester Aniline Benzene, nitroBenzo-[b]-naphtho-[2,3-d]-furan (8Cl 9Cl) Benzoic acid, 2-hydroxy-, phenyl ester Benzoic acid, 2-hydroxy-methyl ester Cyclohexadecane Cyclotetradecane Ekalux Hexadecanoic acid Hexadecanoic acid, methyl ester Hydrocarbons > C20 Hydrochloride of 2-(Dimethylamino)-4,5-diphenylimid Nitrophenols Octadecanoic acid, methyl ester Pentadecanoic acid, 14-methyl-, methyl ester Phenol Phenol, 2-aminoPhenols, phenoxyPhthalates Pyrido[3, 2-d]pyrimidine-2, 4 (1H,3H)-dione Sulphur Undecane
A*
B*
C*
D*
+ + +
0
1
4
7
11
+
+
+ + +
+
+
+ +
+
+ +
+ + +
+ +
+ +
+
+ +
+ + + + +
+ +
+ + +
+
+ + +
+
+
+ + +
+ + + + + +
+
+
+ +
+ + + +
+ +
+
+
+
+
19
+
+ + +
14
+ +
+ +
+ +
+ + + +
+
+ + + + + +
+
+
+
+ +
+ +
+
+ +
+
+ +
+ +
+ +
+
+ +
Phenol removal using GBAC
substances and metabolites, or of their reactions taking place in the system. Bioregeneration can hardly be a way of avoiding deterioration of the adsorbent quality. A positive characteristic is that this operation can prolong the usage time, thus making the process less costly. Substances adsorbed on carbon either during the system's operation, or in the process of its bioregeneration, represent a serious danger to the euent quality, as their desorption may take place. Desorption of 4-nitrophenol and ekalux that occurred in stage III of the experiment con®rmed the supposed deterioration of the euent quality by desorption of the already removed matter, postulated on the basis of a negative value of the intercept in the calculation of the coecient of total rate of substrate removal. CONCLUSION
By carrying out several separate operations of GBAC bioregeneration in the biosorption system for removal of phenols from a wastewater model it was shown that a high eciency of phenols removal (exceeding 99.5%) can be achieved, even with the carbon that has been used and bioregenerated several times. However, it appears that the rate of substrate removal showed a slower increase with increasing phenolic load with the bioregenerated than with fresh carbon. The contribution of adsorption to the process of phenols removal was greater in the system with fresh carbon. The dependence of the substrate removal rate and the coecient of speci®c rates of oxygen consumption in the bio®lm on phenolic load indicated the occurrence of an inhibitory eect of substituted phenols. Namely, the coecient of total rates of substrate removal was 124 dayÿ1 when the substrate was phenol alone, and about 45 dayÿ1 in the case of a mixture of substituted phenols. It was found that the bioregeneration process is faster with the carbon that was previously used just once than with that used and bioregenerated several times. The process was also in¯uenced by the nature of the substrate involved in the stage of system's operation. The qualitative GC/MS analysis showed that deterioration of adsorbent quality occurred both in the stage of system's operation and its bioregeneration. Hence, it is not plausible to carry out bioregeneration as a separate operation. Instead it is better to carry out the process at an optimal phenolic load ensuring a high microbiological activity and thus preserving the functional adsorption capacity of the adsorbent. In our experiment this was achieved for each separate stage at approximate loads of 10, 1, 2 and 2 mg gÿ1 dayÿ1 for stages I, IIa, IIb and III, respectively. Special attention should be paid to the possible desorption of the already adsorbed matter and lysed biomass.
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However, it is still an open question of the applicability of separate bioregeneration in a battery of several biosorption systems linked in series, one of them always working as bioregenerator. AcknowledgementsÐThe authors acknowledge ®nancial support by the Ministry for Science and Technology of Serbia.
REFERENCES
APHA-AWWA-WPCF (1989) Standard Methods for the Examination of Water and Wastewater, 17th edition, American Public Health Association, Washington. Beltrame P., Beltrame P. L., Carniti P. and Lanzetta C. (1988) New results on the inhibiting action of chloroand nitro-substituted phenols on the biodegradation of phenol. Chemosphere 17(2), 235±242. Bigley F. P. and Grob R. L. (1985) Determination of phenols in water and wastewater by post-column reaction detection high-performance liquid chromatography. J. Chromatogr. 350, 407±416. Bryant S. E. and Schultz T. W. (1994) Toxicological assessment of biotransformation products of pentachlorophenol: Tetrahymena population growth impairment. Arch. Environ. Contam. Toxicol. 26, 299±303. Cooney D. O. and Xi Z. (1994) Activated carbon catalyzes reactions of phenolics during liquidÐphase adsorption. AIChE J. 40(2), 361±364. Dalmacija B., Tamas Z., Miskovic D., Karlovic E. and Petrovic O. (1992) The biosorption of selected pesticides from water by biologically activated carbon. Wat. Sci. Technol. 26(9-11), 1991±1994. Dalmacija B., Tamas Z., Karlovic E. and Miskovic D. (1996) Tertiary treatment of oil-®eld brine in a biosorption system with granulated activated carbon. Wat. Res. 30(5), 1065±1068. Ehrhardt H. M. and Rehm H. J. (1985) Phenol degradation by microorganisms adsorbed on activated carbon. Appl. Microbiol. Biotechnol. 21, 32±36. Grant T. M. and King C. J. (1990) Mechanism of irreversible adsorption of phenolic compounds by activated carbons. Ind. Eng. Chem. Res. 29, 264±271. Karlovic E. Dalmacija B. Tamas Z., Petrovic O. and Murgul Lj. (1994) Removal of synthetic dyes using a biosorption system with granular activated carbon. Proceedings of the Meeting ``Water Protection `94'' Igalo, May 25±27, Yugoslavia, pp. 407±411. KuÈhn R., Pattard M., Pernak K. D. and Winter A. (1989) Results of the harmful eects of selected water pollutants (anilines, phenols, aliphatic compounds) to Daphnia Magna. Wat. Res. 23(4), 495±499. Metcalf, and Eddy, Inc. (1979) Wastewater Engineering: Treatment Disposal Reuse. 2nd Edition, New York: McGraw-Hill Book Company. MoÈrsen A. and Rehm H. J. (1990) Degradation of phenol by a de®ned mixed culture immobilized by adsorption on activated carbon and sintered glass. Appl. Microbiol. Biotechnol. 33, 206±212. Petrovic O. (1984) Microorganisms in wastewater puri®cation with a special reference to activated carbon procedure. Ph.D. thesis, University of Novi Sad, Yugoslavia. Servais P., Billen G., Ventresque C. and Bablon P. (1991) Microbial activity in GAC ®lters at the Choisy-le-Roi treatment plant. J. AWWA 83(2), 62±68. Sutton P. M. and Mishra P. N. (1994) Activated carbon based biological ¯uidized beds for contaminated water and wastewater treatment: a state-of-the-art review. Wat. Sci. Tech. 29(10-11), 309±317.
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Vidic R. D., Suidan M. T., Sorial G. A. and Brenner R. C. (1994) Eect of molecular oxygen on adsorptive capacity and extraction eciency of granulated activated carbon for three ortho-substituted phenols. J. Hazard. Mater. 38, 373±388.
Xiaojian Z., Zhansheng W. and Xiasheng G. (1991) Simple combination of biodegradation and carbon adsorptionÐthe mechanism of the biological activated carbon process. Wat. Res. 25(2), 165±172.
Wat. Res. Vol. 32, No. 4, pp. 1085±1094, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00
REUSE OF BIOLOGICALLY REGENERATED ACTIVATED CARBON FOR PHENOL REMOVAL IVANA IVANCEV-TUMBAS*, BOZO DALMACIJA, ZAGORKA TAMAS and ELVIRA KARLOVIC Institute of Chemistry, Faculty of Sciences, 21000 Novi Sad, Trg D. Obradovica, 3, Yugoslavia (First received November 1996; accepted in revised form August 1997) AbstractÐGranular biologically activated carbon (GBAC) is in principle suited for removal of phenolic substrate from wastewater. In this study a wastewater model was used and special attention was paid to the operation of separate aerobic bioregeneration of carbon and reuse of bioregenerated carbon in a new stage of the experiment. In order to assess the eectiveness of bioregeneration, the eciency of the biosorption system both before and after its bioregeneration was monitored at the substrate concentrations ranging from 1.9 to 1053 mg lÿ1. All kinetic parameters for the biosorption system were determined for both fresh and already used GBAC. A 92±100% eciency of phenol removal was achieved irrespective of whether the fresh or bioregenerated GBAC was used. The process of bioregeneration for the once-used GBAC was faster than for the one that was used several times. It was also faster when phenol alone was used as substrate than in the case of a phenol mixture. Respirometric measurements indicated that a mixture of substituted phenols showed inhibition eects on microorganisms. Calculations based on these measurements, as well as the ®ndings of GC/MS analysis, indicated that deterioration of the adsorbent quality was taking place continuously. It was concluded that bioregeneration, carried out as a separate operation, could not be recommended for practice. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐbioregeneration, granulated biologically activated carbon, phenol, biodegradation, respirometry, biosorption
INTRODUCTION
Numerous ecotoxicological studies showed high toxicity of phenols (KuÈhn et al., 1989; Bryant and Schultz, 1994), 11 of them being included in the USEPA Priority Pollutant list (Bigley and Grob, 1985). Phenols appear in water as a result of both humi®cation process and anthropogenic activities. Use of a biosorption system with granular biologically activated carbon (GBAC) in water treatment has become a subject of intensive research in the last decades (Sutton and Mishra, 1994). Ehrhardt and Rehm (1985) proved that Pseudomonas sp. and Candida sp. immobilized onto activated carbon are more resistant to phenol than the suspended cells of these species. GBAC represents a kind of a depot of the substrate and oxygen needed for the microorganisms present on the adsorbent, and protects them from too high concentrations of the substrate or toxic substances. Microorganisms regenerate surface of activated carbon using organic substrate as a source of food and energy. The process of substrate removal includes the adsorption onto activated carbon surface, adsorption onto the bio®lm, and *Author to whom all correspondence should be addressed.
microbiological oxidation (MoÈrsen and Rehm, 1990; Xiaojian et al., 1991). Thermal regeneration of activated carbon substantially increases the costs of water treatment. On the other hand, the bioregeneration in situ extends the time of potential usage of activated carbon, and thus reduces the costs of the system's operation (Servais et al., 1991). The aim of this work was to study the possibility of separately reusing bioregenerated GBAC for continuous removal of phenolic substrate from a wastewater model, and to compare its eciency with that of fresh GBAC. The eciency of phenol removal and the kinetic parameters of the biosorption system was determined before and after the bioregeneration. Potential applicability of respirometric methods for monitoring microbiological regeneration of activated carbon was also investigated.
MATERIALS AND METHODS
Apparatus and experimental procedure All experiments were carried out on the laboratory setup schematically presented in Fig. 1. The apparatus consisted of a glass column, (i.d. 6 cm and height 150 cm), ®lled up with GBAC ``NORIT ROW 0.8 SUPRA'', a tank of model wastewater, and a pump to ensure an upward water ¯ow in the column. Air was intro-
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Ivana Ivancev-Tumbas et al. euent had become equal.Bioregeneration was performed under aerobic conditions. After the bioregeneration, the carbons from stages IIa and b were mixed and used in the third stage of the experiment. Total mass of GBAC in the column was 990 g. Phenol was used as substrate in a concentration range of 85.2±105.9 mg lÿ1. The applied water ¯ow rates were 23.2, 77.5 and 96.9 l dayÿ1. When phenol concentrations in the in¯uent and euent were equal, the water ¯ow was stopped and another aerobic bioregeneration of GBAC was performed. Analytical procedures
Fig. 1. Schematic of the setup. I, Wastewater tank; II, Pump; III, Column with GBAC; IV, Sintered glass plate; 1, 2, and 3: Taps for sampling.
duced through a sintered glass plate at the bottom part of the column with the aid of a compressor. Before beginning the experiment, GAC was colonized by the microorganisms previously adapted to phenol. The adaptation was performed in a laboratory setup by activated sludge procedure: microorganisms were cultured on special supports (Petrovic, 1984), and then put into a continuous active sludge reactor. The water ¯owing through the reactor contained phenol and the necessary nitrogen and phosphorous nutrients. The adaptation lasted 2 months. After that the contents of the reactor were decanted and the liquid phase was put into the GAC column. The experiment consisted of 3 stages. In each of them water containing phenols was passed continuously, and after each stage water ¯ow was stopped without breaking aeration which was continued for a certain period of time. In this way, separate operations of carbon bioregeneration were performed in the column. In stage I of the experiment the glass column was ®lled up with 1350 g of bioregenerated carbon, used in previous investigations to remove ekalux (Dalmacija et al., 1992) and methyl orange (Karlovic et al., 1994) from water. The model wastewater was introduced into the column at a ¯ow rate of 20 l dayÿ1. Concentrations of phenol were 1.9; 45.4; 105.5; 236.2; 455.5 mg lÿ1. For each concentration, the experiment lasted 10 days. After that the water was introduced into the column at a ¯ow rate of 41 l dayÿ1 at a substrate concentration of 1053 mg lÿ1. When the phenol concentrations in the in¯uent and euent had become equal, the water ¯ow was stopped and aeration continued. The column was aerated during a 2-month period, and after that the second stage of the experiment started. Two setups of identical construction to that in stage I were used in the second stage. In stage IIa, the column was ®lled with 980 g of bioregenerated carbon, remaining from stage I (the rest was used for respirometric measurements and determination of phenol content on carbon during previous stage). The second column was ®lled with 500 g of fresh carbon (phase IIb). The model wastewater containing a mixture of phenols (phenol 60%, 4-nitrophenol 2.8%, 2,4-diaminophenol 10%, pyrogallol 2% and 3-nitrophenol 0.2%) was introduced in both columns at the same time. Total phenol concentration was in the range of 10±40 mg lÿ1. Flow rates through the ®rst column (stage IIa) were 16.4, 24.1, 47.1 and 92.1 l dayÿ1, and through the second (stage IIb) 11.1, 27.1, 50.1, 60.3 and 96.5 l dayÿ1. Each ¯ow was applied for 15 days. The water-¯ow was stopped at the highest ¯ow rate in both setups, when the phenol concentrations in the in¯uent and
Phenol concentrations in the in¯uent, euent and on carbon were monitored by spectrophotometric method, using 4-aminoantipyrine (APHA, 1989). Activity of microorganisms present on carbon was estimated using Warburg's apparatus (model V 166, B. Braun) (Dalmacija et al., 1996). An amount of carbon (1± 3 g) was taken from the column for each applied concentration (or ¯ow rate) during water puri®cation, and several times during each stage of bioregeneration. Samples were placed in the bottle with 40 ml of dilution water for BOD5 (APHA, 1989) and 1 ml of 10% KOH solution. Oxygen consumption by the bio®lm attached onto carbon was monitored for 5 to 7 days for each carbon sample. Carbon samples (5 g) were taken from the column before the beginning of stage III, during the course of stage III, and in the last bioregeneration. They were extracted using the Soxhlet apparatus with 50 ml of hexane, methylene chloride and methanol successively, 8 h with each solvent. Extracts were evaporated to dryness under the nitrogen atmosphere and dissolved in 5 ml of the solvent. One part of each of the methanolic extracts was silylated (BSTFA: TMCS: TMSi = 3:2:3). At the end of stage III of the experiment, an euent sample (1 l) was taken for qualitative analysis by GC/MS technique. It was acidi®ed to pH < 2 with sulfuric acid and extracted 3 times with 60 ml of methylene chloride. Extracts were concentrated to 1 ml by roto-evaporation and 1 mL was injected in gas chromatograph. All GC/MS analyses were performed on a HewlettPackard 5890 Series II gas chromatograph with mass selective detector Hewlett-Packard 5971A, using Ultra 2 column (25 m 0.2 mm 0.33 mm). The injection temperature and detector temperature were 2508C and 2808C, respectively. Helium ¯ow rate was 1 ml minÿ1. The results were compared to mass spectra from the commercial WILEY library. Only those spectra with correlations above 70% were taken into account. In the course of the last bioregeneration, microbiological tests by standard cultivation methods on nutritive supports were carried out (Petrovic, 1984).
RESULTS AND DISCUSSION
The rate and eciency of phenol removal were calculated from the results presented in Table 1 and the amount of carbon ®lling for each stage, given in Experimental Procedure. Phenol concentrations presented in Table 1 are average values of individual daily measurements for each applied concentration of phenol in the ®rst stage and for each applied ¯ow rate in the second and third stages of the experiment. Coecients of speci®c rates of oxygen consumption in the bio®lm were calculated as a slope of the linear dependence (r = 0.90±0.99) of the speci®c oxygen consumption in the bio®lm (in mg O2 gÿ1
Phenol removal using GBAC Table 1. Results of phenol removal by the biosorption system Phenol concentration (mg lÿ1) Phase of system's operation I
P (l dayÿ1) 20 20 20 20 20 41.0 16.4 24.1 47.1 92.1 11.1 27.1 50.1 60.3 96.5 23.2 77.5 96.9
IIa
IIb
III
In¯uent
Euent
1.9 45.4 105.5 236.2 455.5 1053 25.8 12.9 21.7 36.2 24.4 13.5 26.8 27.6 37.6 85.2 105.9 92.5
0.14 0.19 0.17 0.50 1.46 480.3 0.08 0.04 0.63 22.3 0.11 0.25 0.83 10.7 22.3 0.002 0.35 51.4
carbon) during days 5±7 of the experiment. Table 2 shows the data obtained for the ®rst stage of the experiment. Similar results were obtained for stages II and III. Figures 2±5 illustrate how the coecient of speci®c rate of oxygen consumption in the bio®lm, (k), the rate and eciency of substrate removal and phenol content on carbon depend on phenolic load in the respective stages (I, IIa, IIb and III) of the experiment. With increasing phenolic load in stage I in the range of 0.04±31.98 mg gÿ1, the rate of substrate removal increased in the range of 0.03±17.40 mg gÿ1 dayÿ1. The eciency of substrate removal was in the range of 92.5±99.7% up to the phenolic load of 9.38 mg gÿ1 dayÿ1, and afterwards showed a decrease. The activity of microorganisms, determined on the basis of oxygen consumption, increased with increasing phenolic load, as well as the content of phenol on carbon. It can be concluded that phenolic load above 9.38 mg gÿ1 dayÿ1 was too high to achieve good eciency, despite of a high rate of substrate removal and well-developed micro¯ora.
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In stage IIa, an increase in the phenolic load from 0.43 to 3.40 mg gÿ1 dayÿ1 was accompanied by an increase in the rate of substrate removal in the range of 0.43±1.30 mg gÿ1 dayÿ1, and decrease in the removal eciency from 99.7, through 97.1 to 38.3%. It was found that the rate of removal of the mixture of phenols in stage IIa was 13 times lower compared to the removal of phenol alone. The activity of microorganisms showed ®rst an increase with increasing phenolic load, and then started to decrease in parallel with decrease in the substrate removal eciency, which can be a consequence of an inhibition process caused by substituted phenols. In stage IIb, an increase in the phenolic load in the range of 0.54±7.25 mg gÿ1 dayÿ1 caused an increase in the removal rate in the range of 0.54± 2.94 mg gÿ1 dayÿ1, whereas the process eciency dropped from 99.4 to 40.6%. During this stage, there was a certain period of microorganisms adaptation to the conditions in the biosorption system, and their activity increased when the system's eciency started to decrease (phenolic load of 3 mg gÿ1 dayÿ1), whereas the concentration of phenols on carbon showed a constant increase. This can be explained by the use of fresh carbon, on which micro¯ora introduced into the system was in the stage of biocenose formation. By comparing the operation of stages IIa and IIb it can be concluded that the system with the fresher GBAC attained a higher rate of substrate removal at the same phenolic load, which can be explained by incomplete reactivation of carbon surface in the course of its bioregeneration. In stage IIb there was no fall in the activity of microorganisms, which can be explained by higher adsorptive and protective activity of the fresh carbon. In stage III of the experiment, at increased phenolic load in the range of 0.01±9.06 mg gÿ1 dayÿ1 the rate of phenol removal increased in the range of 0.01±4.03 mg gÿ1 dayÿ1, whereas the removal eciency decreased from 100% through 99.7 to 44.5%. A longer period of adaptation of microorganisms, which is evident from Fig. 5, is probably a consequence of another change in the substrate.
Table 2. Results of respirometric measurements in stage I of the experiment Phenol concentration in the in¯uent (mg lÿ1) 45.4 time (h) 2 24 28 49 73 96 118 Ð Ð Ð Ð
105.5
mg O2 gÿ1 carbon 0.208 0.731 1.551 2.222 2.539 2.539 2.964 Ð Ð Ð Ð
236.2
time (h)
mg O2 gÿ1 carbon
1.5 5.5 10.5 25.5 30.5 48.5 73.5 93.5 Ð Ð Ð
0.357 0.998 1.606 2.709 3.195 4.055 5.109 6.082 Ð Ð Ð
455.5
time (h)
mg O2 gÿ1 carbon
4.0 10.0 21.5 31.0 46.5 69.5 101 120 Ð Ð Ð
0.931 1.467 1.467 1.915 2.31 3.828 4.993 5.513 Ð Ð Ð
1053
time (h)
mg O2 gÿ1 carbon
time (h)
mg O2 gÿ1 carbon
4.0 9.0 14.7 23.0 33.0 37.8 50.5 60.2 72.3 86.2 96.0
0.742 2.655 5.432 8.198 10.026 10.756 12.598 14.606 15.116 15.570 16.023
3.0 10.0 22.0 27.0 32.5 46.0 52.0 58.0 71.0 Ð Ð
0.346 0.788 2.835 5.478 8.143 10.387 11.792 12.977 14.828 Ð Ð
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Ivana Ivancev-Tumbas et al.
Fig. 2. Dependence of the rate of substrate removal, eciency of phenol removal, phenol content on carbon, and the coecient of speci®c rate of oxygen consumption in the bio®lm on the phenolic load in stage I of the experiment.
The activity of microorganisms upon this adaptation period increased again and reached a plateau of 0.1 hÿ1. During this stage, the phenol concentration on carbon showed an increase. By comparing the processes in stages III and IIb it can be concluded that, at the same phenolic load, the system attains a higher removal rate when phenol alone is involved (stage III), regardless of bioregenerated carbon. This can be explained again by the supposition that some substituted phenols present in the mixture can exhibit an inhibitory eect (Beltrame et al., 1988). The lack of correlation between the phenol concentration on carbon and the phenolic load of the system indicates that phenol adsorption on carbon and its microbiological oxidation take place simultaneously, which is also evident from Figs 2±5.
For stages II and III, the coecients of total rate of substrate removal (ks) were calculated on the basis of the Atkinson model and the Eckenfelder equation for the bio®lter (Metcalf and Eddy, Inc., 1979): Se =S0 eÿksSa ZA=P Se=substrate concentration in the euent S0=substrate concentration in the in¯uent Sa=speci®c surface of the ®lling Z=adsorber's height ks=coecient of total rate of substrate removal P=¯ow rate of the wastewater
The logarithmic form of the equation gives a linearized dependence: ÿln Se =S0 ks V=P where V is the bio®lter volume.
Fig. 3. Dependence of the rate of substrate removal, eciency of phenol removal, phenol content on carbon, and the coecient of speci®c rate of oxygen consumption in the bio®lm on the phenolic load in stage IIa of the experiment.
Phenol removal using GBAC
1089
Fig. 4. Dependence of the rate of substrate removal, eciency of phenol removal, phenol content on carbon, and the coecient of speci®c rate of oxygen consumption in the bio®lm on the phenolic load in stage IIb of the experiment.
Fig. 5. Dependence of the rate of substrate removal, eciency of phenol removal, phenol content on carbon, and the coecient of speci®c rate of oxygen consumption in the bio®lm on the phenolic load in stage III of the experiment.
Graphical presentation of the latter expression rendered the data given in Table 3, with the coecient ks, being the slope of the obtained straight line. Speci®c carbon surface was not taken into account. The values for ks are approximately equal for stages IIa and IIb of the system's operation, because the same substrate (mixture of phenols)
was involved. For stage III, where the substrate was only phenol, ks increased to 124.5 dayÿ1, which supports the hypothesis on the inhibitory eect of substituted phenols. According to the Atkinson model, the intercept should have the value 0, but this happens only when organic matter is removed by sorption on the bio®lm and microbiological oxidation.
Table 3. Coecient of total rate of substrate removal Stage -ln(Se/So) V/P (day) ks (dayÿ1) Intercept R
IIa 5.7 0.13
5.8 3.5 0.09 0.05 47.2 0.46 0.91
IIb 0.5 0.02
5.4 0.10
4.0 0.04
3.5 0.02 46.9 1.1 0.82
III 1.0 0.02
0.5 0.01
10.8 0.09
5.7 0.03 124.5 ÿ0.11 0.92
0.6 0.02
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Ivana Ivancev-Tumbas et al.
Table 4. Coecient of speci®c rate of oxygen consumption Regeneration time (day) 75 89 96
k (hÿ1) bottom
middle
top
0.015 0.029 0.098
0.006 0.078 0.075
0.008 Ð 0.025
If there is another way of organic matter removal involved (e.g. adsorption), the straight line has a positive intercept. If, however, the euent pollution takes place, then the resulting straight line has a negative intercept. For stages II and III the intercept has a positive value, which means that, apart from microbiological process, adsorption is also involved in phenol removal. The latter process was less pronounced in the case of the biologically regenerated carbon (intercept: 0.46), than in the case of a fresh carbon (intercept: 1.1). However, a negative value of ÿ0.11 obtained for the intercept for stage III of the system's operation suggests the occurrence of deterioration of the euent quality, probably caused by lysed biomass, or by desorption of the already removed material. Euent in stage III had become yellow, and both 4-nitrophenol and ekalux were identi®ed by GC/MS analysis. After stage I, and a 75-day long interruption in the system's operation, samples of carbon from the bottom, middle and top of the column were taken for respirometric analysis. It appeared that the microbiological activity on the carbon was still well developed. The results of respirometric analysis are given in Table 4. An increase in the coecient of speci®c rate of oxygen consumption in the absence
of organic load indicates the presence of a live microbiological activity. During the bioregeneration operation after stages II and III of the experiment samples of carbon were taken after phase IIa from the top, after phase IIb from the bottom, and after phase III from the middle of the column. The results of monitoring the bioregeneration process are presented in Figs 6±8 for each regeneration stage, respectively. A decreasing trend is evident for both the phenol content on carbon and microbiological activity. When the wastewater in¯ow was stopped, the microorganisms continued oxidation of phenols adsorbed on carbon surface (under aerobic conditions). In this way, the surface of the biosorption system ®lling was regenerated. The coecient of speci®c rates of oxygen consumption has gradually approached the value obtained in respirometric measurements for the unused carbon sample (blank assay) of 0.004 hÿ1. The rate of microbiological regeneration can be described by the formula: y aeÿkx where y and a can have a twofold meaning, namely y=content of phenol on carbon (solid line in Figs 6±8), or the constant of the speci®c rate of bio®lm oxygen consumption (dashed line in Figs 6±8); a=initial phenol content on carbon, or the initial constant of the speci®c rate of bio®lm oxygen consumption; and k=coecient of microbiological regeneration of activated carbon; x=time of microbiological regeneration.
The results of statistical data treatment are presented in Table 5. Coecient rates of microbiological regeneration of the once used carbon (IIb stage)
Fig. 6. Phenol content on carbon and the coecient of speci®c rate of oxygen consumption during the bioregeneration after phase IIa of the experiment.
Phenol removal using GBAC
1091
Fig. 7. Phenol content on carbon and the coecient of speci®c rate of oxygen consumption during the bioregeneration after phase IIb of the experiment.
Fig. 8. Phenol content on carbon and the coecient of speci®c rate of oxygen consumption during the bioregeneration after phase III of the experiment.
are 2±3 times higher than the coecient for the already regenerated carbon (IIa stage). The process of biological regeneration of carbon was slower when the substrate was a mixture of phenols than phenol alone (stage III). Also, it can be concluded that the coecient of speci®c rates of oxygen con-
sumption better describes the process of microbiological regeneration (R = 0.86±0.97) than the phenol content on carbon. This is probably related to the problem of phenol desorption, its polymerization on carbon (Grant and King, 1990, Cooney and Xi, 1994; Vidic et al., 1994), reaction with the poten-
Table 5. Rate coecient for microbiological regeneration of activated carbon case 1 Regeneration after the stage IIa IIb III
case 2
k(dayÿ1)
R
k(dayÿ1)
R
0.047 0.082 0.206
0.75 0.75 0.92
0.048 0.122 0.159
0.94 0.97 0.86
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Ivana Ivancev-Tumbas et al.
possible. On the other hand, proteolytes, saccharolytes, and lipolytes probably used the lysed biomass as substrate. Findings of qualitative GC/MS analyses carried out before the beginning of stage III, during the course of stage III, and in the last bioregeneration stages, are summarized in Table 6. Identi®cation of aminophenol and nitrophenol suggests that these compounds are hard biodegradables, as the time elapsed from their use was more than 4 months. An indication for this may also be the calculated coecient of total rate of substrate removal (ks), which for stage II was about 47 dayÿ1 and for stage III almost 3 times higher. The metabolic products of phenol, mentioned in the literature, were not identi®ed, which could be explained by complete degradation of phenol to carbon (IV)oxide and water. Phenol polymerization was evidenced by the presence of phenoxyphenol and biphenyl, fouling a portion of the adsorbent surface. The presence of ekalux and sulphur was due to previous usage of the same carbon. The origin of fatty acids and their methyl esters (probably formed by methylation in the Soxhlet apparatus) may be the lysed biomass serving as substrate for lipolytes in the system. All the above observations indicate, the deterioration of GBAC surface quality takes place not only in the course of the system's operation, but also in its bioregeneration. The cause is either the adsorption of the lysed cells, hard-biodegradable
Fig. 9. Population of aerobic heterotrophs in the last bioregeneration phase.
tially present less-biodegradable compounds, or with some metabolic products. These ®ndings show that respirometric methods can be used to monitor the process of microbiological regeneration of carbon in the biosorption system. Population of microorganisms on GBAC was monitored during the last period of regeneration. The results are presented in Fig. 9. Count of heterotrophs showed ®rstly an increase, then a decrease, and then a further increase. The count of phenol-oxidizing bacteria showed an increase, which can be explained by phenol desorption from carbon, taking place for as long as
Table 6. Compounds identi®ed on carbon. A±D denote respectively the pure unused carbon (blank), carbon bioregenerated after stage IIa, after stage IIb, and in the course of stage III Day of bioregeneration Compound [1,1'-Biphenyl]-4,4'diol 1-Hexanol, 2-ethyl2-Butenoic acid 8-Quinolinol, 2-methyl9-Hexadecenoic acid, methyl ester 9-Octadecenoic acid 9-Octadecenoic acid, methyl ester Aniline Benzene, nitroBenzo-[b]-naphtho-[2,3-d]-furan (8Cl 9Cl) Benzoic acid, 2-hydroxy-, phenyl ester Benzoic acid, 2-hydroxy-methyl ester Cyclohexadecane Cyclotetradecane Ekalux Hexadecanoic acid Hexadecanoic acid, methyl ester Hydrocarbons > C20 Hydrochloride of 2-(Dimethylamino)-4,5-diphenylimid Nitrophenols Octadecanoic acid, methyl ester Pentadecanoic acid, 14-methyl-, methyl ester Phenol Phenol, 2-aminoPhenols, phenoxyPhthalates Pyrido[3, 2-d]pyrimidine-2, 4 (1H,3H)-dione Sulphur Undecane
A*
B*
C*
D*
+ + +
0
1
4
7
11
+
+
+ + +
+
+
+ +
+
+ +
+ + +
+ +
+ +
+
+ +
+ + + + +
+ +
+ + +
+
+ + +
+
+
+ + +
+ + + + + +
+
+
+ +
+ + + +
+ +
+
+
+
+
19
+
+ + +
14
+ +
+ +
+ +
+ + + +
+
+ + + + + +
+
+
+
+ +
+ +
+
+ +
+
+ +
+ +
+ +
+
+ +
Phenol removal using GBAC
substances and metabolites, or of their reactions taking place in the system. Bioregeneration can hardly be a way of avoiding deterioration of the adsorbent quality. A positive characteristic is that this operation can prolong the usage time, thus making the process less costly. Substances adsorbed on carbon either during the system's operation, or in the process of its bioregeneration, represent a serious danger to the euent quality, as their desorption may take place. Desorption of 4-nitrophenol and ekalux that occurred in stage III of the experiment con®rmed the supposed deterioration of the euent quality by desorption of the already removed matter, postulated on the basis of a negative value of the intercept in the calculation of the coecient of total rate of substrate removal. CONCLUSION
By carrying out several separate operations of GBAC bioregeneration in the biosorption system for removal of phenols from a wastewater model it was shown that a high eciency of phenols removal (exceeding 99.5%) can be achieved, even with the carbon that has been used and bioregenerated several times. However, it appears that the rate of substrate removal showed a slower increase with increasing phenolic load with the bioregenerated than with fresh carbon. The contribution of adsorption to the process of phenols removal was greater in the system with fresh carbon. The dependence of the substrate removal rate and the coecient of speci®c rates of oxygen consumption in the bio®lm on phenolic load indicated the occurrence of an inhibitory eect of substituted phenols. Namely, the coecient of total rates of substrate removal was 124 dayÿ1 when the substrate was phenol alone, and about 45 dayÿ1 in the case of a mixture of substituted phenols. It was found that the bioregeneration process is faster with the carbon that was previously used just once than with that used and bioregenerated several times. The process was also in¯uenced by the nature of the substrate involved in the stage of system's operation. The qualitative GC/MS analysis showed that deterioration of adsorbent quality occurred both in the stage of system's operation and its bioregeneration. Hence, it is not plausible to carry out bioregeneration as a separate operation. Instead it is better to carry out the process at an optimal phenolic load ensuring a high microbiological activity and thus preserving the functional adsorption capacity of the adsorbent. In our experiment this was achieved for each separate stage at approximate loads of 10, 1, 2 and 2 mg gÿ1 dayÿ1 for stages I, IIa, IIb and III, respectively. Special attention should be paid to the possible desorption of the already adsorbed matter and lysed biomass.
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However, it is still an open question of the applicability of separate bioregeneration in a battery of several biosorption systems linked in series, one of them always working as bioregenerator. AcknowledgementsÐThe authors acknowledge ®nancial support by the Ministry for Science and Technology of Serbia.
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