Environ. Sci. Technol. 1993, 27, 1783-1789
Removal of Chlorophenolics and Toxicity during High-Rate Anaerobic Treatment of Segregated Kraft Mill Bleach Plant Effluents Wayne J. Parker,. Grahame J. Farquhar, and Erlc I?.Hall
Enviromega Ltd., P.O.Box 1249, Burlington, Ontario L7R 4L8, Canada The behavior of selected chlorinated phenols, guaiacols, catechols, and vanillins during the high-rate anaerobic treatment of segregated kraft mill bleach plant effluents was investigated. Eleven of the 12 detected chlorinated catechols, guaiacols, and vanillins were removed at efficiencies of greater than 80% from an 80% v/v dilution of the wastewater, at a hydraulic retention time of 6 h when the influent was cosubstrate supplemented. The behavior of chlorinated phenols was influenced by the chlorine position and degree of chlorination. Supplementation of the reactor influent with a synthetic evaporator condensate containing methanol and ethanol improved the removal of a number of the compounds by greater than 80%. Removal efficiencies were insensitive to wastewater dilution over a range of 40-80% v/v and to hydraulic retention time over a range of 6-48 h. High-rate anaerobic treatment was observed to reduce the toxicity resulting from chlorophenolics, as determined by toxicity equivalence factors, by 93 % when cosubstrate supplementation was employed and by 65% in the absence of cosubstrate.
Introduction Chlorinated organic matter emitted from kraft mills employing chlorine or chlorine dioxide in the bleaching process has become an increasing environmental concern. Certain compounds present in the effluents have been determined to be either recalcitrant, toxic to aquatic species, genotoxic, or lipophilic ( I ) . Virtually all of the chlorinated organic matter produced at bleached kraft pulp mills originated in the bleaching process with the initial chlorination and extraction stages contributing the largest quantities (2). These wastewaters contain a broad range of chlorinated organicsextending from low molecular weight compounds such as chloroform to complex lignin derivatives (3). A class of compounds of particular interest is the chlorinated phenolics since they have been shown to be deleterious to the environment. For example, trichloroguaiacol and tetrachlorguaiacol are known to have median lethal concentrations (96-h L C ~ Oto) rainbow trout that fall within the ranges of 0.1-1.0 and 0.2-0.4 mg/L, respectively ( 4 ) . In addition, these compounds are known to exert sublethal chronic toxicity at lower concentrations (5).
External treatment at bleached kraft pulp mills has most commonly involved aerobic treatment of the combined mill effluent in either aerated lagoons or activated sludge plants. The removal of chlorinated phenolics in these processes typically ranges from 50 to 70% (6-8). Chlorinated catechols have proven to be particularly difficult to remove in aerobic systems (6). In general, aerobic microbial processes become less effective in biodegrading compounds as the degree of chlorination increases (9). Therefore, anaerobic pretreatment of the segregated bleach plant effluents, which contain a majority of the chlorinated 0013-936X193/0927-1783$04.00/0
0 1993 American Chemical Society
compounds, is a potential option for removal of chlorinated organics. Along with the ability to remove biochemical oxygen demand (BOD) at low cost, the treatment of these streams in anaerobic reactors offers the potential for removal of organochlorines at reduced retention times via reductive dechlorination. Many researchers have investigated the behavior of specific organochlorines in anaerobic processes; however, few have studied such complex mixtures as are present in kraft mill bleach plant effluents. The removal of chloroguaiacols, chlorocatechols, and chlorovanillins under anaerobic conditions has been demonstrated in sediment samples (10). Higher chlorinated phenols and guaiacols have been observed to be removed in a nonmethanogenic anaerobic reactor at efficiencies of 99% (11). The production of intermediate metabolites was not reported. The accumulation of intermediate metabolites has been observed elsewhere (121,and such behavior could significantly reduce the overall efficacy of a treatment process. The behavior of a synthetic mixture of chlorophenols, chloroguaiacols, chlorocatechols, and chloroveratroles in a bench-scale anaerobic sludge blanket reactor has been reported (13). Polysubstituted chlorophenolicswere found to be reductively degraded, but little removal of monochlorophenols was detected. The position of the chlorine substitutions was found to be a significant factor in determining whether a compound could be biodegraded. Also, chloroguaiacols were found be transformed to chlorocatechols before reductive dechlorination occurred. The research reported here addresses the behavior of chlorophenolics during high-rate anaerobic treatment of segregated kraft mill bleach plant effluents. The reactors were operated with the goal of assessing dechlorination of the chlorinated organic matter. Actual bleaching effluents containing both poly- and monochlorinated phenolics were studied to assess removal efficiencies and the possible accumulation of intermediate metabolites. The effects of wastewater dilution, cosubstrate supplementation, and hydraulic retention time (HRT) on the behavior of chlorophenolics were examined. The removal of toxicity through anaerobic treatment was determined by the use of toxicity equivalence factors (TEF).
Materials and Methods Bleach Plant Effluent. The wastewaters used in this study originated from a softwood bleach plant with a bleaching sequence of CDE,DEpD. The chlorine multiple employed resulted in an elemental chlorine charge of 4-6 % in the CD stage with a ClOz substitution of 17-25%. A description of bleaching technology and terminology has been previously summarized (3). The acid chlorination and caustic extraction effluents were sampled and shipped separately, stored at 4-10 "C,and mixed in a 4:l ratio just prior to use. Typical values for the mixed bleach plant wastewater chemical oxygen demand (COD), total Environ. Sci. Technol., Vol. 27,
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1993
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Table I. Mixed Bleach Plant Effluent Characteristics parameter
no. of samples
mean
std. de".
COD (mg/L) TOC (mg/L) BOD (mg/L) AOX (mg/L)
3 I 6
1190 391 219 87.0
144
7
49 80 7.6
O'fQOS
Reactor Liquid
Effiuent
-
wastewater volumetric fraction compd
40 %
60%
80%
NH&I KH~POI NaHCOa methanol ethanol
100 12 1200 400' 40'
150
200
18
24 2400 800' 80'
1800 600' 60'
Anasterisk(*) denotesonlvineosubstrate-suoolementedreactor. Gas-Solids-Liquid sepomtor
STREAM
FIgure 1.
Table 11. Concentrations of Nutrients and Buffers at Volumetric Fractions Employed (mg/L)*
Reactor configuration
organiccarbon(TOC),biochemicaloxygendemand (BOD), and adsorbable organic halogen (AOX) are presented in Table I. Reactor Design and Operation. Two identical reactors, designed as modified sludge blanket units (Figure 1) and constructed of polyvinyl chloride (PVC) and stainless steel, were used in the study. Each reactor bed was 10 cm in diameter, and the height from the inlet to the gas-solid-liquid separator was 61.7 cm. This provided an effective bed volume of 5.0 L. A t the time of experimentation,the sludge blanket depth in the reactors was approximately 40 cm. The biomass consisted of a dense granular sludge that originated from a pilot-scale upflow anaerobic sludge blanket unit located a t the Wastewater Technology Centre. The sludge had a volatile suspended solids concentration of approximately 30 500 mg/L. The reactors were acclimated to kraft mill bleach plant wastewater for a period of 9 months prior to the onset of this research and were actively producing methane throughout the entire experimental period. Both anaerobicreactorswereoperated with a companion streamcontaining nutrients and buffer,which constituted 20% vlvofthefeedflowtothereactors. Oneofthereactor influents was supplemented with methanol and ethanol as a cosubstrate. The levels of methanol and ethanol were chosen to simulate addition of the relative quantities of evaporator condensates present at a kraft mill. The concentrations of the nutrients and buffers in the feed for the various operational conditions are provided in Table 11. Tap water was used for those experiments in which the wastewater was diluted. Thereactors were operated at hydraulic retention times (HRT), based on the reactor empty bed volume, of 6,24, and 48 h at each wastewater dilution. The experiments were performed in a randomized pattern with respect to HRT at each wastewater dilution. Wastewater dilution settings were blocked with the block at 40% completed first and that at 80% completed last. A wastewater dilution consisted of the specified percentage ofwastewater in tap water. A period of 3 HRTs was allowed before 1784 Envhon. ScI. Technd., Vol. 27.
No. 9, 1993
sampling at any particular condition to attain hydraulic steady state. Single grab samples of the reactor influents and effluents were taken at each operating condition. The experiments were replicated after a complete set of experimental runs at all combinations of wastewater dilution and HRT had been completed. The removal efficiency of all chlorophenolics was assessed at each operating condition. Analyses. Samples analyzed for chlorophenolicswere initially extracted with petroleum ether and acetylated with acetic anhydride in the presence of potassium carbonate (14). The extracts were concentrated by evaporation and then analyzed by gas chromatography-electron capture detection for higher chlorinated compounds and gas chromatography-mass spectroscopy for monosubstituted phenols, guaiacols, and catechols. A list of target compounds quantitated by this method is presented in Table 111. The detection limit for each compound by this method was approximately 0.5 pg/L.
Results and Discussion The continuous-flowhigh-rate anaerobic reactors demonstrated consistent methane production throughout the entire experimental period, indicating an absence of significantinhibitionofthe methanogenicpopulation.This behavior contradicts the results of batch treatability studies (15) in which 60% vlv bleach plant wastewater severelyinhibited an unacclimated methanogenic biomass. Inhibition was indicated by lag periods to biogas production that were greater than 20 times those observed in controls. The extended solids retention times present in the continuous-flowreactors that allowed for development of an acclimated biomass is the most likely explanation for the lackof inhibition in the present study. The impact of anaerobic treatment on whole and ultrafiltered TOC and AOX has been previously reported (16). Of the 30 chlorophenolic compounds on the analytical target list, only 20 were detected regularly in the influents or effluentsfrom thereactors. The averageconcentrations of these compounds in the raw bleach plant wastewater are presented in Table IV. The chlorinated catechols, gnaiacols, and vanillins, with individual compound concentrations in the range of 5&250 pglL, were present in the greatest quantities. The chlorinated phenols, with the exception of 2,4,6-trichlorophenoI, were generally present at lower concentrations. The chlorinated phenolics constituted approximately 1% of the total chlorinated organic matter as measured by the adsorbable organic halogen (AOX) method (16). The measured influent and effluent chlorophenolic concentrations exhibited substantial variability. This is
Table 111. ChlorophenolicCompounds Detected by Analytical Technique 3-chlorophenol 2-chlorophenol 2,4-dichlorophenol 2,6-dichlorophenol 3,4-dichlorophenol 2,3-dichlorophenol 2,3,5-trichlorophenol 2,3,6-trichlorophenol 2,3,5,6-tetrachlorophenol 3,4,5-trichlorophenol 4-chloroguaiacol pentachlorophenol 3,4,5-trichloroguaiacol 4,5-dichloroguaiacol 4-chlorocatechol 3,4,5,6-tetrachloroguaiacol 3,4,5,6-tetrachlorocatechol 4,5-dichlorocatechol 6-chlorovanillin 3,4,5-trichlorocatechol
4-chlorophenol 3,5-dichlorophenol 2,4,6-trichlorophenoi 2,4,5-trichlorophenol 2,3,4,5-tetrachlorophenol 4,6-dichloroguaiacol 4,5,6-trichloroguaiacol 3,5-dichlorocatechol 3,4,5-trichlorocateel1ol 5,6-dichlorovanillin
Table IV. Concentrations of Chlorophenolicsin MBPE (5 Samples) compd
mean
2-chlorophenol 4-chlorophenol 2,6-dichlorophenol 2,4-dichlorophenol 3,5-dichlorophenol 2,4,6-trichlorophenol 2,4,5-trichlorophenol pentachlorophenol 6-chlorovanillin 5,6-dichlorovanillin
13.9 23.4 8.3 22.1 3.9 41.6 6.3 2.6 82.1 53.8
concn (&L) std.dev. 8.3 10.3 4.4 11.9 1.9 20.9 3.1 2.0 25.9 16.3
mean
4-chloroguaiacol 4,5-dichloroguaiacol 3,4,5-trichloroguaiacol 4,5,6-trichloroguaiacol 3,4,5,6-tetrachloroguaiacol 4-chlorocatechol 3,5-dichlorocatechol 4,5-dichlorocatechol 3,4,5-trichlorocatechol
12.0 57.8 237.9 38.8 171.8 20.4 29.3 66.1 186.6 96.9
3,4,5,6-tetrachlorocatechol
-Avq. 0 40 r
0
Feed Conc. Reactor with Cosubstrate Reactor without Cosubstrate
0
-
150
0
-
100
-
ci
ci
10
--
oi
0
W
t 200 V
9.4 45.1 137.5 23.7 104.3 13.2 10.5 8.4 6.1 12.9
Avg. Feed Conc. Reactor with Cosubstrate Reactor without Cosubstrate
0
5
2 30-
concn !ua/L) std.dev.
compd
n
0 V
0
8
-
0 0 ‘ 0
I
10
I
I
20
30
I
I
40
50
HRT (hr)
Behavior of 2,4dichlorophenol.
Figure 2.
__ Avg. Feed Conc. 0
Reactor with Cosubstrate 125
5 W
2ol
0
-
0
301
0
10
0
0
10
t
0 I
I
I
20
30
40
25 01 0
0 n “50
HRT (hr) Flgure 3.
Behavior of 3,5-dlchlorophenoL
demonstrated in Figures 2-6 where the concentrations of five representative compounds are presented. Since similar trends were observed at all wastewater fractions, only data for the 80% vlv wastewater experiments are presented. To assist in data analysis, the influent contaminant concentrations for each wastewater volumetric fraction were averaged to obtain a mean influent con-
Y
Reactor with Cosubstrate Reactor without Cosubstrate
10
30
40
50
HRT (hr)
Figure 5.
Behavior of 4,5dichlorocatechol.
centration from which removal efficiencieswere calculated. For those cases where a compound was not detected in the effluent, the detection limit was used to calculate a conservative estimate of the removal efficiency. The removal efficiencies of each chlorophenoliccompound were then employed in an analysis of variance (ANOVA) procedure which assessed the impact of wastewater Environ. Scl. Technol..Vol. 27, No. 9, 1993
1785
80
__ Avg. Feed Conc. 0
0
Reactor with Cosubstrote Reactor without Cosubstrde
B 0
10
20
50
HRT (hr) Flgure 0. Behavior of 5,6dichlorovanillin.
Table V. Pooled Removal Efficiencies Compounds with Statistically Significant Cosubstrate Effects removal efficiencies (%) compd
cosubstrate supplemented
3,5-dichlorophenol 2,4,6-trichlorophenol 4-chlorocatechol 4,5-dichlorocatechol 3,4,5-trichlorocatechol 3,4,5,6-tetrachlorocatechol 3,4,5-trichloroguaiacol 3,4,5,6-tetrachloroguaiacol 6-chlorovmillin 5,6-dichlorovanillin
-4463 86.6 93.4 98.4 97.5 93.9 98.1 96.5 98.6 94.6
cosubstrate absent NRn NR NR -39.3 NR 84.0 97.0 95.1 91.9 86.6
Compounds without Statistically Significant Cosubstrate Effects compd removal efficiency (7%) 2,6-dichlorophenol 2,4,5-trichlorophenol 4-chloroguaiacol 4,5-dichloroguaiacol 4,5,6-trichloroguaiacol (I
74.1 72.7 83.8 97.3 93.8
NR = concentrations not Significantlychanged with treatment.
dilution, HRT, and cosubstrate-supplemented operation versus operation with bleach plant wastewater alone. The ANOVA technique indicated that, at a 95% level of significance, wastewater dilution and HRT had no effect on the measured removal efficiencies of any of the compounds. However, in some cases, significant differences in removal efficiencies were identified depending on whether cosubstrate was present or absent. Since the behavior of the chlorophenolics studied was found to be independent of HRT, the pooled influent and effluent concentrations of each compound were compared with t-tests to determine if the concentrations were significantly changed by treatment in the reactors. The removal efficiencies for compound which underwent significant concentration changes through either of the reactors are presented in Table V. The removal efficiencies presented are pooled over HRT and wastewater dilution since the ANOVA technique indicated no significant impact of these factors. The concentrations of 2-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, pentachlorophenol, and 3,5-dichlorocatechol were not significantly changed in either reactor and are not presented in Table V. 1788 Envlron. Scl. Technol., Vol. 27, No. 9, 1993
The results presented in Table Vindicate several trends in behavior which are outlined in the following sections which describe the impact of functional group type, position of chlorine substitutions, and cosubstrate use. Abiotic Removal Mechanisms. Volatilization to the biogas and sorption to biomass were considered as potential removal mechanisms. Henry’s law coefficients are not readily available for many of the compounds present in the bleach plant effluent, and therefore, this value was estimated (17). The estimated values for the dimensionless Henry’s law coefficient ranged from 10-3 to 104 L/L. A conservative estimate of the loss to volatilization was obtained by assuming that the reactor biogas was saturated with respect to the influent concentration of the most volatile compound and an average value of 5 L/d of biogas production. In this scenario, 0.1% of the most volatile compound (2,4-dichlorophenol)would have been removed by volatilization. Volatilization was, therefore, considered to be a negligible removal mechanism. Sorption of chlorophenolics to biomass in the reactors was not directly quantified in the experiments and was, therefore, calculated with linear sorption partitioning coefficients that were estimated from the compounds’ octanol-water partition coefficients (18). As with the Henry’s law coefficients, the octanol-water partitioning coefficients were not available for many of the compounds and were therefore estimated (17). The calculated partition coefficients ranged from 0.4 to 3.9 L/g. A conservative estimate of the mass of each compound sorbed to biomass was calculated by assuming that sorption was in equilibrium with the reactor influent concentration. For the compound with the highest partitioning coefficient (pentachlorophenol), the sorbed mass was compared to the total mass which was treated during the 9-month startup period. The mass of pentachlorophenol fed to the reactors over this period exceeded the mass that was estimated to have been sorbed to the biomass by a factor of 7.5. I t was concluded that, upon initiation of the experiments detailed in this study, the biomass was saturated with respect to sorption of the chlorophenolics and that removal of the chlorophenolics by sorption onto the biomass was negligible. Impact of Functional Group on Behavior. Substantial differences in behavior between the various chlorinated phenols were observed. Four chlorinated phenols were not significantly affected by anaerobic treatment (2-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, and pentachlorophenol). An example of this behavior is illustrated for 2,4-dichlorophenol in Figure 2 where effluent concentrations were either slightly above, slightly below, or equal to the influent concentrations. These results are inconsistent with literature accounts in which biodegradation of 2-chlorophenol, 4-chlorophenol, and pentachlorophenol (19) and 2,4-dichlorophenol (12) has been reported. Several factors are likely responsible for the reduced net removal efficiencies of the four compounds in this study as compared to that of the stated literature. The degradation of polychlorinated compounds probably produced breakdown metabolites which masked the behavior of the less-substituted compounds. This was apparent in the results for 3,5-dichlorophenol (Figure 3), which indicated an increase in concentration through the cosubstratesupplemented reactor. In the literature reports, polychlorinated compounds were absent, and therefore,
accumulation of metabolites would not occur. In this study, although the less-substituted compounds may have also been degraded, simultaneous production from the degradation of more highly substituted compounds made it impossible to detect any net reduction. These results do indicate, however, the compounds which are most likely to persist after anaerobic treatment in high-rate reactors. The hydraulic retention times employed in this study resulted in substantially shorter contact times between the biomass and the contaminants than in previous studies on the degradability of chlorinated phenols. For example, in the studies reported by Boyd and co-workers, contact periods of several weeks were employed in contrast to the longest contact time of 48 h used in this study. This reduced contact time likely contributed to the reduced extent of degradation of some of the compounds in this study as compared to previous reports. It should be noted that competitive inhibition by any of the number of chlorinated phenolics present in the wastewater cannot be ruled out as a cause of the reduced degradation, as compared to the literature. Concentrations of 3,5-dichlorophenol were not substantially changed in the reactor operated without cosubstrate, but actually increased with treatment at all dilution levels and retention times in the cosubstratesupplemented reactor (Figure 3). The behavior of 3,5dichlorophenol was consistent with that reported elsewhere (12)where insignificant degradation of this compound was observed after incubation for 6 weeks. Removals of 3,5dichlorophenol were reported only after long-term exposure in serum bottles and after cross-acclimation to monochlorophenols (12).Increases in the concentration of 3,5-dichlorophenol in the cosubstrate-supplemented reactor were likely due to the production of intermediate metabolites resulting from the degradation of higher chlorinated compounds as previously described. The chlorophenol compounds which were significantly removed during anaerobic treatment included 2,6-dichlorophenol, 2,4,5-trichlorophenol, and 2,4,6-trichlorophenol with overall efficiencies of 74,73, and 87 % , respectively. The concentration of 2,4,6-trichlorophenol was, however, only reduced to a significant extent in the reactor with cosubstrate supplementation. The removals of 2,6dichlorophenol and 2,4,6-trichlorophenol are consistent with those observed in anaerobic serum bottle studies elsewhere (12,19).The removal of 2,4,5-trichlorophenol was greater than that observed by Buisson et al. (20),who noted insignificant degradation after incubation for 32 days in serum bottles. The improved removal efficiencies observed in this study cannot be readily explained as differing biomass sources; cosubstrates and acclimation techniques were employed in the two studies. Guaiacols at all levels of substitution were almost completely removed in both reactors as indicated in Table V. Figure 4 demonstrates the virtual elimination of 3,4,5,6tetrachloroguaiacol under all experimental conditions. These results are consistent with previous observations (13,101 which indicated that chloroguaiacolscan be rapidly demethylated to form the respective chlorocatecholsunder anaerobic conditions. All the chlorocatechols with the exception of 3,5dichlorocatechol were efficiently removed from the reactor receiving cosubstrate. However, the reactor without cosubstrate did not perform as well with respect to catechols. Without cosubstrate supplementation, 3,4,5,6-tetrachlo-
rocatechol was effectively removed but the remaining four catechols were not. In fact, 4,5-dichlorocatechol apparently increased in concentration in this reactor in a number of experiments (Figure 5). This behavior contrasts with that observed in the cosubstrate-supplemented reactor where 4,5-chlorocatechol was virtually eliminated. The transformation of chloroguaiacolsand also the degradation of 3,4,5,6-tetrachlorocatecholmay explain the accumulations of 4,5-dichlorocatechol. The chlorovanillins were readily degraded under anaerobic conditions. Figure 6 illustrates the influent and effluent concentrations of 5,6-dichlorovanillin at a wastewater volumetric fraction of 80% v/v. Consistently high removals were observed in both reactors under all conditions. It has been suggested (10) that the initial transformation of chlorovanillins involves demethylation of the methoxy group followed by carboxylation and subsequent cleavage of the aldehyde group to produce chlorinated catechols. This could not be confirmed in this research. However,the fact that the monochlorinated vanillins were consistently removed would suggest that, as with the chloroguaiacols,the removals were due to initial transformations other than reductive dechlorination. Impact of Chlorine Positions on Behavior. The specificity of degradation with respect to the position of chlorine substitution was apparent in the behavior of the chlorinated phenols in this study. Although 2,6-dichlorophenol was substantially removed, 2,4-dichlorophenol was not significantly changed in concentration. These results are inconsistent with other research where the degradation of both compounds by reduction of the orthochlorine has been reported (12,13). It may however be that the removal of 2,4-dichlorophenol was being masked by formation of this compound as a result of the reduction of 2,4,6-trichlorophenol which was present in the wastewater in substantial quantities. It has been reported that, whereas 2,4-dichlorophenol is produced from 2,4,6trichlorophenol, 2,6-dichlorophenol is not (13). I t is likely that the two dichlorophenols were being degraded at similar rates, but the formation of 2,4-dichlorophenol concealed its degradation. The anaerobic degradation of chlorophenols with only meta- and para-chlorine substitutions has been reported to be difficult (13). This observation was confirmed in the present study since 3,5-dichlorophenol was observed to accumulate to some degree in both reactors. The effect of these substitutions on the other phenolics appeared to be variable. The meta-meta substitution seemed to create a compound which was difficult to degrade since 3,5dichlorocatechol was not well removed in either reactor. The other chlorocatechols which had only meta and para substitutions were 4,5-dichlorocatechol and 3,4,5-trichlorocatechol. These compounds were removed well in the cosubstrate-supplemented reactor but poorly in the reactor without cosubstrate. The specificity of removal with position of substitution suggests that the degradation was biologically mediated. Impact of Cosubstrate Use on Chlorophenolic Behavior. The supplementation with an easily degraded cosubstrate appeared to create an environment in which at least some compounds were more effectively dechlorinated than without cosubstrate. Of the 20 compounds detected most frequently, nine were removed to a greater extent in the reactor with cosubstrate supplementation. One compound, 3,5-dichlorophenol, accumulated to a Environ. Sci. Technol., Vol. 27, No. 9, 1993
1787
Table VI. TEQ Values for Raw and Treated Effluents parameter
influent
cosubstrate supplemented
cosubstrate absent
av (5 samples) std. dev. % removed
212.9 70.1 NAU
15.4 5.8 92.7
75.6 19.6 64.5
NA = not applicable.
greater extent in the reactor with cosubstrate. The segregated effluents from kraft mill bleach plants are generally poorly biodegradable with BOD/COD ratios in the order of 0.1. Values of this ratio below 0.5 are believed to indicate low biodegradability (15). The polymeric nature of many of the contaminants and the high degree of chlorine substitution (3) are likely responsible for the low biodegradability. The superior performance observed with anaerobic treatment with cosubstrate supplementation might result from the fact that the cosubstrate acts as an easily available electron source for the reduction of the chlorinated compounds. The presence of these extra electrons would serve to increase the reaction kinetics for the dechlorination process. It may also be possible that a larger, more viable population of biomass was present in the cosubstrate-fed reactor, thereby increasing the overall dechlorination rate. It was impossible to differentiate between the two hypothesized mechanisms by considering only the results from this study. It should be noted that these mechanisms are not mutually exclusive. Toxicity Reduction. Chlorophenolics are a known source of toxicity in bleached kraft pulp mill effluents ( 4 ) . Phenolic compounds with a greater number of chlorine substitutions typically demonstrate a higher toxicity to aquatic biota. Amethod by which the impact of anaerobic treatment on toxicity of effluents may be determined is through the use of toxicity equivalence factors (TEF). These factors relate the toxicity of a family of compounds to the compound with the greatest toxicity. For chlorinated phenolics this reference toxicant is pentachlorophenol. By using TEF numbers, predictions of the potential combined toxicity can be made using simple additivity of the toxicity of individual compounds. This is done by multiplying the concentration of each compound by its corresponding TEF value and adding the results to provide a single number which is the toxicity equivalency (TEQ). Changes in the TEQ values through a process provide a measure of the reduction of potential toxicity in the wastewater stream. TEF values of 0.05, 0.11, 0.23, 0.48, and 1.00 for monochloro-, dichloro-, trichloro-, tetrachloro-, and pentachloro-substituted phenolic compounds, respectively, have been proposed ( 5 ) . Toxicity of the chlorinated phenols, catechols, guaiacols, and vanillins were considered to be identical and were derived from studies performed with fathead minnows, rainbow trout, American zebrafish, and sea urchins. In this study, TEQ values for the untreated and treated wastewaters were calculated for the case in which 80% v/v wastewater was studied. The calculated mean TEQ values along with corresponding standard deviations are presented in Table VI. It is apparent from Table VI that a substantial fraction of the toxicity resulting from chlorophenols was removed during high-rate anaerobic treatment. Treatment in the presence of cosubstrate, with an average removal efficiency of 92.7 % ,was significantly more effective than treatment 1788
Environ. Scl. Technol., Vol. 27, No. 9, 1993
without cosubstrate where 64.5% of the toxicity due to chlorophenolics was removed. These results demonstrate the efficacy of high-rate anaerobic pretreatment for dechlorination and, hence, detoxification of effluents from kraft pulp mills employing chlorine in the bleaching process. Conclusions
High-rate anaerobic reactors treating segregated kraft mill bleach plant effluents and operating under methanogenic conditions were able to successfully degrade a number of chlorophenolic compounds. Of the 20 chlorinated phenolic compounds routinely detected, 10exhibited removal efficiencies greater than 90%. Mono- and di-substituted chlorophenols were generally poorly degraded or accumulated in the reactors, while trisubstituted phenols were removed at efficiencies greater than 80%. Chlorinated guaiacols were consistently removed at efficiencies greater than 95 %. Chlorinated catechols, with the exception of 3,5-chlorocatechol were removed at efficienciesgreater than 95 % in a cosubstratefed reactor. Chlorovanillins were removed at efficiencies greater than 95 % . The presence of a methanol and ethanol cosubstrate, simulating a kraft mill evaporator condensate, significantly improved removal of a number of chlorophenolic compounds. The combination of meta-meta substitution appeared to produce a compound which was difficult to degrade. This was observed in the chlorophenols as well as the chlorocatechols. The removal of chlorophenolics was not affected by hydraulic retention time over a range of 6-48 h. This indicated that the reactors could have been operated at lower retention times before deterioration of removal efficiencies occurred. High-rate anaerobic treatment substantially reduced the acute toxicity contribution of chlorophenolics as determined by toxicity equivalency factors. The reactor operated with cosubstrate supplementation reduced this factor by 92.7 % while the reactor operated without cosubstrate supplementation reduced the toxicity by 64.5 96. Literature Cited (1) Suntio, L. R.; Shiu, W. Y.; MacKay, D.Chemosphere 1988, 17, 1249. (2) Hall, E. R.; Fraser, J.; Garden, S.; Cornacchio, L.-A. Pulp Pap. Cancl. 1989, 90, T421. (3) Kringstad, P. K.; Lindstrom, K. Environ, Sci. Technol. 1984, 18, 236A. (4) Leach, J. M. Water Chlorination. Environmental Impact and Health Effects; Ann Arbor Science Publishers: Ann Arbor, MI, 1980, Vol. 3, p 325. (5) Kovacs, T. G.; Martel, P. H.; Voss, R. H.; Wrist, P. E.; Willes, R. F. Submitted to Environ. Sci. Chem. (6) Gergov, M.; Priha, M.; Talka, E.; Valttila, 0.;Kangas, A.; Kukkonen, K. Tuppi J . 1988,175. (7) Saunamaki, R.; Jokinen, K.; Jarvinen, R.; Savolainen, M. Water Sci. Technol. 1991, 314, 295. (8) Stuthbridge, T. R.; Campin, D. N.; Langdon, A. G.; Mackie, K. L.; McFarlane, P. N.; Wilkins, A. L. Water Sci. Technol. 1991, 314, 309. (9) Vogel, T. M.; Criddle, C. S.; McCarty, P. L. Environ. Sci. Technol. 1987, 21, 722.
(10) Hakulinen,R.; Salkinoja-Salonen,M. InBiological Fluidized Bed Treatment of Water and Wastewater; Ellis Horwood Ltd: New York, 1981; pp 374-382. (11) Neilson,A.H.;Allard,A.;Lindgren,C.;Remberger,M. Appl. Environ. Microbiol. 1987, 53, 2511. (12) Boyd, S. A,; Shelton, D. R. Appl. Enuiron. Microbiol. 1984, 47, 272. (13) Woods, S. L.; Ferguson, J. F.; Benjamin, M. M . Enuiron. Sci. Technol. 1989, 23, 62. (14) Lee, H.-B.; Hong-You,R. L.; Fowlie, P. J. A. J.-Assoc. O f f . Anal. Chem. 1989, 72,979. (15) Hall, E. R.; Cornacchio, L.-A. Environment Canada Wastewater Technology Centre, Burlington, Ontario, Report WTC-BIO-O1,1988.
(16) Parker, W. J.; Hall, E. R.; Farquhar, G. J. Accepted for publication by Water Environ. Res. (17) Lyman, W. J.; Reehl, W. F.; Rosenblatt, D. H. Handbook of Chemical Property Estimation Methods; American Chemical Society: Washington, DC, 1982. (18) Dobbs, R. A.; Wang, L.; Govind, R. Environ. Sci. Technol. 1989,23, 1092. (19) Mikesell,M. D.; Boyd, S. A. J . Environ. Qual. 1985,14,337. (20) Buisson, R. S. K.; Kirk, P. W. W.; Lester, J. N.; Campbell, J. A. Water Pollut. Control 1986, 387. Received for review July 30,1992. Revised manuscript received March 3, 1993.Accepted May 24, 1993.
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