Environ. Sci. Technol. 2006, 40, 3395-3401
Organic Pollutant Removal versus Toxicity Reduction in Industrial Wastewater Treatment: The Example of Wastewater from Fluorescent Whitening Agent Production ANNETTE KO ¨ H L E R , * ,† STEFANIE HELLWEG,† BEATE I. ESCHER,‡ AND KONRAD HUNGERBU ¨ HLER† ETH Zu ¨ rich, Safety and Environmental Technology Group, 8093 Zu ¨ rich, Switzerland, and Swiss Federal Institute of Aquatic Science and Technology (Eawag), U ¨ berlandstrasse 133, 8600 Du ¨ bendorf, Switzerland
Industrial wastewater treatment in the chemical industry aims at eliminating organic contaminants, as these pollutants may be persistent and ecotoxic. In a case study performed in collaboration with the chemical industry, we investigated the removal of a fluorescent whitening agent and its side products in the wastewater-treatment system. Adsorption to activated carbon and biological treatment were simulated in laboratory tests. Algae toxicity tests were performed to quantify the toxicity of the wastewater mixture and of single components. The contaminants identified accounted for up to 82% of the wastewater’s total organic carbon (TOC). Adsorption to activated carbon eliminated the TOC and the single contaminants only slightly. Nevertheless, the toxicity of the wastewater decreased by 40%. In contrast, biological treatment reduced the TOC by up to 80%, and the whole effluent toxicity increased. These results indicate that new ecotoxic metabolites were formed during the biological treatment. They also illustrate that mere reduction of the TOC in the wastewater-treatment system is not sufficient for ensuring a reduction of environmental impact. Therefore, simultaneously conducting TOC measurements and toxicity tests, as demonstrated in the current work, is recommended.
Introduction Industrial wastewaters are generally very complex, heterogeneous, and poorly characterized mixtures of a large number of contaminants. Wastewaters from the organic chemical industry generally contain remaining portions of reactants and auxiliaries, products which pass the product-isolation processes, and side products. The latter-formed side products might contribute a significant share to the wastewaters’ organic load. Although the wastewaters are purified in chemical, physical, and biological treatment processes, * Corresponding author phone: +41 44 823 4208; fax: +41 44 823 4009; e-mail:
[email protected]. † ETH Zu ¨ rich, Safety and Environmental Technology Group. ‡ Swiss Federal Institute of Aquatic Science and Technology (Eawag). 10.1021/es060555f CCC: $33.50 Published on Web 04/06/2006
2006 American Chemical Society
chemical transformation products and metabolites from microbial degradation add to the organic composition of the final effluent. The majority of such compounds is harmless to the aquatic environment, but there might be exceptions. Because of the heterogeneous composition of wastewaters, wastewater regulations focus particularly on the removal of the organic load as a whole (1-3). The elimination of organics is therefore mainly controlled by measuring composite parameters, particularly, the total organic carbon (TOC) and the not (readily) biodegradable refractory total organic carbon (TOCref). Additionally, the amounts of single organic contaminants of high concern and the whole effluent toxicity (WET) of the system’s final effluent are monitored (1-2, 4-6). The reduction of specific pollutants and toxicity by the various wastewater-treatment processes, however, is rarely monitored. Although numerous studies have examined the relationship between chemical assessments of priority pollutants and the mixture toxicity in final effluents, there are only very few analytical and toxicological investigations throughout the wastewater-treatment system (7-9). Also, biodegradation tests and whole effluent toxicity tests are seldom compared, although such comparisons may help to evaluate the efficiency of the contaminant elimination (6) and performance of the wastewater-treatment processes more comprehensively. Such information is especially valuable when industrial wastewater-treatment systems are changed because of process optimization or the introduction of new product-isolation processes. The main goal of this study is to compare processintegrated and end-of-pipe systems for industrial wastewater treatment by applying both chemical analyses and biological toxicity testing. For the example of fluorescent whitening agent (FWA) production, wastewaters from different productisolation processes and their succeeding purification were investigated to illustrate to what extent the TOC composition and the mixture toxicity of the industrial wastewaters can be attributed to individual pollutants. Thus, the study evaluates whether the TOC removal rate as the main performance indicator is sufficient for process control and performance evaluation or whether further toxicity related criteria are needed.
Materials and Methods Treatment of Wastewaters from the Production of a Fluorescent Whitening Agent. Two treatment systems for processing wastewaters from the production of a FWA were examined (for FWA structure and its production side products, see Figure 1). The first system includes adsorption to activated carbon with subsequent activated sludge treatment (Figure 2). This system, which represents a classical end-of-pipe system, purifies FWA wastewater from a conventional filter-press product-isolation process. The wastewater consists of a mixture of the filtration mother liquor and the washing water. Originally, these wastewaters were treated by an array of reverse osmosis, high-pressure wet-air oxidation, and biological treatment, until a new productisolation process for FWA was introduced (second system, Figure 2). The treatment combination of activated carbon adsorption and biological purification of filter-press wastewater came into operation during the technology transition phase. The second system involves an ultrafiltration process, specifically developed for the FWA isolation and implemented into the production chain to achieve a higher product yield and diminish the organic carbon load in the wastewaters. Thus, the new membrane filtration functions as a processVOL. 40, NO. 10, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Structure of the fluorescent whitening agent (FWA) and its production side products. Numerous side products with different aliphatic, aromatic, and chloro substituents (Rx) are created during synthesis and are therefore contained in the production wastewaters.
FIGURE 2. Wastewater-treatment systems. integrated measure for contaminant reduction at source. Because the smaller TOC load of the membrane-filtration permeates is more biodegradable than the TOC from the filter-press process waters, the permeates are solely purified in the activated sludge treatment (see system 2 in Figure 2). Wastewater Sampling. Wastewater samples were collected as composite samples from product-isolation processes, the filter-press, and the membrane-filtration process. As the membrane-filtration process is newly established, composite samples were taken during the startup phase as well as during the full-scale production. All samples were stored at -20 °C between each sample processing step. Simulation of Wastewater Treatment. In an in-house simulation test, the wastewater samples were blended with an amount of activated carbon equal to the quantity of adsorbent used in full-scale treatment and mixed for 2 h at room temperature. The TOC percentage adsorption was quantified after a 2 h treatment (for simulation procedures, see Supporting Information). The activated sludge treatment was simulated at laboratory scale with the Zahn-Wellens test according to ISO 9888 (10). This test was applied for determining the TOC elimination rate after 28 days due to biodegradation and to adsorption to activated sludge. Elimination is reported as the percentage TOC removal in 5% steps. The standard test duration of 28 days was reduced to 13 days because the maximum TOC elimination in all wastewater samples was reached within this time interval. Activated sludge from the wastewater system’s biological treatment unit was administered as inoculum. After the elimination test, all samples were filtered through a 0.45 mm Millipore filter to remove suspended particulate matter. Chemical Analyses. The analytical characterization of the wastewaters before and after each purification process covered the following parameters: TOC, stilbene compounds, FWA, the reactant diethanolamine (DEA), as well as the sodium chloride content. Additionally, the reaction solvent 3396
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methyl ethyl ketone (MEK) was analyzed in the original filtration wastewaters. TOC and salt content were measured using laboratory standard methods (11, 12). Stilbene side products were quantified as FWA active substance equivalents with UV-VIS spectroscopy (13). High-performance liquid chromatography (HPLC) with fluorescence detection was applied for the FWA active agent quantification. The quantification of the major FWA synthesis reactant diethanolamine and the solvent methyl ethyl ketone was conducted by means of gas chromatography coupled with flame-ionization detection (GC/FID). For analytical protocols, see the Supporting Information. All analyses were carried out at the laboratories of Ciba Specialty Chemicals, Grenzach, Germany. Ecotoxicity Testing. Algae toxicity testing was performed for two reasons. First, several studies suggest that the algae growth inhibition test is often a more sensitive acute bioassay than the standard daphnia and fish acute toxicity tests (1417). Second, potential adverse effects of triazine side products contained in the wastewaters should be covered. These triazines exhibit structural similarity to s-triazine herbicides that are specifically and highly toxic to algae (18, 19). Therefore, aquatic plant species were used as biomarkers for hazard assessment in a screening approach (for whole effluent toxicity testing in hazard assessment, see Chapman, ref 20). For evaluating ecotoxic effects of the complete wastewater samples as well as the FWA, the algae growth inhibition test was carried out according to the OECD guideline 201 with the test species Pseudokirchneriella subcapitata (21). All toxicity measurements were performed by a subcontractor under GLP conditions. For each sample, three to six concentrations in triplicate with 10 controls were measured. To eradicate effects of sodium chloride, wastewater samples with high salt content were diluted to a maximum salt concentration of 3 g/L. This toxicity threshold was previously identified in a range-finding test (see Supporting Information). Inhibition of algae growth was expressed as the biomass integral measured after 72 h of exposure. The concentration-
response relationships were fitted by a log-logistic model using nonlinear regression (Prism 4.0) (22) (see Supporting Information, Figures S1, S2, and S3). The concentrations resulting in a 50% inhibitory effect (effect concentration, EC50) were used as robust endpoints expressed in two different units. Algal toxicity changes during wastewater purification were assigned EC50 values in terms of dilution factors (df) (eq 1). This approach enables the comparison of
df )
volume of wastewater sample [-] volume in the biotest
[mgTOC,ref/L] (2)
where CTOC,ref is the refractory TOC concentration in the final effluents (mgTOC,ref/L). For diethanolamine and the reaction solvent methyl ethyl ketone (MEK), algae toxicity data were retrieved from the literature. As no EC50 value was available for MEK, the 50% effect level had to be extrapolated from the lowest-observed effect concentration (LOEC) available from ref 24 by multiplying with a factor of 2. We chose the factor of 2 as a conservative estimate because LOEC values are often in the order of the 20% effect level and there is a ratio of roughly 2 between EC20 and EC50 for concentration-effect curves with a slope of around 2, which is typical for algal toxicity experiments. The concept of relative potency (RP) estimates was applied to calculate the wastewaters’ predicted mixture toxicity (2527). The relative potency scheme weighs the toxicity of toxic compounds as fractions of the toxicity of a reference substance. It can be applied under the assumption of a similar mode of toxic action and, therefore, concentration addition. We cannot check whether concentration addition really applies, but it is the only concept that is easy to compute and is useful as a realistic worst-case estimate as evidenced in two recent studies (28, 29). The FWA was chosen as a reference for the normalization because it is the product that the process waters originate from and its algae toxicity has been well defined. Each contaminant i is attributed to a specific relative potency, denoted as RPFWA (eq 3). This factor i indicates the degree of toxicity compared to the reference FWA, which is given as a reference value of RP ) 1. The FWA toxic equivalent concentration (EQFWA ) of any substance i i was calculated according to eq 4. EQFWA represents the FWA concentration that would elicit the same toxic effect as the concentration Ci of pollutant i.
RPFWA ) i
EC50FWA [-] EC50i
EQFWA ) RPFWA Ci i i
[
FWA EQpredicted )
(3)
]
mgFWA-equiv L
where Ci is the concentration of pollutant i in the wastewater (mg/L). The sum of EQFWA of all known constituents of the i wastewater is a measure of the mixture toxicity of the wastewater (eq 5). This predicted toxicity was compared to the measured FWA toxic equivalent concentration of the wastewaters, EQFWA ww (eq 6), to check the plausibility of the
∑ EQ
FWA i
i
EQFWA ww )
(1)
ecotoxicity along the wastewater-treatment chain. Because the volume of wastewater from the filter-press process is different from the wastewater volume of the membrane filtration, we chose to compare the toxicity of final effluents on the basis of EC50 values expressed in mgTOC,ref/L units (eq 2). Thus, the whole effluent toxicity could be compared with the classification scheme of dangerous substances (23).
EC50final effluent(mgTOC,ref/L) ) EC50final effluent(df)‚CTOC,ref
analysis and to understand the causes for the toxicity of the wastewaters. For calculation of the confidence intervals as well as the minimum and maximum values, see the Supporting Information.
EC50FWA EC50ww(df)
[
]
mgFWA-equiv
[
L
(5)
]
mgFWA-equiv L
(6)
where EC50ww is the wastewater’s measured EC50 value given as a dilution factor.
Results TOC and Pollutant Elimination. For the TOC and each contaminant, elimination rates were determined for the two wastewater-treatment processes (Table 1). Adsorption to activated carbon showed only a minor TOC removal ranging between 5 and 10%, whereas the biological treatment resulted in an average TOC reduction rate of 70-80% for both the mixture of mother liquor and washing water and the membrane-filtration permeates. The stilbene compounds were removed up to a maximum of 75% by both activated carbon adsorption and biological treatment. The reactant diethanolamine and the solvent methyl ethyl ketone were almost fully degraded in the biological purification process, whereas the adsorption to activated carbon proved to be fairly ineffective. TOC Composition of the Wastewaters. On the basis of the analysis of the contaminants, the TOC composition of all wastewaters prior to biological treatment accounted for up to a share of more than 60% (Figure 3). The main contributor to the organic carbon content of the untreated as well as the activated carbon treated wastewaters was the reactant diethanolamine, accounting for 40-53% of the TOC. Analyses of the solvent methyl ethyl ketone in the membranefiltration permeates and in filter-press process water revealed that this solvent adds an additional mean proportion of 30% to the permeate TOC and approximately 6% to the filtrate mixture TOC. The TOC of the biologically treated wastewaters was only explained by up to 18%. These wastewaters therefore mainly consist of unknown nonbiodegradable wastewater constituents and refractory degradation products (32). Ecotoxicity of Organic Wastewater Contaminants. None of the individual contaminants studied induced adverse effects to algal growth (Table 2). For the fluorescent whitening agent, an EC50 value of 179 mg/L was derived from a loglogistic fit of the concentration-effect curve. For diethanolamine and methyl ethyl ketone, algae toxicity data were retrieved from literature sources (24, 33, 34) and compared with the IUCLID dataset reports (35, 36) as well as the ECOTOX toxicity data (37) (see Supporting Information, Table S2). For MEK, the data are consistent. However, NOEC and EC50 data of green algae varied greatly for DEA. We excluded the value of 2.2 mg/L for the EC50 (for Pseudokirchneriella subcapitata) for DEA reported in the IUCLID dataset (35) because it was retrieved from a nonstandardized toxicity test and it is considerably lower than the NOEC values given for green algae (24, 33, 35). This left us with an EC50 value of 116 mg/L (for Pseudokirchneriella subcapitata). Effluent Ecotoxicity Changes through Physical and Biological Wastewater Treatment. Complementary to the analytical measurements, changes of algae toxicity were determined for each wastewater-treatment process to study the extent to which toxicity is reduced, as well as to track toxicity sources. The results indicate a statistically significant increase in toxicity after biological purification, with the toxicity rising by a factor of 3 for the filter-press wastewater VOL. 40, NO. 10, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Elimination of Wastewater TOC and Pollutants in Activated Carbon Adsorption and Biological Treatment elimination by activated carbon adsorption (%)a
elimination in biological treatment (%)a
10 5 (not applied)
80;b 70c 70b,c 70b
35-75 no eliminationd 10e
65-75 95 95f
TOC TOC of mother liquor and washing water mixture TOC of permeate 1 (startup phase) TOC of permeate 2 (full-scale production) Contaminants stilbene compounds (FWA and stilbene side products) diethanolamine methyl ethyl ketone
a Elimination rates reported in 5% steps. b Elimination in biological treatment without pretreatment. c Elimination in biological treatment after activated carbon adsorption. d Estimated from log Koc ) 0.6 (30) and the activated carbon concentration in the adsorption treatment. e Calculated on the basis of log Koc ) 1.5 (30) and the activated carbon concentration in the adsorption treatment. For calculation, see Supporting Information. f Removal in biological wastewater treatment: 95% (geometric mean) (31).
FIGURE 3. TOC composition of the wastewaters before and after each treatment process (biological treatment simulated in Zahn-Wellens test and adsorption to activated carbon).
TABLE 2. Algae Growth Inhibition Toxicity Test Results for the Wastewater Pollutants substance FWA diethanolamine methyl ethyl ketone
EC50 (mg/L) [CI]a
NOEC (mg/L)
b RPFWA i
179 [160-201] 116c [104-131] 8600e
71
1
4.4d
1.54
LOEC: 4300 mg/Lf
(0.02)
a CI: 95% confidence intervals for effect concentrations. b RPFWA: i Relative potency indicating the degree of toxicity compared to the c d e reference substance FWA (eq 3). Ref 34. Ref 33. Approximated from the LOEC (multiplication with a factor of 2, see Materials and Methods). f Ref 24.
mixture and by about a factor of 2 for the permeate (Table 3). By contrast, the activated carbon treatment process led to a decline in toxicity of 60% for the mixture of mother liquor and washing water. There are marked differences in ecotoxicity between the final effluents of the process-integrated system with membrane filtration and the classical end-of-pipe system. The filter-press wastewater mixture after adsorption and biologi3398
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cal treatment exhibits a significantly lower algae toxicity (EC50: 30.4 mgTOC,ref/L) compared to the biologically treated permeate (EC50: 12.8 mgTOC,ref/L) (see eq 2). Contribution to the Wastewaters’ Mixture Toxicity. To obtain insight into the causes of the wastewaters’ toxicity, the observed mixture toxicity was compared with the predicted joint toxicity calculated according to the model of concentration addition. Figure 4 illustrates the results expressed in terms of FWA toxic equivalent concentrations (EQFWA, eq 5 and 6). For the filter-press wastewater mixture before and after the adsorption process, the predicted total FWA equivalent concentrations overestimate the actually measured FWA equivalent concentrations, whereas for all other wastewaters, the predicted toxicity underestimates the observed joint toxicity. This is particularly the case for the biologically purified effluents, for which 97% of the measured whole effluent equivalent effect concentration remains unexplained. For the wastewaters prior to biological treatment, the reactant diethanolamine contributes the largest share to the calculated mixture toxicity, ranging between 87 and 98%. The other contaminants are insignificant with respect to their toxicity input. Only in the untreated filter-press wastewater is the mixture toxicity influenced by the product FWA (EQFWA share: 12%).
TABLE 3. Change of Algal Toxicity during Wastewater Treatment (EC50 Given as Dilution Factors)a
wastewater
original
mixture of mother liquor and washing water permeate 1 (startup phase) permeate 2 (full-scale production)
0.034 [0.030-0.039] 0.088 [0.065-0.119] 0.074 [0.060-0.091]
EC50 (Dilution Factor) [CI]b after biological treatment 0.011 [0.004-0.028] 0.038 [0.025-0.057]
after activated carbon adsorption
after adsorption and biological treatment
0.053 [0.004-0.239] 0.115 [n.d.]c -
0.051 [0.011-0.229] -
a Note that the two wastewater mixtures of mother liquor/washing water and permeate should not be directly compared on the basis of the values in the Table because membrane filtration produces higher wastewater volumes than filter press. b EC50: effect concentration for 50% inhibition of algal growth. CI: 95% confidence intervals for EC50. c n.d.: not determined. The test concentration causing the highest inhibition of 41% was used to approximate the EC50 value. Therefore, the quantification of the confidence interval was not possible.
FIGURE 4. Comparison of the predicted total effect and observed mixture toxicity of the wastewaters before and after activated carbon adsorption and biological treatment (joint toxicity expressed as toxic equivalents, EQFWA, in mgFWA-equiv/L). Predicted joint toxicity: error bars show minimum and maximum values calculated. Observed mixture toxicity: error bars show 95% confidence intervals (see Materials and Methods). Wastewater 6: confidence interval not determined for the observed toxicity because of the small number of test concentrations (see Materials and Methods).
Discussion Performance Evaluation of Biological Treatment and Activated Carbon Adsorption. In general, the wastewaters’ TOC elimination rates in the biological simulation tests indicate a substantial organic carbon reduction. The contaminants pursued show a moderate to very good removal. Whereas the stilbene compounds are eliminated primarily by adsorption to activated sludge (38), specifically, DEA and MEK are readily biodegradable, which is in line with the degradation data of Mackay et al. (30). A major share of the unidentified TOC in the biologically treated effluents may stem from refractory triazine metabolites (Figure 3). Ko¨hler (32) showed that FWA triazine side products are almost entirely transformed into refractory degradation products. This finding points to the importance of metabolites for the load of refractory organic carbon that is released into the aquatic environment. Despite the substantial TOC and pollutant reduction through the biological treatment, this purification process clearly brings about an increase in ecotoxicity. This increase in toxicity is statistically significant, as the 95% confidence intervals for the EC50 estimates of the permeate and the filter-press wastewater mixture do not overlap with those of the wastewaters after biological treatment (Table 3, Figure 4). In contrast to the biological purification, the physical activated carbon adsorption eliminated the overall TOC only slightly. Specific contaminants such as the stilbene compounds including the FWA, however, were significantly
removed. This is due to the well-adsorbing aromatic stilbene components in the molecules. Although the EC50 confidence interval of the filter-press wastewater after adsorption and after the combined physical and biological treatment covers fairly broad concentration ranges, the determined EC50 values are considered to be plausible, as the experimental data points were well fitted by the log-logistic concentration-response curves. Accordingly, the approximated EC50 value for the activated carbon treated permeate (Table 3) is regarded as a reasonable lower threshold value for a regression-based EC50 point estimate. The reason for the wide confidence intervals mainly lies in the small number of test concentrations (see Materials and Methods). Therefore, it can be stated that the activated carbon adsorption process efficiently reduces wastewater toxicity by 3156% (Table 3), although it contributes only slightly to the overall TOC elimination (5-10%). This observation can be rationalized by considering the hydrophobicity of the wastewater constituents. More hydrophobic compounds adsorb more strongly to activated carbon but are also generally more toxic than hydrophilic substances in the case of narcotic toxicity. The evaluation of the final effluent EC50 values in terms of refractory TOC with the classification scheme for dangerous substances according to the European Directive 1999/48/ EC (23) indicates that both treatment systems possibly emit contaminants harmful to aquatic organisms with the potential of causing long-term effects to the aquatic environment. This directive classifies not readily degradable subVOL. 40, NO. 10, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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stances with an acute algae toxicity of 10 < EC50 < 100 mg/L as harmful to aquatic life. As these classification thresholds merely identify hazards, the actual consequences of the contaminant discharges to the aquatic environment will depend on many factors such as contaminant dilution in the receiving water body or the presence of ambient pollutants. Historic sediment contamination, for instance, may cause substantial adverse effects to aquatic organisms by contaminant release (6) and thus trigger synergistic effects. Correspondence between Predicted and Observed Mixture Toxicity. In addition to the measurements of the wastewater algal toxicity, the organic pollutants’ toxicity was examined to determine which constituents were responsible for the observed mixture toxicity. All quantified contaminants display EC50 values over 100 mg/L (Table 2) and are therefore classified as “not harmful to aquatic organisms” (23). Also, they most likely do not impose long-term adverse effects to the aquatic environment because of a low bioaccumulation potential and/or low persistence. Concentration addition appears to be an adequate approach to estimate the wastewaters’ joint toxicity because for all wastewaters with well-known TOC composition on a single substance basis the toxicity predictions were close to the mixture toxicity measured. For the calculated joint toxicity, the minimum and maximum values span over a narrow range. The 95% confidence intervals of the experimentally determined mixture toxicity generally indicate larger uncertainties. For the filter-press wastewater prior to and after the activated carbon adsorption, the sum of the predicted toxicity contributions resulted in slightly higher FWA equivalent concentrations. In the case of the activated carbon treated wastewater, this difference was not statistically significant, as uncertainty bars overlapped (Figure 4). Another reason for this slight overestimation may be antagonistic effects of the pollutants present in the mixture. For instance, Pedersen and Peterson (34) showed, in a study investigating the algae growth inhibition (for Pseudokirchneriella subcapitata) of different contaminant mixtures containing diethanolamine as one main constituent, that for green algae the mixture toxicity was partly additive or antagonistic. With regard to the biologically treated effluents, the calculated toxicity was notably smaller than the experimentally measured whole effluent toxicity. Because the only ecotoxicologically relevant contaminant diethanolamine was almost completely removed from the effluent by biological treatment, the majority of the observed effect after biological purification must be due to the formation of toxic metabolites. It is known that the wastewater mixtures additionally contain triazine side products with bulky and hydrophilic substituents contributing up to 20% of the wastewaters’ original TOC (32). Such substituents could be cleaved from the triazine ring so that a more hydrophobic and thus more toxic degradation product could be formed. It is even possible that the biological treatment produces compounds structurally similar to herbicidal s-triazines showing herbicidal activity. s-Triazines with certain alkyl and chloro substituents show specific toxicity in algae as they inhibit the electron transfer in the photosystem II by blocking the quinone-binding site (18, 19, 39, 40). Note, however, that for final conclusions substance identification would be required, which was beyond the scope of the present study. Nevertheless, the formation of biological metabolites alone does not sufficiently explain the decrease in ecotoxicity after activated carbon adsorption or the lower ecotoxicity of the filter-press final effluent compared to the treated permeate (Table 3). Such toxicity elimination through activated carbon treatment can be more likely explained by chlorinated aromatic triazine side products that are created in the FWA synthesis and are present in the wastewaters. Their algal toxicity was not determined experimentally because their 3400
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TOC contribution is minute. Their predicted log KOW values of approximately 4 led us to expect a significant bioaccumulation and baseline toxicity. As such contaminants would be eliminated by adsorption, the toxicity increase after biological treatment would be considerably lower, indicating that biological metabolites of those triazines may have also contributed to the elevated whole effluent toxicity after biological treatment. Comparison of Final Effluents from Process-Integrated and End-of-Pipe Treatment. The mass of refractory TOC emitted from the end-of-pipe system is higher than the refractory TOC load from the process-integrated system by a factor of 1.2. Therefore, the new process-integrated system is more efficient concerning TOC elimination than the endof-pipe system as it already diminishes the TOC content in the permeate by a more effective product isolation via ultrafiltration. Note that TOC removal is desirable, not only for reasons of toxicity but also because it reduces the oxygen demand discharged to natural water bodies. A different picture is obtained when looking at algae toxicity, in terms of EC50 as well as in terms of absolute toxic load. The toxic loads were calculated by multiplying the toxic equivalents (Figure 4) with the wastewater volumes per FWA production batch. The end-of-pipe system is ranked higher, as it discharges only 50% of the membrane-filtration system’s toxic load. Note, however, that the preferable treatment option can only be identified by additionally taking into account further assessment parameters, such as energy consumption and emissions to other environmental media. Such requirements are stated by the European Directive on Integrated Pollution Prevention and Control, which demands the overall prevention and reduction of emissions and other impacts on the environment (41). Taking these additional parameters into account might again change the conclusions. In the present study, both performance indicators of TOC removal and toxicity were important, as both provided important information for process optimization. For instance, the FWA synthesis process is currently optimized to achieve lower traizine side-product amounts in the permeates and thus reduce the refractory TOC emissions from FWA production. The toxicity tests were useful, as they pinpointed toxic substances, which were only present in small amounts. These substances may be candidates for future optimization processes. Thus, we have concluded that an integrated concept of TOC and pollutant measurements combined with toxicity tracking, as demonstrated in the current work, is more adequate than TOC removal alone for more comprehensive process assessment, control, and optimization.
Acknowledgments We gratefully acknowledge the experimental and financial support from Ciba Specialty Chemicals, Grenzach, Germany, as well as the valuable discussion with Ciba experts.
Supporting Information Available Analytical protocols, method background information, and concentration-effect curves. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review March 9, 2006. Accepted March 10, 2006. ES060555F
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