Environ. Sci. Technol. 2003, 37, 3111-3116
Removal of Two Polycyclic Musks in Sewage Treatment Plants: Freely Dissolved and Total Concentrations E L S A A R T O L A - G A R I C A N O , * ,† IRIS BORKENT,† JOOP L. M. HERMENS,† AND WOUTER H. J. VAES‡ Institute for Risk Assessment Sciences, Toxicology Division, Utrecht University, P.O. Box 80.176, NL-3508 TD Utrecht, The Netherlands, and TNO Nutrition and Food Research, P.O. Box 360, NL-3700 AJ Zeist, The Netherlands
In the current study, the removal of slowly degradable hydrophobic chemicals in sewage treatment plants (STPs) has been evaluated with emphasis on the combination of free and total concentration data. Free and total concentrations of two polycyclic musks were determined in each compartment of four STPs. The free concentration of the polycyclic musks remains virtually constant throughout all the compartments of the STPs with values between 0.21 and 0.57 µg/L for AHTN and between 0.79 and 2.0 µg/L for HHCB. Total concentrations of these fragrances are highly dependent on the volatile solids in a given compartment resulting in much more variation with values between 0.42 and 92 µg/L for AHTN and between 1.25 and 258 µg/L for HHCB. It is concluded that free concentrations of these hydrophobic chemicals in the compartments of STPs are mostly biodegradation mediated, while total concentrations are mediated by the concentration of solids. The combination of measurements of free and total concentrations can improve estimations regarding removal efficiency and removal pathways.
Introduction The dramatic increase in production and emission of synthetic organic chemicals for industrial and domestic use has obliged sewage treatment plants (STPs) to improve their efficiency. At present, most STPs consist of two purification systems: a physical and a biological purification step. In the physical purification, removal of the chemical is mostly due to sorption of chemicals to organic carbon. The effectiveness of removal is directly related to the size and density of the particles. In the biological purification, removal is achieved by bacterial biodegradation, which mainly occurs via oxidation. Every chemical that enters the STP and is neither sorbed nor degraded will enter the environment via the effluent or via evaporation from the STP. To evaluate the efficiency of STPs, many studies have been carried out in which the difference between the concentration of the chemical in the influent and that in the effluent has been compared (1-4). During the last 15 yr, several studies have been published in which the influence of organic carbon on toxicity of organic * Corresponding author telephone: +31 30 2535018; fax: +31 30 2535077; e-mail:
[email protected]. † Utrecht University. ‡ TNO Nutrition and Food Research. 10.1021/es020226x CCC: $25.00 Published on Web 06/12/2003
2003 American Chemical Society
chemicals is considered. Different authors (5-7) have shown that, in the presence of organic carbon, a decrease of bioconcentration occurs in aquatic organisms. Also a decrease of the acute toxicity in the presence of dissolved organic carbon (DOC) has been reported (8, 9). From all these studies, it can be concluded that only the freely dissolved concentration is available for passive uptake into organisms. Not only toxicity and bioaccumulation but also biodegradation are highly influenced by the presence of a matrix such as slurry, soil, and biomass (10-14). Thus, in many cases, the determination of the free concentration is more relevant than the total concentration. Generally, organisms are exposed to STP effluents in the mixing zone of the effluent with river water. Several processes (i.e., dilution and sorption/desorption to organic matter) control the free concentration in the environment. To understand or even influence these processes, it is essential to obtain information on the distribution, the removal, and the effluent concentrations of chemicals. However, few studies have reported on the removal of organic chemicals by STPs with a focus on free and total concentrations (15, 16). The objectives of the current work were (i) to study the removal of hydrophobic organic chemicals based on free concentrations, (ii) to compare these results with the classical method of assessing the removal processes (total concentration), and (iii) to study the contribution of both sorption and degradation on the total removal. This study focuses on the behavior of two polycyclic musks, 7-acetyl-1,1,3,4,4,6-hexamethyltetrahydronaphthalene (AHTN) and 7-acetyl-1,1,3,4,4,6-hexahydro-4,6,6,7,8,8hexamethylcyclopenta(g)-2-benzopyrane (HHCB). The hydrophobic properties of these chemicals (log Kow values are about 5.7 (17)) are expected to lead to substantial sorption to organic carbon. Additionally, the biodegradation rate constants have been reported to be low (18, 19). These two substances are frequently used as low-cost fragrances in soaps, perfumes, air fresheners, detergents, and other household cleaning products (20, 21); therefore, concentrations in the environment and in STPs are sufficiently high to study their behavior in STPs directly, without spiking (22-27). During this study, sewage was collected from different compartments of STPs. To validate the reproducibility of the study, four different domestic plants in The Netherlands were studied. As is shown in Figure 1, each plant consists of influent, primary settler, primary sludge, aeration tank, solid/ liquid separator (S/L), effluent, and waste sludge. Sludge that settles in the S/L separator is partly recycled to the oxidation tank. Primary and waste sludge are waste products. Both free and total concentrations of the two musks were measured in all compartments of each STP. Free concentrations were determined using negligible depletion solid-phase microextraction (nd-SPME) according to Vaes et al. (28) and total concentrations using conventional liquid-liquid extraction techniques.
Materials and Methods Chemicals. HHCB (purity >98%) was obtained from International Flavors and Fragrances (IFF) (Hilversum, The Netherlands), and AHTN (purity >98%) was from PFW Aroma Chemical B.V., Hercules Incorporated (Barneveld, The Netherlands). NaN3 was purchased from Merck (Darmstadt, Germany). Materials. SPME fibers of 1-cm length, coated with a 100µm poly(dimethylsiloxane) layer, were purchased from Supelco (Bellefonte, CA). In all cases, fibers were cut to 1-mm VOL. 37, NO. 14, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Different compartments present in a sewage treatment plant.
TABLE 1. Specific Characteristics of the STPs Studieda De Bilt
Q (m3/h) KSLR (kgBOD kgdwt-1 d-1) HRT (h)b SRT (d)b VS aerator (g/L) mode of aeration
800 0.15 5.7 8.0 2.11 surface
Nieuwegein Overvecht 1400 0.054 8.9 20 3.87 surface
4200 0.39 4.8 15 2.75 surface
Zeist 1060 0.12 7.0 22 3.00 bubble
a Q, sewage flow; K SLR, sludge loading rate; HRT, hydraulic retention time; SRT, sludge retention time; VS, volatile solids. b For the aeration tank, obtained from the STP management.
length to obtain negligible depletion in the samples. New fibers were conditioned for 1 h at 250 °C in a GC split injector to desorb all impurities. Measurements of AHTN and HHCB in STPs. The study was carried out in four STPs located in The Netherlands during the period from March 5 to April 24, 2001. STPs were located in De Bilt, Zeist, Nieuwegein, and Overvecht (Utrecht). Table 1 gives detailed operating characteristics of these four STPs. Samples were collected from different compartments (influent, primary settler, aeration tank, effluent from the S/L separator, primary sludge, and waste sludge) of the STPs. The Overvecht plant was equipped with an extra primary aeration tank between the influent and the primary settler. Samples were immediately handled for the determination of the free and total concentrations and volatile solids (VS) in each compartment. To establish the withinday variation of free and total musk concentrations in the influent, samples were collected every 2 h during 24 h in the STP in De Bilt. Determination of Free Concentrations. Extractions were carried out using nd-SPME according to Vaes et al. (28). This technique used the fact that, by extraction of a minor (negligible) fraction of the freely dissolved chemical, the partitioning equilibrium of the chemical under study between the aqueous and the organic phase will not be perturbed. Therefore, the extracted fraction is related to the truly freely dissolved concentration. nd-SPME is derived from solidphase microextraction (SPME), as introduced by Arthur and Pawliszyn (29). In particular, SPME methods have been applied successfully by Heberer et al. to the analysis of musks in surface waters and STP effluents (30). From each STP compartment, three 10-mL wastewater aliquots were sampled. Subsequently, 1 mL of 10 mM NaN3 was added to inhibit microbial degradation. Analysis of all samples was carried out on a Varian Star 3600 CX GC equipped with a temperature-controlled (20 °C) Varian 8200 CX SPME autosampler (Varian, Palo Alto, CA). This autosampler incorporates an agitation device for SPME sampling. The GC was equipped with a split/splitless injector, a 30 m × 0.32 mm fused silica DB 5.625 column with a 0.25 µm film thickness (J&W Scientific, Folson, CA) and a Saturn 2000 ion trap mass spectrometer. The injector was maintained at 3112
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250 °C. The fiber was exposed to the sample for 10 min, and immediately after exposure, it was desorbed in the injector for another 10 min. The extraction efficiency was determined as described before (28) and was 1.7%, meaning that only a negligible amount was extracted. During desorption, the column was maintained at 50 °C. Subsequently, the injector was switched to the split mode, the fiber was released from the injector, and the column was raised by 40 °C/min to 150 °C and by 4 °C/min to 210 °C. The ion trap was operated in full-scan mode (m/z 100-300) at 3 scans/s. The m/z ratios of 159, 243, 258 and 213, 243, 258 for AHTN and HHCB, respectively, were used as qualifiers for the identification of the compounds. Quantification was carried out for m/z 243. The concentrations were determined by external calibration using calibration standards in water. Quality control samples with a concentration of about 1 µg/L were included in all series and gave an average repeatability and reproducibility of 6% and 8%, respectively. The limit of quantification was 0.1 µg/L for both chemicals. Determination of Total Concentrations. Total concentrations were determined using conventional liquid/liquid extraction followed by a cleanup. Extractions were carried out in triplicate in the following way. A 10-mL sample of the sewage from the different compartments of each STP, to which 1 mL of 10 mM NaN3 was added directly after sampling, was extracted with 6 mL of cyclohexane (J. T. Baker) during 2 h of shaking. After centrifugation of the samples at 3000 rpm for 10 min, 4 mL of the solvent was collected and concentrated to about 1 mL under a gentle stream of nitrogen. Subsequently, the cleanup of the extract was carried out using 6-mL SPE columns filled with 500 mg of silica (J&W, Folsom, CA), which were washed with 5 mL of ethyl acetate and after that conditioned with 15 mL of cyclohexane. After transfer of the concentrated extract, the column was eluted with 6 mL of cyclohexane:ethyl acetate (98:2). Extracts for the determination of total concentrations were analyzed on a Carlo Erba 5300 GC (Milan, Italy) equipped with a split/splitless injector and a 30 m × 0.25 mm fused silica DB-5MS column (J&W Scientific, Folson, CA) with a 0.25-µm film thickness. The detector was a QMD 1000 mass spectrometer (Carlo Erba Instruments, Milan, Italy). Analyses were carried out by injecting a volume of 1 µL into a splitless injector at 225 °C. The column temperature was maintained at 90 °C for 1 min and raised by 30 °C/min to 150 °C. To obtain a good separation between both chemicals, the column temperature was raised subsequently by 4 °C/min to 210 °C. The MS was used in SIM mode at m/z 243, and 3 scans/s was recorded. Recoveries were determined using sludge spiked with both compounds. The spiked sludge was allowed to equilibrate for 24 h before extraction was started. Additionally, consecutive extractions on nonspiked sludge samples were performed. Recoveries of the procedure were between 85 and 106% for both chemicals. Average repeatability and reproducibility were 8% and 12%, respectively. Limit of quantification was 0.1 µg/L for both chemicals.
FIGURE 2. Within-day variation of total (9) and free (0) concentrations (( SEM, sum of HHCB and AHTN) in the STP in De Bilt in The Netherlands. Determination of Volatile Solids (VS). Samples were taken from all compartments of the STPs to determine the VS of the sludge. First the samples were freeze-dried, and a known amount of the dried sample was deposited in a crucible. Subsequently, the crucible was inserted in an oven, and the sample was burned at 550 °C for 2 h. When the temperature of the oven had decreased to about 150 °C, the crucibles were transferred to an exsiccator. Once the samples reached ambient temperature, the VS of the sample was determined gravimetrically.
Results and Discussion Grab versus Composite Sampling. In the current study, grab samples were collected from all compartments of four different STPs. Similar studies have been criticized for drawing conclusions based on grab sample data mainly because of the variance of chemical concentrations in time, which was specifically shown for alkyl ethoxylate surfactants (31) and fragrance materials (27). Results from the current study, concerning the within-day variation of AHTN and HHCB in the influent of the STP in De Bilt are given in Figure 2. The variation that was found by sampling every 2 h was approximately 19% for total concentrations of the musks, while it was
