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On the use of PAH molecular diagnostic ratios in sewage sludge for the understanding of the PAH sources. Is this use appropriate?
Chemosphere 69 (2007) 1337–1339 www.elsevier.com/locate/chemosphere
Discussion
On the use of PAH molecular diagnostic ratios in sewage sludge for the understanding of the PAH sources. Is this use appropriate? Athanasios Katsoyiannis
a,*
, Eleni Terzi b, Quan-Ying Cai
c
a
b
EC – Joint Research Centre, Institute for Health and Consumer Protection, Physical and Chemical Exposure Unit, Ispra (VA), TP-281, Via E. Fermi 1, I-21020, Italy Aristotle University of Thessaloniki, Department of Chemistry, Environmental Pollution Control Laboratory, Thessaloniki GR-54124, Greece c College of Resources and Environment, South China Agricultural University, Guangzhou 510642, China Received 27 March 2007; received in revised form 11 May 2007; accepted 31 May 2007 Available online 17 July 2007
Abstract The concentrations ratios of specific pairs of polycyclic aromatic hydrocarbons (PAHs) are widely used for the qualitative determination of the PAHs sources. These ratios are called PAHs molecular diagnostic ratios and are commonly used for PAHs concentrations in air, soils and sediments. Some scientists have extended the use of these ratios also for sewage sludges, suggesting that calculation of these ratios by individual PAHs concentrations can be as effective as in soils or sediments. This paper describes the reason why the PAH molecular ratios calculated from sewage sludge concentrations should not be used for the understanding of the PAH sources. 2007 Elsevier Ltd. All rights reserved. Keywords: PAHs; Sewage sludge; Molecular diagnostic ratios; Wastewater treatment plant
1. Introduction In many studies, the sources of polycyclic aromatic hydrocarbons (PAHs) in various environmental samples have been qualitatively determined by the various molecular diagnostic ratios. For example, a widely used concentration ratio is the anthracene (Ant)/[Ant + phenanthrene (Phe)], and if this is lower than 0.10, it is taken as indicative of non-burnt fossil fuel, whereas if this ratio exceeds 0.10, this suggests combustion sources. Similarly, many other ratios exist (Table 1), and are used to reveal petrogenic or pyrogenic origin, fuel or wood combustion, or traffic related sources (Brandli et al., 2007; Cai et al., 2007). When these ratios are used to determine the sources, it is hypothesized that paired chemicals are diluted to a similar extent and that the ratios remain constant en route from sources to receptors, although in many cases it has been proven that this does not happen (Zhang et al., 2005). *
Corresponding author. Tel.: +39 0332 786517; fax: +39 0332 786012. E-mail address: [email protected] (A. Katsoyiannis).
0045-6535/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2007.05.084
Recently, Dai et al. (2007; but also in the past, i.e. Blanchard et al., 2004), used these ratios for the characterization of sources of PAHs ending in the sewage sludge. The authors of the present correspondence claim that such use is not appropriate based on two main reasons: (1) The mixing and homogenization that takes place during the transportation of wastewaters; (2) The treatment that the wastewaters undergo at the various treatment steps. These two parameters will be discussed to prove the afore-mentioned statement about the improper use of PAH molecular diagnostic ratios in sewage sludges. 1.1. Wastewater mixing and homogenization A WWTP may receive municipal, domestic, or industrial wastewater, effluents from small business (e.g. car washing) or medical facilities, plus street runoff, drainage water, wet and dry deposition from the atmosphere (Karvelas et al., 2003). Furthermore, big WWTPs may receive effluents from entire cities, thus from places that are very far from each other, and where different PAH sources may exist.
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A. Katsoyiannis et al. / Chemosphere 69 (2007) 1337–1339
Table 1 Characteristic PAH molecular diagnostic ratios (Brandli et al., 2007)
Ant/(Ant + Phe) BaA/(BaA + Chr) Flt/(Flt + Pyr) IPY/(IPY + BPE)
Flt/(Flt + Pyr) IPY/(IPY + BPE)
BaP/BPE
Petrogenic
Pyrogenic
<0.1 <0.2 <0.4 <0.2
>0.1 >0.35 >0.4 >0.2
Fuel combustion
Grass/coal/wood combustion
0.4–0.5 0.2–0.5
>0.5 >0.5
Non-traffic
Traffic
<0.6
>0.6
Ant: Anthracene; Phe: Phenanthrene; BaA: Benzo[a]anthracene; Chr: Chrysene; Flt: Fluoranthene; Pyr: Pyrene; IPY: Indeno[1,2,3-cd]pyrene; BPE: Benzo[g,h,i]perylene; BaP: Benzo[a]pyrene.
After reaching the WWTP, the effluents will at first undergo the primary sedimentation step, where the wastewaters are mixed and the suspended solids are allowed to stand. Afterwards, the primary sludge will enter the sludge line, and the supernatant wastewater will enter the secondary treatment tank, where it will be treated biologically and then homogenized. The suspended solids of this wastewater will again be allowed to stand and produce the secondary sludge, a part of which will be mixed with the primary sludge. After the mixing, the sludge will be dewatered and anaerobically digested, producing finally what is called the final sewage sludge. Due to this mixing of different wastewaters, and the continuous homogenization, the final sludge is unlikely to have characteristics that correspond to those of the aforementioned categories of the wastewaters that reach the WWTP, nor to the various point sources that contribute to the overall PAH burden that is found in wastewaters. 1.2. Wastewater treatment During the wastewater treatment process (especially for activated sludge WWTPs), the fate of hydrophobic organic compounds (HOCs) is governed by their physico-chemical properties, the design and the operating conditions of the WWTP (Byrns, 2001; Katsoyiannis et al., 2006). The HOCs removal mechanisms that take place during the treatment process are advection, sorption, volatilization, air-stripping and biotransformation. Sorption on the particulate matter is the main removal mechanism that takes place in the primary sedimentation tank and is theoretically removing all HOCs. For very lipophilic chemicals, like PAHs, this removal should be quasi quantitative (Byrns, 2001). Especially for chemicals with similar lipophilicity (octanol-water partition coefficients, Kow), like Ant and Phe (log Kow of 4.54 and 4.46, respectively; Huckins et al., 1999), the sorption should occur in the same extent. The suspended solids (SS), on which the HOCs are sorbed, are deposited (primary sludge) and after mixing with the secondary sludge and the consequent treatment,
these solids become the final sewage sludge. In reality, however, the HOCs are not removed quantitatively during the primary treatment. An important fraction of HOCs remains at the wastewater and passes via advection at the secondary treatment and may also be discharged with the final effluent (Katsoyiannis and Samara, 2004; Busetti et al., 2006). Volatilization is a mechanism that will remove significantly semi-volatile and volatile chemicals. According to Byrns (2001), the extent to which this process will take place is governed by the Henry’s Law constant (H) and for chemicals with log H > 0.97 Pa m3 mol 1, the losses due to this mechanism are significant. The log H of Phe and Ant are 0.48 and 1.29 (Zhang et al., 2005), respectively, suggesting that the former will not be affected by volatilization, while the latter will probably have losses of 10% (rough estimation, based on Fig. 4 of the paper of Byrns, 2001). Ant and Phe are three-ring PAHs, have similar boiling points (340 C) and Kows (4.5), but, apart from H, other important physico-chemical properties vary widely as well. For example, their aqueous solubilities (mg l 1) are 0.076 (Ant) vs 1.18 (Phe) (Huckins et al., 1999) and their vapor pressures (Pa at 25 C) are 2.0 · 10 4 (Ant) vs 1.8 · 10 2 (Phe) (Antizar-Ladislao et al., 2004). The increased solubility of a chemical is expected to affect also its volatilization, since volatilization takes place from the dissolved phase. Biodegradation is also expected to occur from the dissolved fraction of the chemicals and not from the sorbed. According to the US-EPA FATE (Fate and treatibility Estimator) model (US EPA, 1990), there is difference between the biodegradation rates of these two chemicals (log biodegradation rate: 2.3 for Phe and 3.0 for Ant). It is clear that each removal mechanism has a different effect on each chemical and any molecular ratio that exists in the incoming wastewater will not be found in the final effluent, or in the sludge. Apart from the ratio Ant/ (Ant + Phe), the same also stands for the other molecular diagnostic ratios. Several authors have presented concentrations of PAHs in various treatment steps of a WWTP. At the study of Busetti et al. (2006) (see supplementary data), the Ant/ (Ant + Phe) ratio at the incoming wastewater was 0.46, and increased to 0.63 at the final effluent. At the final sludge of that study, the ratio (0.46) is similar to the raw wastewater, but this can be considered only as accidental, since in the secondary sludge the same ratio is 0.52. The Flt/(Flt + Pyr) ratio at the same study was quite constant between the raw wastewater and the final sludge, but in their ‘‘sampling site 3’’, where industrial effluents are received, the same ratio was 0.78. This is suggesting the obvious, thus, that different wastewaters, with different pollutant compositions and sources are mixed in a WWTP, and that the final sludge is produced after this mixing. Therefore, the accuracy of molecular diagnostic ratios for the PAH source identification, using the sludge concentrations of individual PAHs is questionable.
A. Katsoyiannis et al. / Chemosphere 69 (2007) 1337–1339
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.chemosphere. 2007. 05.084. References Antizar-Ladislao, B., Lopez-Real, J.M., Beck, A.J., 2004. Bioremediation of polycyclic aromatic hydrocarbon (PAH)-contaminated waste using composting approaches. Crit. Rev. Environ. Sci. Technol. 34, 249–289. Blanchard, M., Teil, M.J., Ollivon, D., Legenti, L., Chevreuil, M., 2004. Polycyclic aromatic hydrocarbons and polychlorobiphenyls in wastewaters and sewage sludges from the Paris area (France). Environ. Res. 95, 184–197. Brandli, R., Bucheli, T.D., Kupper, T., Mayer, J., Stadelmann, F.X., Taradellas, J., 2007. Fate of PCBs, PAHs and their source characteristic ratios during composting and digestion of source-separated organic waste in full-scale plants. Environ. Pollut.. doi:10.1016/ j.envpol.2006.11.021. Busetti, F., Heitz, A., Cuomo, M., Badoer, S., Traverso, P., 2006. Determination of sixteen polycyclic aromatic hydrocarbons in aqueous and solid samples from an Italian wastewater treatment plant. J. Chromatogr. A 1102, 104–115. Byrns, G., 2001. The fate of xenobiotic organic compounds in wastewater treatment plants. Water Res. 35 (10), 2523–2533.
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Cai, Q.Y., Mo, C.H., Li, Y.H., Zeng, Q.Y., Katsoyiannis, A., Wu, Q.T., Ferard, J.F., 2007. Occurrence and assessment of polycyclic aromatic hydrocarbons in soils from vegetable fields of the Pearl River Delta, South China. Chemosphere 68, 159–168. Dai, J., Xu, M., Chen, J., Yang, X., Ke, Z., 2007. PCDD/F, PAH and heavy metals in the sewage sludge from six wastewater treatment plants in Beijing, China. Chemosphere 66, 353–361. Huckins, J.N., Petty, J.D., Orazio, C.E., Lebo, J.A., Clark, R.C., Gibson, V.L., Gala, W.R., Echols, K.R., 1999. Determination of uptake kinetics (sampling rates) by liquid-containing semipermeable membrane devices (SPMDs) for polycyclic aromatic hydrocarbons (PAHs) in water. Environ. Sci. Technol. 33, 3918–3923. Karvelas, M., Katsoyiannis, A., Samara, C., 2003. Occurrence and fate of heavy metals in the wastewater treatment process. Chemosphere 53, 1201–1210. Katsoyiannis, A., Samara, C., 2004. Persistent organic pollutants (POPs) in the sewage treatment plant of Thessaloniki, northern Greece: occurrence and removal. Water Res. 38, 2685–2698. Katsoyiannis, A., Zouboulis, A., Samara, C., 2006. Persistent organic pollutants (POPs) in the conventional activated sludge treatment process: model predictions against experimental values. Chemosphere 65, 1634–1641. US EPA, 1990. CERCLA site discharges to POTWS. Treatibility manual. EPA 540/2-90-007, Washington DC 20460. Zhang, X.L., Tao, S., Liu, W.X., Yang, Y., Zuo, Q., Liu, S.Z., 2005. Source diagnostics of polycyclic aromatic hydrocarbons based on species ratios. A multimedia approach. Environ. Sci. Technol. 39, 9109–9114.
Discussion
On the use of PAH molecular diagnostic ratios in sewage sludge for the understanding of the PAH sources. Is this use appropriate? Athanasios Katsoyiannis
a,*
, Eleni Terzi b, Quan-Ying Cai
c
a
b
EC – Joint Research Centre, Institute for Health and Consumer Protection, Physical and Chemical Exposure Unit, Ispra (VA), TP-281, Via E. Fermi 1, I-21020, Italy Aristotle University of Thessaloniki, Department of Chemistry, Environmental Pollution Control Laboratory, Thessaloniki GR-54124, Greece c College of Resources and Environment, South China Agricultural University, Guangzhou 510642, China Received 27 March 2007; received in revised form 11 May 2007; accepted 31 May 2007 Available online 17 July 2007
Abstract The concentrations ratios of specific pairs of polycyclic aromatic hydrocarbons (PAHs) are widely used for the qualitative determination of the PAHs sources. These ratios are called PAHs molecular diagnostic ratios and are commonly used for PAHs concentrations in air, soils and sediments. Some scientists have extended the use of these ratios also for sewage sludges, suggesting that calculation of these ratios by individual PAHs concentrations can be as effective as in soils or sediments. This paper describes the reason why the PAH molecular ratios calculated from sewage sludge concentrations should not be used for the understanding of the PAH sources. 2007 Elsevier Ltd. All rights reserved. Keywords: PAHs; Sewage sludge; Molecular diagnostic ratios; Wastewater treatment plant
1. Introduction In many studies, the sources of polycyclic aromatic hydrocarbons (PAHs) in various environmental samples have been qualitatively determined by the various molecular diagnostic ratios. For example, a widely used concentration ratio is the anthracene (Ant)/[Ant + phenanthrene (Phe)], and if this is lower than 0.10, it is taken as indicative of non-burnt fossil fuel, whereas if this ratio exceeds 0.10, this suggests combustion sources. Similarly, many other ratios exist (Table 1), and are used to reveal petrogenic or pyrogenic origin, fuel or wood combustion, or traffic related sources (Brandli et al., 2007; Cai et al., 2007). When these ratios are used to determine the sources, it is hypothesized that paired chemicals are diluted to a similar extent and that the ratios remain constant en route from sources to receptors, although in many cases it has been proven that this does not happen (Zhang et al., 2005). *
Corresponding author. Tel.: +39 0332 786517; fax: +39 0332 786012. E-mail address: [email protected] (A. Katsoyiannis).
0045-6535/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2007.05.084
Recently, Dai et al. (2007; but also in the past, i.e. Blanchard et al., 2004), used these ratios for the characterization of sources of PAHs ending in the sewage sludge. The authors of the present correspondence claim that such use is not appropriate based on two main reasons: (1) The mixing and homogenization that takes place during the transportation of wastewaters; (2) The treatment that the wastewaters undergo at the various treatment steps. These two parameters will be discussed to prove the afore-mentioned statement about the improper use of PAH molecular diagnostic ratios in sewage sludges. 1.1. Wastewater mixing and homogenization A WWTP may receive municipal, domestic, or industrial wastewater, effluents from small business (e.g. car washing) or medical facilities, plus street runoff, drainage water, wet and dry deposition from the atmosphere (Karvelas et al., 2003). Furthermore, big WWTPs may receive effluents from entire cities, thus from places that are very far from each other, and where different PAH sources may exist.
1338
A. Katsoyiannis et al. / Chemosphere 69 (2007) 1337–1339
Table 1 Characteristic PAH molecular diagnostic ratios (Brandli et al., 2007)
Ant/(Ant + Phe) BaA/(BaA + Chr) Flt/(Flt + Pyr) IPY/(IPY + BPE)
Flt/(Flt + Pyr) IPY/(IPY + BPE)
BaP/BPE
Petrogenic
Pyrogenic
<0.1 <0.2 <0.4 <0.2
>0.1 >0.35 >0.4 >0.2
Fuel combustion
Grass/coal/wood combustion
0.4–0.5 0.2–0.5
>0.5 >0.5
Non-traffic
Traffic
<0.6
>0.6
Ant: Anthracene; Phe: Phenanthrene; BaA: Benzo[a]anthracene; Chr: Chrysene; Flt: Fluoranthene; Pyr: Pyrene; IPY: Indeno[1,2,3-cd]pyrene; BPE: Benzo[g,h,i]perylene; BaP: Benzo[a]pyrene.
After reaching the WWTP, the effluents will at first undergo the primary sedimentation step, where the wastewaters are mixed and the suspended solids are allowed to stand. Afterwards, the primary sludge will enter the sludge line, and the supernatant wastewater will enter the secondary treatment tank, where it will be treated biologically and then homogenized. The suspended solids of this wastewater will again be allowed to stand and produce the secondary sludge, a part of which will be mixed with the primary sludge. After the mixing, the sludge will be dewatered and anaerobically digested, producing finally what is called the final sewage sludge. Due to this mixing of different wastewaters, and the continuous homogenization, the final sludge is unlikely to have characteristics that correspond to those of the aforementioned categories of the wastewaters that reach the WWTP, nor to the various point sources that contribute to the overall PAH burden that is found in wastewaters. 1.2. Wastewater treatment During the wastewater treatment process (especially for activated sludge WWTPs), the fate of hydrophobic organic compounds (HOCs) is governed by their physico-chemical properties, the design and the operating conditions of the WWTP (Byrns, 2001; Katsoyiannis et al., 2006). The HOCs removal mechanisms that take place during the treatment process are advection, sorption, volatilization, air-stripping and biotransformation. Sorption on the particulate matter is the main removal mechanism that takes place in the primary sedimentation tank and is theoretically removing all HOCs. For very lipophilic chemicals, like PAHs, this removal should be quasi quantitative (Byrns, 2001). Especially for chemicals with similar lipophilicity (octanol-water partition coefficients, Kow), like Ant and Phe (log Kow of 4.54 and 4.46, respectively; Huckins et al., 1999), the sorption should occur in the same extent. The suspended solids (SS), on which the HOCs are sorbed, are deposited (primary sludge) and after mixing with the secondary sludge and the consequent treatment,
these solids become the final sewage sludge. In reality, however, the HOCs are not removed quantitatively during the primary treatment. An important fraction of HOCs remains at the wastewater and passes via advection at the secondary treatment and may also be discharged with the final effluent (Katsoyiannis and Samara, 2004; Busetti et al., 2006). Volatilization is a mechanism that will remove significantly semi-volatile and volatile chemicals. According to Byrns (2001), the extent to which this process will take place is governed by the Henry’s Law constant (H) and for chemicals with log H > 0.97 Pa m3 mol 1, the losses due to this mechanism are significant. The log H of Phe and Ant are 0.48 and 1.29 (Zhang et al., 2005), respectively, suggesting that the former will not be affected by volatilization, while the latter will probably have losses of 10% (rough estimation, based on Fig. 4 of the paper of Byrns, 2001). Ant and Phe are three-ring PAHs, have similar boiling points (340 C) and Kows (4.5), but, apart from H, other important physico-chemical properties vary widely as well. For example, their aqueous solubilities (mg l 1) are 0.076 (Ant) vs 1.18 (Phe) (Huckins et al., 1999) and their vapor pressures (Pa at 25 C) are 2.0 · 10 4 (Ant) vs 1.8 · 10 2 (Phe) (Antizar-Ladislao et al., 2004). The increased solubility of a chemical is expected to affect also its volatilization, since volatilization takes place from the dissolved phase. Biodegradation is also expected to occur from the dissolved fraction of the chemicals and not from the sorbed. According to the US-EPA FATE (Fate and treatibility Estimator) model (US EPA, 1990), there is difference between the biodegradation rates of these two chemicals (log biodegradation rate: 2.3 for Phe and 3.0 for Ant). It is clear that each removal mechanism has a different effect on each chemical and any molecular ratio that exists in the incoming wastewater will not be found in the final effluent, or in the sludge. Apart from the ratio Ant/ (Ant + Phe), the same also stands for the other molecular diagnostic ratios. Several authors have presented concentrations of PAHs in various treatment steps of a WWTP. At the study of Busetti et al. (2006) (see supplementary data), the Ant/ (Ant + Phe) ratio at the incoming wastewater was 0.46, and increased to 0.63 at the final effluent. At the final sludge of that study, the ratio (0.46) is similar to the raw wastewater, but this can be considered only as accidental, since in the secondary sludge the same ratio is 0.52. The Flt/(Flt + Pyr) ratio at the same study was quite constant between the raw wastewater and the final sludge, but in their ‘‘sampling site 3’’, where industrial effluents are received, the same ratio was 0.78. This is suggesting the obvious, thus, that different wastewaters, with different pollutant compositions and sources are mixed in a WWTP, and that the final sludge is produced after this mixing. Therefore, the accuracy of molecular diagnostic ratios for the PAH source identification, using the sludge concentrations of individual PAHs is questionable.
A. Katsoyiannis et al. / Chemosphere 69 (2007) 1337–1339
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.chemosphere. 2007. 05.084. References Antizar-Ladislao, B., Lopez-Real, J.M., Beck, A.J., 2004. Bioremediation of polycyclic aromatic hydrocarbon (PAH)-contaminated waste using composting approaches. Crit. Rev. Environ. Sci. Technol. 34, 249–289. Blanchard, M., Teil, M.J., Ollivon, D., Legenti, L., Chevreuil, M., 2004. Polycyclic aromatic hydrocarbons and polychlorobiphenyls in wastewaters and sewage sludges from the Paris area (France). Environ. Res. 95, 184–197. Brandli, R., Bucheli, T.D., Kupper, T., Mayer, J., Stadelmann, F.X., Taradellas, J., 2007. Fate of PCBs, PAHs and their source characteristic ratios during composting and digestion of source-separated organic waste in full-scale plants. Environ. Pollut.. doi:10.1016/ j.envpol.2006.11.021. Busetti, F., Heitz, A., Cuomo, M., Badoer, S., Traverso, P., 2006. Determination of sixteen polycyclic aromatic hydrocarbons in aqueous and solid samples from an Italian wastewater treatment plant. J. Chromatogr. A 1102, 104–115. Byrns, G., 2001. The fate of xenobiotic organic compounds in wastewater treatment plants. Water Res. 35 (10), 2523–2533.
1339
Cai, Q.Y., Mo, C.H., Li, Y.H., Zeng, Q.Y., Katsoyiannis, A., Wu, Q.T., Ferard, J.F., 2007. Occurrence and assessment of polycyclic aromatic hydrocarbons in soils from vegetable fields of the Pearl River Delta, South China. Chemosphere 68, 159–168. Dai, J., Xu, M., Chen, J., Yang, X., Ke, Z., 2007. PCDD/F, PAH and heavy metals in the sewage sludge from six wastewater treatment plants in Beijing, China. Chemosphere 66, 353–361. Huckins, J.N., Petty, J.D., Orazio, C.E., Lebo, J.A., Clark, R.C., Gibson, V.L., Gala, W.R., Echols, K.R., 1999. Determination of uptake kinetics (sampling rates) by liquid-containing semipermeable membrane devices (SPMDs) for polycyclic aromatic hydrocarbons (PAHs) in water. Environ. Sci. Technol. 33, 3918–3923. Karvelas, M., Katsoyiannis, A., Samara, C., 2003. Occurrence and fate of heavy metals in the wastewater treatment process. Chemosphere 53, 1201–1210. Katsoyiannis, A., Samara, C., 2004. Persistent organic pollutants (POPs) in the sewage treatment plant of Thessaloniki, northern Greece: occurrence and removal. Water Res. 38, 2685–2698. Katsoyiannis, A., Zouboulis, A., Samara, C., 2006. Persistent organic pollutants (POPs) in the conventional activated sludge treatment process: model predictions against experimental values. Chemosphere 65, 1634–1641. US EPA, 1990. CERCLA site discharges to POTWS. Treatibility manual. EPA 540/2-90-007, Washington DC 20460. Zhang, X.L., Tao, S., Liu, W.X., Yang, Y., Zuo, Q., Liu, S.Z., 2005. Source diagnostics of polycyclic aromatic hydrocarbons based on species ratios. A multimedia approach. Environ. Sci. Technol. 39, 9109–9114.