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Ozone pre-treatment as improver of PAH removal during anaerobic digestion of urban sludge
Chemosphere 68 (2007) 1013–1019 www.elsevier.com/locate/chemosphere
Ozone pre-treatment as improver of PAH removal during anaerobic digestion of urban sludge Arodi Bernal-Martinez, He´le`ne Carre`re *, Dominique Patureau, Jean-Philippe Delgene`s INRA, UR050, Laboratoire de Biotechnologie de l’Environnement, Avenue des Etangs, Narbonne, F-11100, France Received 15 September 2006; received in revised form 7 February 2007; accepted 7 February 2007 Available online 26 March 2007
Abstract Polycyclic aromatic hydrocarbons are ubiquitous persistent pollutants. They may accumulate in sludge during wastewater treatment because of their low biodegradability and their hydrophobic characteristics. Combination of ozonation and anaerobic digestion may be efficient to remove PAHs naturally present in sludge. The objective of this study was to investigate the impact of ozone pre-treatment, with and without surfactant addition, on the anaerobic degradation of 12 PAHs (from low to high molecular weight). Under anaerobic digestion without ozonation pre-treatment, the highest removals were obtained for the lightest PAHs (3-aromatic rings). Ozonation pretreatment of sludge allowed to increase biodegradability or bioavailability of each PAH, and the PAH removals were well correlated to the PAH solubility. Finally, addition of tyloxapol before sludge ozone pre-treatment had antagonist effects on PAH removal during anaerobic digestion: negative impact on anaerobic ecosystem activity and improvement of PAH bioaccessibility (particularly the PAHs with the highest octanol water partition coefficients). 2007 Elsevier Ltd. All rights reserved. Keywords: Polycyclic aromatic hydrocarbons; Methanisation; Ozonation; Sludge treatment; Persistent organic pollutant
1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are formed during the incomplete combustion of organic material or during pyrolisis processes. Though they may have natural origins, the main pollution sources for the PAHs are anthropic (combustion of fossil materials, motor vehicle, industrial combustion, smoke of cigarettes, etc.). This wide spectrum of sources allows to explain their ubiquity in the environment. They are carried out to wastewater treatment plants by effluent discharge and runoff waters. As they present low solubility in water and are highly lipophilic, they adsorb and accumulate in sludge throughout the wastewater treatment (Wild et al., 1990). Moreover, the PAHs are highly toxic with carcinogenic and mutagenic properties. They are also persistent and are considered as priority pollutants in the US EPA and EU lists. PAH *
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0045-6535/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2007.02.019
concentrations are thus monitored in sludge in order to evaluate environmental risk associated with sludge spreading on the agricultural soils (Smith et al., 2001; Abad et al., 2005). French regulation has set 3 PAHs maximum concentrations for sludge spreading: 5, 2.5 and 2 mg kg 1 total solids (TS) for respectively fluoranthene, benzo(b)fluoranthene and benzo(a)pyrene. Lower levels are anticipated in the frame of current European policies. Indeed, a limit value of 6 mg kg 1 TS for the sum of 11 PAHs from acenaphthene to indeno(123cd)pyrene is under discussion. If PAH degradation by anaerobic digestion has been already investigated in aqueous media, soils or sediments (Bamforth and Singleton, 2005), very few works concern PAHs adsorbed on sludge. In these complex matrices, the existing studies have only focussed on low molecular weight PAHs and particularly on simplified or spiked ecosystems. For example, naphthalene and acenaphthene degradation was established in denitrifying conditions by Milhelcic and Luthy (1988) in soil–water systems. Moreover, Coates et al. (1996) showed the degradation of the
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same PAHs under sulfate reduction conditions. Beside, according to Chang et al. (2002), sulfate reducing conditions were more favourable than methanogenic conditions for PAH removal from soils. However, Trably et al. (2003) showed a 50% removal of 13 PAHs under methanogenic conditions during urban sludge treatment. The authors have demonstrated that PAH removal was directly linked to total solids removal and that bioavailability was the limiting factor of biodegradation because of strong adsorption of PAHs onto sludge organic matter. Moreover, the thermodynamic feasibility of PAH degradation in methanogenic systems was demonstrated (Christensen et al., 2004) and Chang et al. (2006) provided evidence that Archaea and thus methanogenesis are involved in the anaerobic degradation pathway of naphthalene and phenanthrene.
Table 1 Properties and molecular structure of PAHs considered in this study from Young and Cerniglia (1995) Compounds
Phenanthrene
Structure
M (g mol 1)
Water solubility at 25 C (mg l 1)
log (Kow)
178
1.0
4.46
In order to improve PAH biodegradation, BernalMartı´nez et al. (2006) investigated combined processes including anaerobic digestion and ozonation. The serial combination anaerobic digestion/ozonation/anaerobic digestion (Bernal-Martı´nez et al., 2006) allowed to obtain higher performance in PAH reduction (total of 12 PAH concentrations) than simple anaerobic digestion. Total PAH concentration was sharply reduced (up to 83%) due to the ozonation step. Addition of surfactant during this ozonation step led to the best performance of the serial combination due to higher gas–liquid mass transfer for ozone and solid–liquid mass transfer for PAHs which have to be desorbed from sludge (Carre`re et al., 2006). The present study is focused on the anaerobic digestion of digested-ozonated sludge. Its objective was to investigate the impact of ozone pre-treatment, with and without surfactant addition, on the anaerobic degradation of each of the 12 PAHs (from low to high molecular weight, Table 1). Discussions are centered on the impact of PAH structures and physical properties (Table 1) on their degradation during anaerobic digestion. 2. Material and methods 2.1. Sludge
Anthracene
178
0.045
4.5
Fluoranthene
202
0.206
4.9
Pyrene
202
0.132
4.88
Benzo(a)anthracene
228
0.0094
5.63
Chrysene
228
0.0018
5.63
Benzo(b)fluoranthene
252
0.0015
6.04
Benzo(k)fluoranthene
252
0.0080
6.21
Benzo(a)pyrene
252
0.0016
6.06
Dibenzo(ah)anthracene
278
0.0050
6.86
Benzo(ghi)perylene
276
0.0007
6.78
Indeno(123cd)pyrene
276
0.0002
6.58
The studied sludge was collected from a French urban wastewater treatment plant which has been contaminated by PAHs for more than 10 years. It was a mixture of primary and excess waste activated sludge (50:50, v:v). It was stored at 20 C and was digested in a 20 l completely mixed continuous anaerobic reactor where the sludge retention time (SRT) was 40 d and temperature was 35 C. This first anaerobic digestion allowed 50% TS reduction, 58% volatile solid (VS) reduction and 50% of total PAH removal (Bernal-Martı´nez et al., 2006). Characteristics of the digested sludge are shown in Table 2.
Table 2 Characteristics and PAH concentration of the digested sludge TS (g l 1) VS (g l 1) Total COD (g l 1) Soluble COD (g l 1) PAHsa (lg l 1) Phenanthrene (lg l 1) Anthracene (lg l 1) Fluoranthene (lg l 1) Pyrene (lg l 1) Benzo(a)anthracene (lg l 1) Chrysene (lg l 1) Benzo(b)fluoranthene (lg l 1) Benzo(k)fluoranthene (lg l 1) Benzo(a)pyrene (lg l 1) Dibenzo(ah)anthracene (lg l 1) Benzo(ghi)perylene (lg l 1) Indeno(123cd)pyrene (lg l 1) a
Total of 12 PAHs concentrations.
15.6 ± 0.3 9.3 ± 0.4 16.7 ± 0.1 0.60 ± 0.01 474 ± 5 24.3 ± 0.3 6.1 ± 0.3 89.3 ± 0.6 67.4 ± 0.7 34.4 ± 0.5 42.5 ± 0.8 51.0 ± 0.4 23.7 ± 0.4 42.3 ± 0.1 5.6 ± 0.1 40.4 ± 0.5 47.2 ± 0.1
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the ozonated digested sludge and the third one with the digested sludge which had been ozonated with tyloxapol.
2.2. Ozonation The digested sludge with a known PAH concentration was ozonated in a 2 l fed batch reactor. A sludge volume (0.45 l) was introduced in the reactor and the ozone was fed continuously. The ozone was generated from pure oxygen by an Ozat CFSI generator and injected into the bottom of the reactor through a thin bubble diffuser. The gas phase ozone concentrations before and after the reaction with sludge were determined each 30 s using an UV BMT 963 analyzer in order to calculate the ozone consumption. The gas flow rate was 1 l min 1, O3 inlet concentration in oxygen varied between 50 and 60 mg l 1. The surfactant (tyloxapol, Acros Organics) was added to the digested sludge at the concentration of 1 g l 1 and mixed during 30 min before the ozonation. Transferred ozone dose was equal to 0.1 g O3 g 1 TS, according to the optimal dose determined in Bernal-Martı´nez et al. (2005). Before its introduction into anaerobic reactors, ozonated sludge were slowly bubbled with nitrogen during 10 min to assure the anaerobic conditions. The ozonation of the digested sludge was realised once for week.
2.3. Anaerobic digestion Anaerobic digestion was carried out in 5 l laboratoryscale continuous stirred tank reactors (CSTR). The 3 reactors were run during 195 d with a SRT of 40 d. The feeding tank and tubes were cooled in order to limit the microbial degradation. The biogas outlet was cooled (4 C) to avoid water or PAH losses. The temperature was maintained at 35 C with a water bath. The pH was not regulated but was relatively constant in the digestors during the stabilised period. It was equal to 8.3 ± 0.1 in the control reactor and 8.2 ± 0.2 in the other ones. The three anaerobic digestors were fed with different sludge samples (Fig. 1). The first one (the control anaerobic digester) was fed with the digested sludge which characteristics shown in Table 2. The second reactor was fed with
Anaerobic Digestion AD1
Digested sludge
Digested sludge
Digested sludge
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OZONE 2 0.1 g O3 g-1 TS
OZONE 3 0.1 g O3 g-1 TS tyloxapol
Anaerobic Digestion AD2
Anaerobic Digestion AD3
Fig. 1. Feeding of the three anaerobic digestors.
2.4. PAH analysis Analytical methods were previously tested and validated in the laboratory by the use of certified matrix (Trably et al., 2004). The minimum volume of sample for PAH analysis was about 350 ml. TS were determined by drying two sludge samples of 20 ml in an oven at 110 C for 24 h. The remaining other 300 ml of the sludge sample were first centrifuged (20 000g, 25 min). Liquid phases were stored at 20 C for further solid-phase extraction on PAH-affinity column (Supelco ENVI-18TM) according to the Supelco recommendations. Solid pellets were grounded with glass beads (diameter 4 mm) and were dried in ventilated oven (60 h – 40 C). Dry samples were sieved on grid (diameter 2 mm) and were stored at 20 C for further accelerated solvent extraction with an ASE-200 system (DIONEXTM). The extracting conditions are reported in Trably et al. (2004). The diverse extracts were then slowly evaporated under nitrogen flow to dryness. Residues were dissolved in acetonitrile and analyzed by Reverse Phase-High Performance Liquid Chromatography (Trably et al., 2004), measurement errors were lower than 2%. Standard solution of PAHs (PAHmix 9) was purchased from Cluzeau Info Labo. 2.5. Other analysis TS, VS and COD were determined according to the standard methods (APHA, 1992). 2.6. Analysis of results Removals were defined as the difference between inlet and outlet concentrations divided by the inlet concentration. They were calculated after reactors stabilisation (after 160 d of operations which corresponded to 4 SRT). Results were the average of 5 points, corresponding to 5 weeks of operation. Series of results were compared by a one factor analysis of variance test (ANOVA). If ANOVA test resulted in non significantly different average values, they were compared two by two using a t-test under a Student law at 5% assuming the variance equality, normality and independence of repetitions. Confidence interval was fixed at 95%, thus probability to reject the null hypothesis was equal to 5%. 3. Results and discussion Macroscopic anaerobic performances of the three reactors are detailed in Bernal-Martı´nez et al. (2006). The main results are summarised here. High sludge retention times (SRT = 40 d) were applied to anaerobic digesters in order to have no kinetics limitation for PAH biodegradation. For the three reactors, the methane yield was around 410 ml CH4 g 1 VSremoved, showing good working
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biodegradable matter was removed during the first anaerobic digestion. PAH removal performances in reactors AD1, AD2 and AD3 are shown in Fig. 2. Taken as a whole, the best performance was obtained with ozonated sludge, the process without tyloxapol being slightly better. PAH removal represents the reduction of PAH concentration in the liquid sludge (lg l 1). However, legislation for land application considers PAH concentration in dry sludge (mg g 1 TS). In order to represent the potential of anaerobic digestion to reduce the concentration in dry sludge, an efficiency factor was defined as the ratio between the PAH removal and TS removal (Fig. 3). When ozonation was not used (AD1), anaerobic digestion efficiency factors were around 1, showing the absence of reduction of PAH concentration in dry sludge, excepted for the lightest ones. In counterpart, the efficiency factors were higher when ozonation was used as pre-treatment, particularly for the lightest PAHs. This means that PAHs were more reduced than TS underlying the fact that ozonation enhanced anaer-
conditions of all anaerobic reactors. VS and TS removals were 23% and 19%, respectively for AD1 fed with digested sludge, 26% and 24% for AD2 fed with digested and ozonated sludge and 21% and 18% for AD3, fed with digested and ozonated sludge in the presence of surfactant. Thus, ozonation pre-treatment did not significantly improve solids removal performance. This can be explained by the application of high sludge retention time (40 d). Moreover, the addition of tyloxapol or by-products of tyloxapol ozonation seemed to have a negative impact on solids reduction in comparison to the performances without surfactant addition. It has to be reminded that the sludge used to feed these three reactors was already digested, and, obviously, the performance of the first anaerobic digestion reactor was higher than the performance of the second digestion reactors (Bernal-Martı´nez et al., 2006). For example, TS and total PAH removals reached 50% and 51% during the first anaerobic digestion and 19% and 20% in the second digestion reactor (AD1) with no ozonation. The most readily
80
Phenanthrene Anthracene Fluoranthene Pyrene
60
Removal (%)
Benzo(a)Anthracene Chrysene Benzo(b)Fluoranthene Benzo(k)Fluoranthene
40
Benzo(a)Pyrene Dibenzo(a,h)Anthracene Benzo(g,h,i)Perylene Indeno(1,2,3,cd)Pyrene
20
0 AD1
AD2
AD3
Fig. 2. PAH removals obtained during anaerobic digestion of digested sludge (AD1), of digested-ozonated sludge (AD2) and of digested-ozonated sludge with tyloxapol (AD3).
4 AD2
AD3
3 2
Indeno(1,2,3,cd)Pyrene
Benzo(g,h,i)Perylene
Dibenzo(a,h)Anthracene
Benzo(a)Pyrene
Benzo(k)Fluoranthene
Benzo(b)Fluoranthene
Pyrene
Fluoranthene
Anthracene
Phenanthrene
0
Chrysene
1
Benzo(a)Anthracene
Efficiency factor
AD1
Fig. 3. Efficiency factors obtained during anaerobic digestion of digested sludge (AD1), of digested-ozonated sludge (AD2) and of digested-ozonated sludge with tyloxapol (AD3).
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3.1. Anaerobic digestion of digested sludge
3.2. Anaerobic digestion of digested-ozonated sludge In the case of anaerobic digestion of digested-ozonated sludge, four groups of PAH removals were determined by Table 3 Results of Anova test on PAH removals in AD1 fed with digested sludge
Phenanthrene Anthracene Fluoranthene Pyrene Benzo(a)anthracene Chrysene Benzo(b)fluoranthene Benzo(k)fluoranthene Benzo(a)pyrene Dibenzo(ah)anthracene Benzo(ghi)perylene Indeno(123cd)pyrene
AD1
60
AD2
50 40 30 20 10
Results of ANOVA analysis are shown in Table 3. Two groups of PAHs can be observed. The first group gathers the 3 PAHs which present the highest removals (27 ± 2%). It is formed by phenanthrene, anthracene and fluoranthene. These 3 PAHs are the smallest studied molecules and are composed of three aromatic rings. PAHs constituting the second group (removal of 20 ± 2%) are composed of 4, 5 or 6 aromatic rings. Low molecular weight PAH degradation in soils (mainly under aerobic condition) is generally reported to be faster and more extensive than that of high molecular weight PAHs (Nam et al., 2001; Goi and Trapido, 2004; Aichberger et al., 2006). For example, Nam and Kukor (2000) working on PAH biodegradation in soil, observed a removal of about 90% for 2 or 3-rings PAHs (naphthalene, fluorene, phenanthrene), about 65% removal for 4-rings PAHs (pyrene and chrysene) and only 15% removal for benzo(a)pyrene (5-rings). The low biodegradability of high molecular weight PAHs is often explained by low solubilities of PAHs in water which in turn reduces the accessibility of PAHs for metabolism by microbial cells in soils (Straube et al., 1999; Bamforth and Singleton, 2005). If we consider the present results (Fig. 4), the highest anaerobic removals are also linked to the highest PAH solubilities. However, pyrene (21% removal) has a higher solubility than anthracene (28% removal). More than the solubility in water, the number of aromatic rings seems thus to be another important parameter to explain higher rate of anaerobic degradation for the first group of PAHs designed by ANOVA.
Group 1
70
Removal (%)
obic PAH biodegradation through probably bioavailability enhancement. Highest efficiency factors were calculated for the heaviest PAHs in the presence of tyloxapol. Statistical analysis is thus needed to better compare the different processes and highlight the key factors for degradation of each PAH.
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0 0.0001
· · · · · · · · ·
0.01
0.1
1
Solubility (mg l-1)
Fig. 4. PAH removals obtained during anaerobic digestion of digested sludge (AD1) and of digested-ozonated sludge (AD2) versus PAH solubility (results presented for the reversed order of PAH given in Table 1).
ANOVA analysis (Table 4). The first group was formed by phenanthrene (63% removal), the most soluble studied PAH. The second group was composed of fluoranthene (46% removal), which also presents the second highest solubility. The third group was formed by anthracene, pyrene and benzo(k)fluoranthene (38 ± 2% removal). It has to be noticed that benzo(k)fluoranthene has a solubility lower than the two other PAHs. However, this group was composed of 3 or 4-aromatic-rings PAHs. The last group (27 ± 2% removal) represented PAHs with the lowest solubilities (4,5,6-aromatic-rings PAHs). Thus, except for benzo(k)fluoranthene, PAH removals observed during anaerobic digestion of digested-ozonated sludge were well correlated to PAH solubilities (Fig. 4). If PAH removals during AD1 and AD2 are compared, it can be seen that ozonation of the sludge led to an average improvement of 68% of PAH removals. The highest improvement was observed for phenanthrene (119% improvement, from 29% to 63%) and the lowest improvement was observed for dibenzo(ah)anthracene (28% improvement). But no simple correlation was shown between the PAH structure or properties and the removal improvement due to ozonation. Thus it can be concluded that ozonation pre-treatment led to the enhancement of PAH biodegradability through the enhancement of bioavailability. Such an improvement Table 4 Results of Anova test on PAH removals in AD2 fed with digestedozonated sludge
Group 2
· · ·
0.001
Group 1 Phenanthrene Anthracene Fluoranthene Pyrene Benzo(a)anthracene Chrysene Benzo(b)fluoranthene Benzo(k)fluoranthene Benzo(a)pyrene Dibenzo(ah)anthracene Benzo(ghi)perylene Indeno(123cd)pyrene
Group 2
Group 3
Group 4
· · · · · · · · · · · ·
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of PAH biodegradability by oxidation (by ozone or Fenton reagent) was already observed during the treatment of contaminated soils (Goi and Trapido, 2004; Kulik et al., 2006). 3.3. Anaerobic digestion of digested-ozonated sludge in presence of tyloxapol ANOVA analysis led to the formation of 4 PAH removal groups (Table 5). Once again, the first group consisted of phenanthrene (50% removal), the most soluble studied PAH. Surprisingly, the second group (31 ± 1% removal) consisted of dibenzo(ah)anthracene, benzo(ghi)perylene and indeno(123cd)pyrene, the 3 heaviest PAHs which also have the highest value of log(Kow). The third and fourth groups (27 ± 1% and 22 ± 1% removals, respectively) were formed by intermediate molecular weight PAHs. As already observed in Fig. 2, addition of tyloxapol has a particular effect on the PAH removal with an overall decrease of the performances. Only the removals of the 3 heaviest PAHs were enhanced in presence of tyloxapol. This can be seen in Fig. 5 where the ratios of removal during anaerobic digestion of digested-ozonated sludge with surfactant divided by removal during anaerobic digestion of digested-ozonated sludge without surfactant are plotted versus the PAH Kow. This ratio was higher than 1 only for the 3 highest values of Kow (heaviest PAHs). Moreover, this ratio could be linked to Kow by a semi-logarithmic correlation (Fig. 5). This shows the presence of several antagonist mechanisms due to the surfactant. On one hand, it led to the increase of the bioaccessibility of PAHs (mainly the heaviest ones) but on the other hand, it disturbed the anaerobic ecosystem. Indeed, TS and VS removal during anaerobic digestion AD3 was reduced compared to anaerobic digestion AD2 (Bernal-Martı´nez et al., 2006). Moreover, Trably (2002) used tyloxapol during anaerobic digestion of sewage sludge in order to increase the bioavailability of the PAHs. This author mentioned that anaerobic digestion was disturbed either by the presence of a supplementary source of carbon or by a possible toxicity of the surfactant. AddiTable 5 Results of Anova test on PAH removals in AD3 fed with digestedozonated sludge with tyloxapol Group 1 Phenanthrene Anthracene Fluoranthene Pyrene Benzo(a)anthracene Chrysene Benzo(b)fluoranthene Benzo(k)fluoranthene Benzo(a)pyrene Dibenzo(ah)anthracene Benzo(ghi)perylene Indeno(123cd)pyrene
Group 2
Group 3
Group 4
· · · · · · · · · · · ·
Removal AD3/ Removal AD2
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1.4 1.2 1 0.8 0.6 0.4 0.2 0 4
4.5
5
5.5
6
6.5
7
log(Kow)
Fig. 5. Impact of addition of tyloxapol before ozonation: ratio of removal during anaerobic digestion of digested-ozonated sludge with surfactant divided by removal during anaerobic digestion of digested-ozonated sludge without surfactant.
tionally, Sartoros et al. (2005) have shown an antagonist effect of another nonionic surfactant (Tergitol) on PAH mineralisation. When the impact was positive the enhancement was higher for pyrene (heaviest studied PAH: from 8% to 28%) than for anthracene (from 17% to 33%). Thus, the surfactant allowed to improve PAH bioaccessibility, the impact being more important on heavy PAHs. It has to be noted that the initial surfactant concentration (1 g l 1) was higher than the critical micellar concentration of tyloxapol (about 75 mg l 1, Boonchan et al., 1998). However, one cannot assess the value of the surfactant effective concentration in the solution in the anaerobic reactors as a part of the surfactant may be adsorbed on the sludge and it may also be attacked by ozone. 3.4. Comparison of performance of the three processes Simple anaerobic digestion allowed to remove about 20% of PAHs but as TS removal was 19%, PAH concentration in dry solids (mg kg 1 TS) was not reduced. Efficiency in PAH removal was improved by sludge pre-ozonation with or without tyloxapol addition. Indeed, PAH removals were increased but not TS removals because anaerobic digestion was operated with high SRT. Ozonation without tyloxapol led to slightly higher efficiencies for PAH removal, except for the three heaviest PAHs for which removals were enhanced in presence of tyloxapol. In the three studied processes, the highest removal was obtained for phenanthrene, the most soluble PAH. In simple anaerobic digestion or in anaerobic digestion with pre-ozonation without tyloxapol, the light PAHs were more biodegraded than the heavy ones. On the other hand, ozonation in presence of tyloxapol favoured the removal of the heaviest PAHs. If the objective of the process is to reduce PAH concentration in sludge, one should advise ozonation of sludge without surfactant before anaerobic digestion. However, if PAH distribution is such as heavy PAHs (Molar mass > 270 g mol 1) are majority, tyloxapol should be added before ozonation. Nevertheless, toxicity of by-products of ozonation of PAHs and of tyloxapol will have to be measured. However,
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the present results seem to show that by-products generated by PAH ozonation had no impact on anaerobic ecosystem whereas tyloxapol or by-products of ozonation of the mixture surfactant-sludge generated compounds that disturbed the anaerobic ecosystem and in consequence the elimination of the PAHs. 4. Conclusions Classic anaerobic digestion (without ozonation pretreatment) led to average PAH removals of the same order as sludge total solids removals. The highest removals were obtained for the smallest (3-aromatic rings) PAHs. Ozonation pre-treatment of sludge allowed to increase PAH biodegradability or bioaccessibility leading to the enhancement of PAH removals during sludge anaerobic digestion. The PAH removals during anaerobic digestion of ozonated sludge were classified according PAH solubility in water: the higher the solubility, the higher the removal. Addition of tyloxapol before sludge ozone pre-treatment had two antagonist effects on PAH removal during anaerobic digestion: it had a negative impact on anaerobic ecosystem activity and it enhanced the bioaccessibility of PAHs (mainly the ones with the highest octanol water partition coefficients). In consequence, in comparison of anaerobic digestion of ozonated sludge without surfactant, the presence of surfactant was positive for the heaviest PAHs and negative for the lightest ones. References Abad, E., Martinez, K., Planas, C., Palacios, O., Caixach, J., Rivera, J., 2005. Priority organic pollutant assessment of sludges for agricultural purposes. Chemosphere 61, 1358–1369. Aichberger, H., Loibner, A.P., Celis, R., Bertrand, C., Ottner, F., Rost, H., 2006. Assessment of factors governing biodegradability of PAHs in three soils aged under field conditions. Soil Sediment. Contam. 15, 73–85. APHA, American Public Health Association, American Water Works Association, Water Pollution Control Federation, 1992. In: Clesceri, L.S., Greenberg, A.E., Trussel, R.R. (Eds.), Standard Methods For The Examination of Water and Wastewater, 18th ed. APHA. Bamforth, S.M., Singleton, I., 2005. Bioremediation of polycyclic aromatic hydrocarbons: current knowledge and future directions. J. Chem. Technol. Biotechnol. 80, 723–736. Bernal-Martı´nez, A., Carre`re, H., Patureau, D., Delgene`s, J.P., 2005. Combining anaerobic digestion and ozonation to remove PAHs from urban sludge. Process Biochem. 40, 3244–3250. Bernal-Martı´nez, A., Carre`re, H., Patureau, D., Delgene`s, J.P., 2006. Removal of polycyclic aromatic hydrocarbons by a serial combination of anaerobic digestion and ozonation, sustainable sludge management: state of the art, challenges and perspectives. In: IWA Specialized Conference, Moscow, Russia, 29–31 May 2006, pp. 592–598.
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Boonchan, S., Britz, M.L., Stanley, G.A., 1998. Surfactant-enhanced biodegradation of high molecular weight polycyclic aromatic hydrocarbons by Stenotrophomonas maltophilia. Biotechnol. Bioeng. 59, 482–494. Carre`re, H., Bernal-Martı´nez, A., Patureau, D., Delgene`s, J.P., 2006. Parameters explaining PAHs removal from sewage sludge by ozonation. AIChE J. 52, 3612–3620. Chang, B.V., Shiung, C.L., Yuan, S.Y., 2002. Anaerobic biodegradation of polycyclic aromatic hydrocarbon in soil. Chemosphere 48, 717–724. Chang, W., Um, Y., Holoman, T.R.P., 2006. Polycyclic aromatic hydrocarbon (PAH) degradation coupled to methanogenesis. Biotechnol. Lett. 28, 425–430. Christensen, N., Batstone, D.J., He, Z., Angelidaki, I., Schmidt, J.E., 2004. Removal of polycyclic aromatic hydrocarbons (PAHs) from sewage sludge by anaerobic degradation. Water Sci. Technol. 50 (9), 237–244. Coates, J.D., Anderson, R.T., Lovley, D.R., 1996. Oxidation of polycyclic aromatic hydrocarbons under sulfate-reducing conditions. Appl. Environ. Microbiol. 62, 1099–1101. Goi, A., Trapido, M., 2004. Degradation of polycyclic aromatics hydrocarbons in soil: the Fenton reagent versus ozonation. Environ. Technol. 25, 155–164. Kulik, N., Goi, A., Trapido, M., Tuhkanen, T., 2006. Degradation of polycyclic aromatic hydrocarbons by combined chemical pre-oxidation and bioremediation in creosote contaminated soil. J. Environ. Manage. 78, 382–391. Milhelcic, J.R., Luthy, R.G., 1988. Microbial degradation of acenaphthene and naphthalene under denitrification conditions in soil–water systems. Appl. Environ. Microbiol. 54, 1188–1198. Nam, K., Kukor, J.J., 2000. Combined ozonation and biodegradation for remediation of mixtures of polycyclic aromatic hydrocarbons in soil. Biodegradation 11, 1–9. Nam, K., Rodriguez, W., Kukor, J.J., 2001. Enhanced degradation of polycyclic aromatic hydrocarbons by biodegradation combined with a modified Fenton reaction. Chemosphere 45, 11–20. Sartoros, C., Yerushalmi, L., Beron, P., Guiot, S.R., 2005. Effects of surfactant and temperature on biotransformation kinetics of anthracene and pyrene. Chemosphere 61, 1042–1050. Smith, K.E.C., Green, M., Thomas, G.O., Jones, K.C., 2001. Behavior of sewage sludge-derived PAHs on pasture. Environ. Sci. Technol. 35, 2141–2150. Straube, W.L., Jones-Meehan, J., Pritchard, P.H., Jones, W.R., 1999. Bench-scale optimization of bioaugmentation strategies for treatment of soils contaminated with high molecular weight polyaromatic hydrocarbons. Resour. Conserv. Recycl. 27, 27–37. Trably, E., 2002. Study and optimization of the biodegradation of polycyclic aromatic hydrocarbons (PAHs) and of polychlorobiphenyls (PCBs) during anaerobic and aerobic digestion of contaminated urban sludge. PhD thesis, University of Montpellier II, France (in French). Trably, E., Patureau, D., Delgene`s, J.P., 2003. Enhancement of polycyclic aromatic hydrocarbons removal during anaerobic treatment of urban sludge. Water Sci. Technol. 48 (4), 53–60. Trably, E., Delgene`s, N., Patureau, D., Delgene`s, J.P., 2004. Statistical tools for the optimization of a highly reproducible method for the analysis of polycyclic aromatic hydrocarbons in sludge samples. Int. J. Environ. Anal. Chem. 84, 995–1008. Wild, S.R., McGrath, S.P., Jones, K.C., 1990. The polynuclear aromatic hydrocarbon (PAH) content of archived sewage sludges. Chemosphere 20, 703–716. Young, L.Y., Cerniglia, C.E., 1995. Microbial Transformation and Degradation of Organic Chemicals. John Wiley, New York.
Ozone pre-treatment as improver of PAH removal during anaerobic digestion of urban sludge Arodi Bernal-Martinez, He´le`ne Carre`re *, Dominique Patureau, Jean-Philippe Delgene`s INRA, UR050, Laboratoire de Biotechnologie de l’Environnement, Avenue des Etangs, Narbonne, F-11100, France Received 15 September 2006; received in revised form 7 February 2007; accepted 7 February 2007 Available online 26 March 2007
Abstract Polycyclic aromatic hydrocarbons are ubiquitous persistent pollutants. They may accumulate in sludge during wastewater treatment because of their low biodegradability and their hydrophobic characteristics. Combination of ozonation and anaerobic digestion may be efficient to remove PAHs naturally present in sludge. The objective of this study was to investigate the impact of ozone pre-treatment, with and without surfactant addition, on the anaerobic degradation of 12 PAHs (from low to high molecular weight). Under anaerobic digestion without ozonation pre-treatment, the highest removals were obtained for the lightest PAHs (3-aromatic rings). Ozonation pretreatment of sludge allowed to increase biodegradability or bioavailability of each PAH, and the PAH removals were well correlated to the PAH solubility. Finally, addition of tyloxapol before sludge ozone pre-treatment had antagonist effects on PAH removal during anaerobic digestion: negative impact on anaerobic ecosystem activity and improvement of PAH bioaccessibility (particularly the PAHs with the highest octanol water partition coefficients). 2007 Elsevier Ltd. All rights reserved. Keywords: Polycyclic aromatic hydrocarbons; Methanisation; Ozonation; Sludge treatment; Persistent organic pollutant
1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are formed during the incomplete combustion of organic material or during pyrolisis processes. Though they may have natural origins, the main pollution sources for the PAHs are anthropic (combustion of fossil materials, motor vehicle, industrial combustion, smoke of cigarettes, etc.). This wide spectrum of sources allows to explain their ubiquity in the environment. They are carried out to wastewater treatment plants by effluent discharge and runoff waters. As they present low solubility in water and are highly lipophilic, they adsorb and accumulate in sludge throughout the wastewater treatment (Wild et al., 1990). Moreover, the PAHs are highly toxic with carcinogenic and mutagenic properties. They are also persistent and are considered as priority pollutants in the US EPA and EU lists. PAH *
Corresponding author. Tel.: +33 4 68 42 51 68; fax: +33 4 68 42 51 60. E-mail address: [email protected] (H. Carre`re).
0045-6535/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2007.02.019
concentrations are thus monitored in sludge in order to evaluate environmental risk associated with sludge spreading on the agricultural soils (Smith et al., 2001; Abad et al., 2005). French regulation has set 3 PAHs maximum concentrations for sludge spreading: 5, 2.5 and 2 mg kg 1 total solids (TS) for respectively fluoranthene, benzo(b)fluoranthene and benzo(a)pyrene. Lower levels are anticipated in the frame of current European policies. Indeed, a limit value of 6 mg kg 1 TS for the sum of 11 PAHs from acenaphthene to indeno(123cd)pyrene is under discussion. If PAH degradation by anaerobic digestion has been already investigated in aqueous media, soils or sediments (Bamforth and Singleton, 2005), very few works concern PAHs adsorbed on sludge. In these complex matrices, the existing studies have only focussed on low molecular weight PAHs and particularly on simplified or spiked ecosystems. For example, naphthalene and acenaphthene degradation was established in denitrifying conditions by Milhelcic and Luthy (1988) in soil–water systems. Moreover, Coates et al. (1996) showed the degradation of the
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same PAHs under sulfate reduction conditions. Beside, according to Chang et al. (2002), sulfate reducing conditions were more favourable than methanogenic conditions for PAH removal from soils. However, Trably et al. (2003) showed a 50% removal of 13 PAHs under methanogenic conditions during urban sludge treatment. The authors have demonstrated that PAH removal was directly linked to total solids removal and that bioavailability was the limiting factor of biodegradation because of strong adsorption of PAHs onto sludge organic matter. Moreover, the thermodynamic feasibility of PAH degradation in methanogenic systems was demonstrated (Christensen et al., 2004) and Chang et al. (2006) provided evidence that Archaea and thus methanogenesis are involved in the anaerobic degradation pathway of naphthalene and phenanthrene.
Table 1 Properties and molecular structure of PAHs considered in this study from Young and Cerniglia (1995) Compounds
Phenanthrene
Structure
M (g mol 1)
Water solubility at 25 C (mg l 1)
log (Kow)
178
1.0
4.46
In order to improve PAH biodegradation, BernalMartı´nez et al. (2006) investigated combined processes including anaerobic digestion and ozonation. The serial combination anaerobic digestion/ozonation/anaerobic digestion (Bernal-Martı´nez et al., 2006) allowed to obtain higher performance in PAH reduction (total of 12 PAH concentrations) than simple anaerobic digestion. Total PAH concentration was sharply reduced (up to 83%) due to the ozonation step. Addition of surfactant during this ozonation step led to the best performance of the serial combination due to higher gas–liquid mass transfer for ozone and solid–liquid mass transfer for PAHs which have to be desorbed from sludge (Carre`re et al., 2006). The present study is focused on the anaerobic digestion of digested-ozonated sludge. Its objective was to investigate the impact of ozone pre-treatment, with and without surfactant addition, on the anaerobic degradation of each of the 12 PAHs (from low to high molecular weight, Table 1). Discussions are centered on the impact of PAH structures and physical properties (Table 1) on their degradation during anaerobic digestion. 2. Material and methods 2.1. Sludge
Anthracene
178
0.045
4.5
Fluoranthene
202
0.206
4.9
Pyrene
202
0.132
4.88
Benzo(a)anthracene
228
0.0094
5.63
Chrysene
228
0.0018
5.63
Benzo(b)fluoranthene
252
0.0015
6.04
Benzo(k)fluoranthene
252
0.0080
6.21
Benzo(a)pyrene
252
0.0016
6.06
Dibenzo(ah)anthracene
278
0.0050
6.86
Benzo(ghi)perylene
276
0.0007
6.78
Indeno(123cd)pyrene
276
0.0002
6.58
The studied sludge was collected from a French urban wastewater treatment plant which has been contaminated by PAHs for more than 10 years. It was a mixture of primary and excess waste activated sludge (50:50, v:v). It was stored at 20 C and was digested in a 20 l completely mixed continuous anaerobic reactor where the sludge retention time (SRT) was 40 d and temperature was 35 C. This first anaerobic digestion allowed 50% TS reduction, 58% volatile solid (VS) reduction and 50% of total PAH removal (Bernal-Martı´nez et al., 2006). Characteristics of the digested sludge are shown in Table 2.
Table 2 Characteristics and PAH concentration of the digested sludge TS (g l 1) VS (g l 1) Total COD (g l 1) Soluble COD (g l 1) PAHsa (lg l 1) Phenanthrene (lg l 1) Anthracene (lg l 1) Fluoranthene (lg l 1) Pyrene (lg l 1) Benzo(a)anthracene (lg l 1) Chrysene (lg l 1) Benzo(b)fluoranthene (lg l 1) Benzo(k)fluoranthene (lg l 1) Benzo(a)pyrene (lg l 1) Dibenzo(ah)anthracene (lg l 1) Benzo(ghi)perylene (lg l 1) Indeno(123cd)pyrene (lg l 1) a
Total of 12 PAHs concentrations.
15.6 ± 0.3 9.3 ± 0.4 16.7 ± 0.1 0.60 ± 0.01 474 ± 5 24.3 ± 0.3 6.1 ± 0.3 89.3 ± 0.6 67.4 ± 0.7 34.4 ± 0.5 42.5 ± 0.8 51.0 ± 0.4 23.7 ± 0.4 42.3 ± 0.1 5.6 ± 0.1 40.4 ± 0.5 47.2 ± 0.1
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the ozonated digested sludge and the third one with the digested sludge which had been ozonated with tyloxapol.
2.2. Ozonation The digested sludge with a known PAH concentration was ozonated in a 2 l fed batch reactor. A sludge volume (0.45 l) was introduced in the reactor and the ozone was fed continuously. The ozone was generated from pure oxygen by an Ozat CFSI generator and injected into the bottom of the reactor through a thin bubble diffuser. The gas phase ozone concentrations before and after the reaction with sludge were determined each 30 s using an UV BMT 963 analyzer in order to calculate the ozone consumption. The gas flow rate was 1 l min 1, O3 inlet concentration in oxygen varied between 50 and 60 mg l 1. The surfactant (tyloxapol, Acros Organics) was added to the digested sludge at the concentration of 1 g l 1 and mixed during 30 min before the ozonation. Transferred ozone dose was equal to 0.1 g O3 g 1 TS, according to the optimal dose determined in Bernal-Martı´nez et al. (2005). Before its introduction into anaerobic reactors, ozonated sludge were slowly bubbled with nitrogen during 10 min to assure the anaerobic conditions. The ozonation of the digested sludge was realised once for week.
2.3. Anaerobic digestion Anaerobic digestion was carried out in 5 l laboratoryscale continuous stirred tank reactors (CSTR). The 3 reactors were run during 195 d with a SRT of 40 d. The feeding tank and tubes were cooled in order to limit the microbial degradation. The biogas outlet was cooled (4 C) to avoid water or PAH losses. The temperature was maintained at 35 C with a water bath. The pH was not regulated but was relatively constant in the digestors during the stabilised period. It was equal to 8.3 ± 0.1 in the control reactor and 8.2 ± 0.2 in the other ones. The three anaerobic digestors were fed with different sludge samples (Fig. 1). The first one (the control anaerobic digester) was fed with the digested sludge which characteristics shown in Table 2. The second reactor was fed with
Anaerobic Digestion AD1
Digested sludge
Digested sludge
Digested sludge
1015
OZONE 2 0.1 g O3 g-1 TS
OZONE 3 0.1 g O3 g-1 TS tyloxapol
Anaerobic Digestion AD2
Anaerobic Digestion AD3
Fig. 1. Feeding of the three anaerobic digestors.
2.4. PAH analysis Analytical methods were previously tested and validated in the laboratory by the use of certified matrix (Trably et al., 2004). The minimum volume of sample for PAH analysis was about 350 ml. TS were determined by drying two sludge samples of 20 ml in an oven at 110 C for 24 h. The remaining other 300 ml of the sludge sample were first centrifuged (20 000g, 25 min). Liquid phases were stored at 20 C for further solid-phase extraction on PAH-affinity column (Supelco ENVI-18TM) according to the Supelco recommendations. Solid pellets were grounded with glass beads (diameter 4 mm) and were dried in ventilated oven (60 h – 40 C). Dry samples were sieved on grid (diameter 2 mm) and were stored at 20 C for further accelerated solvent extraction with an ASE-200 system (DIONEXTM). The extracting conditions are reported in Trably et al. (2004). The diverse extracts were then slowly evaporated under nitrogen flow to dryness. Residues were dissolved in acetonitrile and analyzed by Reverse Phase-High Performance Liquid Chromatography (Trably et al., 2004), measurement errors were lower than 2%. Standard solution of PAHs (PAHmix 9) was purchased from Cluzeau Info Labo. 2.5. Other analysis TS, VS and COD were determined according to the standard methods (APHA, 1992). 2.6. Analysis of results Removals were defined as the difference between inlet and outlet concentrations divided by the inlet concentration. They were calculated after reactors stabilisation (after 160 d of operations which corresponded to 4 SRT). Results were the average of 5 points, corresponding to 5 weeks of operation. Series of results were compared by a one factor analysis of variance test (ANOVA). If ANOVA test resulted in non significantly different average values, they were compared two by two using a t-test under a Student law at 5% assuming the variance equality, normality and independence of repetitions. Confidence interval was fixed at 95%, thus probability to reject the null hypothesis was equal to 5%. 3. Results and discussion Macroscopic anaerobic performances of the three reactors are detailed in Bernal-Martı´nez et al. (2006). The main results are summarised here. High sludge retention times (SRT = 40 d) were applied to anaerobic digesters in order to have no kinetics limitation for PAH biodegradation. For the three reactors, the methane yield was around 410 ml CH4 g 1 VSremoved, showing good working
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biodegradable matter was removed during the first anaerobic digestion. PAH removal performances in reactors AD1, AD2 and AD3 are shown in Fig. 2. Taken as a whole, the best performance was obtained with ozonated sludge, the process without tyloxapol being slightly better. PAH removal represents the reduction of PAH concentration in the liquid sludge (lg l 1). However, legislation for land application considers PAH concentration in dry sludge (mg g 1 TS). In order to represent the potential of anaerobic digestion to reduce the concentration in dry sludge, an efficiency factor was defined as the ratio between the PAH removal and TS removal (Fig. 3). When ozonation was not used (AD1), anaerobic digestion efficiency factors were around 1, showing the absence of reduction of PAH concentration in dry sludge, excepted for the lightest ones. In counterpart, the efficiency factors were higher when ozonation was used as pre-treatment, particularly for the lightest PAHs. This means that PAHs were more reduced than TS underlying the fact that ozonation enhanced anaer-
conditions of all anaerobic reactors. VS and TS removals were 23% and 19%, respectively for AD1 fed with digested sludge, 26% and 24% for AD2 fed with digested and ozonated sludge and 21% and 18% for AD3, fed with digested and ozonated sludge in the presence of surfactant. Thus, ozonation pre-treatment did not significantly improve solids removal performance. This can be explained by the application of high sludge retention time (40 d). Moreover, the addition of tyloxapol or by-products of tyloxapol ozonation seemed to have a negative impact on solids reduction in comparison to the performances without surfactant addition. It has to be reminded that the sludge used to feed these three reactors was already digested, and, obviously, the performance of the first anaerobic digestion reactor was higher than the performance of the second digestion reactors (Bernal-Martı´nez et al., 2006). For example, TS and total PAH removals reached 50% and 51% during the first anaerobic digestion and 19% and 20% in the second digestion reactor (AD1) with no ozonation. The most readily
80
Phenanthrene Anthracene Fluoranthene Pyrene
60
Removal (%)
Benzo(a)Anthracene Chrysene Benzo(b)Fluoranthene Benzo(k)Fluoranthene
40
Benzo(a)Pyrene Dibenzo(a,h)Anthracene Benzo(g,h,i)Perylene Indeno(1,2,3,cd)Pyrene
20
0 AD1
AD2
AD3
Fig. 2. PAH removals obtained during anaerobic digestion of digested sludge (AD1), of digested-ozonated sludge (AD2) and of digested-ozonated sludge with tyloxapol (AD3).
4 AD2
AD3
3 2
Indeno(1,2,3,cd)Pyrene
Benzo(g,h,i)Perylene
Dibenzo(a,h)Anthracene
Benzo(a)Pyrene
Benzo(k)Fluoranthene
Benzo(b)Fluoranthene
Pyrene
Fluoranthene
Anthracene
Phenanthrene
0
Chrysene
1
Benzo(a)Anthracene
Efficiency factor
AD1
Fig. 3. Efficiency factors obtained during anaerobic digestion of digested sludge (AD1), of digested-ozonated sludge (AD2) and of digested-ozonated sludge with tyloxapol (AD3).
A. Bernal-Martinez et al. / Chemosphere 68 (2007) 1013–1019
3.1. Anaerobic digestion of digested sludge
3.2. Anaerobic digestion of digested-ozonated sludge In the case of anaerobic digestion of digested-ozonated sludge, four groups of PAH removals were determined by Table 3 Results of Anova test on PAH removals in AD1 fed with digested sludge
Phenanthrene Anthracene Fluoranthene Pyrene Benzo(a)anthracene Chrysene Benzo(b)fluoranthene Benzo(k)fluoranthene Benzo(a)pyrene Dibenzo(ah)anthracene Benzo(ghi)perylene Indeno(123cd)pyrene
AD1
60
AD2
50 40 30 20 10
Results of ANOVA analysis are shown in Table 3. Two groups of PAHs can be observed. The first group gathers the 3 PAHs which present the highest removals (27 ± 2%). It is formed by phenanthrene, anthracene and fluoranthene. These 3 PAHs are the smallest studied molecules and are composed of three aromatic rings. PAHs constituting the second group (removal of 20 ± 2%) are composed of 4, 5 or 6 aromatic rings. Low molecular weight PAH degradation in soils (mainly under aerobic condition) is generally reported to be faster and more extensive than that of high molecular weight PAHs (Nam et al., 2001; Goi and Trapido, 2004; Aichberger et al., 2006). For example, Nam and Kukor (2000) working on PAH biodegradation in soil, observed a removal of about 90% for 2 or 3-rings PAHs (naphthalene, fluorene, phenanthrene), about 65% removal for 4-rings PAHs (pyrene and chrysene) and only 15% removal for benzo(a)pyrene (5-rings). The low biodegradability of high molecular weight PAHs is often explained by low solubilities of PAHs in water which in turn reduces the accessibility of PAHs for metabolism by microbial cells in soils (Straube et al., 1999; Bamforth and Singleton, 2005). If we consider the present results (Fig. 4), the highest anaerobic removals are also linked to the highest PAH solubilities. However, pyrene (21% removal) has a higher solubility than anthracene (28% removal). More than the solubility in water, the number of aromatic rings seems thus to be another important parameter to explain higher rate of anaerobic degradation for the first group of PAHs designed by ANOVA.
Group 1
70
Removal (%)
obic PAH biodegradation through probably bioavailability enhancement. Highest efficiency factors were calculated for the heaviest PAHs in the presence of tyloxapol. Statistical analysis is thus needed to better compare the different processes and highlight the key factors for degradation of each PAH.
1017
0 0.0001
· · · · · · · · ·
0.01
0.1
1
Solubility (mg l-1)
Fig. 4. PAH removals obtained during anaerobic digestion of digested sludge (AD1) and of digested-ozonated sludge (AD2) versus PAH solubility (results presented for the reversed order of PAH given in Table 1).
ANOVA analysis (Table 4). The first group was formed by phenanthrene (63% removal), the most soluble studied PAH. The second group was composed of fluoranthene (46% removal), which also presents the second highest solubility. The third group was formed by anthracene, pyrene and benzo(k)fluoranthene (38 ± 2% removal). It has to be noticed that benzo(k)fluoranthene has a solubility lower than the two other PAHs. However, this group was composed of 3 or 4-aromatic-rings PAHs. The last group (27 ± 2% removal) represented PAHs with the lowest solubilities (4,5,6-aromatic-rings PAHs). Thus, except for benzo(k)fluoranthene, PAH removals observed during anaerobic digestion of digested-ozonated sludge were well correlated to PAH solubilities (Fig. 4). If PAH removals during AD1 and AD2 are compared, it can be seen that ozonation of the sludge led to an average improvement of 68% of PAH removals. The highest improvement was observed for phenanthrene (119% improvement, from 29% to 63%) and the lowest improvement was observed for dibenzo(ah)anthracene (28% improvement). But no simple correlation was shown between the PAH structure or properties and the removal improvement due to ozonation. Thus it can be concluded that ozonation pre-treatment led to the enhancement of PAH biodegradability through the enhancement of bioavailability. Such an improvement Table 4 Results of Anova test on PAH removals in AD2 fed with digestedozonated sludge
Group 2
· · ·
0.001
Group 1 Phenanthrene Anthracene Fluoranthene Pyrene Benzo(a)anthracene Chrysene Benzo(b)fluoranthene Benzo(k)fluoranthene Benzo(a)pyrene Dibenzo(ah)anthracene Benzo(ghi)perylene Indeno(123cd)pyrene
Group 2
Group 3
Group 4
· · · · · · · · · · · ·
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of PAH biodegradability by oxidation (by ozone or Fenton reagent) was already observed during the treatment of contaminated soils (Goi and Trapido, 2004; Kulik et al., 2006). 3.3. Anaerobic digestion of digested-ozonated sludge in presence of tyloxapol ANOVA analysis led to the formation of 4 PAH removal groups (Table 5). Once again, the first group consisted of phenanthrene (50% removal), the most soluble studied PAH. Surprisingly, the second group (31 ± 1% removal) consisted of dibenzo(ah)anthracene, benzo(ghi)perylene and indeno(123cd)pyrene, the 3 heaviest PAHs which also have the highest value of log(Kow). The third and fourth groups (27 ± 1% and 22 ± 1% removals, respectively) were formed by intermediate molecular weight PAHs. As already observed in Fig. 2, addition of tyloxapol has a particular effect on the PAH removal with an overall decrease of the performances. Only the removals of the 3 heaviest PAHs were enhanced in presence of tyloxapol. This can be seen in Fig. 5 where the ratios of removal during anaerobic digestion of digested-ozonated sludge with surfactant divided by removal during anaerobic digestion of digested-ozonated sludge without surfactant are plotted versus the PAH Kow. This ratio was higher than 1 only for the 3 highest values of Kow (heaviest PAHs). Moreover, this ratio could be linked to Kow by a semi-logarithmic correlation (Fig. 5). This shows the presence of several antagonist mechanisms due to the surfactant. On one hand, it led to the increase of the bioaccessibility of PAHs (mainly the heaviest ones) but on the other hand, it disturbed the anaerobic ecosystem. Indeed, TS and VS removal during anaerobic digestion AD3 was reduced compared to anaerobic digestion AD2 (Bernal-Martı´nez et al., 2006). Moreover, Trably (2002) used tyloxapol during anaerobic digestion of sewage sludge in order to increase the bioavailability of the PAHs. This author mentioned that anaerobic digestion was disturbed either by the presence of a supplementary source of carbon or by a possible toxicity of the surfactant. AddiTable 5 Results of Anova test on PAH removals in AD3 fed with digestedozonated sludge with tyloxapol Group 1 Phenanthrene Anthracene Fluoranthene Pyrene Benzo(a)anthracene Chrysene Benzo(b)fluoranthene Benzo(k)fluoranthene Benzo(a)pyrene Dibenzo(ah)anthracene Benzo(ghi)perylene Indeno(123cd)pyrene
Group 2
Group 3
Group 4
· · · · · · · · · · · ·
Removal AD3/ Removal AD2
1018
1.4 1.2 1 0.8 0.6 0.4 0.2 0 4
4.5
5
5.5
6
6.5
7
log(Kow)
Fig. 5. Impact of addition of tyloxapol before ozonation: ratio of removal during anaerobic digestion of digested-ozonated sludge with surfactant divided by removal during anaerobic digestion of digested-ozonated sludge without surfactant.
tionally, Sartoros et al. (2005) have shown an antagonist effect of another nonionic surfactant (Tergitol) on PAH mineralisation. When the impact was positive the enhancement was higher for pyrene (heaviest studied PAH: from 8% to 28%) than for anthracene (from 17% to 33%). Thus, the surfactant allowed to improve PAH bioaccessibility, the impact being more important on heavy PAHs. It has to be noted that the initial surfactant concentration (1 g l 1) was higher than the critical micellar concentration of tyloxapol (about 75 mg l 1, Boonchan et al., 1998). However, one cannot assess the value of the surfactant effective concentration in the solution in the anaerobic reactors as a part of the surfactant may be adsorbed on the sludge and it may also be attacked by ozone. 3.4. Comparison of performance of the three processes Simple anaerobic digestion allowed to remove about 20% of PAHs but as TS removal was 19%, PAH concentration in dry solids (mg kg 1 TS) was not reduced. Efficiency in PAH removal was improved by sludge pre-ozonation with or without tyloxapol addition. Indeed, PAH removals were increased but not TS removals because anaerobic digestion was operated with high SRT. Ozonation without tyloxapol led to slightly higher efficiencies for PAH removal, except for the three heaviest PAHs for which removals were enhanced in presence of tyloxapol. In the three studied processes, the highest removal was obtained for phenanthrene, the most soluble PAH. In simple anaerobic digestion or in anaerobic digestion with pre-ozonation without tyloxapol, the light PAHs were more biodegraded than the heavy ones. On the other hand, ozonation in presence of tyloxapol favoured the removal of the heaviest PAHs. If the objective of the process is to reduce PAH concentration in sludge, one should advise ozonation of sludge without surfactant before anaerobic digestion. However, if PAH distribution is such as heavy PAHs (Molar mass > 270 g mol 1) are majority, tyloxapol should be added before ozonation. Nevertheless, toxicity of by-products of ozonation of PAHs and of tyloxapol will have to be measured. However,
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the present results seem to show that by-products generated by PAH ozonation had no impact on anaerobic ecosystem whereas tyloxapol or by-products of ozonation of the mixture surfactant-sludge generated compounds that disturbed the anaerobic ecosystem and in consequence the elimination of the PAHs. 4. Conclusions Classic anaerobic digestion (without ozonation pretreatment) led to average PAH removals of the same order as sludge total solids removals. The highest removals were obtained for the smallest (3-aromatic rings) PAHs. Ozonation pre-treatment of sludge allowed to increase PAH biodegradability or bioaccessibility leading to the enhancement of PAH removals during sludge anaerobic digestion. The PAH removals during anaerobic digestion of ozonated sludge were classified according PAH solubility in water: the higher the solubility, the higher the removal. Addition of tyloxapol before sludge ozone pre-treatment had two antagonist effects on PAH removal during anaerobic digestion: it had a negative impact on anaerobic ecosystem activity and it enhanced the bioaccessibility of PAHs (mainly the ones with the highest octanol water partition coefficients). In consequence, in comparison of anaerobic digestion of ozonated sludge without surfactant, the presence of surfactant was positive for the heaviest PAHs and negative for the lightest ones. References Abad, E., Martinez, K., Planas, C., Palacios, O., Caixach, J., Rivera, J., 2005. Priority organic pollutant assessment of sludges for agricultural purposes. Chemosphere 61, 1358–1369. Aichberger, H., Loibner, A.P., Celis, R., Bertrand, C., Ottner, F., Rost, H., 2006. Assessment of factors governing biodegradability of PAHs in three soils aged under field conditions. Soil Sediment. Contam. 15, 73–85. APHA, American Public Health Association, American Water Works Association, Water Pollution Control Federation, 1992. In: Clesceri, L.S., Greenberg, A.E., Trussel, R.R. (Eds.), Standard Methods For The Examination of Water and Wastewater, 18th ed. APHA. Bamforth, S.M., Singleton, I., 2005. Bioremediation of polycyclic aromatic hydrocarbons: current knowledge and future directions. J. Chem. Technol. Biotechnol. 80, 723–736. Bernal-Martı´nez, A., Carre`re, H., Patureau, D., Delgene`s, J.P., 2005. Combining anaerobic digestion and ozonation to remove PAHs from urban sludge. Process Biochem. 40, 3244–3250. Bernal-Martı´nez, A., Carre`re, H., Patureau, D., Delgene`s, J.P., 2006. Removal of polycyclic aromatic hydrocarbons by a serial combination of anaerobic digestion and ozonation, sustainable sludge management: state of the art, challenges and perspectives. In: IWA Specialized Conference, Moscow, Russia, 29–31 May 2006, pp. 592–598.
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