Influence of ozone pre-treatment on sludge anaerobic digestion: Removal of pharmaceutical and personal care products

Chemosphere 67 (2007) 1444–1452 www.elsevier.com/locate/chemosphere

Influence of ozone pre-treatment on sludge anaerobic digestion: Removal of pharmaceutical and personal care products Marta Carballa a

a,*

, Garbin˜e Manterola b, Luis Larrea b, Thomas Ternes c, Francisco Omil a, Juan M. Lema a

Department of Chemical Engineering, School of Engineering, University of Santiago de Compostela, Rua Lope Go´mez de Marzoa, s/n. E-15782 Santiago de Compostela, Spain b CEIT and Tecnun, University of Navarra, Manuel de Lardiza´bal 15, E-20018 San Sebastian, Spain c Federal Institute of Hydrology (BfG), Am Mainzer Tor 1. D-56068 Koblenz, Germany Received 4 May 2006; received in revised form 28 September 2006; accepted 1 October 2006 Available online 30 November 2006

Abstract The effect of an oxidative pre-treatment with ozone on the removal of Pharmaceutical and Personal Care Products (PPCPs) during the anaerobic digestion of sewage sludge has been investigated. Besides, the digested sludge characteristics in terms of pathogens content, dewatering properties, heavy metals content and linear alkylbenzene sulfonates (LAS) were determined. During ozonation (20 mg O3/ g TSS), about 8% of volatile solids (VS) and 60% of the chemical oxygen demand (COD) were solubilized. However, no mineralization was observed. The elimination of VS and total COD during anaerobic digestion were not affected by ozone treatment with efficiencies ranging from 60% to 65%. All PPCPs considered were removed during anaerobic treatment of sludge, with efficiencies ranging from 20% to 99%. No significant influence of ozone pre-treatment was observed on PPCPs elimination except for carbamazepine. Pathogens, heavy metals and LAS contents after conventional and pre-ozonation treatment of sewage sludge were below the legal requirements. However, the dewatering properties of sludge were deteriorated when the ozone pre-treatment was applied.  2006 Elsevier Ltd. All rights reserved. Keywords: Dewatering; LAS; Ozone; Pathogens; Pharmaceuticals; Sludge anaerobic digestion

1. Introduction Anaerobic digestion is a very useful process for the treatment of insoluble organic matter or high Chemical Oxygen Demand (COD) containing wastewaters, being methanogenesis usually considered as the rate-limiting step of the overall process. However, when considering particulate substrates with low biodegradability as sludge from Sewage Treatment Plants (STPs), hydrolysis results in the slower step and long Sludge Retention Times (SRT) are required to reach moderate efficiencies, around 20–30 days *

Corresponding author. Tel.: +34 981 59 44 88x16016; fax: +34 981 52 80 50. E-mail address: [email protected] (M. Carballa). 0045-6535/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2006.10.004

(Pavlostathis and Gosset, 1986). Therefore, significant effort has been dedicated in recent years to find ways of improving the performance of anaerobic digestion: optimizing process conditions, applying thermophilic temperatures, pre-treating the input sludge or using a co-digestion with other substrates (Dohanyos et al., 2004). The objective of the use of pre-treatments before the biological process is to accelerate the hydrolysis step, which would allow to diminish the stabilization time as well as to increase the degree of degradation. In this way, the volume of sludge to be disposed of would be less, and it is also expected that sedimentation would be improved as well as sludge disinfection. On the other hand, biodegradability might not be improved if generation of refractory or inhibitory substances occurs (Delgenes et al., 2000).

M. Carballa et al. / Chemosphere 67 (2007) 1444–1452

Several disintegration methods have been proposed so far (Mu¨ller, 2000), although the full-scale application depends on the technical conditions and energy demands. Mechanical pre-treatment was shown to be very effective in solubilizing microbial cells but turned out to be rather complicated and expensive (Weemaes and Verstraete, 1998). Sonication can disintegrate sludge cells up to 100%, but with high energy consumption (Dichtl et al., 1997). Chemical and thermochemical pre-treatments have been shown to be efficient in enhancing sludge digestion (Tanaka et al., 1997); however, the aggressive reaction conditions often impose special material requirements. Oxidative treatments have been also applied to disintegrate sludge cells. Among the oxidation processes, the treatment using ozone is of special interest because no oxidant residues are remaining and no increase in salt concentration occurs (Weemaes et al., 2000; Goel et al., 2003). Ozone is a very strong oxidizing agent, which reacts in two different ways: the direct and the indirect reaction, both reactions occurring simultaneously. The indirect reaction is based on the high reactivity of hydroxyl radicals, which do not react specifically, whereas the direct reaction rate with ozone depends more on the structure of the reactants. During sludge pre-treatment, the aim of ozone is to cause the hydrolysis and partial oxidation of the organic matter. A complete oxidation is avoided. Pharmaceutical and Personal Care Products (PPCPs) constitute a group of organic contaminants which has recently gained increasing interest. Due to their hydrophobic properties, some PPCPs adsorb on the primary and secondary sludge. However, quantitative data of PPCPs concentrations in sewage sludge is scarce, probably due to inherent difficulties associated with the analysis of sludge samples. Depending on the efficiency of each sludge treatment technology, remaining PPCPs might be recycled with the liquid supernatant or disposed with the sludge (Carballa et al., in press). If the digested sludge is applied to soils, these contaminants could remain in the soil for a long time (months or even years) because of their sorption and slow rates of biodegradation (Wilson et al., 1997). Effects of some PPCPs, e.g. contraceptives and Diclofenac, in the aquatic environment are well-documented (Jobling et al., 2002; Triebskorn et al., 2004), but not much is known about the behaviour of these compounds in soils. Intake by plants (Wild et al., 1994), leaching into the groundwater (Kreuzinger et al., 2004) and negative impact on the terrestrial organism are not excluded (Jensen et al., 2001). Information dealing specifically with PPCPs behaviour during sludge anaerobic digestion is very scarce and only the EU funded Poseidon project (http://poseidon.bafg.de) has reported some data (Carballa, 2005). Besides, it is known that the application of ozone at doses varying from 2 to 10 mg l1 to biologically treated wastewater is sufficient for oxidizing many PPCPs by 90–99%, except X-ray

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contrast media, which showed little oxidation (Ternes et al., 2003; Huber et al., 2005). 1.1. Objectives The objective of this work is to study the influence of an oxidative pre-treatment with ozone on PPCPs removal during sludge anaerobic digestion in comparison with the conventional process. Besides, the digested sludge was characterised in terms of dewatering properties, pathogens, heavy metals and Linear Alkylbenzene Sulfonates (LAS) content to check if these values fit the legal requirements for final disposal. 2. Material and methods 2.1. Sewage sludge Raw sewage sludge used in this work was collected from a STP located in Galicia (NW of Spain). A mixture (70:30, v/v) of primary and secondary sludge collected from the thickener and the flotator, respectively, was used as feeding of the anaerobic digestion pilot plant. The main characteristics of this feeding are: pH: 5.5–5.8, 35–110 g TS Æ l1, 25– 65 g VS Æ l1, 30–95 g TSS Æ l1, 20–60 g VSS Æ l1, 30–110 g CODtotal Æ l1 and 1–8 g CODsoluble Æ l1. 2.2. PPCPs Two musks, galaxolide (HHCB) and tonalide (AHTN); one antiepileptic, carbamazepine (CBZ); one tranquilizer, diazepam (DZP); two antiphlogistics, ibuprofen (IBP) and diclofenac (DCF); one X-ray contrast medium, iopromide (IPM); one antibiotic, sulfamethoxazole (SMX) and three estrogens, estrone (E1), 17b-estradiol (E2) and 17aethinylestradiol (EE2) were selected in this work. Table 1 shows the physico-chemical properties of the selected PPCPs. Several spiking solutions containing the different substances were added to the sludge mixture before feeding the anaerobic digesters in order to ensure their presence in the raw sludge. The spiked concentrations of PPCPs ranged between 4 and 400 lg l1 and they were selected according to the amounts of each substance associated to solids reported in literature (Carballa, 2005). 2.3. Ozonation of sludge The sludge was ozonated in a 10-l bubble column operated in batch and at room temperature (Fig. 1). The gas flow rate was 240 l Æ h1 with an ozone concentration of around 20 mg O3 Æ l1. The ozone dose was set approximately at 20 mg O3 Æ g TSS1 in the reactor. The time needed to add the exact amount of ozone (around 2 h) was calculated for each experiment based on the initial Total Suspended Solids (TSS) content of the sludge.

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Table 1 Physico-chemical properties of selected PPCPs PPCP

Water solubility (mg Æ l1)a

log Kowa

pKa

HHCB AHTN CBZ DZP IBP DCF IPM SMX E1 E2 EE2

1.8 1.2 17.7 50 21 2.4 23.8 610 30 3.6 11.3

5.9–6.3 4.6–6.4 2.3–2.5 2.5–3.0 3.5–4.5 4.5–4.8 – 0.5–0.9 3.1–3.4 3.9–4.0 2.8–4.2

– – 13.9 3.3–3.4 4.9–5.7 4.0–4.5 – 5.6–6.0 10.4 10.4 10.5–10.7

a b c

a

log Kd (l Æ kg1)b

Rate constants (M1 Æ s1)c

Primary pH: 6.6

Biological pH: 7.5

Ozone pH: 7.0

OH radicals pH: 7.0

3.7 3.7 <1.3 1.6 <1.3 2.7 <0.7 – – – 2.4

3.3 3.4 0.1 1.3 0.9 1.2 1.0 – – – 2.5

– – 3.0 · 105 0.8 ± 0.2 9.6 ± 1.0 3.0 · 106 <0.8 2.5 · 106 – – 7.0 · 109

– – 8.8 ± 1.2 · 109 7.2 ± 1.0 · 109 7.4 ± 1.2 · 109 7.5 ± 1.5 · 109 3.3 ± 0.6 · 109 5.5 ± 0.7 · 109 – – 9.8 ± 1.2 · 109

Carballa (2005). Ternes et al. (2004). Huber et al. (2003).

bed reactor (UASB) operated under mesophilic conditions with saccharose as inoculum. After a 3-month start-up period, the digesters were fed with sludge previously spiked with PPCPs and two stages of operation, with nonozonated (conventional operation) and with ozonated (advanced operation) sludge, were performed in each digester. The mesophilic digester was operated at a SRT of 20 d, whereas the thermophilic reactor was operated at 10 d. Once the steady-state was achieved in each digester (1–2 SRT), 2–3 samples were taken for PPCPs, pathogens, dewatering properties, heavy metals and LAS analysis. All samples were taken as 5-day composite samples preserved by refrigeration (4 C) and, to those for PPCPs analysis, hydrochloric acid was added to pH < 2 in order to stop the biological activity. 2.5. Analytical methods

Fig. 1. Scheme of the batch ozonation unit (Maran˜o´n, 2004).

2.4. Anaerobic digestion pilot plant Two lab-scale (10 l) continuously stirred anaerobic digesters were installed and started up (Fig. 2). Mesophilic (37 C) and thermophilic (55 C) conditions were established in each reactor, respectively. Four parameters were measured online: temperature, pH, stirring speed and biogas production. Other determinations, such as solids, organic matter, alkalinity, Volatile Fatty Acids (VFA) and biogas composition, were measured off-line two or three times per week. The digesters were started-up using a 20% of methanogenic sludge coming from an upflow anaerobic sludge

Solids, organic matter and alkalinity were analyzed according to Standard Methods (APHA-AWWA-WPCF, 1999). Ammonia nitrogen was determined by a colorimetric method based on the reaction of NH3 with HClO and phenol, forming a strong-blue compound (indophenol) which is measured in a spectrophotometer at 635 nm. Biogas production was measured as water displacement. Biogas composition and VFA concentrations were analyzed by a gas chromatograph (HP 5890) equipped with a thermal conductivity or a flame ionization detector, respectively (Ferna´ndez et al., 2001). The soluble content of PPCPs was determined as described in Carballa et al. (2004). Besides, all compounds, except Diazepam, have been also analyzed in sludge samples (Ternes et al., 2002, 2005). 2.6. Digested sludge characteristics 2.6.1. Pathogens Total coliforms, Fecal streptococcus, Escherichia coli and Clostridium perfringens were enumerated according

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5

5 3

4

3

4

6

6

1

1

2

1: Recirculation water bath; 2: Feeding; 3: pH electrode; 4: Thermometer; 5: Biogas outlet; 6: Digested sludge. Fig. 2. Scheme of the anaerobic digestion units.

to APHA (1999), BOE (1987), Araujo et al. (2001), respectively. The presence or absence of Salmonella and Shigella was determined as described in APHA (2001). 2.6.2. Dewatering properties The dewatering properties were studied by determining the Specific Resistance to Filtration (SRF) and the compressibility coefficient (s). SRF was determined at applied vacuum pressures of 150, 450 and 650 mbar using a plot of filtration time/filtrate volume (t/V) versus filtrate volume (V). The compressibility coefficient (s) was calculated from the analysis of specific resistance data obtained at various pressure differentials (Weber, 1972). 2.6.3. Heavy metals content The samples obtained from the reactors were digested in a microwave vessel at 200 C for 30 min with HCl/HNO3 (1:3, v/v). Metals present in extracted solutions were then measured by Atomic Absorption Spectrophotometry in a Perkin Elmer 1100 B (BOE, 1990). 2.6.4. Linear alkylbenzene sulphonates 0.5 g of freeze-dried sludge was sequentially extracted with 10 ml of methanol. After purification through solid phase extraction (SPE) using Strata SAX cartridges and elution with methanol, the samples were analysed by HPLC coupled to fluorescent and UV detection (Petrovic and Barcelo´, 2000). 3. Calculations 3.1. Sludge ozonation effectiveness The pre-treatment effectiveness has been evaluated in terms of COD and solids solubilization (Eqs. (1) and (2)) and COD and solids mineralization (Eqs. (3) and (4)).

CODs;pret  CODs;non-pret  100 CODs;pret VSSnon-pret  VSSpret  100 VSSnon-pret CODt;non-pret  CODt;pret  100 CODt;non-pret VSnon-pret  VSpret  100 VSnon-pret

ð1Þ ð2Þ ð3Þ ð4Þ

where CODs,pret is the soluble COD in the ozonated feeding (g Æ l1); CODs,non-pret is the soluble COD in the nonozonated feeding (g Æ l1); VSSnon-pret is the VSS in the non-ozonated feeding (g Æ l1); VSSpret is the VSS in the ozonated feeding (g Æ l1); CODt,non-pret is the total COD in the non-ozonated feeding (g Æ l1); CODt,pret is the total COD in the ozonated feeding (g Æ l1); VSnon-pret is the VS in the non-ozonated feeding (g Æ l1), and VSpret is the VS in the ozonated feeding (g Æ l1). 3.1.1. PPCPs mass balances To develop the PPCPs mass balances during sludge anaerobic digestion, there are several factors which must be considered in the calculations (Carballa et al., in press). Most of PPCPs studied in this work have been detected in the STP considered, and therefore they were already present in the sludge. The total background concentration (Craw) is the sum of the liquid and sludge contributions. The total inlet concentration (Cin) of each PPCP is the sum of the background (Craw) and the spike (Cspike) added to the inlet stream. Since the digesters are completely stirred, their outlet comprises the digested sludge and the aqueous phase. Both must be considered in the calculation of the total outlet concentration (Cout) of each PPCP. Removal efficiencies for all PPCPs were calculated taking into account the total load at the inlet (min) and that at the outlet (mout), and the error was expressed as the standard deviation when the

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number of data was higher than two or as the average error when only two values were available. Since the data obtained in this work could be seriously affected by analytical and operational factors, a data processing methodology for validating the values measured has been applied (Carballa et al., in press). 4. Results and discussion 4.1. Sludge ozonation During ozonation, about 1% and 8% of volatile solids were mineralized and solubilized, respectively. These results were similar to those obtained by Goel et al. (2003) during the ozonation of activated sludge. Besides, the soluble COD concentration increased from 6 to 16 g l1, which leads to a COD solubilization efficiency of 60% approximately. No COD mineralization was observed during ozone treatment. Weemaes et al. (2000) observed an alteration of the organic matter up to 67% during biosolids ozonation. 4.1.1. Operation of the anaerobic digestion pilot plant A summary of the operation of each digester with ozonated sludge in comparison with the conventional anaerobic digestion is shown in Table 2.

The Organic Loading Rate (OLRin) increased when operating with ozonated sludge, from 2 to 3 kg Æ m3 Æ d1 in the mesophilic digester and from 4 to 6 kg Æ m3 Æ d1 in the thermophilic one. These values were higher than those tested by Goel et al. (2003). The ozonation process did not affect VS and total COD elimination during anaerobic treatment, with removal efficiencies around 60% and 65%, respectively. Goel et al. (2003) also found a slight increase of 10–30% in VS removal when digesting ozonated sludge (0.015 g O3 Æ g TS1) compared to non-ozonated sludge. However, the elimination of soluble COD increased from 65% to 80% in the mesophilic digester, whereas no effect was observed in the thermophilic reactor (around 50%). Similar results were obtained by Weemaes et al. (2000), who observed that ozonation treatment (0.1 g O3 Æ g COD1) enhanced sludge COD degradation from 36% up to 70%. The observed ammonia levels were higher when the reactors were fed with ozonated sludge, thus indicating the higher degradation efficiencies achieved (Goel et al., 2003). The specific methane production was improved with ozone pre-treatment in the mesophilic digester (from 540 ± 160 to 680 ± 90 ml CH4 Æ g VS1 rem ). However, it remained in the same level in the thermophilic range (650 ± 200 ml CH4 Æ g VS1 rem ).

Table 2 Effluent quality and performance of mesophilic and thermophilic digester Mesophilic digester

Thermophilic digester

Non-ozonated

Ozonated

Non-ozonated

Ozonated

OLRin (kg CODt Æ m3 Æ d1) OLRin (kg VS Æ m3 Æ d1)

2.1 ± 0.2 1.7 ± 0.2

3.4 ± 0.3 1.9 ± 0.2

4.3 ± 0.5 3.4 ± 0.4

6.6 ± 0.5 3.8 ± 0.4

Reactor TS (g Æ l1) VS (g Æ l1) TSS (g Æ l1) VSS (g Æ l1) CODtotal (g Æ l1) CODsoluble (g Æ l1) 1 N–NHþ 4 (g Æ l ) pH TA (g Æ l1) VFA/TA VFAa (mg acetic Æ l1) Acetic Propionic

37.3 ± 4.3 12.6 ± 1.2 33.4 ± 3.2 12.3 ± 1.2 16.4 ± 3.0 1.8 ± 0.3 0.7 ± 0.1 7.8 ± 0.3 4.1 ± 0.6 0.33 ± 0.10 0–137 0–136 0–1

28.3 ± 1.0 15.6 ± 0.7 24.6 ± 1.3 14.4 ± 0.7 23.4 ± 1.5 3.0 ± 0.6 1.1 ± 0.1 8.0 ± 0.1 6.2 ± 0.3 0.24 ± 0.02 0–77 0–73 0–4

41.6 ± 6.1 13.2 ± 1.7 41.5 ± 4.8 12.0 ± 1.2 15.0 ± 2.1 3.0 ± 0.3 0.6 ± 0.1 7.7 ± 0.1 4.3 ± 1.0 0.48 ± 0.11 25–835 0–336 25–499

31.1 ± 2.3 16.6 ± 1.2 26.6 ± 1.1 14.5 ± 0.5 24.8 ± 1.4 8.3 ± 0.7 1.3 ± 0.1 8.3 ± 0.1 6.3 ± 0.3 0.24 ± 0.04 62–495 62–190 0–305

Biogas Daily production (l Æ d1) GPR (m3 Æ m3 Æ d1) %CH4 %CO2 SGP (m3 CH4  kg VS1 removed )

10.2 ± 1.9 1.02 ± 0.19 59.2 ± 3.7 33.7 ± 2.1 0.54 ± 0.16

12.8 ± 0.6 1.28 ± 0.06 61.8 ± 1.0 35.2 ± 1.4 0.68 ± 0.09

20.2 ± 2.8 2.02 ± 0.28 58.3 ± 3.0 34.1 ± 3.7 0.65 ± 0.24

23.4 ± 0.4 2.34 ± 0.04 65.2 ± 2.20 32.7 ± 2.4 0.68 ± 0.11

Removal efficiencies (%) VS CODtotal CODsoluble

64 ± 5 62 ± 9 65 ± 11

62 ± 5 66 ± 5 80 ± 5

61 ± 5 65 ± 4 56 ± 6

59 ± 6 64 ± 6 46 ± 6

a Minimum and maximum concentrations. OLR: organic loading rate; TS: total solids; VS: volatile solids; TSS: total suspended solids; VSS: volatile suspended solids; COD: chemical oxygen demand; TA: total alkalinity; VFA: volatile fatty acids; GPR: gas production rate; SPG: specific gas production.

M. Carballa et al. / Chemosphere 67 (2007) 1444–1452

Removal (%)

4.1.2. Fate of PPCPs during anaerobic treatment of sludge Fig. 3 shows the PPCPs removal efficiencies in the mesophilic (a) and thermophilic (b) digester operated with non-ozonated and ozonated sludge. The elimination of PPCPs was likely caused by biodegradation since sorption onto sludge was accounted for in the removal efficiencies calculations, and volatilization is negligible for these substances. All substances were removed in some extent during anaerobic treatment of sewage sludge, except carbamazepine, which was not eliminated during the conventional operation in any of both digesters. The use of ozone seems to favour CBZ removal (up to 60% at 55 C), this fact being explained by the tendency of this substance to stay primarily in the aqueous phase, and thus being available for a direct ozone attack (high rate constant of 3 · 105 M1 Æ s1, Table 1). Very high removal during anaerobic treatment (>80%) was achieved for estrogens and sulfamethoxazole, with no significant influence of ozone pre-treatment. In the case of estrogens, it is probably due to the fact that they are mainly sorbed onto sludge and thus protected from the ozone attack, despite they showed an extremely high second order rate constant for the reaction with ozone (Table 1). Sulfamethoxazole should also react with ozone; however, as it is totally removed during the anaerobic digestion process, the influence of ozone cannot be shown. Musks (except tonalide with pre-ozonation at 55 C) and diclofenac (after a sludge adaptation period) were also eliminated to a significant degree in both digesters (60– 80%), followed by diazepam (50%) and ibuprofen (20– 50%). Iopromide was the substance with the lowest removal efficiency, of around 20%, which is still within the analytical standard deviation. Taking into account the standard deviations of the removal rates, no influence of ozone pre-treatment was observed, except for carbamazepine and tonalide. The explanation could be that most PPCPs are either sorbed onto sludge (e.g. musk fragrances), and thus not being available for ozone attack, or that they posses too slow reaction rates with ozone (e.g. iopromide or ibuprofen). In the case of tonalide, the differences are more likely based on changes in the feeding composition rather than on the operational conditions. The reaction with OH radicals is of minor relevance, since the oxidant should be mainly consumed by the elevated TSS content (Dodd et al., 2006). 100 90 80 70 60 50 40 30 20 10 0 HHCB

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Despite being operated at different SRT, similar results were achieved under mesophilic and thermophilic conditions, thus temperature is not affecting significantly PPCPs removal. Taking into account that the ozone dose applied in this study (20 mg O3 Æ g TSS1) is equivalent (assuming an specific sludge production of 100–400 g TSS  m3 wastewater ) to those usually applied to wastewater (2–8 g O3  m3 wastewater ), the results obtained in this study were similar to those reported in literature (Ternes et al., 2003; Huber et al., 2005) during ozonation of municipal wastewater effluents (most PPCPs were oxidized by more than 90–99%). 4.2. Digested sludge characteristics 4.2.1. Heavy metals content The concentrations of heavy metals in the digested sludge were far below the current limit values established by Directive 86/278/EEC, and also below those proposed in the Working Document on Sludge (EU, 2000). The following concentrations (limit values in brackets) were obtained for the different elements (in mg Æ kg1): Cu, 200–575 (600– 800); Ni, 55–152 (100–200); Cr, 163–318 (600–800); Fe, 12 000–25 000 (no limit); Zn, 540–2100 (1500–2000); Pb, 70–167 (200–500); Cd, 1–2 (2–5); and Hg, 1–2 (2–5). This fact indicates that all the operational conditions tested lead to a digested sludge suitable for land application. 4.2.2. Pathogens content While Salmonella spp. was present in the raw feeding, Shigella spp. was not detected in any sludge sample. All the pathogens considered were significantly removed (>85%) during ozone pre-treatment, with the exception of F. streptococcus (around 63%) and Salmonella spp., which was still present after ozonation (Table 3). According to the feeding, Shigella spp. was not present in any digested sludge sample and Salmonella spp. was inactivated during anaerobic digestion treatment. Total coliforms and E. coli were very well eliminated in both digesters regardless the type of operation, with removal efficiencies higher than 98% and 90%, respectively. F. streptococcus was also very well removed (>90%) in both digesters, except during conventional operation under mesophilic conditions (37%), and the inactivation of C. perfringens increased with temperature and after ozonation. 100 90 80 70 60

AHTN CBZ

DZP IBP

DCF

IPM SMX E1+E2 EE2

50 40 30 20 10 0 HHCB

AHTN CBZ

DZP IBP

DCF

IPM SMX E1+E2 EE2

Fig. 3. PPCPs removal efficiencies (%) during conventional (h) and pre-ozonated (j) operation in mesophilic (a) and thermophilic (b) digester.

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Table 3 Sludge characteristics before and after conventional and advanced anaerobic digestion Raw sludge

Mesophilic digested sludge

Thermophilic digested sludge

Non-ozonated

Ozonated

Non-ozonated

Ozonated

Non-ozonated

Ozonated

Pathogens contenta T. coliforms (MPN Æ g TS1) E. coli (MPN Æ g TS1) F. streptococ. (MPN Æ g TS1) C. Perfringens (CFU Æ g TS1) Salmonella spp.c (PA Æ 50 g TS1) Shigella spp.c (PA Æ 50 g TS1)

2.4 · 105–2.4 · 107 4.6 · 103–1.1 · 106 1.1 · 102–2.4 · 107 8.4 · 104–8.1 · 106 Presence Absence

4.6 · 103 1.1 · 103 2.4 · 106 2.3 · 105 Presence Absence

1.1 · 102 4.6 · 101 1.1 · 102 1.6 · 105 Absence Absence

4.6 · 101 4.3 · 100 1.1 · 104 3.2 · 105 Absence Absence

9.2 · 102 3.0 · 102 1.1 · 101 9.3 · 104 Absence Absence

2.4 · 100 4.0 · 102 2.4 · 100 2.0 · 105 Absence Absence

Dewatering properties SRF Æ 1014 (m Æ kg1) 150 mbar 450 mbar 650 mbar s

5.3–7.9 1.8–4.3 0.1–3.9 0.6–4.7

26.3 ± 0.1 6.0 ± 0.1 3.0 ± 0.1 2.5 ± 0.1

1.8 ± 0.1 1.6 ± 0.1 1.1 ± 0.1 0.6 ± 0.1

81.5 ± 4.0 18.4 ± 0.3 9.9 ± 0.2 2.4 ± 0.1

4.7 ± 0.1 3.8 ± 0.1 3.5 ± 0.1 0.4 ± 0.1

89.6 ± 1.0 19.6 ± 1.0 10.9 ± 0.1 2.4 ± 0.1

Linear alkylbenzene sulfonatesb C10 (mg Æ kg1) C11 (mg Æ kg1) C12 (mg Æ kg1) C13 (mg Æ kg1) CT (mg Æ kg1)

1.2–3.0 31.1–66.4 59.2–112.3 59.8–89.7 151–271

3.3 52.0 87.3 82.4 225

0.8 22.2 43.7 47.5 114

2.4 29.1 47.5 45.1 124

0.8 20.8 46.5 39.7 108

2.3 13.6 20.2 19.1 55.2

a b c

MPN: most probable number (95% confidence); CFU: colony forming units (95% confidence). N = 6 (standard deviation of 2–6%). PA: presence/absence. SRF: specific resistance to filtration; s: compressibility coefficient.

As reported in literature (Nielsen and Petersen, 2000), thermophilic conditions leads to a better sanitizing effect than the mesophilic ones and fulfills the US EPA limit values for Class A sludge as well as the requirements proposed in the Working Document on Sludge for an advanced treatment. 4.2.3. Dewatering properties Higher vacuum levels increased SRFs (Table 3), related to the greater pressure difference in the sludge cake, which leads to clogging problems. Similarly to other studies (Weemaes et al., 2000), the dewatering properties were deteriorated after ozonation pre-treatment in both digesters. During conventional operation, lower SRF were obtained in the mesophilic digested sludge compared to the thermophilic one; however, no differences were observed between both sludges in the operation with pre-ozonation. Concerning the compressibility coefficient (s), similar results were achieved for the mesophilic and thermophilic sludge, with the highest values being obtained for the pre-ozonated sludge. 4.2.4. Linear alkylbenze sulfonates (LAS) The concentrations of the different LAS homologues in the digested sludge after conventional and pre-ozonated operations were (Table 3): 0.8–2.4 mg Æ kg1 (C10), 13.6– 20.8 mg Æ kg1 (C11), 20.2–43.7 mg Æ kg1 (C12) and 19.1– 39.7 mg Æ kg1 (C13). The total LAS content of the digested sludge (55– 125 mg kg1) was much lower than the typical values reported in literature (Waters and Feijtel, 1995) and also

lower than the limit value proposed in the Working Document on Sludge (2600 mg Æ kg1). Besides, these concentrations are far away the threshold value (15 000 mg Æ kg1) found as inhibitory for biogas formation (Battersby and Wilson, 1989). The removal efficiencies of the different LAS homologues were similar in both digesters during all the experiments, ranging from 50% to 90%. The removal efficiencies reported in literature for full-scale anaerobic digesters vary from 18% (Prats et al., 1997) to 35% (Osburn, 1986). However, it is not clear to which process (binding, humification, co-metabolism and anaerobic desulphonation) can be ascribed. 5. Conclusions Ozonation has been used as sludge pre-treatment in order to improve the sludge stabilization by anaerobic digestion. The use of this process leads to an improved COD solubilization of 60%, thus increasing the biogas production and the soluble organic matter removal efficiency during anaerobic digestion. However, the elimination of solids and total COD remains in the same range. The digested sludge characteristics, except the dewatering properties which were deteriorated after ozonation, indicate that it is suitable for final disposal or application as agricultural fertilizer. Concerning PPCPs in the digesters, it was observed: (i) very high removal (>80%) of sulfamethoxazole, 17aethinylestradiol and natural estrogens (estrone and 17bestradiol); (ii) high removal (>60%) of galaxolide, tonalide

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and diclofenac; (iii) medium removal of diazepam (50%) and ibuprofen (20–50%); and, (iv) absence or low elimination (20%) of iopromide. Except carbamazepine, which was only removed with pre-ozonated sludge (up to 60% under thermophilic conditions), the ozonation process did not affect the removal efficiencies of PPCPs during sludge anaerobic digestion. Considering the ozone dose used in this study (20 kg O3 Æ t1 TSS) and assuming a sludge production of 9–10 t Æ d1, the total expected cost for sludge ozonation (for an ozone production cost between 0.8 and 1.6 € Æ kg1) is 144–320 € Æ d1. Compared to the current total costs for sludge treatment 2700–9000 € Æ d1 (for a sludge treatment cost of 300–900 € Æ ton1, depending on the type of treatment), ozonation could represent an economically feasible solution to improve sludge stabilization. However, the real decisions regarding the application of such system will depend on the magnitude of capital investments, energy cost and sludge disposal costs. The proposed system seems to be interesting for larger STPs and for sludge that requires higher disposal costs. Acknowledgements This work has been supported by the European Union (Poseidon Project, EVK1-CT-2000-00047) and the Spanish Ministry of Education and Science (FARMEDAR Project, CTM2004-04475). Special thanks go to Guido Fink and Dirk Lo¨ffler for their support during PPCPs analysis. References APHA-AWWA-WPCF, 1999. Standard methods for the examination of water and wastewater. 20th ed. Clesceri, L.S., Greenberg, A.E. & Eaton, A.D. (Eds.). APHA-AWWA-WPCF, 2001. Compendium of Methods for the Microbiological Examination of Foods. American Public Health Association, Washington, DC. Araujo, M., Sueiro, R.A., Go´mez, M.J., Garrido, M.J., 2001. Evaluation of fluorogenic TSC agar for recovering Clostridium perfringens in groundwater samples. Water Sci. Technol. 43 (12), 201–204. Battersby, N., Wilson, V., 1989. Survey of the anaerobic biodegradation potential of organic chemicals in digesting sludge. Appl. Environ. Microbiol. 55, 433–439. BOE, 1987. Orden del 8 Mayo 1987 por la que se aprueban los me´todos oficiales de ana´lisis microbiolo´gicos para la elaboracio´n, circulacio´n y comercio de aguas de bebida envasadas. Boletı´n Oficial del Estado 114, 13964–13973. BOE, 1990. Real Decreto 1310/1990 de 29 de octubre por el que se regula la utilizaio´n de los lodos de depuracio´n en el sector agrario. Boletı´n Oficial del Estado 282, 32339–33347. Carballa, M., 2005. Fate of pharmaceutical and personal care products (PPCPs) in sewage treatment plants focusing on the anaerobic digestion of sludge. PhD thesis. Department of Chemical Engineering. University of Santiago de Compostela, Spain. ISBN: 978-84-609-9409-0. Carballa, M., Omil, F., Lema, J.M., Llompart, M., Garcı´a-Jares, C., Rodrı´guez, I., Go´mez, M., Ternes, T., 2004. Behavior of pharmaceuticals, cosmetics and hormones in a sewage treatment plant. Water Res. 38, 2918–2926. Carballa, M., Omil, F., Ternes, T., Lema, J.M., in press. Fate of pharmaceutical and personal care products (PPCPs) during anaerobic digestion of sewage sludge. Water Res.

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