Organic Micropollutant Degradation in Sewage Sludge during

Jun 3, 2010 - Table 3 shows AAD for pollutant concentrations (trend represented by best fit straight line) before and after the second turning. Genera...
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Environ. Sci. Technol. 2010, 44, 5086–5091

Organic Micropollutant Degradation in Sewage Sludge during Composting under Thermophilic Conditions T J A L F E G . P O U L S E N * ,† A N D K A I B E S T E R ‡ Department of Biotechnology, Chemistry, and Environmental Engineering, Aalborg University, Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark, and Department of Environmental Chemistry and Microbiology, National Environmental Research Institute, Aarhus University, Frederiksborgsvej 399, 4000 Roskilde, Denmark

Received January 4, 2010. Revised manuscript received May 11, 2010. Accepted May 17, 2010.

Degradation of 12 common organic micropollutants in sewage sludge representing bactericides, flame retardants, fragrances, vulcanizers, and plasticizers (part of many common products) during thermophilic composting was investigated. Micropollutant concentrations, compost temperature, water content, and organic matter content were measured over 24 days in a full-scale compost windrow made from digested sewage sludge, yard waste, and horse manure. Composting took place indoors, and the windrow was turned several times during the experimental period. Concentrations of all 12 micropollutants decreased during composting, and degradation was statistically significant for 7 of the 12 micropollutants. Metabolites (galaxolidone and methyl-triclosan) were produced from two micropollutants (galaxolide and triclosan) during composting, indicating microbial degradation. Pollutant concentrations early in the experiment were more variable than those experienced for the chemical method development. This was likely due to compost heterogeneity. After the second compost turning, concentrations became more stable as compost became more homogeneous.

Introduction Sewage sludge (biosolids) is an important urban organic waste fraction. In Europe alone, more than 8 000 000 t of dry matter (dm) sludge is produced annually (1, 2). As advanced wastewater treatment by the activated sludge process becomes more widespread, global sludge production increases. Sludge contains organic carbon, phosphorus, and nitrogen and is, therefore, an important resource especially with respect to phosphorus. Natural phosphorus deposits of sufficient quality are becoming scarce, prompting rapid price increases. As a result, large quantities of sludge are applied to farmland. Use of organic residues on farmland increases soil organic matter content and water retention capacity (3), reduces effects of salinization in arid areas (4), facilitates easier soil tillage and plant root development, and sequesters carbon * Corresponding author e-mail: [email protected]; phone: +45 9940 9938; fax: +45 9635 0558. † Aalborg University. ‡ Aarhus University. 5086

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in the soil, thereby reducing atmospheric CO2 content and global warming potential (7). Addition of organic matter to soil reduces erosion (5, 6), improves soil structure, and reduces plant diseases (8-16). Organic matter addition is especially important in dry regions where soil organic matter is often low and water retention capacity is poor. The main obstacle for sludge utilization is its heavy metal, organic micropollutant, and pathogen content. Heavy metals mainly originate from industrial sources and are relatively simple to eliminate from sludge via source control. Metals are in many regions including Europe and USA no longer a major concern (17). Pathogens are removed by thermophilic anaerobic digestion, hygienization, or composting, combined with soil application procedures preventing pathogen spreading. Recently, organic micropollutants (detergents, plasticizers, flame retardants, pharmaceuticals, bactericides, etc.) in sludge (17-19) have become a major concern. Their sources are private households and small enterprises (20, 21), and they are, therefore, difficult to eliminate from sludge. They originate from products such as cleaning agents, disinfectants, fragrances, cosmetics, medicines, etc. With a large number of chemicals in use and new chemicals being introduced continuously, the potential number of organic micropollutants in sludge is large. Only a small fraction of these chemicals have been investigated with respect to their presence in sludge, and even fewer, with respect to persistence, plant uptake potential, and adverse effects on ecosystem and human health. The risk that sludge contains unknown potentially hazardous organic micropollutants has caused increasing opposition among farmers and the food industry against the use of sewage sludge. They fear that their land will be contaminated and unsuitable for future food production. Many organic micropollutants degrade microbially, either aerobically (oxidation) or anaerobically (dehalogenation). Presently, information about degradation is available for micropollutants regulated by legislation: polycyclic aromatic hydrocarbons (PAH), linear alkyl sulfonates (LAS), di(2ethylhexyl) phthalate (DEHP), nonyl phenol (NP), nonylphenol ethoxylates (NPE), polychlorinated biphenyls (PCB), and polychlorinated dibenzodioxins (PCDD) (22-30). It is wellknown that such compounds degrade during composting while anaerobic digestion has a much less effect. Apart from a few studies on pesticides (31), very little information about degradation of other organics during sludge treatment is available. Sludge is often treated via composting, treatment in reed beds (low cost treatment), or incineration. Incineration effectively eliminates organic micropollutants but is costly, and reuse of sludge phosphate is difficult. Reed bed treatment is lengthy, typically 10 years, and micropollutant degradation is slow with half-lives of 1000 d for the fragrance HHCB (Table 1) and about 300 d for triclosan (32, 33). The objective of this paper was to investigate and document removal of selected, commonly used organic micropollutants and metabolites during the thermophilic phase of aerobic sewage sludge composting in full-scale windrows. The organic micropollutants investigated were selected to represent major daily use product groups. Groups represented here were bactericides, flame retardants, fragrances, vulcanizers, and plasticizers (Table 1).

Experimental Section Concentrations of 12 organic micropollutants (Table 1) were measured in a full scale compost pile, 4 m wide, 2 m high, 10.1021/es9038243

 2010 American Chemical Society

Published on Web 06/03/2010

TABLE 1. Characteristics of the 12 Organic Micropollutants Investigated

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80 m long, consisting of 42% digested sewage sludge, 30% yard and park waste, 12% straw/horse manure, 7% cardboard, and 9% Troltex (insulation material containing wood shavings and cement) by weight. Composting took place indoors at Komtek, a composting company in southern Denmark. Sampling was initiated immediately following pile construction and continued for 24 days covering 6 pile turnings. Sampling was reduced gradually from 4 times daily in the beginning to once a week at the end of the experiment. Each time, 10 samples of 50 g were taken randomly within the same 10 m windrow section, combined, and mixed thoroughly in a 10 L bucket to make samples more representative. Two 150 g samples of this homogenate were put into amber wide mouth bottles and frozen until further analyses. Temperature and oxygen concentration were measured in situ at the compost pile center using digital probes while water (based on wet weight) and organic matter content were determined using part of the 150 g compost homogenate. Temperature was measured continuously while oxygen concentration and water content were measured at each sampling occasion. Prior to analysis, samples were shredded and mixed thoroughly in a 3 L steel blender for 3 min. Extraction similar to that in ref 34 was utilized: Samples were lyophilized overnight at 0.05 bar and -50 °C with a Christ lyophillisator (Christ Alpha 1-2 LD plus, Osterode, Germany), and 8-10 g of dried sample was extracted for 4 h with 25 mL of ethyl acetate, p.a, Merck, Darmstadt, Germany, using a Soxhlet type extraction device (fexIKA 50; IKA, Staufen, Germany). An aliquot of 100 ng of D15 musk xylene (Ehrenstorfer, Augsburg, Germany) was added as internal standard. Internal standard addition was done after extraction because the behavior of freshly added standard in compost material is likely very different from the target compounds that have been present for a much longer time. One milliliter of toluene was added after extraction, and extracts were condensed at 55 °C and 80 mBar. The condensate was cleaned following the procedure in ref 33 using silica (silica 60, Merck Darmstadt, Germany) (particle removal by eluting with ethyl acetate), size exclusion, and polarity fractionation. Here, size exclusion was performed using a 300 mm × 21.2 mm Phenogel column with a particle size of 5 µm and pore size of 100 Å (Phenomenex, Torrance, CA), together with an Agilent 1100 Series, G1311A quaternary pump with a flow of 3 mL/min of a 1:1 mixture of premixed cyclohexane (p.a., Merck) and ethyl acetate (p.a., Merck). Polarity fractionation was performed using silica 60 eluted with (1) n-hexane with 5% methyl tert-butylether and (2) ethylacetate, successively. Resulting fractions were condensed to 1 mL under a nitrogen flow in a block heater at 50 °C (Stuart Scientific SHT100D, Bie & Berntsen A/S, Denmark) and stored at 2 °C until analysis. Analysis was performed on a gas chromatograph (GC) equipped with a mass spectrometer (MS) (Trace MS, Thermo Scientific, Copenhagen, Denmark). The GC-MS was equipped with a split/splitless injector and A200s auto sampler. A 1 µL splitless injection was performed at 240 °C with a splitless time of 0.55 min. GC separation was performed with a Rxi-5 Sil MS fused-silica capillary column (Restek, Bellefonte, PA), 12 m long, 0.18 mm inner diameter, and 0.18 µm film thickness, and deactivated 1 m precolumn with 0.32 mm inner diameter. Carrier gas was 1.3 mL/min helium (purity 5.0) with a temperature program as follows: 90 °C (1 min) ramp with 50 °C/min to 135 °C ramp with 10 °C/min to 200 °C and ramp with 40 °C/min to 260 °C (5 min). The MS was operated in single ion monitoring mode (SIM) with the ion source set to 160 °C and electron energy of 70 eV while the detector was operated with 450 V. A multistep internal standard calibration with 1/X weighting was performed. The quantification limit was obtained via calibration at the lowest concentration giving a signal-to-noise ratio g 10. The calibration ranged from 3 to 10 000 ng/mL. Two mass 5088

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TABLE 2. Analytical Procedure Data (Gas Chromatographic Retention Time and Mass Fragments Used for the Quantification) retention time [min]

analytical ion [amu]

verifier ion [amu]

AHTN HHCB HHCB-lactone (galaxolidone) OTNE TCS Me-TCS DEHP

5.87 5.52 8.76

243 243 257

258 258 272

4.19 9.00 8.63 9.94

191 288 302 279

TiBP TnBP TCPP TPP MTB

2.98 4.00 5.49 9.70 4.13

155 155 277 325 148

234 290 304 149 167 211 211 279 326 181

analyte

fragments were used to exclude false positive results, i.e., signals giving the same mass fragment at the specified retention time. An overlap giving quantitatively the same results on two mass fragments is extremely rare. For DEHP, three mass fragments were analyzed as the intensity is extremely different; thus, an enhanced linear range of the instrument can be achieved. Analytical parameters are shown in Table 2. Method validation involved analyzing selected homogenized samples in triplicate and assessing the relative concentration standard deviations (RSD). These ranged from 3 and 11% for TPP and MTB, respectively, to 40 and 43% for DEHP and HHCB-lactone, respectively. This represents the analytical accuracy (including homogenization, extraction, internal standard addition, cleanup, and final detection). Higher variability than these are likely due to heterogeneous original material or temporal variability in micropollutant degradation rates. Because compost organic mass decreases over time, pollutant degradation was assessed on the basis of total pollutant mass present in the compost pile (compost dry mass times pollutant concentration) rather than on pollutant concentrations only. Temporal variability of data within a given time period was quantified by (a) identifying the best fit first or second order polynomial for the data (representing background trend) and (b) calculating average absolute deviation (AAD) as AAD )

1 N

N

- M(t)| ∑ |T(t)M(t)

(1)

i)1

where N is the number of measurements, T(t) is the polynomial, and M(t) is the measurement.

Results and Discussion Figure 1 shows temporal development in compost temperature, oxygen concentration, and gravimetric water content. Compost temperature quickly rises above 60 °C during the first 2 days and remains thermophilic (55-75 °C) throughout the sampling period. The brief temperature drops are associated with pile turnings. Oxygen concentration quickly increases from 10% to between 13 and 19%, indicating sufficient oxygen supply. Compost organic matter content is relatively low as sludge and yard waste had low organic matter. The sludge had been anaerobically digested prior to composting, explaining the low organic matter content. Compost organic matter content decreases from 32% to 24% during the experiment while gravimetric water content (based

FIGURE 1. Temperature (°C), organic matter content (g organic matter g dm-1), water content (g H2O g wet compost-1), oxygen content (%), and turning events in the compost pile as a function of time during the thermophilic composting. on wet weight) increases from about 45% to 60% due to compost irrigation. Water content fluctuates more about the overall increasing trend before the second turning but less after. The average absolute deviation (AAD) calculated using a second order polynomial was 2.2 and 0.7% before and after the second turning, respectively. Fluctuations are likely a result of heterogeneous compost early in the experiment. The pile is constructed by depositing the materials in layers on top of one another and mixing them via turning. Thus, it takes a few turnings before the compost pile becomes homogeneous. For future measurements, it is, therefore, desirable that compost made from different materials is turned 2 to 3 times before sampling. Figure 2 shows concentrations of the 12 organic micropollutants as functions of time, while initial concentrations are shown in Table 3 and range from 11 (MTB) to 30 000 ng/g dm (DEHP), in agreement with earlier studies (32, 33). As duplicate samples were taken at most sampling events, determination of individual sampling event standard deviations was not possible. Instead, each individual measurement in the desired time period was divided by the sampling event mean, and a common standard error for all measurements was then calculated from these numbers. Standard deviations for each sampling event (error bars in Figure 2) were then determined from the common standard error. This was done for the periods both before and after the second turning. Standard deviations represent the average sampling accuracy before and after the second turning, and it is seen that there is generally no tendency for one being higher than the other. Table 3 shows AAD for pollutant concentrations (trend represented by best fit straight line) before and after the second turning. Generally, deviations and, thus, concentration fluctuations are larger before the second turning. As for the water content, the reason is likely incomplete compost material mixing and large differences in organic micropollutant concentrations between different compost materials. Straw/horse manure likely has much lower micropollutant concentrations compared to sludge. As turning increases, pile homogeneity variations decrease. Table 3 shows relative reductions in micropollutant quantity present in the compost during the experiment based on initial (0 d) and final (24 d) mass present. Quantities of all 12 pollutants are reduced, although reduction for some is relatively small. Due to the initial concentration fluctuations, pollutant degradation rates were evaluated using concentration data measured after the second turning. Relative pollutant mass reductions after the second turning are also shown in Table 3. For 8 out of 12 micropollutants, reductions are higher than 75%. First order degradation rate constants (K1) and half-lives (T1/2) were determined, taking into account reductions in compost dry

matter during the experiment. All pollutants except TPP show degradation (Table 3) with K1 values ranging between 0.016 and 0.033 d-1 although apparent degradation rates for HHCB-lactone and TCPP are about a third of that. HHCB-lactone is a HHCB metabolite and, thus, is possibly formed from HHCB during composting while TCPP has been shown to be slowly degradable (35, 36). Half lives are generally 20-40 days with HHCB-lactone and TCPP having half-lives of 74 and 110 days, respectively. Percentile bootstrapping (37) was carried out using the data in Figure 2, to test if degradation rates were significantly larger than 0 (95% confidence level). The average values were used because the deviations between individual measurements at each sampling occasion represent measurement uncertainty rather than temporal concentration variation. Degradation rates for seven micropollutants are significantly larger than 0 (Table 3). For some micropollutants, e.g., HHCB-lactone and Me-triclosan, the apparent lack of degradation can be explained by microbial productions. HHCB-lactone is the product of microbial aerobic degradation of HHCB (38, 39), while Me-triclosan is the product of biomethylation of triclosan (40-42). Figure 2a,b shows HHCB, HHCB-lactone, TCS, and Me-TCS concentrations, respectively, as functions of time during composting. Although there is significant variation in the data, HHCB-lactone and Me-TCS disappear slower than their parent micropollutants HHCB and TCS. HHCB-lactone actually increases during part of the experiment, clearly indicating microbial degradation/transformation. Abiotic photolytic degradation was not likely, as micropollutants are embedded in compost being processed inside dimly lit buildings. Irreversible partitioning into compost solids cannot be completely excluded, although the quantity of TPP (which is also relatively resistant to microbial degradation) did not decrease significantly (Table 3), supporting that irreversible partitioning did not occur. Should irreversible partitioning occur, it will still reduce the problematic mobile fraction and potential exposure and, thus, be an improvement. Micropollutant evaporation probably did not occur due to strong sorption to compost and low air exchange rates during composting. The data presented here, therefore, indicate that composting can significantly contribute to removal of unwanted organic pollutants from biosolids before use in agriculture. Initial DEHP concentrations (Figure 2d) increase when concentrations of most of the other micropollutants decrease and vice versa. This is especially clear on day 2 where DEHP concentration increases sharply while concentrations of the remaining micropollutants decrease. It is possible that the main source of DEHP is not the same as the rest of the micropollutants. The yard waste which was observed to contain plastic fragments (a material commonly containing DEHP) could be the DEHP source. Of the micropollutants investigated, environmental standards exist only for DEHP (100 mg g dm-1 in the EU). Although DEHP concentrations are reduced by more than 90%, the final concentration is about 20 times higher than the standard. However, extensive documentation exists that DEHP also degrades during the composting mesophilic and curing phases, and concentrations will be significantly lower in full length composting. It is also likely that degradation can be optimized by adjustment of temperature, oxygen, and water content conditions. At present, no other kinetic data on the compounds studied here during sludge composting, with the exception of DEHP, are available. For selected pesticides, ref 31 reports concentration decreases of 0-50% for thiazoles and 96-100% for dodemorph, carbendazime, pyrifenox, spiroxamine, terbutryn, diuron, and pirimicarb. Ref 25 reports that some congeners of polychlorinated bipenyls (PCBs) and polyaromatic hydrocarbons (PAHs) are removed during composting. Half VOL. 44, NO. 13, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Concentrations of organic pollutants as functions of time during thermophilic composting, (a) HHCB and HHCB-lactone, (b) TCS and Me-TCS, (c) OTNE and TiBP, (d) DEHP and AHTN, (e) TPP and TCPP, and (f) TnBP and MTB. Error bars indicate one standard deviation, also indicated are turning events.

TABLE 3. Observed Removal of the 12 Organic Micropollutants in Composta micropollutant AHTN HHCB HHCB-lactone (galaxolidone) OTNE TCS Me-TCS DEHP TiBP TnBP TPP TCPP MTB

initial conc. (ng/g)

∆m 0-24 d (%)

∆m 2-24 d (%)

AAD 0-2 d (ng/g)

AAD 2-24 d (ng/g)

K1 2-24 d (d-1)

T1/2 2-24 d (d)

p* 2-24 d (%)

110 1000 120

68 89 59

65 86 56

0.19 0.31 0.20

0.12 0.21 0.16

0.016 0.024 0.009

44 20 74

17 (ns) 13 (ns) >20 (ns)

820 290 68 31000 130 120 80 130 11

88 84 74 84 70 64 13 50 71

88 76 66 93 78 79 33 81 90

0.30 0.23 0.17 0.36 0.16 0.20 0.43 0.18 0.27

0.16 0.18 0.12 0.18 0.09 0.04 0.10 0.31 0.06

0.033 0.028 0.019 0.027 0.018 0.017 0 0.006 0.021

21 25 36 26 38 40 n/a 110 33

3 3 1 3 0.4 20 (ns) >20 (ns) 0 at 95% confidence level. *p < 5% indicates K1 significantly larger than 0 at 95% confidence level. ns: not significant.

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lives for DEHP found in this study are similar to those in ref 43 but somewhat lower than those found in ref 26.

Acknowledgments The authors wish to thank Silvia Escuadra Delso and Patricia Pena Martinez and the staff at the Komtek Composting Company for their help and support in gathering the data for this study. Komtek also sponsored the chemical consumables used in this study.

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