Determination of the aerobic biodegradability of polymeric material in a laboratory controlled composting test

00456535(95)00326-6

Chemosphere, Vol. 31, Nos 11112, pp. 44754487, 1995 Cominht 0 1995 Elsevier Science Ltd Printed-&at Britain. All rights reserved 0045-6535/95 $9.50+0.00

in

DETERMINATION OF TEE AEROBIC BIODEGRADABILITY OF POLYMERIC MATERIAL IN A LABORATORY CONTROLLED COMPOSTING TEST

U.Pagga, * ‘), D .B .Beimborn’), J. Boelens” and B.De Wilde2’

I) BASF AG, Ecology, 67056 Luwigshafcn,Germany *)Organic Waste Systems, 9000 Ghent, Belgium

(Received in Germany 26 June 1995; accepted 16 October 1995)

Abstract A laboratory method is presented for investigating the biodegradation of an organic test material in an aerobic composting system baaed on the evolution of carbon dioxide. In addition to carbon conversion, biodegradation can also be monitored through weight loss and physical disintegration. The test method is different from other biodegradation tests, especially aquatic tests, because of the elevated temperature representative for real composting conditions and also because of enhanced fungal degradation activities. A ring test was run using paper and poly-B-hydroxybutyrate/valerate

as test materials and cellulose powder as a reference material. The

test results and the experience gained by the participants showed that the method is suitable and practicable. Experience with real technical-scale composting facilities confirms that the method provides test results of high predictive value. The test is designed to become a European Standard in connection with determining the compostability of packagings and packaging materials.

Keywords Compost, aerobic biodegradability, paper, poly-B-hydroxybutyrate/valerate, cellulose powder

1. Introduction

In recent years composting has been recognized as a valuable method for solid waste treatment and recycling of organic matter (OECD Report 1994; Biological Waste Management Symposium 1995). In several countries many new composting facilities have been constructed and put into operation. Typical input materials are not only of green waste from gardens and biowaste from households, such as kitchen refuse, but also packagings and other manufactured products based on paper and biodegradable plastics (Evans and Skidar 1990, Jopski 1993, Verstraete et al. 1993, Bardtke et al., 1994). Composting of packagings has also specifically been mentioned in the recently adopted European directive on packaging and packaging waste (1994). Compostable products, especially synthetic plastic materials, need to be identified and classified as being biodegradable and must not * corresponding author 4475

4476 negatively influence the composting process or the quality of the compost produced. For this purpose suitable definitions, test methods and evaluation criteria are required (Augusta et al., 1992, Swift 1992).

A possibility of obtaining basic information on the biodegradability of teti materials is to use aquatic screening tests such us the COZ evolution test (IS0 9439), the manometric respirometry test (IS0 9408) or the two-phase closed bottle test (IS0 10708). These tests can also be used in slightly modiied versions as Wchner (1995) demonstrated, e.g. using suspensions of compost as inoculum and inorganic media with higher buffering capacity and nutrients. However, all these tests have the disadvantage that they are not representative of a real-life composting environment in which fungi, molds and actinomycetes are very active and high temperatures (SO70°C) prevail. These conditions, which for many materials are important prerequisites

for a succesfbl

biodegradation, are completely different from those in normal environmental aquatic systems. For these reasons it was deemed necessary to develop a laboratory-controlled

composting test in which typical composting

conditions were simulated as far as possible while still yielding reproducible and reliable results.

No internationally adopted standardized laboratory method exists for investigating aerobic biodegradability in a cornposting environment, Working Group 2 of the European Committee for Standardization CEN TC 261 SC4 (Degradability of packaging and packaging material) had the task of preparing a European Standard to evaluate the ultimate aerobic biodegradability of organic packagings and packaging materials in a controlled composting system. The test is based on a method developed by De Baere et al. (1994), which is also published as ASTM Standard D 5338 (1992). A ring test was organized by the working group to obtain more practical experience and to improve the method. This publication describes the test method and presents the results of the ring test.

2. Materials and methods

Principle of the test method The test conditions are designed to simulate typical aerobic composting facilities for the treatment of the organic fraction of mixed municipal solid waste. A mixture of mature compost and the test material is introduced into closed vessels and incubated under optimal oxygen, temperature and moisture conditions for a test period of normally 45 days. In parallel, blank vessels with compost only and controls with compost and a reference substance are investigated. The production of carbon dioxide deriving from the degradation of the test material is measured, compared to the theoretical maximal amount and recorded as biodegradation percentage. The process of biodegradation is shown in a curve where carbon dioxide production or biodegradation percentage is plotted as a function of time. As additional information the weight loss of the test material and the disintegration of a compact material can be determined at the end of the test.

4477 Terminology The t&mate biodegradation is the complete utilization of a test material by micro-organisms resulting in the production pf carbon dioxide, water, mineral salts and new microbial cellular constituents (biomass). Typical biodegradation curves consist of a rclg p&se,

which is the time from the start of a test until a distinct

biodegradation (e.g. 10%) can be measured, a biodegradtionphase,

in which the maximum degradation takes

place and a plateau phase, in which it is almost completed. The bioa@ra&tion degree or percentage is the mean level of biodegradation in percent achieved after the plateau phase has been reached. Disintegration is the falling apart or physical decay of a compact test material which is caused by the composting process and can be determined qualitatively or quantitatively. Total dry soiia!s is the amount of solids obtained by taking a known volume of test material or compost and drying it at about 105oC to constant weight and related to the wet weight. Volatile solids (incineration loss) is the amount of solids obtained by subtracting the residues of a known volume of test material or compost after incineration at about 55OoC from the total dry solids content of the same sample and related to the total dry solids. The volatile solids content is an indication of the amount of organic matter of a sample.

Test materials The test and reference materials chosen, were known to be biodegradable (Cooke 1990, Yakabe et al. 1992). (1) Poly-P-hydroxybutyrate, poly+hydroxyvalerate

copolymers (Biopol), supplied as compounded melt extruded

granules by ZENJXA Bio Products, Billingham Cleveland, England. (2) Common packaging paper made from only virgin kr& unbleached softwood pulp cut into pieces of about 2x2 cm obtained from Centre Technique du Papier, Grenoble, France. (3) Microcrystalline cellulose powder (Avicel, Nr. 2330) was obtained from Merck, Darmstadt, Germany. The total organic carbon content, total dry solids and the volatile solids of the test material were determined. The test material was added to the test vessels in a suitable form (e.g. granules or pieces of about 4 cm2).

Test procedure Well aerated compost, 2 to 4 months old, derived from a properly operating aerobic composting plant treating the organic tiaction of municipal solid waste, was used as inoculum. Glass, stones or metal particles were removed after which the compost was sieved through a screen of about 0.5 - 1 cm to obtain a homogenous inoculum of sufficient porosity. Porosity can be improved if necessary by adding structural materials such as small wood particles.

The total dry solids and the volatile solids of the compost inoculum were determined. The total dry solids content of the compost should be between 50 and 55% of the wet solids and the volatile solids more than about 30% of the dry solids. A suspension of the inoculum was prepared and the pH measured at the start and the end

4418

of the test. If required further parameters may be used to characterize the compost quality such as total nitrogen content or amount of fatty acids.

As test vessels glass flasks or bottles were used with a minimum volume of about 2-3 I allowing an even gas purge in an upwards direction. One set of tests contained 3 parallel vessels for the test material, the blank control and the reference substance. The test vessels were filled with mixtures of 600 g total dry solids of compost and 100 g dry solids of the test material to obtain a ratio of about 6: 1. Water was added to obtain the required water content of about 50-55 %. The ratio of nitrogen to organic carbon of the test mixture should be between 1:lO and 1:40. About l/4 of the total volume of the compost vessel should be air in order to have sufficient headspace for regular manual shaking of the test mixtures. Shaking once a week was necessary to prevent extensive channelling and to provide uniform attack of the micro-organisms on the test material.

The vessels were incubated in the dark or in diffise light at a constant temperature of 58 f 2°C or submitted to temperature profile (1 day at 35 oC, 4 days at 58 oC, up to day 28 at 50 oC then at 35 oC until the end of the test) for a period of normally 45 days and aerated with carbon dioxide free air in such a way as to maintain aerobic conditions throughout the test. Air flow was carefully dosed or recorded. The flow rate of air depended on the test systems, in the case of BASF 50-100 ml/min were used. Carbon dioxide was measured in the exhaust air using suitable methods such as continuous or semi-continuous detection with the help of an i&ared analyzer or a gas chromatograph or discontinuous determination by absorption in a sodium hydroxide solution and subsequent determination of the dissolved inorganic carbon (DIG) in a carbon analyzer or absorption in a barium hydroxide solution and titration with hydrochloric acid. A flow diagram of the test set-up is presented in Fig. 1.

It should be ensured by close observation that the humidity of the test mixtures in the compost vessels is neither too high nor too low throughout the test. No free-standing water or clumps of material should be present. Too dry conditions are typically revealed by the absence of condensate in the headspace of the composting vessel. Moisture, which can be measured by suitable instruments, should be kept in a range of about 50%, preferably by aerating with humidified or dry air or, as a more drastic change, by adding water or by drainage via air inlet. Visual observations were recorded with regard to the appearance of the compost quality such as structure, clumping, colour, fimgal development, smell of the exhaust air and disintegration of the test material.

The activity of the compost inoculum was checked during the test by means of a biodegradable reference substance and by measuring the carbon dioxide evolution in the blank vessels. To &lfil the validity criteria the reference substance should degrade by more than 70% within 45 days, the carbon dioxide production of the compost should be between 50 and 150 mg per g of volatile solids after 10 days and the deviation of the percentage of biodegradation for the reference substance in the parallel vessels should be less than 20% at the end of the test

4479

Figure 1 Principle of the laboratory controlled composting test cgmpressed

air

COZfree

1-

exhausted air

air

headspace t

compost and test material system for the production of COZfree

co2

I

determination

air

e.g. NaOH solution

cornposting vessel

system

Additional information on the quality of the,compost can be otained by measuring the pH in an aqueous solution and the volatile fatty acids. If the pH is less than 7.w.2

at the end of the test, measurement of the volatile fatty

acids spectrum is recommended to check souring of the contents in the cornposting vessel. If more than 2 g of volatile fatty acids per kg of total dry solids has been formed, then the test must be regarded as invalid due to aciditlcation and inhibition of the microbial activity.

If the weight loss of test material was used as additional information on compostabiity, the compost vessels with the test mixtures were weighed, samples were taken from all vessels and the total dry solids and the volatile solids were determined.

Cdculation and expression of results The theoretical amount of carbon dioxide (ThC02 in g per vessel) which can be produced by a total oxidation of the added test or reference material was calculated by

where M, is the total dry solids, Ct the relative amount of total organic carbon in the total dry solids, 44 is the molar mass of carbon dioxide and 12 is the atomic mass carbon. From the accumulated amounts of biologically

4480 produced carbon dioxide, measured in the test vessels (CO& and in the blank control

(C%)b

,the

degree

of

biodegradation (Dr in % of ThCo2) of the test material was calculated by

0,

= CO,),

4CO2)b

x 100

nco,

The average percentage of the parallel vessels was calculated if the deviations of the single measurements were less than 20%. To obtain biodegradation

curves the accumulated amounts of carbon dioxide or the

biodegradation percentages were plotted as a tbnction of time. The mean degree of biodegradation was read from the plateau phase of the biodegradation curves and indicated as final test result.

The weight loss of the test material baaed on volatile solids can be calculated, from the added amount at the start of the test and the measured diierence between test and.blank control at the end of the test.

3. Results The test results baaed on Co2 evolution and expressed as percentage of the theoretical are summarized in Table 1. Avicel was degraded to about 84% on average. The mean degree of Biopol biodegradation was 88%. Paper showed a biodegradation of about 80%. The statistical treatment of the measured data is only intended to give a rough idea of the variability and reproducibility, which for such a hind of measured data was rather good. Figure 2 is an example of the accumulative Co2 production of the test and blank vessels. Figure 3 shows biodegradation curves for Avicel, Biopol and Paper (obtained from OWS) using a temperature profile and Figure 4 shows biodegradation curves for the same substances (obtained by BASF) using a constant temperature.

The time (in days) from the beginning of the test until the plateau phase was reached and the biodegradation was * completed is an indication of the time required for adaptation of the micro-organisms and the velocity of the degradation processes in the test system. Depending on the shape of the biodegradation curves this period may be determined fairly accurately, as can be seen in Figure 4 in the case of Biopol, or it may only roughly be estimated, [as the Paper and Avicel examples show. Avicel needed about 30-40 days, Biopol 20-30 days and Paper 35 days to complete degradation. Besides the inherent material characteristics the time for complete biodegradation is also determined by the particle size of the test materials.

The ratio of the CO2 produced by the compost in the blanks per g of volatile solids is an indication of the compost quality and the viability of the micro-organisms. It should be in the range given by the validity criteria to enable better reproducibility of the test results. The mean value of the ring test experiments was about 75 mg/g.

4481

The laboratories used a constant temperature of 5852 oC or a temperature profile (1 day at 35 OC, 4 days at 58 oC, up to day 28 at 50 oC then at 35 oC until the end of the test).

Table 1 Biodegradation of cellulose powder (Avicel), poly-j3-hydroxybutyrata$oly-j3-hydroqvalerate

copoly-

mers (Biopol) and packaging paper (Paper) indicated as biologically produced COZ as percmtages of the theoretical amount, test duration until plateau phase was reached and amount of Ca

evolution in the blank

controls atler 10 days per g of volatile solids (vs) of the compost. Constant test temperature (tempconst)

or

temperature profile(temp.prof.)

Test material Avicel Laboratory

ATO-DLO (3 experiments) (tempconst.) BASF (tempconst.) Bayer

Avicel

Biopol

Paper

Paper

Blank

biodegrad. degree (%)

plateau biodegad. reached degree (%) (d)

plateau biodegad. plateau reached degee (%) reached (d) (d)

E 86 85: 80

70 39 >89

100,93 104, 105

>50 46

65,82 76,70

39 >69

(724sO)

95, 89, 88

18

78,72,61

6

82, 72,78

25

110

85

30

94

40

30

105, 103, 98 93,93,78

20

91, 85, 89

35

60

20

71,61, 73

30

79

100,106, 96 85,84,89

20

80,81, 80

27

20

75,68, 76

30

80

30 (temp.prof ) OWS Dayton (temp.prof) Stockhausen

Biopol

76, 82, 75

27

70, 72, 76

30

CO2 prod. by composIt (mglg vs)

Essen University (temp.prof.) VTT temp. constant (temp.proE)

(56,55)

(52, 58)

60

(50,34)

28

56

-

71,68,74 80,76,75

35 40

95,91,94

88, 88, 87

30 30

67 64

single values mean value stand. deviation

21 84.0 8.5

26 87.7 13.3

11 32.4 16.4

25 79.9 9.2

10 34.3 12.8

8 73.7 17.0

10 39.3 22.2

The short degradation phase and the relatively low biodegradation degree of Biopol in the case of BASF (Figure 4) can be explained by an immediate start and an intensive degradation at this constant incubation temperature. Due to the COr- measurement technique used, absorption in alkaline solution and DIC determination, some COz

4482

may have escaped. VTT performed parallel tests using the temperature profile and the constant temperature. From the test results it may be concluded that in the cases of the constant high temperature the same final degrees of degradation may be reached in a shorter time. The validity criteria were normally fulfilled except for one laboratory which did not reach the 70% pass level for the cellulose reference. This could be an indication of insufficient compost quality. These data were not used to calculate the mean value. A comparison of CO, determination by infrared analysis and a titrimetrical method using barium hydroxide run in parallel by one laboratory (VTT) showed no significant differences.

To obtain additional information on the disintegration of the test material the weight loss based on the volatile solids of compost and test material can be determined. Some participants in the ring test performed these measurements, but as this technique had not been approved and principal and technical problems occured, the data obtained are not suitable for publication. The experience gained can however be used to improve the measurement and calculation procedure. As in this ring test powdered or granulated material and small pieces of paper were used, no reports on the visual disintegration of the test materials can be given,

Figure 2 Accumulated biogenically produced C@ in the controlled composting test

180

T

160 s

140

! 120 b 100 .!

80

-

Aviccl

d P, 6o .

20

25 test duration (d)

30

35

40

45

I 50

4483 Figure 3 Example of biodegradation curves of the mntrokd composting test using a temperature profile: 1 day at 35 “C, 4 days at 58 “C, up to day 28 at SO“C and then 35 “C through to the end of the test

100 90

T

1

80

.g

60

3

50

$

40

2

30 20 10 0 15

20

25

30

35

40

45

50

test duration (d)

Figure 4 Example of ~od~a~on

curyes of the controlled ~~st~g

test at a constant temperature of 58 “C

90

80

20

25 test dmtion (d)

30

35

40

45

50

4484 4. Discussioo and recommendation?

The biodegadation

behaviour of the test substances was as expected and contirm investigations, e.g. by Krupp

and Jewel1 (1992) or Timmins and Lenz (1994). The test results confirm that the test method is suitable for investigating the ultimate aerobic biodegradation of an organic test material in a composting environment. Carbon conversion is an unequivocal indicator of biodegradation processes and can be measured precisely and reproducibly. The big advantage of the test is the simulation of real composting conditions including the action of fungi and thermophilic micro-organisms. The method does not need too much technical equipment. There is however considerable potential for automation and the handling of the test data by computer, which could significantly improve the performance and evaluation of the test data.

For the incubation initially a temperature profile was used to simulate natural composting environments. Test results and experience showed, however, that a constant incubation temperature improves the method and can reduce the test duration. The plateau phase can be reached earlier using a constant temperature but there is no significant difference between the final degradation degrees obtained. Therefore it is suggested to use a constant temperature of about 58?2 oC throughout the whole test in the standardized method.

The determination of CO* is used to obtain a direct measure of the degraded TOC of the test material. Continuous measurement of the exhaust gas requires tight connections, an exact measurement or dosage of the gas flow and a sufficiently exact determination of the carbon dioxide concentration in the exhaust gas e.g. by infrared spectrometry or by gas chromatography. An alternative is the absorption of CO* in sodium hydroxide solution and the determination of the inorganic carbon (IC) content of the absorber solution in a carbon analyzer. The disadvantage of this technique is the frequent handling of concentrated alkaline solutions and the possibility of losing COz if the absorption capacity of the solution is not sufficient or CO2 production is too intensive at the start of the test. One laboratory used COZ-detection with IR analyzer and a titrimetrical method in parallel and showed that there is almost no difference.

A possible problem can be the maintenance of aerobic conditions in the incubation mixtures. Optimal shape of the incubation vessels, the addition of inert materials to improve the porosity, regular mixing of the content, sufficient volume of the test vessels (at least 2 I) and correct air supply from the bottom of the vessels may be helpful. Instead of inert material small structural materials such as wood should be in the compost which may be obtained by sieving through sieves of 0.5-I cm mesh diameter. Another important factor for aerobic conditions and biodegradation rates is the humidity of the compost mixture, which can be influenced by the humidification of the air and the direct addition of water to the compost.

4485 The determination of the weight loss of the test material based on volatile solids at the end of the test compared with the added test material is recommended as an additional parameter. Problems may arise from the weighing of the filled yessels (high tare of the vessels), the loss of moisture and volatile solids during the test, the taking of representative homogeneous samples and the temperature, at which weighing takes place.

The test duration of normally 45 days is sufficient and should only be extended if significant biodegradation is still taking place after a more or less constant plateau phase has been reached. The influence of the inoculum on the test results cannot be predicted and standardized but only described by the quality criteria used. The percentage of biodegradation based on carbon dioxide determination does not include the amount of carbon converted to new cell biomass, which is not in turn metabolized to carbon dioxide during the test. For precise biomass determination additional investigations or separate tests are required. The extension of the test duration until the newly produced biomass is converted in turn to carbon dioxide and water may be a solution but has to show its applicability in additional tests.

Preliminary comparisons with bench-scale and technical-scale composting tests as well as the daily hands-on experience of OWS in monitoring, consulting and operation of full-scale composting plants, have demonstrated that the laboratory controlled composting test yields similar results which can therefore be considered to be relevant and predictive. (Bartioli 1994, De Wilde et al. 1995).

Acknowledeements

The authors are grateful to the participants of the ring test:

F. Degli Innocenti (Novamont S.p.A, Via G. Fauser, 8, I-28100 Novara), (R. Koch and P. Schyns (BAYER AG ZF-FBT Q18, D-51368 Leverkusen), J.Lorenz (Chemische Fabrik Stockhausen GmbH, Postfach 570, D-47705 Krefeld), U.Pagga and D.B.Beimbom (BASF AG

DUU/OM - 2570, D-67056 Ludwigshafen), R.Tillinger

(OWS Inc. 3155 Research Boulevard. Suite 104, Dayton, Ohio USA-45420)

B. De Wilde and J. Boelens

(Organic Waste Systems n.v. , Dok Noord 4, B-9000 Gent), M. van der Zee (Instituut voor Agrotechnologisch Onderzoek ATO-DLO, Bomsesleeg 59, NL-6700 AA Wageningen), L. Streff and W.Bidlingmaier (Universitat Essen - Fachbereich 10, UniversitatsstraDe 5, D-45 141 Essen) and M. Itiivaara (VT’T, Technical Research Centre ofFinland, Tietotie 2, Espoo, P.O. Box 1500 FIN-02044 VTT, Finland)

4486

Rtftrtntq

American Society for Testing and Materials - Standard Test Method for Determining Aerobic Biodegradation of Plastic Materials under Controlled Cornposting Conditiorls - ASTM D 5338-92 Annual Book of ASTM Standards 1993

Augusta J., Muller R.J. and Widdtcke H. (1992). Biologisch abbaubare Kunststoffe:

Testverfahren und

Beurttilungskriterien. Chem.Ing.Tech. 64:310-415

Bardtke D., Mtiller W.R., Puchner P. and Wenger S. (1994). AbschluSbericht uber die Untersuchung zum BMFT Projekt 01 ZV 89072 - Bioabbaubare Kunststoffe: Untersuchung zur mikrobiologischen Abbaubarkeit von Kunststoffen, Entwicklung eines Schnelltests unter praxis&en

Verbindungen. Universitat Stuttgart, Institut

fir Siedlungswasserbau, Wassergtite- und Abfallwirtschafl.

Bartioli C. (1994). The starch based thermoplastics. “Recycle 94 - Global Forum and Exposition”, Davos

Biological Waste Management (1995). Proceedings of the “International Symposium on Biological Waste Management - A Wasted Chance?“, University of Essen and Technical University of Hamburg-Harburg

Cooke, T.F. (1990). Biodegradability of Polymers and Fibers - A Review of the Literature. J. ofPolymerEngin.

9: 171-211

De Baere, L., De Wilde B. and Tillinger R. (1994). Standard test methods for polymer biodegradation in solid waste treatment systems - In Y.Doi and K.Fukuda, Studies in Polymer Science -Biodegradable Plastics and Polymers, Elsevier, Amsterdam: 323-330

De Wilde B., Boelens J. and De Baere L. (1995). Results of laboratory and field studies on wastepaper inclusion in biowaste in view of cornposting. International Symposium “The Science of Composting”, Bologna

European Union (1994). Directive 94/62/EC of the European parliament and the council on packaging and packaging waste

Evans, J.D. and Sir&r, S.K. (1990). Biodegradable plastics: An idea whose time has come? Chemtech January: 38-42

4487

International Standards (ISO). Water Quality - Evaluation in an aqueous medium of the ultimate aerobic biodegradability of organic compounds (IS0 9439 - Method by analysis of released carbon dioxide, IS0 9408 Method by determining the oxygen demand in a closed respirometer, IS0 10708 - Method by determining the biochemical oxygen demand in a two-phase closed bottle test)

Jopski, T (1993). Biologisch abbaubare Kunststoffe. Kunststoffe 83: 748-75 1

Krupp, L.R. and Jewell, W.J. (1992). Biodegradability of Modified Plastic Films in Controlled Biological Environments. Environ.Sci.Technol. 26: 193-198

OECD Report (1994). Biotechnology for a Clean Environment - Prevention, Detection, Remediation

Puchner P. (1995). Screeningtestmethode zur Abbaubarkeit von Kunststoffen unter aeroben und anaeroben Bedingungen. Stuttgarter Berichte zur Abfallwirtschatt, Band 59, Bielefeld

Swift, G (1992). Biodegradability of polymers in the environment: complexities and significance of definitions and measurements. FEMS Microbial. Review 103:339-346

Timmins, M.R. and Lenz, R.W. (1994). Enzymatic Biodegrad. of Polymers. Trends Polymer Science 2: 15-19

Verstraete W., De Baere L. and Seeboth R.G. (1993). Getrenntsammlung und Kompostierung von Bioabfall in Europa. Entsorgungspraxis 3 :124- 132

Yakabe, Y., Mori, E., Hara T. and Nohara K. (1992). Biodegradability of Poly(3-hydroxbutyrate-co-3hydroxyvalerate) in activated sludge. Chemosphere 25:835-841