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Environmental fate & pathways

Biodegradation in water: screening tests

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Reference
Endpoint:
biodegradation in water: ready biodegradability
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
other: According to international guideline(s), GLP-compliant, performed in recognized contract research organization, no restrictions, fully adequate for assessment.
Qualifier:
according to
Guideline:
OECD Guideline 301 F (Ready Biodegradability: Manometric Respirometry Test)
Deviations:
no
Qualifier:
according to
Guideline:
EU Method C.4-D (Determination of the "Ready" Biodegradability - Manometric Respirometry Test)
Deviations:
no
GLP compliance:
yes
Oxygen conditions:
aerobic
Inoculum or test system:
activated sludge, domestic, non-adapted
Details on inoculum:
- Source of inoculum/activated sludge: Totnes Sewage Treatment Works, Totnes, Devon, UK
- Method of cultivation / Preparation of inoculum for exposure: At the laboratory, the activated sludge was kept aerated at room temperature and the pH maintained at 7.0 ± 1.0. Seven days prior to the exposure start the activated sludge was centrifuged, washed and resuspended in the mineral medium and the solids concentration determined. This sludge was then diluted in medium, added to test bottles and stirred until required for use. The pH of the seeded mineral medium was adjusted as necessary to 7.4 ± 0.2 prior to use.
Duration of test (contact time):
28 d
Initial conc.:
100 mg/L
Based on:
test mat.
Initial conc.:
200 mg/L
Based on:
COD
Parameter followed for biodegradation estimation:
O2 consumption
Details on study design:
TEST CONDITIONS
- Composition of medium: mineral salts medium according to guidelines, prepared using reverse osmosis (RO) water.
- Test temperature: 22 ± 1°C. The temperature of the solutions in a selection of test bottles was measured on days 0 and 28. The temperature of the incubator was continuously monitored throughout the study.
- pH: RUN 1: 7.4 at the start of the test and 7.0-8.0 at the end; RUN 2: 7.3 at the start of the test and 7.1 at the end
- pH adjusted: not necessary
- Suspended solids concentration: 30 mg/L
- Continuous darkness: yes

TEST SYSTEM
- Culturing apparatus: The measurement of oxygen uptake was conducted in the Oxitop™ respirometers (Wissenschaftlich-Technische Werkstätten, GmbH, Germany). Each individual unit consisted of a dark glass 500 mL bottle with an Oxitop™ bottle top containing a piezoresistive electronic pressure sensor. Bottles were situated on a magnetic stirrer in a constant temperature incubator. Carbon dioxide produced by microbial respiration was absorbed by potassium hydroxide solution (KOH) placed in a seal cup in the neck of each bottle, and the oxygen taken up was measured as a decrease in pressure. The Oxitop™ controller collected the pressure values from the measuring tops and calculated the BOD (as mg/L).
- Number of culture flasks/concentration: 3

SAMPLING
- Sampling frequency: In Run 1, the oxygen uptake was recorded automatically every 112 minutes during the 28 day experimental period. In Run 2, oxygen uptake was recorded automatically every 112 minutes during days 0 to 6.9. Due to a fault with the transfer of data from the Oxitop™ controller, oxygen uptake was recorded manually once per day, on week days, during days 12 to 28.

CONTROL AND BLANK SYSTEM
- Inoculum blank (3 flasks): inoculated mineral medium
- Reference (3 flasks): inoculated mineral medium + sodium benzoate (100 mg/L)
- Toxicity control: not performed
Reference substance:
benzoic acid, sodium salt
Key result
Parameter:
% degradation (O2 consumption)
Value:
40
Sampling time:
28 d
Remarks on result:
other: Run 1
Key result
Parameter:
% degradation (O2 consumption)
Value:
30
Sampling time:
28 d
Remarks on result:
other: Run 2
Details on results:
The test material attained a maximum mean level of biodegradation (based on the BOD:COD ratio) of 40% in Run 1 and 30% in Run 2 (see Table 1). The increased surface area of the test material in the second run did not improve its degradation.
Less than 60% degradation was achieved within the 10-day window, so the test material cannot be classified as readily biodegradable, however it is not considered likely to persist in the environment.
Results with reference substance:
Sodium benzoate attained a maximum mean level of biodegradation (based on the BOD:ThOD ratio) of 67% in Run 1 and 64% in Run 2 (see Table 2).
More than 60% degradation was achieved within the 10 day window as expected for a biodegradable substance, thus confirming that the activated sludge contained viable organisms.

Table 1: Biodegradation of the test substance

 

% Biodegradation

Day

Run 1

Run 2

Flask 1

Flask 2

Flask 3

Mean

Flask 1

Flask 2

Flask 3

Mean

5

24

24

26

25

14

13

14

14

10/12.1*

32

32

35

33

25

24

25

25

15

36

36

39

37

27

26

27

27

20

39

38

42

40

29

28

29

29

28

39

39

42

40

30

30

30

30

 

*: Run 1/Run 2. Due to a fault with the transfer of data from the Oxitop™ controller during Run 2, oxygen uptake was recorded manually once per day, on week days, during days 12 to 28.

 

 

Table 2: Biodegradation of the reference substance

 

% Biodegradation

Day

Run 1

Run 2

Flask 1

Flask 2

Flask 3

Mean

Flask 1

Flask 2

Flask 3

Mean

5

60

58

60

59

58

56

53

56

10/12.1*

68

63

66

66

63

65

59

62

15

69

65

66

67

64

64

60

63

20

70

65

65

67

64

64

61

63

28

68

65

65

66

66

64

61

64

 

*: Run 1/Run 2. Due to a fault with the transfer of data from the Oxitop™ controller during Run 2, oxygen uptake was recorded manually once per day, on week days, during days 12 to 28.

Validity criteria fulfilled:
yes
Remarks:
rate of degradation of sodium benzoate was 60% of its ThOD within 10 days; difference between replicates was <20%.
Interpretation of results:
inherently biodegradable
Conclusions:
The test material is not considered likely to persist in the environment.

Description of key information

Biodegradation in water: screening tests: aerobic biodegradation 30-40% (COD) in 28 days (OECD 301F, EU C.4-D)

Key value for chemical safety assessment

Biodegradation in water:
inherently biodegradable
Type of water:
freshwater

Additional information

The test material is not considered likely to persist in the environment.