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Biodegradation in water: screening tests

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Reference
Endpoint:
biodegradation in water: inherent biodegradability
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
guideline study without detailed documentation
Qualifier:
according to guideline
Guideline:
OECD Guideline 302 B (Inherent biodegradability: Zahn-Wellens/EMPA Test)
GLP compliance:
not specified
Oxygen conditions:
aerobic
Inoculum or test system:
activated sludge (adaptation not specified)
Details on inoculum:
- Organisms: a mixed population of activated sewage sludge micro-organisms.
- Source: the aeration stage of Severn Trent Water PLC Sewage Treatment Plant, Belper, Derbyshire.
- Preparation: the activated sludge was washed by repeatedly settling and resuspending with the test water.
- Usage rate: equivalent to 400 mg dry weight/l.
Duration of test (contact time):
28 d
Initial conc.:
400 mg/L
Based on:
test mat.
Parameter followed for biodegradation estimation:
DOC removal
Details on study design:
TEST CONDITIONS
- Test temperature: 24 °C
- Aeration of dilution water: by magnetic stirrers and compressed air via narrow bore glass tubes.
- Continuous darkness: yes

SAMPLING
- Evalporation: losses by evaporat'ion were made good by the addition of deionised water immediately prior to sampling.
- Sampling method: 20 ml samples were withdrawn from each vessel and filtered through 0.5 µm disposable filtres. The first 5 ml was discarted.

CONTROL AND BLANK SYSTEM
- Abiotic control: containing the test material and activated sludge poisoned by the addition of mercuric chloride.
Reference substance:
diethylene glycol
Parameter:
% degradation (DOC removal)
Value:
68
Sampling time:
28 d
Remarks on result:
other:
Remarks:
rate corrected for abiotic adsorption
Details on results:
Preliminary investigational work showed that there was a 44 % loss of test material, as DOC, on addition of a test material stock solution to activated sludge. This suggested that the test material was adsorbed to activated sludge and therefore an abiotic test vessel, containing the test material and activated sludge poisoned by the addition of mercuric chloride, was included in the main study in order to correct percentage degradation values for any adsorption that occurred.
The results obtained during the main study indicated that there was no significant immediate adsorption of the test material in the abiotic test vessel and therefore the initial loss of test material, as DOC, observed in the preliminary work conducted and the main study is considered to be due to rapid biodegradation of the test material by the activated sewage sludge micro-organisms present.
Test item attained 91 % degradation, as loss of DOC, after 28 days. The degradation rate corrected for abiotic adsorption of the test material to the activated sewage sludge was 68 % after 28 days.
No definite guidelines are given for pass criteria of test materials in the OECD Guidelines, however, a result of 68 % degradation could be taken as evidence of inherent biodegradabiIity.

pH values did not drop below 6.7 in any culture vessel during the course of the study and were adjusted to pH 7.4 on each sampling occasion. Dissolved oxygen levels remained at approximately 5.6 mg O2/l or above in all cultures.
Results with reference substance:
The standard material, Digol, attained 99 % degnadation after 28 days, thereby confirmi g the suitability of the inoculum and culture conditions.

Overall on test results

Day DOC* value mg/l (mean of 3 assays) % degradation
Test item Test item - abiotic vessel Reference substance Test item Test item - abiotic vessel Reference substance
0 hr 77.44 99.11 100.20 23 1 3
3 hrs 72.69 89.27 101.20 27 11 2
1 65.92 89.21 99.47 34 11 3
2 67.50 85.50 88.50 33 15 16
3 44.00 85.00 83.00 56 15 19
6 26.00 85.00 28.00 74 15 74
8 23.42 80.83 9.24 77 19 95
10 22.40 47.34 9.69 78 53** 93
14 16.44 76.56 6.24 84 23 99
16 14.97 76.31 6.38 85 24 98
21 10.04 80.15 5.34 90 20 99
23 9.02 81.28 4.91 91 19 100
27 12.31 84.14 6.36 90 16 99
28 8.89 77.10 6.05 91 23 99

*All values are blank corrected.

** The high % degradation value is considered to be due to sampling and/or analytical variation.

Interpretation of results:
inherently biodegradable
Conclusions:
Inherently biodegradable
Executive summary:

The biodegradability of test item exposed to a mixed population of activated sewage sludge was investigated under aerobic static conditions. The procedures outlined into the OECD guideline 302 B were followed.

Preliminary investigational work showed that there was a 44 % loss of test material, as DOC, on addition of a test material stock solution to activated sludge. This suggested that the test material was adsorbed to activated sludge and therefore an abiotic test vessel, containing the test material and activated sludge poisoned by the addition of mercuric chloride, was included in the main study in order to correct percentage degradation values for any adsorption that occurred.

The results obtained during the main study indicated that there was no significant immediate adsorption of the test material in the abiotic test vessel and therefore the initial loss of test material, as DOC, observed in the preliminary work conducted and the main study is considered to be due to rapid biodegradation of the test material by the activated sewage sludge micro-organisms present.

Test item attained 91 % degradation, as loss of DOC, after 28 days. The degradation rate corrected for abiotic adsorption of the test material to the activated sewage sludge was 68 % after 28 days.

No definite guidelines are given for pass criteria of test materials in the OECD Guidelines, however, a result of 68 % degradation could be taken as evidence of inherent biodegradabiIity.

pH values did not drop below 6.7 in any culture vessel during the course of the study and were adjusted to pH 7.4 on each sampling occasion. Dissolved oxygen levels remained at approximately 5.6 mg O2/l or above in all cultures.

Conclusion

Inherently biodegradable.

Description of key information

Inherently biodegradable.

Key value for chemical safety assessment

Biodegradation in water:
inherently biodegradable
Type of water:
freshwater

Additional information

The biodegradability of test item exposed to a mixed population of activated sewage sludge was investigated under aerobic static conditions, according to the procedures outlined into the OECD guideline 302 B. Preliminary investigational work showed that there was a 44 % loss of test material, as DOC, on addition of a test material stock solution to activated sludge. This suggested that the test material was adsorbed to activated sludge and therefore an abiotic test vessel, containing the test material and activated sludge poisoned by the addition of mercuric chloride, was included in the main study in order to correct percentage degradation values for any adsorption that occurred.

The results obtained during the main study indicated that there was no significant immediate adsorption of the test material in the abiotic test vessel and therefore the initial loss of test material, as DOC, observed in the preliminary work conducted and the main study is considered to be due to rapid biodegradation of the test material by the activated sewage sludge micro-organisms present. Test item attained 91 % degradation, as loss of DOC, after 28 days. The degradation rate corrected for abiotic adsorption of the test material to the activated sewage sludge was 68 % after 28 days.

No definite guidelines are given for pass criteria of test materials in the OECD Guidelines, however, a result of 68 % degradation could be taken as evidence of inherent biodegradabiIity.