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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:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
equivalent or similar to guideline
Guideline:
ISO 10708 Water quality - Evaluation in an aqueous medium of the ultimate aerobic biodegradability of organic compounds - Determination of biochemical oxygen demand in a two-phase closed bottle test
Principles of method if other than guideline:
A defined mineral solution was inoculated with a mixed bacterial inoculum (30 mg suspended solids/L), stabilised under laboratory conditions for one week and then spiked with an amount of the test item referring to 20 mg ThOD (Theoretical Oxygen Demand).
The test vessels were closed glass bottles with 2/3 of aqueous test mixture and 1/3 air. They were shaken continuously to assure steady state oxygen partitioning between liquid and gas phase. The degradation was followed by weekly measurements of the BOD in the aqueous phase for a 28 d period.
The extent of biodegradation was calculated as the cumulative biochemical oxygen demand (BOD) related to the theoretical oxygen demand.
GLP compliance:
not specified
Oxygen conditions:
aerobic
Inoculum or test system:
activated sludge (adaptation not specified)
Details on inoculum:
- Source of inoculum/activated sludge: aeration tank of the municipal sewage treatment plant of Pforzheim, German
- Pretreatment: washed twice by centrifugation with mineral solution. After decanting the supernatant, the pellet was mixed with mineral solution giving a final sludge concentration of 3.0 g/L.
- Preparation of inoculum for exposure: 2 L Mineral Medium was inoculated with 20 mL activated sludge adjusted to 3.0 g/L. 200 mL of this solution was transferred to 9 test vessels. The bottles were closed with glass stoppers and shaken without test substances for one week in the dark at 20 °C.
- Concentration of sludge: 3.0 g/L
Duration of test (contact time):
28 d
Initial conc.:
20 mg/L
Based on:
ThOD/L
Details on study design:
TEST CONDITIONS
- Composition of medium: 85 mg KH2PO4, 217.5 mg K2HPO4, 334 mg Na2HPO4* 2 H2O, 5 mg NH4Cl, 36.42 mg CaCl2* 2 H2O, 22.5 mg MgSO4* 7 H2O and 0.25 mg FeCl3* 6 H2O
- Test temperature: 20 °C
- pH: 7.5
- Continuous darkness: yes

TEST SYSTEM
- Method used to create aerobic conditions: vessels with the test solutions were aerated for 15 minutes by a membrane air pump with hoses and Pasteur pipettes
- Measuring equipment: oximeter

EVALUATION
- Readings: 6, 13, 21 and 28 days
- Measurement: remaining O2 concentration

CONTROL AND BLANK SYSTEM
- Inoculum blank: yes

STATISTICAL METHODS:
- The consumption rates of the blanks, the reference substance and the test substance were compared, presenting the degradability of the test substance.
Reference substance:
benzoic acid, sodium salt
Remarks:
ThOD: 0.104 mg/mL
Test performance:
The reference substance sodium benzoate was degraded to 78.1 % of its initial concentration.
Key result
Parameter:
% degradation (O2 consumption)
Value:
8.7
Sampling time:
28 d
Remarks on result:
other: mean of 3 replicates
Details on results:
The results can be regarded as valid, since:
- the percentage degradation of the reference substance has reached the level of 60% ThOD within 14 days.
- the total oxygen uptake in the blanks after the first week of the test was lower than 3 mg O2/L and in the following week below 1 mg/L per week.

The degradation rates varied in time:

% degradation (cumulative)
Study group Replicate 6 days 13 days 21 days 28 days
Test substance 1 2.5 2.5 8.9 8.9
2 9.0 9.0 11.8 11.8
3 2.5 3.5 5.3 5.3
Reference substance 1 59.5 74.3 82.1 82.1
2 54.1 67.4 76.0 76.0
3 54.7 62.5 76.3 76.3
Validity criteria fulfilled:
yes
Interpretation of results:
not readily biodegradable
Conclusions:
The test substance demonstrated an average degradability of 8.7% after 28 days. It did not reach a biodegradation of 60% in a 10-day-window and therefore must be regarded as not readily biodegradable.
Executive summary:

The biodegradability of the test item was determined with the BOD test for insoluble substances (BODIS). The degradation process was followed by measuring the oxygen consumption in closed vessels, over a test period of 28 days.

 

In short, a defined mineral solution was inoculated with a mixed bacterial inoculum (30 mg suspended solids/L), stabilised under laboratory conditions for one week and then spiked with an amount of the test item referring to 20 mg ThOD (Theoretical Oxygen Demand). The test vessels were closed glass bottles with 2/3 of aqueous test mixture and 1/3 air. They were shaken continuously to assure steady state oxygen partitioning between liquid and gas phase. The degradation was followed by weekly measurements of the BOD in the aqueous phase for a 28 d period.

The test substance demonstrated an average degradability of 8.7% after 28 days. The reference substance sodium benzoate was degraded to 78.1 % of its initial concentration. The test results can be regarded as valid since the percentage of degradation of the reference substance reached the level of 60 % 
ThOD within 14 days. Moreover, the total oxygen uptake in the blanks after the first week of the test was lower than 3 
mg O2/L and in the following week not >= 1 mg/L per week 


 

The test item did not reach the 60% biodegradation in 28 days. Therefore, the test item must be regarded as not readily biodegradable.

Description of key information

For this endpoint there is one study available in which the readily biodegradability of the test item is assessed according to the BOD test for insoluble substances (BODIS). At the end of the experiment, after 28 days, the test item was found to have degraded 8.7 %. Hence it is concluded that the substance is not readily biodegradable.

Key value for chemical safety assessment

Biodegradation in water:
under test conditions no biodegradation observed

Additional information