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Biodegradation in water: screening tests

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biodegradation in water: inherent biodegradability
Type of information:
experimental study
Adequacy of study:
weight of evidence
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Pre-guideline, but comparable to guideline study. Detailed study report not available.
equivalent or similar to
OECD Guideline 302 B (Inherent biodegradability: Zahn-Wellens/EMPA Test)
; pre guideline study, no duplicate COD/DOC determinations, no test with reference compound, different mineral medium, restricted reporting
Principles of method if other than guideline:
Pre-guideline study; method: original Zahn-Wellens test (see reference 2) and 3)). Comparable to OECD test guideline 302 B ((Zahn-Wellens/EMPA Test)
GLP compliance:
Oxygen conditions:
Inoculum or test system:
activated sludge, non-adapted
Details on inoculum:
- Source of inoculum/activated sludge: source of inoculum/activated sludge unknown
- Preparation of inoculum for exposure: centrifugation for 5 min at 2000 rpm
- Pretreatment: no data
- Concentration of sludge in test medium: 1 g dry substance/L (ca. 12 g wet sludge after centrifugation) for 2 L reaction medium
- Water filtered: no, tap water
Duration of test (contact time):
10 d
Initial conc.:
ca. 1 000 mg/L
Based on:
Parameter followed for biodegradation estimation:
other: COD
Details on study design:
- Composition of medium: mineral medium: 215 mg urea (alternatively 385 mg NH4Cl) and 89 mg NaH2PO4 x H2O in 2 L of water
- Additional substrate: no
- Solubilising agent (type and concentration if used): no
- Test temperature: 22 ± 3°C
- pH: 6.5 - 7.5
- pH adjusted: if necessary
- Aeration of dilution water: yes
- Suspended solids concentration: 1 g dry substance (activated sludge)/L
- Continuous darkness: no data

- Culturing apparatus: 4 L cylindric glas vessel (beaker)
- Number of culture flasks/concentration: 1
- Method used to create aerobic conditions: stirring and introduction of compressed air passed through a cotton wool strainer and a wash-bottle
- Measuring equipment: COD or TOC analyzer
- Test performed in closed vessels due to significant volatility of test substance: no data
- Test performed in open system: no data

- Sampling frequency: daily or other time intervals
- Sampling method: withdrawal of sample from test vessel and filtration of sample through paper filter (Schleicher und Schüll, Blauband Nr. 589) washed three times with distilled water

- Inoculum blank: yes
Reference substance:
not specified
other: COD
> 95
Sampling time:
10 d
other: COD
Sampling time:
5 d

abiotic elimination < 10%

Validity criteria fulfilled:
not specified
Interpretation of results:
inherently biodegradable
Isobutyric acid undergoes >95% biodegration within 15 days based on determination of COD.
Executive summary:

Isobutyric was tested for biodegradability in a pre-guideline test following the original method of Zahn-Wellens. Test realization ist reported with no specific details, but the general procedure followed is described in reference 2, i.e. Zahn and Wellens (1974). The test method is comparable to OECD test guideline 302 B (Zahn-Wellens/EMPA Test).


Non-adapted activated sludge of a sewage treatment plant served as inoculum. Test substance was incubated for 10 days at 22°C. No parallels were set up. Samples were analyzed for COD at various time points.


After 10 days isobutyric acid was degraded to >95% (day 5: 70%).


In a Zahn-Wellens test for biodegradability, isobutryric acid was shown to be biodgradable (Hoechst AG, undated).


This biodegradability study is classified as acceptable. The method used is comparable to OECD test guideline 302 B (Zahn-Wellens/EMPA Test) with minor restrictions (no duplicate COD/DOC determinations, no test with reference compound, different test medium, restricted reporting).

Description of key information

Isobutyric acid has been demonstrated to be inherently biodegradable in a valid degradation test similar to OECD TG 302 B (Hoechst AG, undated; KS) and to be readily biodegradable in biodegradation test similar to OECD TG 301 E (WoE).

Key value for chemical safety assessment

Biodegradation in water:
readily biodegradable

Additional information

The biodegradability of isobutyric acid was tested in four studies. In two of them, anaerobic conditions were applied.


study record





Hoechst AG, undated

Zahn-Wellens test;

similar to OECD TG 302 B


(10 days)

2 (KS)

70% and >95% degradation at day 5 and day 10;

inherently biodegradable

Henkel KGaA, 1983

similar to OECD TG 301 E


(19 days)

3 (WoE)

97% and 98% degradation at day 5 and day 14;

readily biodegradable

Chou, 1978

anaerobic degradation by acetate adapted domestic activated sludge (metha-nogenic culture, repeated additions of substrate)


(not suited for ready biodegradability)

after a 3 day lag period, complete anaerobic degradation of isobutyric acid with a degradation rate of 250 mg/(L*d)

Kameya, 1995

anaerobic biodegradation test using methanogenic bacteria as inoculum


(not suited for ready biodegradability)

complete anaerobic degradation of isobutyric acid within 7 days



The biodegradability of isobutyric acid was tested in a valid Zahn-Wellens test similar to OECD TG 302 B (RL2). After 5 days, the test substance was degraded to 70% and after10 days to >95% demonstrating the inherent biodegradability of the test substance (Hoechst AG, undated).


In a test for ready biodegradability similar to OECD TG 301 E, isobutyric acid was shown to be readily biodegradable (Henkel KGaA, 1983). Due to insufficient documentation, the reliability for this test is low. Nevertheless, the results are estimated to represent the real degradation potential of isobutyric acid. Thus, the results of this test are taken as weight of evidence.


Based on the results of the key study (Hoechst AG, 1979) in combination with the test results for ready biodegradability (Henkel KGaA, 1983) as weight of evidence, isobutyric acid is assessed to be readily biodegradable. This is in accordance with results for the closely related compound butyric acid, which has been shown to be readily biodegradable.


In addition to the results of aerobic biodegradation, isobutyric acid was demonstrated to undergo complete biodegradation under anaerobic conditions as well.