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Environmental fate & pathways

Biodegradation in water: screening tests

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Endpoint:
biodegradation in water: ready biodegradability
Data waiving:
study technically not feasible
Justification for data waiving:
the study does not need to be conducted because the substance is inorganic
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 with acceptable restrictions
Qualifier:
equivalent or similar to guideline
Guideline:
OECD Guideline 302 B (Inherent biodegradability: Zahn-Wellens/EMPA Test)
Deviations:
yes
Remarks:
Exposure duration shortened to 48 h, neither a reference compound was tested to prove the slugde activity, nor a toxicity control was run in parallel
Principles of method if other than guideline:
The study was conducted based on general design features of the OECD Guideline for Testing of Chemicals 302 B " Inherent Biodegradability, Method: Zahn-Wellens/EMPA-Test (July 17, 1992)".
GLP compliance:
yes (incl. QA statement)
Oxygen conditions:
aerobic
Inoculum or test system:
activated sludge, domestic, non-adapted
Details on inoculum:
Type: mixed population of aquatic microorganisms (activated sludge)
Origin: activated sludge of an aeration tank of a wastewater plant treating predominantly domestic sewage (Wupper area water authority, STP Odenthal)
Date of collection: 2010-05-25
Concentration of inoculum (dry weight): 300 mg/L suspended solids (ss)
Duration of test (contact time):
48 h
Initial conc.:
0.5 mg/L
Based on:
test mat.
Initial conc.:
5 mg/L
Based on:
test mat.
Parameter followed for biodegradation estimation:
test mat. analysis
Details on study design:
Pre-treatment of the test item:
1.08 mg and 10.12 mg of the test item were weighed out on aluminium foil. Each was added to the test flasks, which had already been filled with 1800 mL of mineral medium. Afterwards the flask content was made up to 2000 mL with mineral medium containing inoculum to give the two test concentrations of 0.5 and 5.0 mg test item/L.

Exposure conditions
Test volume: 2000 mL
Incubation time: 48 hours
Incubation temperature : 20 - 25 °C
Parameter:
% degradation (test mat. analysis)
Value:
99
Sampling time:
24 h
Remarks on result:
other: initial test item concentration: 0.5 mg/L
Parameter:
% degradation (test mat. analysis)
Value:
99
Sampling time:
24 h
Remarks on result:
other: Initial test item concentration : 5 mg/L
Details on results:
Hydrazine hydrate (0.5 mg/L) showed:
45 % elimination after 2 hours
71 % elimination after 4 hours
88 % elimination after 6 hours
100 % elimination after 24 hours

Hydrazine hydrate (5.0 mg/L) showed:
17 % elimination after 2 hours
33 % elimination after 4 hours
49 % elimination after 6 hours
99 % elimination after 24 hours
99 % elimination after 48 hours

Deviating from the Guideline, neither a reference compound was tested to prove the slugde activity, nor a toxicity control was run in parallel. As Hydrazine hydrate is assumed to be eliminated by physico-chemical or abiotic processes rather than biological ones, the sludge activity and test item toxicity towards microorganisms is not relevant for the test evaluation.

As can be seen, the elimination of hydrazine from activated slugde depends on the initial test item concentration as the elimination at an initial test item concentration of 5 mg/L proceeded slower than at an initial concentration of 0.5 mg/L. The evaluation of the environmental behaviour was thus based on the results from tests with an initial test item concentration of 5 mg/L yielding a first oder rate constant of 0.2 h-1

Validity criteria fulfilled:
not specified
Interpretation of results:
inherently biodegradable
Executive summary:

A test similar to a test on inherent biodegradability (OECD 302 B) showed a hydrazine elimination of 99 % after 48 hours when brought in contact with activated sludge (Currenta, 2010a).

Description of key information

A test similar to a test on inherent biodegradability (OECD 302 B) perfomred with a domestic sewage sludge showed a hydrazine elimination of 99 % after 24 hours when brought in contact with activated sludge (Currenta, 2010a).

Key value for chemical safety assessment

Biodegradation in water:
inherently biodegradable, fulfilling specific criteria

Additional information

Generally, tests on the biodegradability of hydrazine are not required as the test substance is purely inorganic. Nevertheless, a study similar to a test on inherent biodegradability was conducted to refine the description of the environmental behaviour of hydrazine.

The study was designed to investigate the rate of hydrazine elimination bringing contaminated aqueous solutions in contact with activated sewage sludge rather than the kind of degradation contributing to the test item disappearance. No distinction can be made between biotic, abiotic or physico-chemical elimination processes, but the results provide evidence that hydrazine will be removed to a significant extend subjecting hydrazine containing waste waters to a sewage treatment. In contact with activated sludge, hydrazine will rapidly be eliminated with a half-life of about 2.4 to 3.5 hours yielding a first order rate constant of 0.28 to 0.2 h-1. This rate constant is between that of an inherent biodegradation test, fulfilling specific criteria (0.1 h-1) and that of a ready test failing the 10days-window (0.3 h-1) and is therefore considered appropriate. As a worst case assumption, the lower value (0.2 h-1) was taken for calculating the environmental exposure. When modelling the environmental distribution that result enables a refined description of the behaviour of hydrazine contained in waste waters.

The initial test item concentration of 5 mg/L is about three orders of magnitude greater than acceptable environmental concentrations (PNECaqua= 6.0 µg/L). As the elimination rate was shown to depend on the initial test item concentrations, higher concentrations preferably lead to an underestimation of the environmental dissipation rate and thus to a conservative estimation of the environmental behaviour of hydrazine when emitted in a sewage treatment plant.