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Toxicity to microorganisms

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Endpoint:
activated sludge respiration inhibition testing
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to
Guideline:
OECD Guideline 209 (Activated Sludge, Respiration Inhibition Test
GLP compliance:
yes
Analytical monitoring:
no
Details on test solutions:
PREPARATION AND APPLICATION OF TEST SOLUTION (especially for difficult test substances)
- Method: 59 µl of the test substance, corresponding to 50 mg, was pipetted below the surface of the Milli-RO-water (tap water purified by reverse osmosis; Millipore Corp., Bedford, Mass., USA) and stirred briefly to mix (5 minutes). Subsequently, synthetic sewage feed and sludge were added.
Test organisms (species):
activated sludge of a predominantly domestic sewage
Details on inoculum:
- Preparation of inoculum for exposure: Coarsely sieved then washed and diluted with ISO medium.

- Amount suspended solids: 3.8 g/l of sludge, as used for the test
Test type:
static
Water media type:
freshwater
Limit test:
yes
Total exposure duration:
3 h
Post exposure observation period:
After the 3-hour contact time, a well-mixed sample of the contents was poured into a 300 ml oxygen bottle, and the flask was sealed with an oxygen electrode connected to a recorder, forcing the air out of the vessel. Oxygen consumption was measured and recorded for approximately 10 minutes. During measurement, the sample was not aerated but continuously stirred on a magnetic stirrer.
Test temperature:
20.1 - 20.4°C
pH:
8.2 - 8.3
Nominal and measured concentrations:
A loading rate of 100 mg/l was tested in duplicate. Performance of a limit concentration at 100 mg/l is acceptable, because 100 mg/l is far above the water solubility of Decamethyltetrasiloxane and thus exceeds the expected concentration in a Sewage Treatment Plant.
Details on test conditions:
TEST SYSTEM
- Test vessel:
- Material, size, headspace, fill volume: All glass, approximately 300 ml oxygen bottles and 1 litre test bottles.

- Aeration: Strong aeration with clean, oil-free air.

- No. of vessels per concentration (replicates): Loading rates of 100 mg/l tested in duplicate.

- No. of vessels per control (replicates): Two replicates.

- No. of vessels per reference (replicates): Three concentrations, each one replicate.


TEST MEDIUM / WATER PARAMETERS
- Source/preparation of dilution water: Tap-water purified by reverse osmosis (Milli-RO) and subsequently passed over activated carbon and ion-exchange cartridges (Milli-Q) (Millipore Corp., Bedford, Mass., USA).



Reference substance (positive control):
yes
Remarks:
3,5-dichlorophenol
Duration:
3 h
Dose descriptor:
EC50
Effect conc.:
> 100 mg/L
Nominal / measured:
nominal
Conc. based on:
test mat.
Basis for effect:
inhibition of total respiration
Remarks:
respiration rate
Results with reference substance (positive control):
- Results with reference substance valid? Yes. The EC50 of 3,5-dichlorophenol was in the accepted range of 2 to 25 mg/l

Controls (C) and Decamethyltetrasiloxane (T): pH, oxygen concentration, oxygen consumption and percentage inhibition of the respiration rate

Flask

Loading rate

(mg/l)

Oxygen conc.

at the start

(mg O2/l)

Oxygen consumption

(mg O2/l/h)

% Inhibition

respiration

rate

pH

Cstart T

0

7.0

38

-

8.3

Cend T

0

7.9

41

-

8.3

 

∆ 8%

 

T 1

100

5.8

41

-4¹

8.2

T 2

100

6.1

40

-1¹

8.2

¹: Negative values indicate stimulation of the respiration rate of the sludge. The values are considered as not significant.

Validity criteria fulfilled:
yes
Conclusions:
An EC50 value of >100 mg/l for toxicity to microorganisms was determined in a reliable ASRI study conducted according to an appropriate test protocol, and in compliance with GLP.
Endpoint:
activated sludge respiration inhibition testing
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
Justification for type of information:
See endpoint summary for justification of read-across
Reason / purpose:
read-across source
Duration:
3 h
Dose descriptor:
EC50
Effect conc.:
> 100 mg/L
Nominal / measured:
nominal
Conc. based on:
test mat.
Basis for effect:
inhibition of total respiration

Description of key information

Toxicity to microorganisms: 3 hour EC50 >100 mg/l (ASRI), read-across from a structurally-related substance.

Key value for chemical safety assessment

Additional information

There are no reliable microorganism toxicity data available for H-L3.

A category approach is applied to this endpoint and is detailed in the Siloxane Category report (PFA, 2017). The hypothesis for read-across is that siloxane substances exhibit no evidence of significant toxicity to sewage treatment plant microorganisms. This is consistent with a non-polar narcosis mode of action. Therefore, good quality data for the structurally-related substance, decamethyltetrasiloxane, L4, (CAS 141-62-8), have been read across. H-L3 and L4 are structurally-similar substances, both are linear siloxanes.

H-L3 and L4 are members of the Reconsile Siloxanes Category within which, in general, there is no evidence of significant toxicity to microorganisms.

The table below presents microorganism toxicity data available for substances within the Siloxanes Category.

Table: Microorganism toxicity data for substances within the Reconsile Siloxane Category

CAS

Name

Result: E(I) C50

Result: NOEC (or EC10/ EC20)

Guideline Number

Duration

Reliability

540-97-6

Dodecamethylcyclohexasiloxane

>100 mg/l

 

OECD 209

3hr

1a

541-05-9

Hexamethylcyclotrisiloxane

>100 mg/l

>100 mg/l

OECD 209

3hr

1a

541-02-6

Decamethylcyclopentasiloxane

>2000 mg/l

 

C11

3hr

1a

556-67-2

Octamethylcyclotetrasiloxane

>10 000 mg/l

 

ISO 8192 (Test for Inhibition of Oxygen Consumption by Activated Sludge)

3hr

4e (WOE)

556-67-2

Octamethylcyclotetrasiloxane

Little or no toxicity (24 h): 0.1 g/g solution test mat. based on: reduction in number of viable bacteria

 

Toxicity toEscherichia coliandStaphylococcus aureusat 37°C for 24 hours

24hr

4e (WOE)

107-46-0

Hexamethyldisiloxane

>100 mg/l

≥100 mg/l

OECD 209

3hr

1a

141-62-8

Decamethyltetrasiloxane

>100 mg/l

 

OECD 209

3hr

1a

352230-22-9

Siloxanes and Silicones, di-Ph, Ph (trimethylsilyl) oxy

>1000 mg/l

≥1000 mg/l

OECD 209

3hr

1a

2627-95-4

1,1,3,3-tetramethyl-1,3-divinyldisiloxane

>1000 mg/l

≥1000 mg/l

Nitrification control indicated no heterotrophic inhibition of respiration rate

OECD 209

3hr

1a

2554-06-5

2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane

>1000 mg/l

≥1000 mg/l

Nitrification control indicated no heterotrophic inhibition of respiration rate

OECD 209

3hr

1a

70131-69-0

Silsesquioxanes, Phenyl

>1000 mg/l

≥1000 mg/l

Nitrification control indicated no heterotrophic inhibition of respiration rate

OECD 209

3 h

1a

99363-37-8

dimethyl, methyl(3-aminopropyl) siloxane, trimethylsiloxy-terminated

> water soluble fraction

≥ water soluble fraction

OECD 209

3hr

1a

71750-79-3

dimethyl, (aminoethylaminopropyl) methyl siloxane, trimethylsiloxy-terminated

> water soluble fraction

≥ water soluble fraction

OECD 209

3hr

1a

 


It is, therefore, considered valid to read-across the results for L4 to fill the data gap for the registered substance. Additional information is given in a supporting report (Peter Fisk Associates, 2017) attached in Section 13 of the IUCLID 6 dossier.

In the study with L4, an activated sludge respiration inhibition 3h EC50value of >100 mg/l was reported.