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activated sludge respiration inhibition testing
Type of information:
experimental study
Adequacy of study:
key study
Study period:
09 February 2012 - 09 February 2012
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
according to guideline
OECD Guideline 209 (Activated Sludge, Respiration Inhibition Test
according to guideline
EU Method C.11 (Biodegradation: Activated Sludge Respiration Inhibition Test)
according to guideline
ISO 8192 (Water quality - Test for inhibition of oxygen consumption by activated sludge for carbonaceous and ammonium oxidation)
GLP compliance:
yes (incl. QA statement)
Analytical monitoring:
Details on test solutions:
The batch of Amino Functional Alkoxysilanes (tested as received), a clear light yellow liquid, was a mixture including reactants and species consistent with the various reaction products obtained by combining aminopropyltriethoxysilane, glycidoxypropyl-trimethoxysilane and methyltrimethoxysilane (information obtained by the sponsor). Since Amino Functional Alkoxysilanes may have the potential to be volatile (information obtained by the sponsor), the test substance was pipetted below the surface of Milli-RO-water (200 ml, tap water purified by reverse osmosis) and stirred briefly to mix (5 minutes) in closed dark brown bottles. The initial loading rates were 2.5 times the final loading rate. Subsequently, 16 ml synthetic medium, 250 ml sludge and Milli-RO water up to 500 ml were added resulting in the required loading rates. Optimal contact between the test substance and test organisms was ensured applying continuous aeration and stirring.
Test organisms (species):
activated sludge of a predominantly domestic sewage
Details on inoculum:
The sludge was coarsely sieved, washed and diluted with ISO-medium. A small amount of the sludge was weighed and dried overnight at ca. 105°C to determine the amount of suspended solids (3.0 g/l of sludge, as used for the test). The pH was 7.4 on the day of testing. The batch of sludge was used one day after collection; therefore 50 ml of synthetic medium was added per litre of activated sludge at the end of the collection day. The sludge was kept aerated at test temperature until use.
Test type:
Water media type:
Limit test:
Total exposure duration:
3 h
Post exposure observation period:
After the 3-hour contact time, the oxygen consumption was recorded for a period of approximately 10 minutes. During measurement, the sample was not aerated but continuously stirred on a magnetic stirrer. The pH and temperature were determined in the remaining part of the reaction mixture.
Test temperature:
Between 18.1 and 20.3°C
Before addition of sludge: 7.6 - 8.5.
After the 3 hour exposure period: 7.7 - 8.3.

Dissolved oxygen:
The oxygen concentration at the start of measurements was at least 7.5 mg O2/l

Oxygen consumption:
Controls: 48 mg O2/l h (mean value)
Amino Functional Alkoxysilanes 10 mg/l: 53 mg O2/l h
Amino Functional Alkoxysilanes 100 mg/l: 45 mg O2/l h
Amino Functional Alkoxysilanes 1000 mg/l: 51 mg O2/l h (mean value)
Nominal and measured concentrations:
Nominal loading rates: 10, 100 and 1000 mg/l
Details on test conditions:

- Type (delete if not applicable): open

- Material, size, headspace, fill volume: All glass

- Aeration:During exposure with clean, oil-free air

- No. of vessels per concentration (replicates): 1 for 10 and 100 mg/l and 3 for 1000 mg/l

- No. of vessels per control (replicates): 2

- Biomass loading rate:1.5 g/l suspended solids in final test mixture


- Source/preparation of dilution water: Tap-water purified by reverse osmosis (Milli-RO)

EFFECT PARAMETERS MEASURED (with observation intervals if applicable) :

The respiration rate from each vessel, in mg O2/l/hr, was calculated from the linear part of the respiration curve, which was generally between 2 and 7 mg O2/l.


- Test concentrations: limit study at 10, 100 and 1000 mg/l
Key result
3 h
Dose descriptor:
Effect conc.:
>= 1 000 mg/L
Nominal / measured:
Conc. based on:
test mat.
(loading rate)
Basis for effect:
inhibition of total respiration
respiration rate
Key result
3 h
Dose descriptor:
Effect conc.:
> 1 000 mg/L
Nominal / measured:
Conc. based on:
test mat.
(loading rate)
Basis for effect:
inhibition of total respiration
respiration rate
Details on results:
In the combined limit/range-finding test no inhibition of the respiration rate of the sludge was recorded at a loading rate of 1000 mg Amino Functional Alkoxysilanes per litre.

There was no significant oxygen uptake from abiotic processes and the results at 1000 mg/l with a nitrification inhibitor showed no heterotrophic inhibition of the respiration rate.
Results with reference substance (positive control):
The EC50 of 3,5-dichlorophenol was in the accepted range of 2 to 25 mg/l for total respiration (4.0 mg/l).
Reported statistics and error estimates:
For the reference substance the percentage inhibition was plotted against the logarithm of the concentrations and the EC50 was determined using linear regression analysis.

For Amino Functional Alkoxysilanes no EC50 could be calculated because the test substance proved to be non-toxic (EC50 > a loading rate of 1000 mg/l).

NOEC estimation
No statistical analysis of the data was performed as no inhibition of the respiration rate of the sludge was recorded at a loading rate of 1000 mg Amino Functional Alkoxysilanes per litre.
Validity criteria fulfilled:
Under the conditions of this present test Amino Functional Alkoxysilanes was not toxic to waste water (activated sludge) bacteria up to and including a loading rate of 1000 mg/l (NOEC).

The EC50 exceeded a loading rate of 1000 mg/l.

Description of key information

Toxicity to microorganisms (whole substance): 3-hour ASRI EC50 >1000 mg/L, NOEC ≥1000 mg/L (OECD 209) (Desmares-Koopmans, 2012).

Silanol HP-X (produced by parent constituents A, D, E, F and G): 3-hour ASRI EC10 >100 mg/L (loading rate) (OECD 209), read across from trimethoxy(methyl)silane (CAS 1185-55-3).


Silanol HP-Z (produced by parent constituents B): 5.75-hour EC10 13 mg/L (respiration inhibition test), read across from 3-aminopropyl(triethoxy)silane (CAS 919-30-2).


Silanols HP-W and HP-Y (produced by parent constituents C, D, E, F): 3-hour NOEC 220 mg/L, EC10 270 mg/L (OECD 209), read across from 3-(trimethoxysilyl)-N-[3-(trimethoxysilyl)propyl]-1-propanamine (CAS 82985-35-1).


Key value for chemical safety assessment

Additional information

Reliable toxicity to sewage treatment plant microorganisms data are available for the registered substance as a whole. Under the conditions of the test, trimethoxy(methyl)silane and its reaction products with 3-aminopropyltriethoxysilane and [3-(2,3-epoxypropoxy)propyl]trimethoxysilane was not toxic to wastewater (activated sludge) bacteria up to and including a loading rate of 1000 mg/L (NOEC). 

The EC50 exceeded a loading rate of 1000 mg/L. 

These data are used in environmental risk assessment for the substance as a whole.


The constituents of trimethoxy(methyl)silane and its reaction products with 3-aminopropyltriethoxysilane and [3-(2,3-epoxypropoxy)propyl]trimethoxysilane are expected to hydrolyse to some extent within the timescale of the test to silanol hydrolysis products and methanol and ethanol.


The submission substance is a UVCB substance. Therefore, in addition to the data on the whole substance, available data on the constituents and/or analogue substance(s) are also reported.

The substance has been divided into separate parent Blocks (A-G) depending on structure and properties.

All of the constituents share four final silanol hydrolysis products, plus ethanol and methanol, after the hydrolysis of the parent constituents. Please see the attached document in IUCLID Section 13, which gives further details on structure and properties of the parent substance and its hydrolysis products.

For the purposes of risk characterisation it is therefore possible to group all of the constituents into four silanol Assessment Entities: methylsilanetriol (CAS 2445-53-6, EC No. 219-489-9), 3-aminopropylsilanetriol (CAS 58160-99-9, EC 261-145-5), disilyl(alkylamine)-heptol and trisilyl(alkylamine)-undecol. Data for each of the four silanol assessment entities are sourced from: 


Methylsilanetriol (CAS 2445-53-6, EC 219-489-9): Silanol HP-X

An activated sludge respiration inhibition 3 hour EC10value of >100 mg/L (loading rate) was determined for trimethoxy(methyl)silane (CAS 1185-55-3) in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP. In a secondary source to which reliability could not be assigned, an IC50 value of >10000 mg/L was reported for trimethoxy(methyl)silane. Trimethoxy(methyl)silane rapidly hydrolyses to methylsilanetriol, which is Silanol HP-X.


3-Aminopropylsilanetriol (CAS 58160-99-9, EC 261-145-5): Silanol HP-Z

Aa respiration inhibition test in a single bacterial strain is available for 3-aminopropyl(triethoxy)silane (CAS 919-30-2). In this study, a Pseudomonas putida respiration inhibition 5.75 hour EC50 of 43 mg/L and EC10 of 13 mg/L were determined. The study was reliable and conducted in accordance with generally accepted scientific standards and described in sufficient detail and in compliance with GLP with acceptable restrictions. The restrictions were that a reference substance was not used to check the sensitivity of the microorganisms. 3-Aminopropyl(triethoxy)silane hydrolyses to 3-aminopropylsilanetriol, which is Silanol HP-Z.


To support this result for CAS 919-30-2, a second study with an analogous substance, triethoxy(3-isocyanatopropyl)silane (CAS 24801-88-5), is read across. Both substances hydrolyse rapidly and share the same silanol hydrolysis product, 3-aminopropylsilanetriol. The isocyanate in the read-across substance, triethoxy(3-isocyanatatopropyl)silane will undergoes very rapid hydrolysis (half-life of about 5 minutes) to form 3-aminopropyl(triethoxy)silane, CAS 919-30-2 and carbon dioxide. 3-Aminopropyl(triethoxy)silane then undergoes rapid hydrolysis to form 3-aminopropylsilanetriol and ethanol. This study reports an activated sludge respiration inhibition 3 hour NOEC value of 10 mg/L and EC10 of 17 mg/L. An IC50 >5000 mg/L was reported in secondary literature (handbook). The original reference was not available for review and no further information is available. The reliability of this result is not assignable.


Trisilyl(alkylamine)-undecol: Silanol HP-W, and disilyl(alkylamine)-heptol: Silanol HP-Y

There are no reliable respiration inhibition of activated sludge studies for Silanols HP-W and HP-Y. Therefore, good quality data from 3-(trimethoxysilyl)-N-[3-(trimethoxysilyl)propyl]-1-propanamine (CAS 82985-35-1) have been read-across. 

The influence of 3-(trimethoxysilyl)-N-[3-(trimethoxysilyl)propyl]-1-propanamine (CAS 82985-35-1) on the respiration rate of activated sludge was investigated according to OECD 209 and GLP (NOTOX B.V., 2012). The nominal test concentrations were 10, 22, 46, 100, 200, 460 and 1000 mg/L. No inhibition in respiration rate of the sludge was recorded up to 220 mg/L. Hence, the NOEC for 3-(trimethoxysilyl)-N-[3-(trimethoxysilyl)propyl]-1-propanamine is 220 mg/l. The EC10 is 270 mg/l and the EC50 is 1000 mg/L. These results are read-across to Silanols HP-W and HP-Y of the submission substance.


Ethanol and methanol do not have adverse effects on microorganisms (OECD 2004a and OECD 2004b).


Carbon dioxide, which is the by-product of the isocyanate read-across substance hydrolysis, is exempt from registration under REACH Article 2(7) (a) because it is a substance with sufficiently known information and it is considered to cause minimum risk due to its intrinsic properties. Carbon dioxide is therefore not assessed further.


Consideration of Parent Substance Constituents

The registered parent substance has been divided into separate parent Blocks depending on structure and properties: Block A, B, C, D, E, F and G. The constituents of the substance are expected to hydrolyse rapidly. There are measured or reliable predicted hydrolysis data for the constituents of Blocks A and B, indicating rapid hydrolysis. Chemical safety assessment for Blocks A and B is therefore based on the silanol hydrolysis products methylsilanetriol and 3-aminopropylsilanetriol, respectively.

However, there is uncertainty around the hydrolysis rates for constituents C-G, therefore risk assessment has been performed on the parent constituents, in addition to the silanol hydrolysis products. This is discussed further in the ecotoxicological general discussion.

The approach for assessing the toxicity of parent Blocks C-G, and to carry out risk assessment, is therefore as follows:

- Use the existing set of data that is used for the hydrolysis products of the constituents of each Block and base the PNEC for the parent Block on this data, without molecular weight correction where relevant. 

- Where the constituents of the Block produce more than one silanol hydrolysis product, PNECs are calculated from the data for each hydrolysis product and the most conservative PNEC will be assessed. 

- A separate exposure assessment and risk characterisation is carried out using the properties of the parent. 

Data for assessment of the parent Block are as follows:

Constituents A, D, E, F and G: Data for methylsilanetriol (CAS 2445-53-6, EC 219-489-9) (Silanol HP-X) are used.

Constituent B: Data for 3-aminopropylsilanetriol (CAS 58160-99-9, EC 261-145-5) (Silanol HP-Z) are used.

Constituents C, D, E and F: Data for trisilyl(alkylamine)-undecol (Silanol HP-W) and disilyl(alkylamine)-heptol (Silanol HP-Y) are used.



OECD (2004a): SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 18-20 October 2004, Methanol, CAS 67-56-1.

OECD (2004b): SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 19-22 October 2004, Ethanol, CAS 64-17-5.