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Toxicity to microorganisms

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Description of key information

An activated sludge respiration inhibition test reported a 3-hour EC50 of >100 mg/L.

Key value for chemical safety assessment

EC50 for microorganisms:
100 mg/L
EC10 or NOEC for microorganisms:
100 mg/L

Additional information

In a GLP study conducted according to OECD Guideline 209 and EU Method C.11, the test material was studied for its potential inhibitory effect on the respiration of activated sludge microflora from a predominantly domestic waste sewage source.

Since the test material was insoluble, it was prepared in an aqueous solution of tetrapropylenebenzene sulphonate, sodium salt (TPBSNa) and ultrasonicated. Calcium chloride was added to stabilise the dispersion, further sonicated and diluted with distilled water, sonicated then stirred vigorously for 30 minutes. The test material was added to Drechsel bottles containing 300 mL distilled water to give final concentrations of 1, 3.4, 10, 20, 50 or 100 mg/L after addition of the activated sludge inoculum. At 15-minute intervals, 200 mL of activated sludge was added to each bottle and the contents aerated with moist air. The final concentration of flocculated activated sewage sludge suspended solids was 0.8 g/L. Duplicate bottles were prepared for the controls: synthetic sewage, inoculum and water or synthetic sewage, inoculum, water and TPBSNa. After 3 h exposure oxygen consumption was measured using a dissolved oxygen electrode and meter connected to a chart recorder, the respiration rate determined from the linear portion of the graph divided by the concentration of the flocculated activated sewage sludge suspended solids and the percentage inhibition of total oxygen consumption was calculated.

The test material did not inhibit the specific respiration rate at any of the doses tested, which was probably due to its insolubility and lack of bioavailability. No analysis was carried out on of the actual concentrations in the aqueous medium which were likely to be considerably less than the nominal concentrations. The EC50 based on the nominal doses (which includes dissolved, undissolved and adsorbed concentrations of the test material was greater than 100 mg/L (no upper limit was investigated). An EC50 of 20 mg/L (95 % CL 14 to 28 mg/L) was obtained for the positive control by interpolation of the dose-response curve using non-linear regression software. The respiration rates of control and vehicle control were within 15 % of their respective means, demonstrating the validity of the assay.

In conclusion, an EC50 of >100 mg/L was calculated for the test material in an activated sludge respiration inhibition test. However, since the test material was insoluble, the actual concentration bioavailable (if any) was likely to be considerably lower than the nominal concentrations.