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Toxicity to microorganisms

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Description of key information

Key value for chemical safety assessment

EC50 for microorganisms:
2.1 mg/L

Additional information

In the risk assessment report it is reported that the toxicity of DHTDMAC to Pseudomonas putida was investigated in a growth inhibition test according to a German DIN-guideline (Bringmann & Kühn method; UBA (1992). Experimentelle Überprüfung der Aussagekraft von Ergebnissen aus unterschiedlichen Bakterientoxiztätstests sowie vergleichende Untersuchungen zur Übertragbarkeit von Labortests auf reale Abwasser- und Gewässerbedingungen, unpublished report No. UBA-FB 106-03-069). In two tests EC50-values of 48 and 58 mg/L were derived after 18 hours (nominal values, graphically extrapolated).

Secondary effluent of a domestic wastewater treatment plant was used as inoculum in a closed bottle inhibition test (OECD 301D, UBA, 1992). The graphically extrapolated EC50-values of two tests were 2.0 and 6.5 mg/L (nominal concentrations) after a test duration of 5 days.

In an activated sludge respiration inhibition test (OECD 209) inoculum from a predominantly domestic wastewater treatment plant was used (UBA, 1992). A 3h EC50 of 520 mg/L was derived graphically from the dose response curve. The corresponding statistically derived value was 267

mg/L (nominal concentrations).

The toxicity of DHTDMAC to nitrifying bacteria enriched in a laboratory wastewater treatment plant (domestic sludge originally) was investigated in a manometric respirometer test (Wagner & Kayser, 1990). The test duration in the reference was referred to between 119 and 254 hours for different substances and was stopped when the nitrification of the controls was completed. The IC50 for inhibition of respiration was 2.1 mg/L active ingredient of DHTDMAC (a carrier solvent was used).

Anaerobic bacteria from a domestic wastewater treatment plant were exposed to DHTDMAC in an OECD 209 test (Hoechst AG (1989d). Prüfung des biologischen Abbaus von Genamin DSAC im OECD-Confirmatory-Test, unpublished report no. 434/89). The inhibition of respiration was measured after 3 hours and the EC10 was 80 mg/L, the EC50 = 220 mg/L.

Using different safety factors according to the sensitivities of the test systems and the mean effect values the lowest PNEC-values are as follows:

Pseudomonas putida EC50 = 53 mg/L, SF = 100 PNEC = 0.53 mg/L

Nitrifying bacteria EC50= 2.1 mg/L, SF = 10 PNEC = 0.21 mg/L

Secondary effluent EC50= 4.3 mg/L, SF = 100 PNEC = 0.043 mg/L

With all these PNECs it has to be considered that the microorganism toxicity derived in laboratory water tests has to be handled with care as a high influence of the composition of the wastewater (e.g. suspended particles, complexing agents) can be assumed, which is the same phenomenon as in surface water tests. Moreover the lowest PNECmicroorganisms of 0.043 mg/L seems to be unrealistic as it is reported that wastewater treatment plants operate at DHTDMAC concentrations of 3 to 8 mg/L. However, it is not documented whether the treatment process would be more effective without this DHTDMAC load in the influent and how less adapted plants might react. Nitrifying bacteria were found to be the most sensitive microorganisms with the lowest EC50 of 2.1 mg/L on which the risk assessment should be based (PNECmicroorganisms = 0.21 mg/L) to ensure that the most sensitive treatment process can take place.