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Toxicity to microorganisms

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Description of key information

An OECD 209 study carried out according to GLP is available. It shows a EC50-3h = 77.1 mg available chlorine/L. The EC10-3h = 46.9 mg available chlorine/L (for information, NOEC = 41.1 mg/L).
It has to be underlined that hypochlorite is frequently used in wastewater treatment plants experiencing bulking episode in order to restore a good activated sludge quality.

Key value for chemical safety assessment

EC50 for microorganisms:
77.1 mg/L
EC10 or NOEC for microorganisms:
46.9 mg/L

Additional information

A study has recently been done under the standardised conditions of OECD 209 ang GLP (Eisner, 2013). The EC50 for inhibition of respiration of activated sludge and 3 hours exposure = 77.1 mg available chlorine/L The EC10 -3h = 46.9 and the NOEC = 41.1 mg available chlorine/L. This is considered as the key study for this endpoint.

Despite the strong antimicrobial potential of hypochlorite, concern for inhibition effects on biological sewage treatment at the current hypochlorite utilization pattern is undue. Activated sludge flocs are not very sensitive to NaClO, probably due to protection by their glycocalix made out of polysaccharides.

The EC50 for the inhibition of activated sludge respiration is situated around 3000 μg/l for a continuous hypochlorite dosage, with a LOEC of ca. 375 μg/l (Raff et al., 1987). The sludge concentration used and the pH were not mentioned in the paper. The result obtained is very much dependent on the sludge concentration used (i.e. the organic content in the test vessel). Therefore the figure mentioned is just a figure and different data will be obtained using different sludge concentrations. Also different pH values will probably generate different data. The tentative LOEC of 375μg/l derived by the authors should only be used as an indication for potential effects on a sewage treatment plant with the limitations mentioned before (Raff et al., 1987).

Marstaller et al. (Environmental Technology, 13, 1077 -1084, 1992) reported a decrease of nitrifying activity as of batch doses of 1500 μg NaClO/mg sludge suspended solids, but they mention also that chlorine is commonly used in Sewage Treament Plants as a mean to control proliferation of filamentous bacteria, that increase Sludge Volume Index (poor settleability).