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Description of key information

The NOEC for inhibition of nitrification is 0.025 mg Ag/L.

Key value for chemical safety assessment

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The PNECmicroorganisms is intended to protect the degradation functions and process performance and efficiency of domestic and industrial sewage treatment plants (STP). ECHA guidance (ECHA 2008) allows the derivation of a PNECmicroorganisms from a wide range of laboratory tests utilising microorganisms, including standardised biodegradation tests, respiration inhibition tests, activated sludge growth inhibition tests, pilot scale activated sludge simulation tests and tests performed with specific bacterial populations, such as nitrifying bacteria or ciliates. Data on the toxicity of silver in Microtox assays (e.g. Hsieh et al. 2004) are not considered relevant to STP functioning. Nitrifying bacteria are considered to be the most sensitive component of the microbial fauna found in STP and tests investigating effects on these fauna are preferred to more generic microbial testing when deriving a PNECmicroorganisms. An AF of 1 is applied to NOECs from these types of tests when deriving a PNECmicroorganisms (the AF for other microbial tests range from 10 – 100).

Two studies (Choi et al. 2008a,b) report the effects of ionic silver on nitrification inhibition. Additional methodological information was obtained for both of these studies from Hu et al. (2002). Both studies used a population of nitrifying bacteria initially obtained from a municipal STP (Missouri, USA) and each reports inhibition of nitrification of between 10 and 20% at the lowest test concentration (LOEC) of silver tested. Where a LOEC is associated with effects >10% but <20% ECHA guidance (ECHA 2008) allows a NOEC to be calculated as LOEC/2. This results in NOECs of 0.025 and 0.05 mg Ag/L for Choi et al (2008a) and Choi et al. (2008b), respectively.

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