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Toxicity to microorganisms

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Description of key information

The range of loading rate varies between ca. 1 – 100 g Fe/m3 waste water. The reported typical overall average usage (15 g Fe/m3 waste water) is reported to result in concentrations of 200-500 mg Fe/L in activated sludge (Kronos, Phosphate elimination by simultaneous precipitation pamphlet). Higher levels have been reported but this may be a seasonal or occasional effect; loading rates of iron vary significantly for different applications at different stages of the wastewater treatment processing, and the maximum concentration to which activated sludges are regularly exposed could easily be much higher than this. Overall there is substantial evidence to assume a NOEC of 500 mg Fe/L.

Key value for chemical safety assessment

Additional information

A number of results for iron salts are available for microbial inhibition, in several species and with varying reliability. In some cases the acidic pH (4-5) was noted by the reporting authors. The observed effects may be associated with the intrinsic acidity of Fe (III) salts.

Ferrous and ferric iron salts in this category are used as flocculation and precipitation agents in mechanical and biological waste water treatment units. According to experience, under normal conditions, these applications of these substances cause no disturbances in the normal operation of the biological degradation, which is clear evidence of an absence of inhibitory effects within the normal loading range.

 

In acute toxicity studies in micro-organisms, an IC50result is available for ferrous sulfate of 40 mg Fe/l in a reliable Microtox study using Vibrio fischeri. The pH was not adjusted as part of this assay and the effects may well be attributable, at least in part, to acidification.

 

A short term study specifically investigating effects on nitrifying micro-organisms in WWTP sludge concluded an EC50 of 95 mg/L. The study is reliable (with some methodological shortcomings) and pH control was good. However it is apparent that under neutral conditions the majority of iron present will be undissolved; it is not cler how account is taken of this in the study. The IC50 that is concluded is well below the loading which is routinely used in real wastewater treatment plants, and so it would be misleading to base the PNEC on this result.

 

Inhibition of cellular respiration in activated sludge biomass was observed, with an IC50of ca. 500 mg FeCl3/L (equivalent to ca. 170 mg Fe(III)/l) under conditions where the pH was not adjusted to neutralise pH effects (Broglio, 1987). This was a non-standard shortened test and some details of test conduct and conditions are not reported, so the result is of non-assignable reliability. The author concluded that effects on aquatic micro-organisms appear to be related to the pH of the test medium, which decreases significantly as more iron is added; pH fell to ca. 5.7 at the 500 mg/L loading from a baseline pH of ca. 7.4.

 

Results in the range of 100-183 mg/l have been reported in studies of uncertain reliability using Pseudomonas putida, P. fluorescens and Photobacterium phosphoreum.

 

Several IC50values (1-10 mg/l) have been measured for Fe(Cl)3using 14 micro-organism species, in studies of uncertain reliability. Available data on test conditions is minimal but the acidic pH (4-5) was noted.