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Toxicity to microorganisms

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Four tests of sludge respiration are available. Hansvelt and Schoonmade reported an EC20 of 112 mg B/L (95% CI 87 to 144 mg B/L), and an unbounded EC50 which exceed the maximum exposure of 175 mg B/L. Muller and Burns (2001) tested amorphous boron and found a NOEC of >10000 mg/L. Gerike et al. (1976) evaluated methylene blue activity reductions (MBAS test) and COD reduction (OECD, 1971) and reported that addition of 20 mg B/L had no adverse effects on a model STP. Webber et al. (1977) found a NOEC of 100 mg B/L (effect criterion: respiration rate of a municipal activated sludge).


Effects of boron on nitrifying microorganisms are reported by Buchheister & Winter (2003). At a starting concentration of 200 mg/L NH4-N the NH4-N concentrations in the effluent were less than 1,0mg/l. However, when 600 mg/L B was supplied, nitrification activity was reduced to 55%.


Growth inhibition NOECs between 10 and 20 mg/L were identified for the protozoans Entosiphon sulcatum, Paramecium caudatum and Opercularia bimarginata (Guhl, 2000).


Monitoring data from studies on a pilot-scale plant indicate that once microorganisms have been adapted to the presence of boron they can tolerate boron at a concentration of at least 3 mg B/L (Umweltbundesamt BE121, 2000). Unfortunately, boron data for full-scale plants which include good performance data are currently not available.

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Additional information

Respiration tests:

The sludge respiration test that was reported by Hansvelt and Schoonmade (2002, using boric acid as test substance, was conducted according to the OECD 209 guidelines. The purpose of this guideline is to provide a rapid screening method, which is not designed to derive EC10 values. The actual inhibition at the maximum exposure was 24%. A NOEC of 17.5 mg B/L and LOEC of 56 mg B/L were also reported by these authors, although this may not be based on a statistical measure as the systems were not replicated.

The test conducted by Gerike et al. (1976) showed that MBAS and COD removal was enhanced by boron relative to controls. No higher boron concentrations were tested. The actual substance tested was a sodium perborate which had been boiled to remove oxygen.

Assessment of the respiration rate of a municipal activated sludge by Webber et al. (1977) was based on Warburg oxygen uptake measurements. In later work, these authors observed no difference in COD removal in laboratory-scale activated sludge plants operating with 0, 100 or 300 mg B/L.


Nitrification tests:

The study by Buchheister & Winter (2003) showed that in a continuously run fixed bed reactor a nitrification efficiency of 99.5% was achieved up to boric acid concentrations of 500 mg B/l. As indicated, however, nitrification activity was reduced to 55% when 600 mg/L B was supplied.


Protozoan toxicity data

The protozoans that were used by Guhl (2000) have been found in activated sludge from sewage treatment plants in Brazil, France, Germany and Spain (Amann et al. 1998, Motta et al. 1998 and Pérez-Uz et al. 2010) and are therefore considered relevant when assessment the potential toxic impact on substance on the microbiological community of an STP. 


Monitoring data

The arithmetic average influent concentrations that were measured in the UBA-study (Umweltbundesamt BE121, 2000) were 3.0 mg B/L (min. 1.6; max. 4.9 mg B/L) in unfiltered samples collected after primary treatment. The function (nitrogen and carbon - removal) of the pilot-scale plant was not affected. Zessner et al. (2003) monitored average boron concentrations in the effluent of two Austrian full-scale STPs. The average concentration was 1.31 mg B/L for STP1 and 0.78 mg B/L for STP2. ECETOC (1997) reported influent and effluent concentrations between 0.27-0.78 and 0.39 -0.75 mg B/L.

These studies do not permit conclusions about higher boron concentrations, but suggest that, under current use conditions, boron concentrations are below those seen for all the laboratory studies described above.