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Toxicity to microorganisms

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Description of key information

Conclusions. Taking into account the extensive information on the toxicity of boron to microorganism in STP, i.e.
- 3 respiration studies using activated sludge, showing NOEC between 17.5 and 300 mg B/L,
- 1 nitrification study in a fixed bed reactor, showing no effect up to 500 mg B/L, and
- 3 studies of growth of protozoans relevant for the functioning of STPs, show NOEC data between 10 and 20 mg B/L.
Therefore, it was decided to base the PNECstp on the lowest NOEC from the available toxicity data. The lowest NOEC for the protozoan Opercularia bimarginata on growth is 10 mg B/L (Guhl. 2000). Since the studies of STP functioning showed no adverse effects at higher concentrations, no additional assessment factor was used, therefore resulting in a PNECstp of 10 mg B/L.

Key value for chemical safety assessment

EC10 or NOEC for microorganisms:
10 mg/L

Additional information

Different test methods are available to derive a PNEC for sewage treatment plants (STPs). Preferred methods are tests which use activated sludge and measure functional endpoints, for example, oxygen consumption or nitrification. Less preferred, but still acceptable are studies of effects on sludge micro-organisms, for example protozoans.

 

7.4.1.1 Respiration Inhibition.Three tests of sludge respiration are available. Hansvelt and Schoonmade (2002) tested the effect of boric acid on sludge respiration 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. 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. Actual inhibition at the maximum exposure was 24%. They reported a NOEC of 17.5 mg B/L and LOEC of 56 mg B/L, although this may not be based on a statistical measure as the systems were not replicated..

 

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. In fact, 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.

 

Webber et al. (1977) conducted Warburg oxygen uptake measurements to evaluate the effect boron would have on the respiration rate of a municipal activated sludge. They found a NOEC of 100 mg B/L. In later work, they observed no difference in COD removal in laboratory-scale activated sludge plants operating with 0, 100 or 300 mg B/L.

 

7.4.1.2 Nitrification efficiency. Effects of boron on nitrifying microorganisms are reported by Buchheister & Winter (2003). This study shows that in a continuously run fixed bed reactor a nitrification efficiency of 99,5% wasachieved up to boric acid concentrations of 500 mg B/l. At a starting concentration of 200 mg/L NH4-N the NH4-Nconcentrations in the effluent were less than 1,0mg/l. However, when 600 mg/L B was supplied, nitrification activity was reduced to 55%.

 

7.4.1.3 Micro-organism toxicity. Toxicity data on protozoans (ciliates and flagellates) which are relevant for STP show NOECs between 10 and 20 mg/L (Guhl 2000). The protozoans for which toxicity data are available on the growth inhibition areEntosiphon sulcatum, Paramecium caudatumandOpercularia bimarginata. These protozoans 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).

 

7.4.1.4 STP data. 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). The measured arithmetic average influent concentrations 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. Unfortunately, boron data for full-scale plants which include good performance data are missing. 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.