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EC number: 234-522-7
CAS number: 12007-92-0
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.
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.
126.96.36.199 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.
188.8.131.52 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%.
184.108.40.206 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).
220.127.116.11 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.
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