Sodium dichromate

Toxicity to microorganisms

Link to relevant study record(s)

Reference

Endpoint:

activated sludge respiration inhibition testing

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study with acceptable restrictions

Remarks:

Test concentrations are not specified and the concentration response curve is not shown.

Qualifier:

according to guideline

Guideline:

OECD Guideline 209 (Activated Sludge, Respiration Inhibition Test

Version / remarks:

1981

GLP compliance:

not specified

Remarks:

not specified in publication

Analytical monitoring:

no

Vehicle:

no

Details on test solutions:

No information.

Test organisms (species):

activated sludge, domestic

Details on inoculum:

- Name and location of sewage treatment plant where inoculum was collected: local municipal wastewater treatment plant, not further specified
- Method of cultivation: On return to the laboratory, the solids were allowed to settle and the waste liquor was discarded. The solids were then transferred into a 9-L laboratory-scale, semi-continuous activated sludge cylinder and diluted to approximately 8.5 L with deionized
water. The system was initially mixed by aerating at a rate of approximately 0.5 L/min, and the concentration of mixed liquor suspended solids was determined by gravimetric analysis. The solids were then allowed to settle in the cylinder and the upper layer of waste liquor was
discarded. The activated sludge was washed three times with appropriate volumes of deionized water. After washing, the system was adjusted to contain 4000±400 mg of mixed liquor suspended solids (dry weight) per L. The system was then aerated continuously at a rate of 0.5 L/min and incubated at ambient temperature (21°C). The system was supplemented daily with 50 mL of a synthetic sewage stock solution per L of activated sludge.
- Initial biomass concentration: 4000±400 mg of mixed liquor suspended solids (dry weight)/L

Test type:

static

Water media type:

freshwater

Limit test:

no

Total exposure duration:

3 h

Hardness:

Not reported

Test temperature:

21 °C

pH:

7.4-8.0

Nominal and measured concentrations:

Not reported

Details on test conditions:

TEST SYSTEM
- Test vessel: 1-L bottle
- Fill volume: 500 mL
- Aeration: The reaction mixture was aerated at a rate between 0.5 and 1.0 L/min to ensure complete mixing using a Pasteur pipette aeration device.
- No. of vessels per concentration (replicates): not reported
- No. of vessels per control (replicates): not reported
- Sludge concentration (weight of dry solids per volume): 1600 mg/L
- Nutrients provided for bacteria: synthetic sewage

OTHER TEST CONDITIONS
- Adjustment of pH: pH 7.5 ± 0.5 (by addition of H3PO4)

EFFECT PARAMETERS MEASURED (with observation intervals if applicable) :
- oxygen consumption, at 3 h

Reference substance (positive control):

yes

Remarks:

3,5-dichlorophenol

Duration:

3 h

Dose descriptor:

IC50

Effect conc.:

30.1 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Remarks:

Cr

Basis for effect:

inhibition of total respiration

Remarks on result:

other: Based on graphic analysis

Duration:

3 h

Dose descriptor:

IC50

Effect conc.:

21.8 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Remarks:

Cr

Basis for effect:

inhibition of total respiration

Remarks on result:

other: Based on moving average analysis

Key result

Duration:

3 h

Dose descriptor:

IC50

Effect conc.:

28 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Remarks:

Cr

Basis for effect:

inhibition of total respiration

Remarks on result:

other: Based on Probit-transformation analysis

Remarks:

95% CI: 6.9-114.2 mg/L

Details on results:

- Blank controls oxygen uptake rate: 0.96 ± 0.052 mg O2/L*min (corresponding to 36 ± 1.95 mg O2/g suspended solids per h); 0.96 ± .059 mg O2/L*min (corresponding to 36 ± 2.2 mg O2/g suspended solids per h)
- Coefficient of variation of oxygen uptake rate in control replicates: 5.4-6.1%

Results with reference substance (positive control):

- Results with reference substance valid? yes
- Relevant effect levels: 3 h IC50: 12.2 ± 2.2 mg/L (CoV 18%; municipal sludge); 3 h IC50: 11.4 ± 1.5 mg/L (CoV 13%; industrial sludge)

Reported statistics and error estimates:

For the derivation of the IC50, three methods were applied: Graphical analysis, Thompson's method of moving averages and a probit-transformation model.

Validity criteria fulfilled:

yes

Conclusions:

In a 3-h activated sludge, respiration inhibition test with sodium dichromate (Na2Cr2O7) according to OECD guideline 209 (1981), an EC50 of 21.8 mg Cr/L was determined using the moving average method. Applying graphical interpolation or probit-transformation analysis, respective 3 h EC50 values of 30.1 and 28 mg Cr/L were derived. Since the probit-transformation analysis is the only method that derived an EC50 value including the associated lower and upper 95% confidence limit as required by OECD TG 209, the respective 3 h EC50 of 28 mf Cr/L ( 95% CI: 6.9-114.2 mg/L) appears to be the only guideline-conform effect concentration.

Description of key information

In a 3-h respiration inhibition test of activated sewage sludge with sodium dichromate (Na2Cr2O7) according to OECD TG 209 (1981), 3 h EC50 values of 21.8, 30.1 and 28 mg Cr/L were determined using the moving average method, graphical interpolation or probit-transformation analysis, respectively. Since the probit-transformation analysis is the only method that derived an EC50 value including the associated lower and upper 95% confidence limit as required by OECD TG 209, the respective 3 h EC50 of 28 mf Cr/L (95% CI: 6.9-114.2 mg/L) appears to be the only guideline-conform effect concentration and was used as key value.

Key value for chemical safety assessment

EC50 for microorganisms:

28 mg/L