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Ecotoxicological information

Toxicity to microorganisms

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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:
other: Not a guideline study, but well reported and conducted study
Qualifier:
equivalent or similar to guideline
Guideline:
other: APHA (1989): Standard methods for the examination of water and wastewater, 17th ed. American Public Health Association
Principles of method if other than guideline:
The study investigated the effect of arsenic (As(III)) and As(V)) on the respiration rate of activated heterotrophic sludge microorganisms.
GLP compliance:
not specified
Specific details on test material used for the study:
Additionally, analytical grade arsenic pentoxide (As2O5), purchased from Merck, was tested.
Analytical monitoring:
no
Vehicle:
no
Test organisms (species):
activated sludge, domestic
Details on inoculum:
A sequencing batch reactor (SBR) was used to simulate the activated sludge process and to provide biomass for the toxicity tests. The SBR was operated in a 24-h fill and draw cycle, each cycle consisted of four stages: FILL (10 min), REACT (22.5 h with aeration), SETTLE (1 h) and DECANT (20 min). Activated sludge from a municipal wastewater treatment plant was used to seed the reactor. Synthetic wastewater with a known COD concentration was used as a feed and phosphate buffer was added to maintain a constant pH. When biomass was acclimatized to the feed and steady state had been attained, activated sludge of known
concentration of suspended solids was mixed in a 300 mL BOD bottle with the appropriate amount of feed to yield a food-to-microorganisms (F/M) ratio of 0.1 mg COD/ mg MLVSS, and the test compound at various concentrations.
Test type:
static
Water media type:
freshwater
Limit test:
no
Total exposure duration:
10 d
Test temperature:
20 ± 1 °C
pH:
7.0 ± 0.2
Nominal and measured concentrations:
Not reported
Details on test conditions:
TEST SYSTEM
- Test vessel: 300 ml BOD bottle
- Aeration: yes
- No. of vessels per concentration (replicates): 3
- No. of vessels per control (replicates): 3
- Sludge concentration (weight of dry solids per volume): not reported
- Nutrients provided for bacteria: Synthetic wastewater (ratio of 0.1 mg COD/mg MLVSS)
- Nitrification inhibitor used: Thiourea (at a final concentration of 10 mg/L)

OTHER TEST CONDITIONS
- Adjustment of pH: phosphate buffer was added to maintain a stable pH

EFFECT PARAMETERS MEASURED:
- Respirometric measurements of specific oxygen uptake rate (sOUR) using an oxygen probe
Reference substance (positive control):
no
Key result
Duration:
2.5 d
Dose descriptor:
EC50
Effect conc.:
6.1 mg/L
Nominal / measured:
nominal
Conc. based on:
element
Basis for effect:
inhibition of total respiration
Remarks:
respiration rate
Remarks on result:
other: lowest reported EC50 for As (III)
Duration:
5 d
Dose descriptor:
EC50
Effect conc.:
7.3 mg/L
Nominal / measured:
nominal
Conc. based on:
element
Basis for effect:
inhibition of total respiration
Remarks:
respiration rate
Remarks on result:
other: for As (III)
Duration:
10 d
Dose descriptor:
EC50
Effect conc.:
10.6 mg/L
Nominal / measured:
nominal
Conc. based on:
element
Basis for effect:
inhibition of total respiration
Remarks:
respiration rate
Remarks on result:
other: for As (III)
Duration:
2.5 d
Dose descriptor:
EC50
Effect conc.:
81 mg/L
Nominal / measured:
nominal
Conc. based on:
element
Basis for effect:
inhibition of total respiration
Remarks:
respiration rate
Remarks on result:
other: lowest reported EC50 for As(V)
Duration:
5 d
Dose descriptor:
EC50
Effect conc.:
97 mg/L
Nominal / measured:
nominal
Conc. based on:
element
Basis for effect:
inhibition of total respiration
Remarks:
respiration rate
Remarks on result:
other: for As(V)
Duration:
10 d
Dose descriptor:
EC50
Effect conc.:
198 mg/L
Nominal / measured:
nominal
Conc. based on:
element
Basis for effect:
inhibition of total respiration
Remarks:
respiration rate
Remarks on result:
other: for As(V)
Validity criteria fulfilled:
not specified
Remarks:
Not assessable (control OUR not reported; no reference toxicant)
Conclusions:
In an activated sludge respiration inhibition test, the lowest EC50 for respiration inhibition was reported for 2.5-d old sludge at 6.1 mg As/L (nominal, As(III).

Description of key information

In an activated sludge respiration inhibition test, the lowest EC50 for respiration inhibition was reported for 2.5-d old sludge at 6.1 mg As/L (nominal, As(III).

Key value for chemical safety assessment

EC50 for microorganisms:
6.1 mg/L

Additional information