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Ecotoxicological information

Toxicity to microorganisms

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Reference
Endpoint:
toxicity to microorganisms
Type of information:
experimental study
Adequacy of study:
supporting study
Study period:
Not specified
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Method given but not done to a recognised guideline. Limited information on test material. No information on GLP.
Qualifier:
no guideline followed
Principles of method if other than guideline:
The activated sludge used for all tests was obtained from a sequential batch reactor that was operated at sludge ages (θc) of 10 and 20 days. The batch reactor was operated in a 24 h fill-and-draw cycle and was fed on synthetic sewage of fixed COD, 840 mg/L (Table 1).
Once steady state had been attained at each sludge age, activated sludge containing known concentrations of suspended solids was mixed in a 300 mL BOD bottle with the appropriate amount of feed to yield a food-to-microorganism (F/M) ratio of 0.1 mg COD mg^-1 MLVSS and the test material at various concentrations. Moreover, to inhibit autotrophic nitrifying bacterial respiration, thiourea was added to a final concentration of 10 mg/L. The control sample was prepared in the same manner but contained no organotin compounds. The flask was hermetically sealed by the probe of a membrane oxygen meter. The O2 concentration was recorded as a function of time up to concentrations of about 2 ppm, while the activated sludge was continuously mixed by means of a magnetic stirrer.
To estimate the effect of exposure time on inhibition of respiration rate, the above procedure was repeated but the addition of feed and the determination of respiration rate were done after the chosen exposure time had passed. In all tests pH was adjusted to 7.2 ± 0.2 and the temperature was 18 ± 0.5 °C.
GLP compliance:
not specified
Analytical monitoring:
not specified
Vehicle:
not specified
Test organisms (species):
activated sludge
Test type:
not specified
Water media type:
not specified
Limit test:
no
Total exposure duration:
20 d
Duration:
10 d
Dose descriptor:
other: Ki
Effect conc.:
4.1 mg/L
Nominal / measured:
meas. (not specified)
Conc. based on:
test mat.
Basis for effect:
other: Inhibition of respiration rate
Duration:
20 d
Dose descriptor:
other: Ki
Effect conc.:
42.5 mg/L
Nominal / measured:
meas. (not specified)
Conc. based on:
test mat.
Basis for effect:
other: Inhibition of respiration rate

The inhibition coefficient, Ki was determined for the test material according to Eq. (3) using a concentration of suspended solids equal to 1000 mg/L MLVSS and sludge age (θc) of 10 and 20 days. In all tests, the respiration rate was calculated for direct contact (t = 0) of inhibitor with biomass. The measured Ki values are shown in Table 2.

The values of Ki for the test material were higher in tests with a sludge age of 20 days than in those with a sludge age of 10 days, indicating a decrease in toxicity with increase in sludge age.

Validity criteria fulfilled:
not specified
Conclusions:
The Ki values of activated sludge after 10 and 20 days are 4.1 and 42.5 mg/L respectively.
Executive summary:

Inhibition of respiration rate of activated sludge heterotrophic microorganisms was determined for the test material.

An increase in sludge age or in the concentration of suspended solids reduces the observed inibition. Longer exposure seems to increase the inhibition of the test material during the first hours of exposure, while later the inhibition remains constant.

The Ki values of activated sludge after 10 and 20 days are 4.1 and 42.5 mg/L respectively.

Description of key information

Key value for chemical safety assessment

Additional information

In a supporting study for this endpoint (Stasinakis et al, 2001), inhibition of respiration rate of activated sludge heterotrophic microorganisms was determined for the test material.

Inhibition of respiration rate of activated sludge heterotrophic microorganisms was determined for the test material.

An increase in sludge age or in the concentration of suspended solids reduces the observed inibition. Longer exposure was found to increase the inhibition of the test material during the first hours of exposure, while later the inhibition remains constant. The Ki (inhibition of respiration rate) values of activated sludge after 10 and 20 days are 4.1 and 42.5 mg/L respectively.

A reliability rating of 2 was assigned to this study, according to the criteria of Klimisch, 1997 as although there was limited information on test material and no information on GLP, the methodolgy is well documented.