MP99

Toxicity to microorganisms

Description of key information

The lowest valid result, which leads to the most critical PNEC, was found in m-cresol:
EC75 (4 h) = 11.4 mg/L on activated sludge of predominantly domestic sewage (inhibition of nitrification, similar to ISO/DIS 9509)

Additional information

o-cresol

No standard tests concerning the respiration inhibition on microorganisms in sewage treatment systems are available. However, several tests are available to evaluate the toxicity of o-cresol to microorganisms.

Tomlinson (1966) described the influence of o-cresol on the nitrification process in domestic wastewater. A purified activated sludge was incubated for 2 to 4 hours to the wastewater (BSB5 = 250 mg/L, 50 to 80 mg NH4-N/L) fortified with different concentrations of o-cresol. The effect on the inhibition of the nitrification was determined by photometric analysis of nitrite and nitrate. An EC75 -value of 12.8 mg/L was given for the inhibition of the reaction step of ammonia to nitrite.

The toxicity of o-cresol to Pseudomonas putida was assessed in a cell multiplication inhibition test according to Bringmann (1976). Bacteria were exposed for 16 hours at a temperature of 25 °C to the test substance. A toxicity threshold concentration of 33 mg/L was determined after 16 hours exposure. This result is equivalent to an EC3 or a NOEC. The endpoint biomass is measured by turbidity determination with a photometric detector.

In another test by Bringmann (1978), the protozoa Entosiphon sulcatum was exposed to o-cresol for 72 hours using the cell multiplication inhibition test method. A toxicity threshold of 17 mg/L was obtained which is equivalent to a NOEC or an EC3.

 

m-cresol

The sensitivity of activated sludge to m-cresol was determined by a respiration inhibition test according to OECD Guideline 209.The 3 h-IC50 was 462 mg/L (Klecka and Landi 1985). In a non-guideline study the effect of m-cresol on activated sludge was determined by a respiration inhibition test. A 49 h-IC50 of 440 mg/L was obtained (Blum and Speece 1991). Nitrification inhibition by m-cresol was measured by a method similar to ISO/DIS 9509 yielding an EC75 of 11.4 mg/L during a 2-4 h incubation period (Tomlinson, Boon, and Trotman, 1966). In a cell multiplication inhibition test conducted with Entosiphon sulcatum a 72 h-NOEC of 31 mg/L was determined (Bringmann, 1978). For Pseudomonas putida a NOEC of 53 mg/L was obtained in a cell multiplication inhibition test during 16 hours of exposure (Bringmann, 1976).

 

p-cresol

The sensitivity of activated sludge to p-cresol was determined by a respiration inhibition test according to OECD Guideline 209. The 2 h-IC50 for respiration inhibition of microorganisms was 440 mg/L (Chan et al. 1999). The effect of p-cresol on activity of Nitrosomonas was measured by a nitrification inhibition method comparable to ISO/DIS 9509. The 24 h-IC50 was 27 mg/L (Blum and Speece 1991). Inhibition of respiration was measured in a test comparable to ISO 8192. A 49 h-IC50 of 260 mg/L was found for respiration rate. Nitrification inhibition by p-cresol was also measured by a similar method yielding an EC75 of 16.5 mg/L during a 2-4 h incubation period (Tomlinson, Boon, and Trotman, 1966). Testing growth inhibition of Tetrahymena pyriformis an EC50 of 157 mg/L was found after a testing period of 48 h (Schultz, 1996).