Abstract: | The ability to distinguish between true non-biodegradability, and inhibition of biodegradability due to the toxic effects of the compound tested present problems. It results in possible false identification of substances as non-biodegradable.Six chemicals were evaluated in a variety of tests to assess their toxicity to aerobic sludge organisms. The tests employed were: BOD5 and Closed Bottle inhibition tests, inhibition of respiration of activated sludge, growth inhibition of activated sludge, light emission from
(MICROTOX test) and the repetitive die-away test (RDA). Results from these tests were compared with results obtained from a number of ready biodegradability tests using the compounds listed above at anticipated non-toxic and toxic concentrations.No test method evaluated consistently forecast toxicity due to the chemicals tested. The MICROTOX and nitrification inhibition tests were too sensitive. A combination of sludge respiration rate and/or growth tests seem most appropriate. To avoid toxicity to the sludge, biodegradability testing should be made at 10% of the EC50 value. Compounds with an EC50 value greater than 300 mg/l are unlikely to be toxic in ready biodegradability tests. Compounds with an EC50 value of less than 20 mg/l may pose problems necessitating the use of the stringent closed bottle test, or the use of C14 labelled test materials. Compounds with an intermediate EC50 that is between 20–300 mg/l, need to be evaluated at a range of concentrations in biodegradability tests, or may need to be evaluated carefully to define the precise no-effect level. Procedure recommendations to distinguish between the inhibition of biodegradability and inertness of the test substance are made. |