A comparison of five rapid direct toxicity assessment methods to determine toxicity of pollutants to activated sludge

Five rapid direct toxicity assessment methods were used in three European partner countries to determine the toxicity of single toxicants, mixed toxicants and real industrial wastes. The final aim was to protect microbial degradation of organic wastes in biological treatment processes and hence enhance the quality of treated effluents to be discharged to the environment. Nitrification inhibition, Respirometry, Adenosine triphosphate luminescence and Enzyme inhibition were tested utilising activated sludge as the testing matrix. The Vibrio fischeri toxicity test was used as a surrogate to compare the various microbial bioassays. The IC50 (toxicant concentration eliciting a 50% inhibitory effect) was determined for a number of pollutants including single toxicants Cd, Cr, Cu, Zn, 3,5-dichlorophenol, toluene and linear alkylbenzenesulphonate (LAS); a standard mixture of metals and LAS; a standard mixture of organics and LAS, and 16 industrial effluents. The V. fischeri bioassay was also chosen in order to assess quality control of toxicant preparation during testing in the different laboratories of the partner countries. Comparisons of sensitivity, cost of implementation, cost per test, relevance, and ease of use were made. The most sensitive bioassays were V. fischeri and Nitrification inhibition, however, this depended in the main on the pollutant and mixtures tested. It is recommended that during assessment of wastewater toxicity a suite of tests be used rather than reliance on one particular test.

Evaluation of nitrification-inhibition measurements

A screening method for nitrification-inhibition determination has been evaluated at two laboratories, one in Sweden and one in Spain. Allyl-thiourea (ATU), methanol, chromium and zinc were used as reference toxicants in combination with different kinds of full-scale sludge and sludge produced in the laboratory. Different results were obtained with different combinations of activated sludge and toxicants. It was found that activated sludge often exposed to small amounts of a variety of toxic substances may build up a general resistance to toxic substances. Specific adaptation to certain substances was also observed. Domestic activated sludge responded in the same way to ATU regardless of whether it originated from Sweden or Spain. Synthetic sludge and domestic sludge exhibited the same inhibition of nitrification when tested with ATU as a toxicant. Synthetic activated sludge from two laboratory-scale plants run in parallel did not exhibit exactly the same sensitivity to certain toxicants. The variation in repeated tests with the synthetic sludges was about the same as for repeated tests with full-scale sludge and for other biological toxicity tests. Limits of detection were calculated to be about 5% for a single sample with three blanks as references at both the laboratories.