Report No DWI0663

Jun 1986


Using the results of tests such as the inhibition of respiration of activated sludge (SCA 1982) to predict the effects of a chemical in the environment, eg on the activated sludge process, can lead to wrong conclusions. The principal reason for this is the greater effect some chemicals have on growth than on metabolic activity, such as oxygen uptake, of preformed cells. Since it is not possible to predict which chemicals fall into this category, it is necessary to carry out a test to ascertain the effect of chemicals on growth. (There are other reasons for predictions of environmental behaviour not being able to be made accurately from respiration tests t adsorption onto solids, acclimatisation leading to tolerance and/or biodegradation. For such chemicals growth-inhibition tests are not satisfactory and only simulation tests will decide on their true behaviour.)

The Standing Committee of Analysts (UK) has devised a test to assess inhibition of growth, based on that described by Alsop et al (1980). The original method, in which the turbidity of a medium inoculated with settled sewage was measured, gave inconsistencies and was not reproducible because of the variable nature of sewage from day to day and source to source. The present method greatly reduces this variability by using an inoculum containing a more constant number of cells which are in the logarithmic phase of growth and which contains less inert colloidal matter. This inoculum was achieved by overnight incubation of portions of medium containing various amounts of settled sewage and selecting that culture which gave a pre-determined optical density at the end of 16 h incubation.

A limited ring test of the UK method, which is soon to be published, has shown that the reproducibility is of the same order as that of the inhibition to respiration method.

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