through tubing and fittings made of PTFE. Analysis was undertaken by the Warren Spring Laboratory of the Department of Trade and Industry, according to the method described by Bailey and Bedbo rough The results are shown in Table IV. and plotted in Fig. 3. and 4. Table IV. Variation of odour strength of extracted samples with volune of eluted air Volume of air Strength of odour samples passing through (dilutions) sludge before sampling (1/1) Raw sludge Digested sludge 0 154 000 9 900 11.1 53 000 350 22.2 30 600 270 55.6 15 500 190 111 8 200 160 It is clear from these results that there is considerable die-off of odour strength with time, and that, as would be expected, the anaerobic digestion of sludge can reduce the odour potential by at least one order of magnitude. To illustrate the importance of this die-off effect, the results have been re-plotted in Fig. 5. in a cunulative form; that is to say as cumulative percentage of the eventual colour release against volume of air. In the case of the raw sewage sludge, 38% of the ultimate odour was carried in the first odour sample, and 90% of the odour had been extracted by the passage of about 200 1. In the case of the anaerobically digested sludge, the same effect is much more marked; 72% of the ultimate odour was carried by the first sample, and thereafter the strength of the odour fell off very rapidly. There are two possible explanations for this. First, it can be postulated that as it is known that many of the important odorous chemical species are highly volatile, they may be only physically trapped in the sludge, and need little encouragement to transfer to the atmosphere. An alternative explanation concerns the existence of two equilibria. As the vapour/liquid equilibrium is disturbed by the passage of air, the concentration of dissolved compounds in the liquid phase falls, disturbing the ’solid’/liquid equilibrium The kinetics of transfer across this latter phase boundary are much slower than for the liquid/vapour transfer, so that the extraction of odour becomes limited by the rate of diffusion into the liquid phase. Two observations may be cited as evidence for this latter view. First, when sludge is applied to land, there is a rapid tail-off of odour nuisance after spreading. Hie incidence of rain after a dry period is known to result in an increased evolution of odour. Second, in earlier experiments samples of sludge were centrifuged, and the supernatant liquor discarded and replaced by tap water, before being used in the standard odour potential test. Some re-extraction of odour from the samples was rapidly found. In practice, both postulated mechanisms are probably at work, especially if the concept of ’solid/liquid equilibrium’ be extended to