DECOMPOSITION IN SEWAGE EFFLUENT. 681 



oxvgen, this being taken as one-fifth of the oxygen consumed \'alue 

 as determined by the five-minute boihng method. 



With these data the following provisional criteria of putres- 

 cibihty were found to hold for the experiments in question : — (1) 

 When the consumed oxygen value was equal to or in excess of 

 dissolved oxygen figure, and there are no nitrates and nitrites 

 present, the sample will putrefy. (2) When the consumed rxygen 

 value is equal to or slightly less than the amount of oxygen in the 

 effluent in the form of nitrates, nitrites and dissolved oxygen, the 

 sample may or may not putrefy. (3) When the consumed oxygen 

 value is less than the oxygen contained in the effluent in the form 

 of nitrates and nitrites, under ordinary conditions the sample will 

 not putrefy. 



It may be noted that these investigators determined putre- 

 faction by the presence of sulphuretted hydrogen or other odom"s 

 as revealed by smell. Further reference to this paper will be made 

 later. 



More recently (1906) the use of methylene blue has been in- 

 troduced by Spitta and Weldert as an indicator of putrefaction. 

 This substance is an extremely sensitive indicator for sulphuretted 

 hydrogen and other reducing bodies, being discolourised at once 

 in the presence of even small traces. The test is made by adding 

 one c.c. of a -1 per cent, solution to theeffluent in a glass-stoppered 

 bottle. The sample is then incubated. The blue colour of the 

 solution remains practically un hanged until the available oxygen 

 used in it is used up and putrefactive conditions arise. At this 

 point, the dye is reduced and discolourised. Phelps and Winslow 

 found^ that, if the incubation temperature were 37^C., a positive 

 result could be obtained in half the time that would be occupied 

 were the samples kept at 20'^'C. 



It must be obvious that all incubator tests suffer from a common 

 disadvantage, viz., the time occupied in the test. It frequently 

 happens that a report on a sewage effluent is required without delay, 

 and it is my object in this communication to indicate how it may 

 be possible to arrive at a conclusion as to the probability or other- 

 wise of decomposition occurring in a given effluent from data which 

 are afforded by ordinary analysis. The first point to decide was 

 what evidence could be accepted as to the existence of putrefaction. 

 Early in the inquiry the Manchester method was followed, samples, 

 however, being incubated at 37°C. No notice was taken of the 

 nitrates or nitrites in the incubated sample. It was soon found, 

 however, that the results were both unsatisfactory and apparently 

 contradictory. Samples, which obviousl}- did not decompose so 

 far as such condition could be determined by appearance and smell, 

 showed a high degi-ee of putrescibility by the oxygen absorbed in 

 three minutes reading. On investigating this point. I found 



\ Journal Infectious Diseases, 1907, Spt. III., pa^c I. 



