MICROBIOLOGICAL ASSAY 



133 



He summarises the importance of oxidation-reduction potential measurements in 

 sewage plant control as follows : — 



(1) Detection of deposits and growths in sewer and pumping systems leading 



to septic conditions and sulphide production. 



(2) Detection of industrial wastes and toxic conditions. 



(3) Establishment of proper sludge pumping schedules. 



(4) Detection of stagnation in primary tanlcs. 



(5) Determination of internal conditions in filters and activated sludge 



aeration tanks. 



(6) Detection of excessive sludge blanket in setthng tanks. 



(7) Determination of conditions in sludge digestion tanks. 



(8) Examination of influents and effluents. 



High Ef, 



Fluid Sewa ge 



S edimentatio n 

 Tank 



Fig. 37 

 Oxidation-reduction potentials in sewage sedimentation tank 

 The oxidation-reduction potentials developed in sewage and activated sludge 

 passing through septic tanks have been followed. When subjected to sHght aeration 

 the Eb of sewage dropped slightly at first and then rose. When not aerated the 

 potential dropped from +0-28 to — 0-1 volts in 10 hours. When aerated the Eh again 

 rose, but only to -f-0-24 v, ; after each further anaerobic period in the septic tank 

 aeration raised the potential less. Activated sludge in a septic tank developed a 

 potential of volt and after agitation with nitrogen this fell to — 0-3 v. (Moore and 

 Ruchoft, 1943). 



The potentials in sea-water and their relation to the decomposition of organic 

 matter particularly in different depths of water have been studied by Zobell and 

 by Cooper. (1937.) 



MICROBIOLOGICAL ASSAY 



It will have become evident from earlier sections that the metaboHc requirements 

 of bacteria are frequently similar to those of higher organisms. Hence in the assay of 

 vitamins it is less common to use large, expensive and difficultly controlled animals 

 for many vitamin determinations. Instead of having to rear animals under standard 

 conditions and plan supplies for months ahead it is now possible in many cases to 

 carry out vitamin assays on bacterial cultures. Results are obtained much more 

 rapidly and imiformly, and so are the statistical requirements of adequate numbers in 



