The relative stability method may be employed to obtain a measure of the
putrescible material in sewages and effluents in terms of oxygen demand.
_Procedure for effluents._—Divide the total available oxygen, including
the oxygen of nitrite and nitrate, by the relative stability expressed
as a decimal.
_Procedure for sewages._—Make one or two dilutions with fully aerated
distilled water of known dissolved oxygen content. Tap water may be
employed if it is free from nitrates. Vary the relative proportions of
sewage and water to be employed to give a relative stability of 50 to
75. Unless seals[1b][2b][52a] are used bring the water as well as the
sewage to the temperature at which the mixtures are to be incubated
before preparing the dilutions. During the manipulation avoid aeration.
Having made the proper dilutions, determine the relative stability of
each.
Calculate the oxygen demand in parts per million by the formula: