The quantity of river mud most suitable for the determination of the
biochemical oxygen demand ranges within certain limits, largely
according to the amount of oxidizable matter present. For examinations
of river mud prepare a 1 per cent stock suspension in distilled water or
tap water saturated with oxygen and free from nitrate; use in the test a
dilution of this stock suspension equivalent to a concentration of 1 to
10 grams per liter of mud. For examinations of fresh sewage sludge
prepare a 1 per cent stock suspension in a similar manner, but use in
the test a dilution equivalent to only 0.1 to 1.0 gram per liter of wet
material. For examinations of dried sludges, which have undergone more
or less oxidation higher concentrations may be required.
_Procedure._—Place a measured portion of the sample, or the proper
amount of the 1 per cent stock suspension of the sample, in a 300 cc.
narrow-mouth glass-stoppered bottle, and dilute it to the desired
concentration with water saturated with oxygen. Determine the oxygen
content at 20° C. of the waters that are used for dilution. This
determination must be made before the mud or sludge is added because
iron sulfide in the mud or sludge rapidly consumes part of the dissolved
oxygen. Incubate at 20° C. for five days.
Shortly before the determination of the oxygen remaining in solution at
the end of five days rotate the bottle once or twice to mix its contents
and allow sedimentation for about 30 minutes. Siphon the greater part of
the liquid through a narrow-bore siphon into a 150 cc. bottle, which has
been filled with carbon dioxide. Reject the first 25 cc. of the siphoned
liquid and allow a little to overflow at the end of siphoning. Determine
the oxygen content of the solution in the bottle in the usual way (pp.
65–68). Report the oxygen demand in percentage of the dried mud or
sludge.