ANALYSIS OF SLUDGE 355 
iccator and weigh. If it is desirable, particularly with certain trade 
wastes, to determine the quantity of soap fat, determine the fats with 
and without the addition of acid. The difference between the amounts 
found by the two determinations is the amount of soap fat present. 
Ferrous Sulphide. The liberation of hydrogen sulphide on adding 
dilute hydrochloric acid to a sludge indicates the presence of ferrous 
sulphide. As ferrous sulphide quickly oxidizes on exposure to air, a 
quantitative determination of this constituent must be made imme- 
diately after collection of the sample. 
Procedure. Heat a definite portion of the sludge with hydrochloride 
acid in a flask. Pass the liberated gas through bromine water or hydro- 
gen peroxide. Determine gravimetrically the sulphate in the oxidizing 
solution, and calculate the equivalent of ferrous sulphide by multiplying 
the weight of barium sulphate by 0.376. 
Biochemical Oxygen Demand. 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 deoxygenating 
matter present. For examinations of river mud prepare a 1 per cent 
stock solution in distilled water or tap water saturated with oxygen 
and free from nitrate; use in the test a dilution of this stock solution 
equivalent to a concentration of 1 to 10 gms. per liter of mud. For 
examinations of fresh sewage sludge prepare a 1 per cent stock solution 
in a similar manner, but use in the test a dilution equivalent to only 0.1 
to 1.0 gm. per liter of wet material. For examinations of dried sludges, 
which have undergone more or less mineralization, higher concentrations 
may be required. 
Procedure. Place a measured portion of the sample, or the proper 
amount of the 1 per cent stock solution of the sample, in a 300 c.c. 
narrow-mouth, glass-stoppered bottle, and dilute it to the desired dilu- 
tion 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 sulphide 
in the mud or sludge rapidly consumes part of the dissolved oxygen. 
Inevbate 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 thirty minutes. Siphon the greater 
part of the liquid through a narrow-bore siphon into a 150 c.c. bottle, 
which has been filled with carbon dioxide. Reject the first 25 c.c. 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 
