ANALYSIS OF SLUDGE 353 
sample only the surface layer. Samples may be analyzed either sep- 
arately or as composites of careful mixtures. After the sample has 
settled a few minutes roughly drain or siphon the excess water. Allow 
sewage sludge to stand for one hour before draining it free from excess 
water unless it is essential to determine the moisture content of the 
sample originally collected. If sludge cannot be analyzed within 
twenty-four hours it is better not to use air-tight bottles and to add 
small quantities of chloroform to retard decomposition. At the time 
of collection carefully examine mud from the bottom of surface water 
for evidence of sewage pollution and macroscopic and microscopic 
animal and plant organisms. Record the predominant species. Note 
the physical appearance of the material, particularly its color, odor, and 
consistency. Express all analytical results in percentage on a dry basis. 
Specific Gramty. Weigh to the nearest tenth of a gram a wide- 
mouthed flask of 100 to 300 c.c. capacity, according to the quantity of 
material available. Then completely fill the flask with distilled water 
to the brim, and weigh it again. Empty and fill the flask completely 
with fresh sewage sludge or mud. If the material is of such consistency 
that it flows readily, fill the flask to the brim and weigh. The specific 
gravity is equal to the weight of the sludge or mud divided by the 
weight of an equal volume of distilled water. 
If the material does not flow readily, fill the weighed flask as com- 
pletely as possible without exerting pressure during the procedure. 
Weigh and then fill the flask to the brim with distilled water. Let it 
stand for a few minutes, until trapped air has escaped, then add more 
water if necessary and weigh. The specific gravity is equal to the 
weight of the material divided by the weight of the distilled water less 
the weight of the water added. Record the specific gravity only to the 
second decimal place. 
Moisture. Heat approximately 25 gms. of sludge or mud in a 
weighed nickel dish on the water bath until it is fairly dry. Dry the 
residue in an oven at 100° C., cool, and weigh. Repeat to approximate 
constant weight. The loss in weight is moisture. 
Volatile and Fixed Matter. Ignite, in a hood, the residue from the 
determination of moisture until all the carbon has disappeared. Cool 
the residue in a desiccator and weigh it. The residue is the fixed matter. 
The volatile matter is the difference in weight between the original dried 
sludge and the ignited sludge. 
Total Organic Nitrogen. Preparation of Sample. For the deter- 
mination of organic nitrogen and fat dry approximately 50 to 75 gms. 
of the sludge or mud in a porcelain dish first on the water bath and 
