B02 WATER HYGIENE 
nitrate. If there is free oxygen left, the bottle containing the sodium 
nitrate solution may be discarded. If there 1s no free oxygen determine 
residual nitrite and nitrate as duected under the procedure for sewage, 
and caleulate the oxygen demand. (Standard Methods of Water 
Analysis.) 
Examination of Sewage. The chemical examination of sewage and 
sewage effluents is probably more valuable than the bacterial examina- 
tion. The problem involved in controlling a sewage treatment plant 
is quite different than that involved in the control of a water treatment 
plant. As Lederer and Bachmann (1911) have stated the effluent from 
a water treatment plant is subjected to different requirements than 
the effluent from a sewage treatment plant. In the first case the bac- 
teriologist is concerned in the presence of certain types of bacteria and 
their concentration. Little attention is given to the daily variations in 
the chemical content of the effluent. With the sewage effluent, however, 
the question is a different one. It is necessary to watch carefully the 
variations in the chemical content and keep the organic matter reduced 
to a minimum. The question of stream pollution is involved if the 
effluent is to be emptied into a stream. It must not be putrescible and 
thus cause the nuisances which result from too heavily polluting a 
stream. Lederer and Bachmann have stated some of the fallacies 
connected with the bacterial control of sewage plants. They have 
stated that “to try to obtain reliable information on the number of 
bacteria in effluents from various devices in a large plant would require 
a staff of trained men entirely out of proportion to the value of the 
results obtained.” The difficulties in securing careful sampling and the 
necessity of immediate plating since bacteria in sewage die very rapidly 
when stored at icebox temperatures, are also mentioned. For these 
reasons and others more attention is given to the determination of the 
organic matter content and the biochemical oxygen demand according 
to the methods which have been given before. 
The deposits from rivers and sewage sludges are examined in the 
following manner according to Standard Methods for the Examination 
of Water and Sewage, 1917. 
ANALYSIS OF SEWAGE SLUDGE AND Mup Deposits 
Collection of the Sample. Collect a representative sample. In 
general, more than one sample should be taken from a spot and a large 
number of samples should be collected rather than a few large samples. 
If the surface layer is darker and a lower layer consists of pure clay 
