STREAM POLLUTION 
431 
In practical tests the writer has found that wastes taken directly from their sources in the 
plant, i. e., not from sewers or flumes, differ widely in toxicity to aquatic life, at different plants 
using the same general process. Several factors contributed to these differences. For example, 
the process of tanning hides with chromium compounds is basically the same in all establishments 
using this process, but as is well known the plant chemists in the interests of economy and efficiency 
frequently make minor or at times even drastic changes in the process, with, of course, resultant 
changes in the effluents. Variations in raw material, emergencies in plant operation, and many 
other conditions also call for changes in procedure which result in changes in effluent composition 
and concentration. There is, therefore, no standard hide vat liquor waste or pickle liquor or 
winery effluent, even when taken at the source, which may be used with either fairness or accuracy 
in the estimation of the probable hazards of another plant engaged in the same work. 
Besides, as the waste moves away from its source through conduits and flumes, these differences 
become greater owing to the addition of cooling water, wash waters, and other effluents from differ- 
ent parts of the plant; so that the final mixture as poured into the stream varies not only in concen- 
tration, but also in composition as the result of the sequence in which the different effluents were 
mixed. 
However, from the studies of various wastes before mixing, the general changes which these 
effluents will produce in the stream complex and the types of their actions on aquatic life can be 
ascertained with considerable accuracy. In table 14 the usual effects of 29 common industrial 
wastes and of municipal sewage on the aquatic environment and the critical features of these actions 
as shown by a large series of field studies have been pointed out. 
