354 Messrs Purvis and Black, Oxygen content of the rwer Ca 
The quality of the river or stream as the receiver of sewage 
or an effluent is a very important factor to be considered in the 
disposal of sewage or sewage effluents; and the Commissioners 
have classified rivers into groups so that, for example: 
Very clean take up 0-1 gram of dissolved oxygen in 100,000 parts of the 
stream in 5 days. 
Clean 0:2 ,, of dissolved oxygen in 100,000 parts of the 
stream in 5 days. 
Fairly clean ,, 0-3 ,, of dissolved oxygen in 100,000 parts of the 
stream in 5 days. 
Doubtful iy 0-5 ,, of dissolved oxygen in 100,000 parts of the | 
stream in 5 days. 
Bad a 1:0 ,, of dissolved oxygen in 100,000 parts of the 
stream in 5 days. 
They consider that, under ordinary conditions, the average | 
quality of the diluting water, for the purpose of arriving at) 
a standard, should be represented by 0:2, that is, by a “clean | 
TUNE Tee 
But the most important factor is the degree of dilution | 
afforded by a river receiving the discharge; because there are 
numerous instances in which the degree of dilution is sufficient to 
dispose of the sewage by natural agencies without cost or injury 
to the community. a 
Considering, however, the various methods of sewage treat-_ 
ment, as, for example, tank treatment with or without chemical | 
precipitation, or artificially constructed filters or sewage farms, 
the Commissioners recommend that, in those cases where a. 
complete system of sewage disposal is necessary, the sewage 
effluent shall not contain more than 3 grams of suspended matter ~ 
per 100,000, and that, including its suspended matters, it should | 
not take up more than 2 grams of dissolved oxygen in five days » 
at 60° F. (18°3° C.). 
The Report also discusses the cases where, owing to the) 
relatively small volume of the river, a more stringent standard 1s: 
necessary ; and, on the other hand, conditions of dilution which » 
indicate that a relaxation of the normal standard may be allowed. . 
For example, a claim for a relaxed standard may be considered — 
when (1) the particular river water when mixed with sewage or | 
sewage effluent does not take up more than 0'4 gram of dissolved | 
oxygen per 100,000 in five days, and (2) when it can be shown) 
that the river will receive no further pollution until it has- 
recovered itself so far as not to take up in five days an amount 
of dissolved oxygen much in excess of that which it took up before 
receiving the first discharge. 
In view of this Report, the authors have studied the con- 
dition of the sewage effluent poured into the Cam from the 
