BIOCHEMICAL OXYGEN DEMAND Bol 
is based on the biochemical consumption of oxygen from sodium nitrate 
by a sewage or polluted water during an incubation period of ten days 
at 20° C. A reasonable excess of sodium nitrate does not give a higher 
oxygen demand, as do higher dilutions with aerated water. The oxygen 
absorbed from the air in applying the method to sewages is negligible. 
Reagent. Sodium nitrate solution. Dissolve 26.56 gms. of pure 
sodium nitrate in | liter of distilled water. One c.c. of this solution in 
250 c.c. of sewage represents fifty parts per million of available oxygen. 
The strength of the sodium nitrate solution may be varied to suit con- 
ditions. 
Procedure for Sewages. Ordinarily disregard the initial available 
oxygen as it is very small compared with the total biochemical oxygen 
demand. Add measured amounts of the sodium nitrate solution to the 
sewage in bottles holding approximately 250 c.c’which have been com- 
pletely filled and stoppered. Incubate for ten days at 20° C. A seal 
is not required during incubation. The appearance of a black sediment 
and the development of a putrid odor during incubation indicates that 
too little sodium nitrate has been added. Methylene blue solution in 
proper proportion may be added at the start to serve as an indicator 
during the incubation. Domestic sewage usually varies in its oxygen 
demand from 100 to 300 parts per million, approximately 30 per cent 
of which is used up at 20° C. in the first twenty-four hours. At the 
end of the incubation period determine the residual nitrite and nitrate. 
Determine the nitrate by the aluminium reduction method, followed 
by direct Nesslerization. To convert the nitrogen into oxygen equiv- 
alents, multiply the nitrite nitrogen by 1.7 and the nitrate nitrogen 
by 2.9. The difference between the available oxygen added as sodium 
nitrate and that found as nitrite and nitrate at the end of the incuba- 
tion period is the biochemical oxygen demand. 
Procedure for Tradewastes. Employ the same procedure using larger 
quantities of the sodium nitrate solution. Make the reaction alkaline 
to methyl orange and acid to phenolphthalein. Adjust an acid reaction 
with sodium bicarbonate, and a caustic alkaline reaction with weak 
hydrochloric acid. If the liquid is devoid of sewage bacteria seed it 
with sewage after adjusting the reaction. 
Procedure for Polluted River Waters. Determine the initial available 
oxygen. Unless the river water is badly polluted add ten parts per 
million of sodium nitrate oxygen. Collect carefully to avoid aeration, 
three samples in 250 cc. bottles. To one sample add a definite quan- - 
tity of sodium nitrate solution and incubate. Incubate the other two 
samples for the determination of the residual free oxygen, nitrite and 
