For the determination of the biochemical oxygen demand the sodium
nitrate method may be used[60a][60c][60d][52a]. The method 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 grams of pure sodium
nitrate in 1 liter of distilled water. One cc. of this solution in 250
cc. of sewage represents 50 parts per million of available oxygen. The
strength of the sodium nitrate solution may be varied to suit
conditions.
_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 cc. which have been
completely filled and stoppered. Incubate for 10 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 24 hours. At the end of the
incubation period determine the residual nitrite and nitrate. Determine
the nitrate by the aluminium reduction method and direct Nesslerization.
To convert the nitrogen into oxygen equivalents, 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 incubation period is the biochemical
oxygen demand.
_Procedure for industrial wastes._—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 10 parts per
million of sodium nitrate oxygen. Collect carefully, avoiding aeration,
three samples in 250 cc. bottles. To one sample add a definite quantity
of sodium nitrate solution and incubate. Incubate the other two samples
for the determination of the residual free oxygen, nitrite, and nitrate.
If there is free oxygen left, the bottle containing the sodium nitrate
solution may be discarded. If there is no free oxygen determine residual
nitrite and nitrate as directed under the procedure for sewage (p. 72)
and calculate the oxygen demand.