View report page

24
IV.—TABULATION OF THE RESULTS OF THE CHEMICAL EXAMINATION:
OF THE CRUDE SEWAGE AND OF THE EFFLUENTS.
previous Reports the figures representing the results of the analyses of the various
liquids have been given for each day during the time which the Reports covered. This was found
to necessitate the insertion of so large a mass of figures that it has been discontinued, and
in the present Report monthly averages only are submitted. The space thus saved has admitted
oi the insertion of other tables, without unduly increasing the mass of figures.
figures representing the results of analytical estimations are given in parts per
100,000, unless otherwise stated.
(To convert parts per 100,000 into grains per gallon, multiply by 0.7; to convert grains per
gallon into parts per 100,000, multiply by
The following explanation of the headings of the columns in the various tables may be
found useful—
Oxygen absorbed from permanganate in four hours at 80° Fahr.
(a) By the crude liquid (total putrescible matter).
The figures in this column represent the amount of oxygen which the crude
liquid absorbed from potassium permanganate in four hours when kept in a constant
temperature chamber at 80° Fahr. The figures are to some extent a measure
of the putrescibility of the liquid, since the oxygen is absorbed by oxidisable
matter dissolved in the liquid and floating in the liquid as minute solid particles
(total putrescible matter). When sewage is discharged into sea-water it absorbs
three times as much of the dissolved oxygen as it absorbs from permanganate, and
it would thus very seriously reduce the aeration of the water. (See Report of the
Royal Commission on Sewage Disposal, vol. ii., Dr. Letts' evidence, p. 479.)
(b) By the clear liquid (dissolved putrescible matter).
These figures are obtained in a similar manner to those in the column,
headed "By the crude liquid." The only difference is that the estimations are made
in the clear liquid produced by passing the turbid liquid through ordinary filter
paper; whereas, in the case of the crude liquid estimations, the liquid is dealt
with in the unfiltered state.
Nitric and nitrous nitrogen.
The figures in these columns represent the amount of nitrogen which was
found to be present in the form of nitrite and of nitrate. These compounds are
produced in the liquid by the oxidising action of nitrifying bacteria in the cokebed.
They indicate, to some extent, the amount of oxidation and purification which
has taken place. Nitrates are fully oxidised products and nitrites are only partially
oxidised products. Both bodies are produced in sewage effluents by the
oxidation of nitrogenous compounds present in the sewage. Their presence and
permanence indicate that purification of the sewage has taken place, since they
cannot exist in impure liquids, as such liquids lead to their deoxidation and destruction.
It has been shown that the presence of nitrates in an effluent may prevent
the putrefaction of a certain amount of organic substances from occurring in
another effluent with which the nitrate effluent has been mixed.
Table I.—Averages of Figures relating to the Treatment of Sewage Effluent in the One-acre
Coke-bed at the Northern Outfall Works.
This table gives the monthly averages of the daily estimations of the amount of oxygen
absorbed from permanganate by the crude liquids (total putrescible matter) and by the clear
liquids (dissolved putrescible matter). The liquids dealt with were—(a) The sewage as it
arrived at the works and before treatment of any kind; (b) the effluent from the chemically
treated and sedimented sewage as it was supplied to the coke-bed; (c) the effluent from the cokebed.
The table also contains the monthly averages of the daily estimations of the quantity
of nitrous and nitric nitrogen in the liquid supplied to the coke-bed and in the effluent from
the bed. It should be noted that the averages of the crude sewage were calculated from the
results of the daily analysis of a sample of sewage which was obtained by mixing together
samples of equal volume collected every hour throughout the twenty-four hours, whereas, the
samples of the "chemical" effluent and of the coke-bed effluent, which were taken for analysis,
corresponded with only a portion of the day's sewage. It will therefore be seen that the figures
for the crude sewage are not strictly comparable with the figures for the "chemical" effluent
and for the coke-bed effluent, although any error likely to arise from this cause is minimised
by the averages extending over long periods.
No estimations were made of the nitrous and nitric nitrogen in the sewage, since nitrogen
in these conditions is more often absent than present, and even when present its quantity is
usually too small to be worthy of consideration.
The records of the analyses of the "chemical" effluent supplied to this bed and of the
coke-bed effluent, extending from May 12th, 1898 (the date of the commencement of the experiments
after the bed had been deepened to 6 feet), to December 31st, 1899, were published on
pp. 25 to 34 of the Third Report on the Bacterial Treatment of Sewage.