Messrs. Dibdin and Thudiehum give their solution of the problem as follows:

' In the recognition of the fact that all processes of sewage-purification must be made subservient to the requirements of the various micro-organisms, lies the

-"rank sewage can .. be reenderd as chemically pure as a high class drinking-water", is, to say the least, curious.

Reference to Dr, Houston's remarks in Tables XXXVI. and XXXVII. shows that he cannot speak quite so highly of the purification; "the high-class drinking-water" of Col Ducat is according to Dr. Houston, only "practically" odourless, and "practically" free from cloudy appearance, and contains suspended matter, although ■ very little ".

1 Journal of the Sanitary institute. Vol XVIII., part IV

Table XXXVL Chemical Analysis Of The "Effluent" From Colonel Ducat's Aerobic Cultivation Filter

Results expressed as Parts per 100,000. To obtain grains per gallon, multiply by 0'7.

Date.

Total Solids

Ammonia

Albuminoid Ammonia.

Oxygen abeorhed from Permanganate in :-

Oxidized Nitrogen.

Remarks

1 hr. at 23" C.

5 hrs. at 23" C.

Collected,

3/8/97.

The effluent was remarkably clear and transparent, and entirely free from objectionable odour.

136

0.95

008

056

0.64

2. 28

The cloudy appearance usually associated with Sewage effluents was here practically absent.

Examined,

4/8/97.

The suspended matter appeared to be very small in amount.

Table XXXVII. Comparison Between The Crude Sewage, And The Effluent From Colonel Ducat's Aerobic Cultivation Filter

Samples Collected

3/8/97, Examined,

4/8/97

Ammonia.

Oxygen aboardd from permanganate.

Remarks.

free

Ammonia.

Albuminoid Ammonia.

sum of the two

Ammonias.

In 1 hr at

23' c.

In 5 hrs. at

23' c.

Crude Sewage,...

12. 00

7 60

19.60

2900

59.00

Very turbid, full of solid matters, having an extremely offensive odour.

Effluent, .....

095

0.08

1.03

0.56

0.64

Parctically odourless, clear and transparent. very little suspended matter visible,

Difference per cent,

92

94. 7

98

98.9

Difference per cent

whole secret of success. All sterilizing or antiseptic agents must be strictly excluded; a condition of neutrality or slight alkalinity must be maintained; oxygen. anvhow in the final Stages, must be freely supplied; the temperature must be kept above the freezing-point of water; and the amount of food must be proportioned to the powers of the organisms."

There are two stages in the purification of sewage - one in which the or-ganisms break down and liquefy solid organic matter, and the second, in which the organisms deal with the sewage when in solution (the effluent). The ganisms may again be divided into two classes, (a) those which do their work in the presence of air, and (b) those that thrive in its absence. The latter are those which are at work under the "Septic" Tank system already described, and the others are those which are doing their work in the thoroughly-aerated filter-beds at Sutton near London, where simple filter-beds, thoroughly aerated, are said to be successfully purifying a sewage-effluent.

Mr. Dibdin arrived at these conclusions after a series of exhaustive experiments on London sewage, carried out by him under the direction of the London County Council at the Metropolitan Northern Outfall Works. In March, 1895, Mr. Dibdin presented a voluminous report on the subject, which is given in The surceyon. Vo1. VIII., No. 198, and contains a vast amount of most useful information upon this important and disputable subject. For further information, the reader may consult the book on the subject recently published by Mr. Dibdin.