The analysis of the effluent from the filter of one of these works as taken by Dr. Rideal, was as follows:-

Parts per 100,000.

Total solids, .........

76.8

Mineral matter.

571

Organic loss on ignition. ..

19 7

Chlorine,

728

Nitrogen as nitrates.

372

Nitrites, ...........

strong

Free ammonia,

00124

Albuminoid ammonia,

0044

Oxygen consumed in four hours at 80 F.,

0.324

Total organic nitrogen,1 ..

0066

Dr. Rideal, in a paper which he read before the Sanitary Institute in Decem-ber, 1896 on "The Purification of Swage by Bacteria", stated, in connection with Mr. Cameron's septic process, that radical changes take place in the tanks, produced by the bacteria which are present in the raw sewage, and whose growth is favoured by the absence of light, air, and comparative absence of movement. He summarized the results of a series of experimental analyses, which he had carried out. as follows: -

"(1) A marked increase in the total solids in solution or fine suspension.

(2) A redaction of about 33 per cent of the organic matter as measured by the oxygen consumed.

(3) An increase of about 33 per cent in the free ammonia.

(4) A reduction of about 54 per cent in the organic or albuminoid ammonia, or 50 per cent of the organic nitrogen.

(5) A slight production of oxidized nitrogen, and a disappearance of a small amount of the total nitrogen."

These changes are effected by means of the bacteria, enzymes, or spontaneous chemical decomposition in the, tank, and Dr. Rideal was of opinion that the septic tank effected as much purification as an average chemical precipitation process, or as slow upward filtration, and "that the solid feces and other matter in suspension pass into solution in the septic tank".

1 The Rivers Pollution commissioners allow up to 0.3 organic nitrogen in an effluent passed into a river.

The effluent from the tank is arranged to flow in a thin stream over the edge of an "aerator", or trough, into a receptacle communicating with the filters. At the aerator", the effluent, which emerges free from dissolved oxygen, takes up atmospheric oxygen amounting to 0.56 c.c per litre, or about 10 per cent of the quantity theoretically possible, before passing into the filter-beds. This preliminary aeration is a new departure, and worthy of every consideration, as it obviously is an important aid in supplying the necessary oxygen for the successful working of the filters.

The construction of the filter-beds and the material used in them do not call for any special remarks, as they have been constructed on well-known lines, and follow the conditions for successful working which have been established by the Massachusetts experiments, and more recently in this country by the London County Council, under Mr. Dibden's advice, at Crossness. In the latter case, the removal of 75 per cent of the oxidizable organic matters in solution in the chemically-treated London sewage was effected by filtering one million gallons of effluent through a coke-breeze filter of one acre per day; 50,000 gallons of sewage require 200 cubic yards per day, and this is approximately the filtering volume available at Exeter, excluding a reserve filter-bed of about one-quarter the above volume.

The effect of the filtration is to again considerably reduce the organic matter, as well as a large proportion of the free ammonia, and to produce a large amount of oxidized nitrogen in the form of nitrate. The following figures show the percentage of purification effected in the two series, calculated on the raw sewage and on the tank-effluents: -

(1) Purification due io filters -

Oxygen consumed, 73 per cent; alhuminoid ammonia, 60 per cent.

(2) Purification due to septic tank and filters -

Oxygen consumed, 82 per cent; alhuminoid ammonia, 77 per cent.

In other words, the process is almost identical in its results with that of the combined chemical treatment and filters as used at Crossness, the septic tank effecting 29 per cent purification as against the Rivers Pollution Commissioners' mean result of 28.4 per cent removal of organic carbon by all the best-known chemical methods, and the filters yielding 73 per cent purification on the tank-effluent as compared with 75 per cent purification obtained by Mr. Dibden in the same way on a chemically-treated effluent.

"A filtrate in an active bacterial state can, it is said, be discharged into a river or water-course without danger, and our present knowledge certainly warrants some modification in the standard of purity of such discharges. The standard of 03 part per 100,000 of organic nitrogen some time insisted upon was never legalized, and has seldom been lived up to, even on well-managed sewage-farms, at all seasons of the year, and it is not adopted by the Thames Conservancy, which at present may be regarded as the most exacting authority in England. Adenev has suggested a simple standard which conforms to modern requirements, and which might with advantage be adopted by authorities. It is as follows: - The limit of impurity to be allowed to a given water should be such that, when a given volume of it is mixed with a given volume of fully aerated water, and the mixture kept in a little out of contact with air for a sufficient length of time, a decided oxidation of the ammonia originally present in the mixture into nitrous or nitric acid shall be indicated. If some such standard as this were rally adopted, it would tend to greater uniformity of results, and give a broader basis for judging as to the merits or demerits of any system of sewage-purification than we have at present It would further show that it is possible to produce satisfactory effluents without the use of land, and would permit of the Local Government Board sanctioning schemes which would not only be more econmical to execute, but be of a more satisfactory character than many sewage-work- of the present time." These are Mr. Cameron's words.

In the author's opinion, there is a great future before the "Septic" system, or some system embodying the same principles, such as the Bacterial or Aerobic methods of purification advocated by Col. Walter M. Ducat, (late) R.E., and Professor W. J. Dibdin, F.I.C., etc, which are practically on the same lines of allowing natural forces to act on the sewage by oxidation. The filter in each case is so constructed that the effluent from the sewage may be treated with as much air as possible.