Case B. - Somewhat similar to A, but 15 cwts. of milk of lime are added to the swage the precipitation, however, is done in thirty tanks, each tank having a cubic capacity of 50,000 gallons, or l million gallons in all. Four tanks are simultaneously filled, taking 10 minutes, and after 20 minutes of complete rest the top liquid is run off through a floating exit-pipe, from which the sewage panes over a weir in a thin layer, and then downwards through a coke filter to a depth of 2 feet, after which it passes upwards through a similar layer. The coke in these filters is changed every 3 months. The process of drawing off from a tank takes 2 hours, hence the necessity for such a large number of tanks.

Case . C - Here the sewage of 50,000 inhabitant-, amounting to 2 million gallons daily, is treated with 13 cwts. of quick or 1 ton of slaked lime, and 18 cwts, of sulphate of alumina, the latter being applied first and then the lime, and a thorough mixture being made in a tank. The treated sewage flows into eight tanks, arranged in parallel series, each tank being 5 feet 6 inches in depth and having a capacity of 120,000 gallons. The of passage is slow .

owing to the large tank-capacity, amounting to nearly a million gallons. Precipitation is well effected in these tanks, as two out of the eight tanks are emptied daily, and yield 74 tons of wet sludge. The effluent then passes on to a plot of land only 8 acres in extent, and laid out as intermittent filters, underdrained to a depth of about 6 feet The effluent from the land is good, but might he improved if more land were used.

The failure of lime to arrest decomposition is very interesting, as showing how Nature is always trying to work by her own laws, and how difficult it is to act contrary to these laws. More will be said on this point when the question of the "septic" instead of the "antiseptic system of sewage-treatment is dealt with, but meanwhile it may be well to briefly sketch a few more chemical prone, which have been tried from time to time with varying results. Hille's Process consists of the application of the following mixture as a precipitant: - Lime, 100 lbs.; gas-tar, 3 lbs.; chloride of magnesium, 17 lbs. This mixture is made into a paste by adding 180 lbs. of water, and is added to the sewage in the proportion of from 3 lbs. to 10 lbs. per thousand gallons of sewage, according to the character of the sewage and the purity of the effluent desired. It is stated to have no better effect upon the sewage than the ordinary lime treatment The Amines Process is the invention of Dr. Wollheim. and consists of tin-addition of ordinary herring-brine to the lime; this addition is said to destroy all the micro-organisms abounding in the sewage. It is stated that by the action of the lime on certain organic bases belonging to the "Amine" group, a soluble gas is produced which spreads throughout the liquid, and which is antagonistic to the existence and multiplication of the micro-organisms contained in the sewage; and it was found that ordinary herring-brine, a cheap waste product, contained "trimethylamine", the exact chemical which, in contact with lime, produces this gas so fatal to the putrefactive organisms. The proportions used are only 3 grains of herring-brine and 30 to 50 grains of lime per gallon of sewage treated, and Mr. Santo Crimp, who tried this process at Wimbledon, states that the clarification of the sewage was very rapid and complete, and that the sludge produced was inodorous, and did not afterwards become putrid. The effluent in this case was passed over land, but Dr. Klein was of opinion that the effluent was quite sterilized. It is not apparent why this process was abandoned at Wimbledon, or why it has not hitherto Wen largely adopted

Spence's Alumino-ferric Process is also known as the Sulphate of Alumina Process, and is used with or without the addition of lime.1 Messrs. Spence and

1 Alumino-ferric is said to contain the following chemical bases:-

Soluble alumina.

14.00

parts.

Peroxide of iron.

•75

"

Sulphuric acid,

3381

"

Water. .. ...

51-44

"

Total, ...

100.00

parts.

Sons,. in a letter written to The Contract Journal in 1891, thus describe that methods: - "The alumino-ferric is produced in solid cakes measuring 21 inches by 12 inches by 5 inches, and weighing about 56 lbs. each. These are placed either in a stout wire cage, or a cask or box freely perforated with large holes. This is immersed in the conduit or stream of sewage leading to the precipitating tanks. As the cakes dissolve fresh ones are added. This is all the labour required, and need not occupy an ordinary labourer more than a few minutes per day. No machinery or plant of any kind is necessary beyond what we have named We recommend in many cases the use of a small amount of lime along with the altunino-ferrie. As the proportion required depends upon the character of the sewage, a few experiments with an average sample of the sewage will indicate the- proper quantity. We do not recommend the quiescent system of using the tanks, - i.e. the plan of filling up a tank, and then shutting off the flow till its sludge has settled, - but the continuous system, the raw sewage with its proportion of alumino-ferric and lime going in at one end of the series of tanks, and the purified effluent flowing out over the last lip at the other. If there is adequate tank-capacity, complete precipitation will take place daring the passage through the tanks. . . When a cake of alumino-ferric is immersed in a stream of sewage, a remarkable change takes place in the liquid, which is quite evident on comparing a portion of it after passing the cake with a sample taken before. The sewage before passing the cake is dark and muddy, and on standing does not improve. After passing the cake, it is seen to be separated into two portions - a clear, bright, inodorous effluent, and a curdy precipitate which rapidly settles. . . . The alumino-ferric, by the action both of its bases and acids, throws down the suspended matters and destroys the smell."

This process has been fairly successful where the effluent is passed over the land; notably at Wimbledon, where Mr. Santo Crimp says of it, "nearly every known re-agent has been tried at Wimbledon, some in solution, others in suspension, and some in the form of filters of every conceivable shape, but up to the present (189(0 the system now in operation has been found to best meet all the exigencies of the case".1