1 At Swinton, near Manchester, this rate is about 680 gallons of effluent per 24 hours, per square yard of surface of filter.

The following is an analysis of this filtered effluent, made by the eminent chemist, Sir Henry E. Roscoe: -

Table XXXI. Analysis Of Sewage-Effluent After Passing Through A Polarite Filter-Bed

Grs. per Gallon.

Parts per 100,000

Chlorine, ... ....

4-480

6.400

Free ammonia, ......

0.284

0.407

Albuminoid ammonia, ... ....

0.018

0.022

Oxygen absorbed from permanganate in 4 hours,

0.228

0.325

The results of this system of sewage-purification, as carried out at Acton and elsewhere, are said to be highly satisfactory and economical in working. It appears to be essential with these filters, as with all others, that periods of rest must be given to the filters to ensure satisfactory results.

Fig 427  Section of Part of a Polarite Sewage filter.

Fig 427 -Section of Part of a Polarite Sewage filter.

Lowcock's System of Sewage-Filtration, as designed by Mr. Sidney R. Low cock, A.M.I.C.E. is now carried on by the Sewage and Effluent-water Filtration Company. The object of the system is the rapid removal of the dissolved oxygen in a filter to assist nitrification. The matters in suspension are precipitated from the Mirage before its application to the filter, to prevent, as far as possible, the clogging of the surface, The sewage is then run evenly on to the surface of the filter by means of sunk channels. The top layer of the filter is of sand, which retains any matters left in suspension, and only permits the liquid to pass slowly down into the body of the filter. After passing through the sand, liquid travels somewhat faster in thin films over the grains of the coarser material below, and thus presents an enormous surface to the purifying organisms and to the air contained in the interstitial spaces.

The air is supplied continuously into the body of the filter by a blower, which discharges it through perforated pipes into a specially porous layer in the upper part (see Fig. 428), from which it is distributed through the whole filter, and passes down and out with the purified liquid through the uuder-drains. The result of this is that the aeration, which is the all-important feature in filtration, is rendered entirely independent of the surface, the filter is worked continuously, and the depth can be increased and the necessary area thus reduced At the same time, the ample supply of air prevents the deoxidizing of the nitrates already formed, a process which, in the absence of air and the presence of organic matter, proceeds for more rapidly than the nitrification process. It is estimated that 1 horse-power will be sufficient to supply the air necessary for the purification of a million gallons of sewage.

Table XXXI Analysis Of Sewage Effluent After Passi 4005

9 int. Hand. 41 ins. Pea Gravel 4 int Bean Gravel. 12 ins Pebbles with Air-pipes.

2 ft. 6 int. Pea Gravel.

A int. Bean Gravel. 12 ins. Pebbles with efflu-ent Drains.

Fig 428. - Lowcock's towage-filter: Section of one Division of Filter-bed. Scale 1/6 In. to a foot.

Frost does not affect the operation of the filter, - except in a very small degree by lowering the temperature and thus reducing the action of the bacteria,

- as the surface is always covered with a sufficient depth of sewage to prevent it being frozen, and the sewage can pass under any ice formed, and down through the filter without hindrance.

The surface is divided up into small areas by divisions extending a short distance below the top layer of sand, as shown in Fig. 428. The liquid to In-filtered can thus be diverted at will from any of these spaces by shutting down the sluices in the divisions, so as to allow of the surface of any section or sections being cleaned, without interfering with the working of the lower part of tin-filter. Of course the more efficient the preliminary precipitation, the lees frequently this cleansing has to be performed, but its neglect does not impair tin-purity of the effluent. If the surface is allowed to become clogged the quantity passed is reduced, but the time occupied in passing is increased, and the purity of the resulting effluent is increased also.

It is claimed for this filter that it absolutely purifies the effluent and renders it sterile. The following table of analyses represents the result of the working at Wolverhampton. All the analyses were taken from an average of 13 hours' flow, the liquid to be analysed being collected continuously for this period; the collection of the filtrate was started six hours after that of the tank effluent.

Table XXXIL Analyses Of Tank And Filter Effluents: Lowcock's System

Date.

Tank Effluent.

Filter Effluent.

REDUCTION PERCENT

Parts per 100,000.

Grains per

Parts per 100,000

Grains per Gallon.

Free

Ammonia.

Albuminoid

Ammonia.

Nitrates and

Nitrates

Oxygen consumed.

Chlorine.

Amnionia

Albuminoid Ammonia.

Nitrites

Oxygen consumed.

Chlorine

Ammonia

Albuminoid

Ammonia

Oxygen consumed

1805.

July 18.

60

.6

traces.

1.9

24.5

1 43

.081

2.56

•6

24.4

75.6

86 5

68

August 3.

6 0

.7

...

4.6

22.3

1.2

.084

2.72

1.4

22.4

80

88

70

October 8.

40

.35

traces.

1.7

140

1.20

.07

2.68

•40

168

70

80

77

Sand and gravel alone do not make very efficient sewage-effluent filters; fine cinders or coke should be interposed, as shown in Fig. 429. A filter of this description will successfully filter sewage-effluent at the rate of 500 gallons per diem per square yard of surface-area, with a head of 4 feet 6 inches of fluid upon its surface, but of course it must have periods of rest to revivify it, varying according to the work it has to do and the degree of purity to which it is expected to act. The top surface of sand must occasionally be cleared off to a depth of half an inch, which can then be worked and replaced. Such a filter, with proper treatment, will continue for some years to produce a satisfactory effluent

There can be no doubt that filtration of the effluent of crude sewage, where the solid matters have been deposited or arrested, may be successfully carried out. and indeed it seems almost essential to the process of sewage-purification that, whatever chemical or other process is resorted to, the effluent must undergo some sort of filtration before it can be raised to a proper standard of purification. Not only must the solid matter be arrested before passing on to the filter-beds, but also the floeeulent particles, which otherwise will spread themselves over the surface of the filter, and in a short time stop the passage of the liquid into the filter and thus render it inoperative.

Table XXXIL Analyses Of Tank And Filter Effluents  4006

12 ins Fine sand

6 ins. Fine cinders, or coke Breeze.

6 ins. Fine sand.

3 ins. Pea Gravel.

6 ins ordinary Gravel, containing drain-pipes ventilated to surface of filter.

Fig 429 - Section of Sewage filter composed of coke and Sand.