INTERMITTENT CONTINUOUS FILTRATION SYSTEM.*

BY T. AIRD MURRAY, C.E.

The object of this paper is to deal as concisely as possible with the leading features appertaining to a complete sewage purification plant for a small town of about 2,000 inhabitants. The method described is that known as the "Intermittent Continuous Filtration System," accompanied by preparatory liquefying tanks.

In general and in detail it is up-to-date, in accordance with the practice in Great Britain, and is of such a nature that it would receive the sanction of the British Local Government Board, which must always be obtained before money can be loaned for such purposes by any local sanitary authority in the Old Country.

A water supply is assumed at 60 gallons per head per day of twenty-four hours. This gives a dry weather flow of sewage of 120,000 gallons per day to be dealt with. The system as described will be capable, however, of taking five times the dry weather flow. Any surplus over this amount may be treated as storm water on land or flow direct into effluent channel.

Such a system as I shall now describe more in detail is planned with every consideration of economy to meet the requirements of localities not over-wealthy, and yet produce an effluent of a harmless nature.

The main characteristics of the system are as follows:

(a) A Screening Chamber.

(b) A Storm Overflow.

(c) Duplicate Liquefying Tanks.

(d) A Dosing Chamber.

(e) Three Continuous Sprinkler Filters.

(f) Effluent Drain.

SCREENING CHAMBER.

The sewage first enters a screening chamber 10 feet x 6 feet x 8 feet deep. Here a wrought iron screen of 3-8 inch mesh is provided to keep back heavy solids. There is no fixed size for this tank, but it is desirable that it be comparatively small, the main object being that the force of the incoming sewage may produce a boiling or swirling action, tending to break up solids by

* Read at meeting of Ontario Medical Association, Hamilton, May, 1908.

disintegration as far as possible; while the screen will keep back such obstacles as cans, scrubbing brushes, etc., the usual accompaniment of domestic sewage.

STORM OVERFLOW.

This apparatus is arranged between the above chamber and the liquefying tanks. The communicating pipe to the liquefying tanks is of such a diameter and gradient that when running full it will only take five times the dry weather flow, viz., 600,000 gallons per twenty-four hours, or 425 gallons per minute. During heavy storms the surplus passes over a longitudinal weir, level with the top of the above outlet drain, and passes away down the storm overflow pipe of a diameter equal to the difference between the above outlet drain and the main sewer entering the works.

This presents a simple method of dividing what may fairly be termed storm water of such a diluted character that special treatment is unnecessary. However, in cases where this overflow might pollute a drinking water source, it would be wise to provide some form of filter either on land or by means of a coarse filter tank.

LIQUEFYING TANKS.

Presenting the third stage, are sometimes called septic tanks, this being a trade name, which has been applied to a covered-in cesspool. These tanks are arranged in duplicate, to allow of repairs and cleaning when necessary; otherwise they are in use in conjunction.

The joint capacity is made equal to twenty-four hours dry weather flow, the sizes in each case being 27 feet x 15 feet x 8 feet deep (10 feet at inlet end and 6 feet at outlet). The tanks may be either covered or otherwise. In frosty climates it is as well to provide a covering. It should however, be observed that a cover in no way assists the tanks in the duty they have to perform.

The work of the liquefying tanks, as their name proclaims, is to reduce organic solid compounds into their liquid forms by putrefaction. Heavy matter is precipitated by gravity to the tank floor, while light matter, such as grease, floats and forms a surface scum. The tank effluent being in a liquid state, is in a highly suitable form for treatment in nitrifying filters, without the liability of filtering media becoming choked.

In the construction of the tanks the principal object is to pre

vent as far as possible any undue disturbance and present a condition of stagnant quietude.

The sewage enters over a weir in the form of a thin film, the weir being the full breadth of the tank. The sewage, after it passes over the weir, is at once met by a projecting scum board, which dips about 2 feet into the tank. This prevents any surface disturbance of the tanks, while the thin film of sewage finds its way into the body of the tank 2 feet below the surface, presenting practically no disturbing influence to the precipitated matter on the floor. The outlet is arranged on precisely similar lines; the liquid sewage passing over a longitudinal weir with a protecting scum board.

Now, it should here be most emphatically stated that there is very little sewage purification takes place in the above tanks. A prevalent opinion exists that such tanks are sufficient for sewage purification. They, however, accomplish no such desirable an object. A useful liquefying action is certainly accomplished, with the aid of the bacteria, which assist in breaking up solid organic compounds. The effluent sewage, however, on analysis practically contains the same amount of chlorides, free and albumenoid ammonia, without any trace of either nitrates or nitrites, while the oxygen absorbed is practically the same. The effluent, in fact, is putrescent, and should on no account be turned into any natural water course, if the object be to avoid nuisance.

It is necessary to lay stress upon this point. Many towns are persuaded that, if they adopt liquefying tanks, they have solved the sewage problem, whereas they have only reduced a mass of unseemly sewage into a temporary appearance of seemliness, without reducing its dangerous condition as far as the health of the community is concerned.

CONTINUOUS FILTERS AND DOSING TANK.

We now arrive at the principal stage in the sewage purification treatment. "Continuous filters" are so called to distinguish them from "contact filters." In the latter case the sewage was retained in contact with the filtering media in saturation for a given length of time in order to allow the nitrifying organisms a prolonged opportunity of acting upon the organic compounds. By the continuous method no such period of contact is allowed, apart from the sewage liquid percolating in driblets slowly through the media.

The main object is to obtain slow and even percolation and prevent flushing the filters with a weight of sewage. This is done by providing a dosing chamber, by means of which the sewage is

presented to the filters in the form of intermittent doses. On the size and arrangement of this dosing chamber the successful working of the filters entirely depends.

With the older form of contact filters, it was generally determined that strong sewage, representing 30 gallons per capita per day of water supply, required one cubic yard of filtering media for each 168 gallons of sewage per 24 hours. With the continuous filter, however, accompanied with a dosing tank, one cubic yard of filtering media is capable of dealing with 500 gallons of sewage, and first-class results are produced.

As the main expense of sewage works appertains to the filters, their construction and media, it will be readily seen that an enormous saving is effected by the adoption of this more recent method of treatment.

It should be here pointed out that figure data does not always absolutely apply in every particular case. Sewage may vary less or more in its strength and constituents. The information supplied by the analyst is of the utmost importance in determining data, both as to construction and sizes of the various parts of a plant. The presence and character of trades effluents, if any, are also subjects for serious consideration. Let deputations hesitate, and all amateurs ponder, and not conclude because they have seen a sewage system working with satisfaction at the city of A. it will also without modifications be satisfactory for the city of B.

The figures here are, therefore, average in nature, and apply to a comparatively weak domestic sewage representing a water supply of 60 gallons per capita per day, as before stated.

The dosing tank should have a capacity equal to two gallons of sewage per super yard of filtering media, or, in other words, equal to 1-2 inch rainfall at each dose. The discharge is brought about by a simple form of automatic measuring valve. The tank should be made shallow, so as to expose the sewage to as much air as possible and allow area for settlement of any further organic matter in suspension.

Penstocks are fixed at each outlet to the filters, so that any one filter may be out of use on occasion.

There are three separate filters provided, each 50 feet diameter, with 8 feet depth of filtering media. The media should be of any hard, indissoluble character, such as furnace clinker, river bed gravel, etc. The top layer, about 3 feet of 4 inch to 5 inch cubes; the centre layer about 4 feet 1 inch to 2 inch cubes, and a base layer for draining purposes of about 1 foot of 3 inch cubes surrounding radiating tile or floor drains.

It is quite satisfactory to build the outer wall circle of the beds

with large blocks of the filtering media. This should take the form of dry rubble walling, as open as possible to allow free access of air, and air pipes should be provided horizontally throughout the filter body at depths of 18 inches.

To obtain an even discharge over the whole surface of the beds the usual and most efficient method is by means of automatic revolving sprinklers.

The sewage from the dosing chamber passes, by means of iron piping, to the centre of each filter, and then up through an upright standard turbine construction, from whence it delivers into radiating perforated arms, which, revolving, act as spreaders, sprinkling the sewage over the whole surface of the bed. The power for this action is obtained from a hydraulic head by fixing the dosing tanks from 1 foot 6 inches to 2 feet above the level of the spreaders.

This practically completes what would be taken as an up-to-date sewage purification plant, and only an effluent drain has to be provided to discharge the water either to a water course or for purposes of irrigation. In Canada such filters would be the better of some wood covering to keep off severe frost. A system on the above lines was recently adopted at Berlin (Europe), and last year was unaffected during a winter showing 7 degrees below zero Fahr., apart from a covering to the outlet channel to the filters, when the sewage temperature showed a reduction to freezing point.

The cost of a plant of the above size and character for a population of 2,000 would run to about $10,000, viz., $5 per head of population; this apart from outfall sewer and cost of land.

THE TREATMENT OF DIFFUSE SUPPURATIVE PERITONITIS WITHOUT DRAINAGE.*

DR. MACKINNON (Guelph.) Before saying anything in the way of discussing the paper of Dr. Moore on this subject, I wish to express to him my real sympathy as to the result in the four cases he refers to. No surgeon who has operated for septic peritonitis can fail to realize the grave nature of the situation. If the disease be well advanced-acute, general peritonitis of many hours' duration that death will result is the rule, and recovery the exception. This result does not necessarily imply any fault in the operator or in his methods. It will occur, no matter what method of operation -and without any deference to the skill or experience of the operator. I do firmly believe that the only hope for success in these cases is that the operation shall follow the infection as speedily as possible-not even a delay of hours, not to speak of days-and that

Discussion on Dr. C. F. Moore's paper at Ontario Medical Association at Hamilton, May, 1908. (Paper published in August issue.)