The water overflows the edges of the troughs which are laid as nearly horizontal as possible. The covers prevent the dirty water from clogging the troughs during washing, and also aid in regulating the water distribution. The height of the main pipes above the sand surface regulates the proportion of the dirty water which can be removed by the pump. Several inches are left for the purpose of "re-seeding" the filter because, as has already been stated, clean sand alone is incapable of properly filtering water, a certain amount of bacterial or equivalent deposit being necessary to start the purification and therefore to render possible the continuous operation contemplated by this plant. The energy required to propel the scrapers in this filter is very small, and the economy of operation is therefore correspondingly great. The knives being adjustable, may not only be regulated to remove the exact amount of surface sand desired to produce the best results, but at the same time they level off the bed in the most perfect manner. No sand is lost, however fine it be, because just sufficient time may be left between stirring up the deposit and pumping out to permit of the resettlement of the sand but not of the lighter silt and other undesirable matter.

Automatic washers have been invented, as stated, for washing the sand in place. But those with which the writer is familiar operate to agitate and wash the deposit in such a manner as to involve the loss of some of the valuable sand of the filter, and they do not provide any means of automatically reseeding the filter, which must therefore be subsequently done by hand or by running to waste the filtrate for a number of days to permit of a reformation of the required amount of bacterial gelatinous deposit to restore effectual working by natural process.

Portable Slow Sand Filters. Figs. 643 to 646 represent a portable "sprinkling" filter for house use designed by the writer.

The sand is held in one or more sections of earthenware pipe and the raw water enters a sprinkler through four small pipes, at the top, and thence discharges by means of a sprinkling trough over the surface of the sand in numerous very small streams, the amount in aggregate being equivalent to one drop per second for every square inch of surface of the filter. Air enters the space between the bottom of the sand vessel and the pure water receptacle forming the base of the apparatus, and rises through the sand as the water descends. A tight cover connected with a heated ventilating flue produces a forced draught, and an effective aeration.

The result of this slow percolation of the water through the sand with the accompanying constant and abundant aeration is to cultivate aerobic bacteria in sufficient force and activity to completely dispose of all organic matter, including all harmful or pathogenic germs, and their spores, in the raw water, and yield an effluent of the utmost possible purity. The bacteria themselves develop in numbers just sufficient to consume the food supplied by the water, and live as long as this food or organic matter continues. An increase or diminution of organic impurity in the raw water, or the quantity of water filtered, results in a corresponding change in the number of these friendly bacteria, and the surface of the sand, therefore, is never clogged so long as the impurities deposited on it are organic. Inorganic matter is easily removed by scraping at intervals of considerable length, depending upon the nature of the water to be filtered.

Fig. 643. Section of Writer's Pot table Filter.

Fig. 643. Section of Writer's Pot table Filter.

The efficiency of a filter working upon this principle is very largely dependent upon the equal distribution of the raw water over the sand. This distribution is effected in the filter under consideration by means of perforated troughs connected together and designed as shown in the drawings, and fed from the water supply pipe through one or more small supply pipes connected directly or indirectly with a regulating stop-cock. The main supply pipe is controlled in an auxiliary float cistern connected with a pure water collecting cistern. These small feed pipes are made large enough to ensure against any possible stoppage by sediment, and to furnish the maximum amount of water required at any time of the largest demand.

Fig. 644. Distributing Trough.

Fig. 644. Distributing Trough.

Fig. 645. Another Form of the Distribution.

Fig. 645. Another Form of the Distribution.

In hotels, the collecting cistern if installed for drinking water supply, should be fitted up with galvanized iron coils from the refrigerating plant, in which case it would usually be better to have a separate cistern for the regulating float.

Such a plant as this ensures the utmost possible purity of drinking water, uncontaminated by ice, which should never be used for refrigerating unless it is made of filtered water, or contained in a separate vessel, whereby the drinking water is cooled without the ice coming in direct contact with it. The illustration shows such a plant built by the writer this summer in the Charlesgate Hotel, Boston.

Fig. 646. Another Form of Distributor.

Fig. 646. Another Form of Distributor.

Fig. 647. Detail of Distributor.

Fig. 647. Detail of Distributor.

The distribution of the water over the sand is effected as follows: A ring of concrete, reinforced by iron wires, is moulded on the inside of the hub shoulder of the pipe forming a water channel all around nearly as deep as the height of the hub. The water suppy-pipe, with its float-valve discharges into this channel, and keeps its water level up to that of the four little outlet pipes moulded into the concrete ring, a little distance above its bottom. This forms a water seal for the enamelled iron or copper ventilating cap of the filter, as shown, and prevents the draught of the ventilating flue from being weakened by the entrance of any air which has not passed through the sand from bottom to top.

Four small pipes, whose united capacity is equal to that of the main supply pipe, lead the water from the water channel, through the concrete ring, into the "distributor" proper. This distributor consists of a number of perforated interconnected distributing troughs, into which the four small pipes discharge. The troughs are V-shaped in section, set with the apex down, and the perforations are evenly distributed along their bottoms. Small brass plungers having ends fit into these holes, and not only regulate and equalize the water distribution through them, but also conduct the water from each hole directly downwards to the sand. The plungers are fastened to a frame, and arranged upon it in such a manner and position that all may be raised or lowered together and equally by means of thumb-screws set at opposite corners of the frame. The holes in the troughs are oblong in shape, measuring about 1-10 by 1-4 of an inch. The plungers are of the same dimensions at the top, but reduced at the lower end to 1-10 of an inch. Their lower ends extend through, and a little below, the bottom of the holes leaving, always, at least one-half of the holes open when the amount of water required is large. When less water is required through each hole, the frame is lowered by the thumb-screws and the bevel on the plungers gradually reduces the capacity of the holes until the proper size is attained.

Should the holes become more or less clogged by sediment they may be easily and quickly cleared by simply screwing down the frame until the plungers completely fill the holes and force out the obstructions. They are then raised again to their proper operating position.

The V-shape of the troughs provides perfect distribution without obstructing the upward movement of the ventilating air through the sand surface. The distributor is stamped out of sheet copper, or of sheet iron afterwards porcelain enamelled.

During the hours of most rapid use the float in the collecting cistern, or its auxiliary float tank will be lowered so as to open the raw water supply valve full bore, and water will then enter the distributor troughs at its maximum rapidity, and rise in the troughs until a water head is developed in them sufficiently high to force this greater quantity of water through the perforations into the filters.

The entire operation thus becomes absolutely automatic, and a sufficient number of bacteria live in the filter between maximum supplies to take care of all organic impurity when it comes. The water dropping from each plunger point distributes itself in the sand in an inverted cone-shaped area - so that at a distance below the sand, inversely proportional to the number of points, the cones unite. It is therefore important that the points should be as near together as possible to produce the maximum of efficiency.

Inorganic impurities in the raw water, deposited upon the sand from the points, for a long time, really aid the working of the filter by forming underneath each point a small saucer-shaped auxiliary distributor which may gradually be converted into an inverted cone or stalagmite. Therefore the sand surface may, with certain kinds of raw water, as a fact, never have to be scraped or*cleaned at all, or, if scraped at very long intervals, the labor involved is so very slight as to be practically negligible.