This filter consists of two large wooden tanks, one of which is placed above the other.

The upper tank B contains a layer of gravel and a layer of sand separated by a piece of coarse table cloth f, folded in four thicknesses. The lower tank D is almost entirely filled with charcoal. The water is first admitted into the reservoir E from the main through the supply pipe H, the flow being regulated by the float t. The cock G being open, the water from the tank E flows into the conical perforated vessel Y, from which it passes into the tank B in the form of spray.

Fig. 15.   The Billich Filter

Fig. 15. - The Billich Filter.

The object of delivering the water into these tanks in the form of spray is to avoid making holes in the layer of sand by the flow of a continuous stream of water. The water, having filtered through the sand and gravel, which arrest any solid impurities, collects in the chamber formed by the perforated metal plate d, whence it flows through the pipe b to a receiving tank 0, The supply in this tank is automatically regulated by the float u. From the tank 0 the water passes to the charcoal tank D, through the pipe Q, and is then discharged through the discharge pipe i, from the chamber formed by the perforated metal plate h.

In order to cleanse this filter, shut off the supply of water by closing the cock G, and also shut off the discharge by closing the cock b. Then open the cocks L and I. A stream of water will now enter the lower chamber d of the filter and will force its way up through the gravel and charcoal, carrying the retained solid impurities to the surface. These are afterward discharged through the pipe I into the sink J.

This cleansing operation would be more effective if the water, instead of coming from the reservoir, would directly enter from the main service pipe under pressure, thereby agitating the filtering material and cleansing it so much more. Never mind about the sand and charcoal getting mixed; sand alone in the upper filter would be sufficient. The lower filter should also have an arrangement for a reverse current; this would be an improvement. This whole filtering arrangement is a very practical device, and adapted even for a large water supply, when charcoal filtering is to be adopted. The cleansing or rinsing operation by the reverse current should be applied every day; the charcoal must be removed frequently, whenever it ceases to do its work, which should be ascertained by testing the filtered water in regard to its purity. In general, experiments with fresh charcoal filters prove that the removal of pollution and the retention of bacteria diminish with every day that the filter is in use. The results show that filters when first used successfully, accomplish the purification of water, but if the cleansing of the filter is neglected, it will rapidly become clogged with colonies of growth and actually contaminate, instead of purifying the water. For instance, unfiltered water contains thirty-six colonies of growth, while the filtered water, from a neglected filter, shows the presence of colonies to the number of 117,000 after 17 days. (Report of Dr. G. T. Swarts, to the Rhode Island Medical Society, 1887.)