The conditions which control the efficiency of mechanical filters have been discussed in connection with coagulation. With sufficient coagulant the removal of turbidity or mud is complete. Color also can be removed with these filters. The bacterial efficiencies secured with them have been discussed at length in connection with the Pittsburg experiments.
With careful coagulation and manipulation it is possible to get 98 per cent bacterial efficiency without difficulty. The results are somewhat irregular, for reasons not as yet fully understood. On some occasions higher bacterial efficiencies are secured with smaller quantities of coagulant, while at other times the efficiencies are less without apparent reason. There seems to be a limit to the bacterial efficiency which can be secured with any amount of sulphate of alumina and rapid filtration, and it is doubtful if a plant could be operated to regularly secure as high a bacterial efficiency as 99 per cent with any amount of sulphate of alumina.
PRESSURE FILTERS.
Pressure mechanical filters are constructed in entirely closed receptacles, through which the water is forced under pressure and not by gravity. Many of the earlier mechanical filters were of this type. In small plants this system has the distinct advantage that the water can be pumped from a river or other source of supply through a filter direct to the reservoir or into the mains, while any other system would involve a second pumping. Pressure filters are extensively used for hotel supplies, etc., where, from the conditions, gravity filters are impossible. The practical objections to this system have been found to be so great that it is rarely used under other conditions. Some experiments were made at Louisville with a filter of this type, but they were not long continued, and aside from them there is no precise information as to what can be accomplished with filters of this type.
CHAPTER XI.
OTHER METHODS OF FILTRATION.
WORMS TILE SYSTEM.
This system, invented and patented by Director Fischer of the Worms water-works, consists of the filtration of water through artificial hollow sandstone tiles, made by heating a mixture of broken glass and sand, sifted to determined sizes, to a point just below the melting-point of the glass, in suitable moulds or forms. The glass softens and adheres to the sand, forming a strong porous substance through which water can be passed. These tiles are made hollow and are immersed in the water to be treated, the effluent being removed from the centre of each tile. They are connected together in groups corresponding in size to the units of a sand-filtration plant. They are washed by a reverse current of filtered water. These tiles have been used for some years at Worms, Germany, and at a number of smaller places, and were investigated experimentally at Pittsburg. Some difficulty has been experienced in getting tiles with pores small enough to yield an effluent of the desired purity, and at the same time large enough to allow a reasonable quantity of water to pass. In fact, with other than quite clear waters, it has not been found feasible to accomplish both objects at the same time, and it has been necessary to treat the water with coagulants and preliminary sedimentation or filtration before applying it to the tiles. The problem of making the joints between the tiles and the collection-pipes water-tight when surrounded by the raw water also is a matter of some difficulty.
THE USE OF ASBESTOS.
It has been suggested by Mr. P. A. Maignen that the surface of sand filters should be covered with a thin layer of asbestos, applied in the form of a pulp, with the first water put onto the filter after scraping. The asbestos forms a sort of a paper on the sand which intercepts the sediment of the passing water. The advantage of the process is in the cleaning. When dried to the right consistency this asbestos can be rolled up like a carpet, and taken from the filter without removing any of the sand.
This procedure is almost identical with that which has occurred naturally in iron-removal plants, where algæ grow in the water upon the filters, and form a fibrous substance with the ferric oxide removed from the water, which can be rolled up and removed in the same way as the asbestos. The advantages of the process, from an economical standpoint, are less clear.