This tendency would be fatal were it not for the revolving rake which loosens and mixes the sand and largely corrects it. The correction, however, is imperfect, and some parts of the filter are washed more than others.
The rake is also necessary to prevent the separation of sand into coarser and finer particles. It is practically impossible to get filter sand the grains of which are all of the same size. When a filter is washed the tendency is for the wash water to go up in limited areas. The larger sand grains tend to collect at these points while the finer grains collect in places where there is no upward current, or where it is less rapid. In many filters this tendency is very strong. The revolving rake is necessary to correct it, and to keep the sand thoroughly mixed, otherwise when a filter is put in operation after washing, the frictional resistance through the coarse sand being less, the bulk of the water goes through it, with the result that a part of the area, and the part which is least efficient as a filter, passes nearly all of the water, and with inferior results.
In the Jewell filter provision is made for the distribution of the wash water over the whole area in another way. The strainers have areas at the surface amounting to 1.2 to 1.4 per cent of the whole area, but the water before reaching them passes through throats much smaller in size than the strainer outlets, and amounting in the aggregate to only about 0.07 per cent of the filter area. When washing at a rate of seven gallons per square foot per minute, water passes through these necks at a velocity of 22 feet per second. The friction and velocity head in passing these necks is estimated to be about 30 vertical feet, and is so much greater than the friction of the outlets proper, and of the sand, that the water passes through each strainer with approximately the same velocity, and the wash water is equally distributed over the whole area of the bottom of the filter.
This result is accomplished, however, at a great loss of head in the wash water. When a filter is washed from the pressure-mains without separate pumping, the pressure is usually sufficient and there is no disadvantage in the arrangement. When, however, the water is specially pumped for washing, the required head is much greater than would otherwise be necessary.
Mechanical Filters at Elmira, N. Y. Outlet to Filters with Controller and Pure-water Flume.
[To face page 174.]
It would not be possible to increase the size of the necks, thereby decreasing the friction, without increasing very largely the size of the pipes in the underdrainage system into which the strainers are fastened. These pipes are so small that during washing the velocity in them is about 13 feet per second, and if the throats of the necks were increased without also enlarging these pipes, the friction would be so reduced that most of the water would go through the necks nearest the supply, thus failing to reach the object to be attained.
A more rational system would be to increase the sizes of all the waterways in the outlet and wash-water system. The Jewell filter is also provided with a rake to keep the sand mixed during washing, as this is necessary even with the complete distribution of wash-water over the area of the filter.
Both the Warren and the Jewell filters are provided with receptacles through which the water passes after receiving the coagulant, and before entering the filter. In the Jewell filter the receptacle, called a sedimentation-basin, is of such size as to hold as much water as is filtered in 15 minutes. In the Warren filter the receptacle is entirely independent and larger, holding about an hour’s supply.