Filter Sand Maintenance

Abstract

Many of the ills to which filter beds are subject are of purely local origin, for example, a large amount of iron or manganese in the water being filtered. Most lime softening plants find that the sand grains increase in size by accumulating a deposit of calcium carbonate. But even when local conditions are eliminated in the ordinary rapid filter plant, using alum coagulation with a fairly turbid water, the filter bed develops certain types of disabilities that are widespread in occurrence. The most frequently encountered troubles of this nature are the formation of mud balls, cracking of the filter surface, and shrinkage away from the side wall. Many times two or three of these elements occur together and in all cases the condition is likely to become progressively worse as time goes on until the filter bed must be reconditioned. Reconditioning means usually the removal of the sand and its cleaning by some mechanical device, a costly and timeconsuming procedure. All rapid filter plants are provided with a washing system designed to keep the sand reasonably free from accumulations of foreign matter. But until recently it has been considered desirable to encourage the accumulation of coating on the sand grains in the belief that filtration efficiency would thereby be improved. The filter bed troubles that have been described almost invariably occur in these so-called wellseasoned filter beds. The cause is the accumulation of foreign matter on the sand far beyond the point where it might possibly be of value in filtration. Each sand grain accumulates a coating of aluminum floe combined with the material suspended in the water, a gelatinous coating, cohesive and easily compressed. It is this accumulation of dirt that encourages the formulation of mud balls, surface cracks and wall shrinkage. The real preventative for these maladies is the maintenance of the sand in a clean condition. When the sand is removed from a filter for cleaning and then put back the troubles disappear for a time.