SAND FILTERS.
It would probably be more correct to use the term “fine-grain filters” to describe the alternative methods occasionally adopted to deal with the effluents from percolating filters, as they do not always consist of sand. Fine clinker, ashes, broken saggars, and similar material, is equally suitable so long as it is of a gritty nature and not wholly dust. The term “sand-filters” is, however, used here, as it is well known in connection with the filtration of drinking water, and the method of construction is practically the same.
Although primarily designed for the purpose of arresting the suspended solids in final effluents, and thus required to act simply as mechanical strainers, sand filters have the additional advantage of increasing the degree of purification, especially from a bacterial point of view. The most important factor to be considered in constructing these filters is the grading of the material. It must not be too fine or contain too great a proportion of dust, or it will rapidly become clogged and involve much labour and expense in cleaning the surface layer. The nearest approach to perfection in material for this purpose is the coarse Leighton Buzzard filter sand. As in the case of the best material for the percolating filters themselves, it may cost a little more than other less satisfactory kinds, but it will generally be found to be the cheapest in the end.
In constructing filters of this type, whether composed of sand or other material, it is essential that the bottom layer should be of a coarser grade, in order to provide free drainage. In a general way it will be found satisfactory to have a series of 2-inch agricultural drain pipes laid on the floor and converging towards the outlet. Over the whole floor should then be laid a layer 6 inches deep of gravel, broken bricks or stones of the size of walnuts, upon this a layer 3 inches deep of pea gravel, and at the top a layer not less than 9 inches deep of suitable sand or other fine grade material. The surface of the filter should be well below the level of the inlet, in order to allow the liquid to pond up on the surface 6 inches to 9 inches in depth without backing up to the level of the floor of the percolating filter. One of the difficulties encountered in operating filters of this type is to secure even distribution over the whole area. The means to be adopted for this purpose should be simple and easily cleaned, and it is usual to find troughs of wood or iron, glazed ware channel-pipes and similar arrangements in use. Unfortunately these do not effectively cover the whole area until the sand has been saturated and the surface slightly coated, thus preventing the liquid from passing through as fast as it comes in. When this occurs, however, the time for cleaning the surface is not far distant, and when the filter is brought into use again the whole preliminary process has to be repeated. The best way of avoiding these difficulties would appear to be to arrange the filter in such a way that the liquid must cover the whole area from the very beginning. This can be accomplished by fixing the normal outlet above the level of the surface of the filter as suggested, [Fig. 135], where the final effluent discharge is normally from the end of the swivel-jointed pipe when in its vertical position. When it becomes necessary to clean the filter and drain it for purposes of aeration, the swivel-jointed pipe is simply lowered to the floor of the outlet-chamber, as shown in plan, and raised again when the filter is brought into operation.
Fig. 135.—Sand Filter.
Filters of this type should never be less than two in number, so that one may be in work while the other is being cleaned. It would probably be advisable to have even three or four filters for schemes of moderate size, so as to provide longer or more frequent intervals of rest for aeration. It will be obvious from the preceding observations that these filters must be substantially constructed and made absolutely watertight. When dealing with a good effluent from percolating filters or contact beds, these final sand filters may be provided at the rate of 1 square yard for every 500 gallons of the daily dry weather flow.
Where ample fall is available, careful consideration should be given to the advisability of operating these sand filters in the same manner as percolating filters, i.e. by using revolving sprinklers for the purpose of distribution without submerging the filtering material. This applies particularly in cases where it is desirable to secure a very high degree of bacterial purification. Recent investigations have shown that sand filters for drinking-water, when operated in this manner, are highly efficient and involve less expense for maintenance. In addition to this they require less cleaning, so that a much smaller area is thrown out of work during cleaning operations, and a smaller total area of filter surface is needed than in the case of similar filters operated on the submerged system. The additional fall required for the revolving sprinklers will usually be a serious difficulty in the case of sewage disposal works, but where it is available, and the extra cost entailed is not of great importance, the idea deserves consideration. Filters of this type should be preceded by an effluent settling tank as previously described.