Practical Methods of Sewage Disposal for Residences, Hotels and Institutions

by Henry N. Ogden; H. Burdett Cleveland

Respecting the quality and relative fineness of sand suitable for sewage filters, it should be noted that certain empirical methods of measurement have been developed for use in comparing the size and uniformity of particles of various sands. These measures are (1) the “effective size,” and (2) the “uniformity coefficient.” The “effective size” is the size of sand particle expressed in millimetres compared to which ten per cent by weight of the particles in the sample is finer. The “uniformity coefficient” is the ratio of the size of grain which has sixty per cent of the sample finer than itself to the size which has ten per cent finer than itself.

Concerning the grades of sand through which sewage may be successfully and properly treated by intermittent filtration, it has been found that the “effective size” should not be less than .20 of a millimetre, nor greater than .50 of a millimetre, and the “uniformity coefficient” should generally be from 1.5 to 3.0, when sewage is applied at the usual rate. If, however, the sand is clean and sharp, but has an “effective size” somewhat smaller than the limit above stated, it may sometimes be found suitable.

In the case of any sewage-disposal project of considerable magnitude, where any doubt exists as to the suitability of the sand available for use in sand filters, analyses of representative samples of the sand should be arranged for, and competent engineering advice should be sought before any large outlay is incurred. In general, however, it may be said that any clean, sharp sand suitable for building use is suitable for sand-filter beds in any situation. Obviously, the coarseness of the sand plays no part in its suitability as a filtering medium if the sand occurs in a natural bed and underdrains are not necessary, since no question of the discharging of an unpurified effluent would ordinarily arise in such cases.

Contact Beds

The treatment of sewage in contact beds consists in distributing the effluent from settling tanks over beds of broken stone, furnace slag, or other similar material contained in water-tight compartments and allowing the beds to fill so that the spaces between the filtering material will be filled with the sewage effluent. These beds are so arranged that the effluent is held in contact with the filtering material for a fixed interval of time and then, usually by means of special siphons called “timed siphons,” or other automatic devices, it is discharged from the beds onto sand filters for further treatment, or into streams, as the case may be.

The process involves, as in intermittent sand filtration, the nitrifying or oxidizing agencies of bacterial action, and differs from intermittent filtration and from treatment of sewage on sprinkling filters principally in the fact that the flow of effluent through the beds is arrested and the liquid sewage held in contact with the filtering material, as noted above.

Much smaller areas of filter beds are required than in the case of sand filters, and for this reason this form of filter will often be found preferable. The conditions which result in its selection are usually either the unsuitable character of the soil or the presence of ground water, making the installation of sub-surface irrigation systems impracticable; or the absence of sand deposits or the high cost in any locality of sand suitable for sand-filtration beds, making their construction difficult or expensive.

The walls and floor of a contact bed are generally constructed of concrete, and the filter should be rectangular in form, as it is easier to distribute the effluent uniformly over a bed of this shape. The details given in Chapter II for constructing the walls and floors of settling tanks will serve as a general guide in the construction of contact beds.

The real work of the contact filter is carried on during the period of “resting empty,” that is, after the effluent has been withdrawn from the bed. While the effluent fills the beds, much of the suspended solid matter, together with a large proportion of the bacteria contained in the sewage, adheres to a gelatinous film which has formed on the surfaces of the stones or other materials forming the beds. This interval of “resting full” should usually be about two hours. Then, when the liquid portion is withdrawn from the bed, air is drawn in between the stones, enabling the nitrifying or aërobic bacteria to do their work of breaking down both the suspended and the partially dissolved organic matters which have been contained in the sewage and which have adhered to the filter material. It is believed, that some oxidation of that portion of the organic matter which is in true solution is also accomplished when the effluent passes over the gelatinous covering of the stones by reason of the oxygen which has been absorbed by this covering.

The interval when the bed is “resting empty” should be considerably longer than the combined intervals when the bed is filling, “resting full,” and emptying. For this reason there should be a series of from three to five beds in order that it will not be necessary to turn the effluent from the settling tank continuously onto one bed, which would result in the clogging of this bed with suspended matters. The additional third (or fifth) bed also gives opportunity for allowing each bed in turn to be thrown out of use for intervals of a week or so at a time, which is also necessary to keep the beds up to their proper efficiency and obviate the necessity of cleaning or renewing the filter material oftener than once in seven or eight years.