apart and 3 to 4 feet below the surface of the bed with a 3/8 inch space between each
tile in a line. The drain is surrounded with 2 or 3 layers of different-sized gravel to
prevent the passage of sand into the lines. The lines usually lead through a single
outlet to a water course.
Figure 2-17. Aerial view of wastewater treatment plant with intermittent sand
filters. (In each set of four beds, distribution troughs for dosing
radiate from a central diversion box.)
(2) Filtration process. Wastewater is filtered through sand as a secondary
treatment following primary settling or for further treatment following other secondary
processes. Discharge to natural sand deposits is sometimes used as a method of
disposal whereby wastewater passes into the ground water and there is no surface
effluent. Sand filters with underdrains usually produce an effluent that is clear,
sparkling, low in nonfiltrable residues and BOD, and highly nitrified. The filter removes
nonfiltrable residues mechanically by straining and oxidizes organic matter by action of
bacterial films on the sand grains.
(a) Dosing. The size of individual doses, rate of application, and
frequency of dosing vary considerably. The rate must be high enough to cover the
entire bed quickly to a 3-inch average depth. Dosage varies from one dose every other
day to three daily. To permit thorough bed aeration, the bed must drain thoroughly and
the flow in the underdrains from the filter must be very low before the bed is used again.
(b) Adding ridges/mounds. In cold climates, freezing may seriously
interfere with sand-filter operation. To avoid such difficulty, the bed may be furrowed
every 3 feet into ridges about 10 inches high; ice is then supported by the ridges. A