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4.2 SUBSURFACE INTAKES 73 The plant operates at 45% recovery. The water is collected in a large wet well that is located
underground and pumped to the plant using submersible pumps.
4.2.4 Infiltration Galleries
Infiltration galleries consist of a submerged slow sand media filtration bed incorporated into the bottom floor of the source surface water body (i.e., ocean, lake, or river). The filtration bed contains a number of equidistant horizontal perforated pipes that convey filtered source water to a wet well of an intake pump station located on shore (see Fig. 4.6).
Infiltration galleries are typically implemented when conventional horizontal or vertical intake wells cannot be used due to unfavorable hydrogeological conditions. For example, they are suitable for intakes where the permeability of the bottom soil formation is relatively low, or in the case of river or seashore filtration, where the thickness of the onshore and offshore sediments or the depth of the coastal aquifer is insufficient to install conventional intake wells.
The filtration bed of the infiltration gallery is sized and configured using the same design criteria as slow sand filters. The design surface-loading rate of the filter media is 0.12e0.25 m3/ m2 h (0.05e0.10 gpm/ft2).
Such subsurface intake systems are only feasible in locations where there is good periodic or continuous natural movement of water over the bed because they rely mainly on wave and current action to remove the solids retained and accumulated in the surface layer of the filtration bed. One potential challenge with infiltration galleries is the biofouling of the filtration media, which could reduce their production capacity over time.
to Treatment plant
Intake well
Pump house
Pump
FIGURE 4.6
Infiltration gallery.
Intake pipes
Water Body
Sand
Gravel
Collector screens
Filtration bed