Infiltration intakes may be an effective control strategy for new intakes serving facilities such as small drinking water plants, where the total volume of water used is modest. Using naturally layered soils or constructed layers, infiltration intakes draw water through porous layers. Apart from construction impacts, infiltration intakes are considered environmentally benign. Several different infiltration intake designs exist, including sand filtration methods and Ranney wells.
Filtration is used in water treatment for removing suspended solids and zebra mussel veligers by use of graded granular media. Common granular materials used for filter media are sand, anthracite coal, activated carbon, resin beads, and garnet. Coarse filter material for the removal of larger particles and debris make up the upstream or top layers. Subsequent layers are composed of finer media with higher specific gravities for the removal of finer particles.
Ranney wells intercept and collect ground water derived principally from surface water infiltration. Ranney wells are most suitable in areas having subsurface sand and gravel deposits which are hydraulically connected to surface sources such as rivers, and lakes. Most designs consist of protected vertical conduit sunk to depths up to 60 m with horizontal perforated pipe extending radially. The filtering capabilities of the soil eliminate zebra mussel infestation..
Regional and local hydraulic/hydrologic conditions, the availability of space, and flow requirements must be addressed when considering infiltration systems. To obtain a large amount of filtered water, infiltration intakes of huge dimensions would have to be constructed. Initial costs, including feasibility studies, design, and implementation, are high. The feasibility study and design process consider variables such as raw water quality, proximity to sources of high turbidity, water volume, and pressure drops. Infiltration intakes are not maintenance free. Granular filters are generally backflushed with filtered water or periodically have the top layer, which captures all extraneous material, removed. For a general discussion of infiltration intakes and other types of filtration systems, see Smythe and Short (1995).
Prospective Control Methods