2 Granular media filtration is widely used to remove particl

2. Granular media filtration is widely used to remove particles from water. (8 points) (a) Name typical filter media used in dual media filters, and describe the depth, size, and density of the two media (relative comparison, specific values are not necessary). (b) Describe the purposes of the underdrain system.

Solution

a)Sand filters are used to remove suspended solids from water and wastewater. Because they have one layer of filter media, sand filters remove only relatively large solids (normally in the range of 40 micron and larger). The effective size of each granule of sand typically varies from 0.35mm – 1.2mm. Larger granules yield coarser filtration. Finer granules yield finer filtration.

b)

Developers and special districts use underdrain systems that drain utility trenches to reduce the disturbance of the pipe bedding around wastewater collection lines. This reduces the potential inflow & infiltration of groundwater into the wastewater collection systems which requires treatment, thus also reducing the potential for illegal discharge connections from sump pumps to the wastewater system. With modern pipeline construction technology, there is diminished benefit to special districts from an inflow & infiltration perspective since new collection systems are sealed and air tested, resulting in a very tighter systems.

Developers benefit from dewatering around building foundations and subsurface infrastructure, allowing construction to proceed and owner financing to be obtained. An underdrain system may also be installed as insurance against future groundwater problems in a new subdivision if groundwater tables change due to drought or other developments.

Homeowners and businesses benefit from the reduction of potentially damaging water infiltration into their foundations and basements.