About the Surface Sand Filter
General Information Performance Data Water Quality Treatment Process Design


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Design

The surface sand filter system tested at the UNHSC is comprised of a sedimentation forebay and a surface sand filtration basin. The common practice of oversizing sand filters to reduce maintenance burden was not applied to this design. This led to an increased hydraulic loading rate, the filtration of a relatively large surface area by a small filter area, and, potentially, a weaker water quality treatment performance.

The basin contains a filter bed composed of coarse to medium grain sand. The filter bed’s thickness is 30 inches, within the 18- to 36-inch range typically used in the design of these systems. It is designed to allow for five feet of ponding—which appears to be excessive and reduce performance—over a 24- to 48-hour period.

The filter bed was not vegetated; however, vegetation of the bed is a design option for these systems. Attempts to grow grass over the sandy surface of the filter bed proved to be a challenge. The surrounding slopes of the system were stabilized over three months, during which no monitoring data was collected.

The forebay holds 25 percent of the water quality volume (WQ_V), and drains slowly through a restrictive standpipe into the surface sand filtration basin, which holds the remaining 75 percent of the WQ_V. The standard drain time is 24 hours and extends to no more than 72 hours to prevent standing water and mosquito habitat. Typically, channel protection volume (CP_V) events are conveyed through the system within 24 to 48 hours, while ten-year events (Q_p) bypass to adjacent conveyance systems.