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The recirculating sand filter (RSF) concept was introduced in the late 1960s and early 70s by public health engineers from Illinois who were experimenting with sand filter designs. An RSF system is a modified version of the single-pass open sand filter. It was designed to alleviate the odor problems associated with open sand filters. The odors were eliminated through recirculation, which increases the oxygen contend in the effluent that is distributed on the filter bed.
Why Use a Recirculating Sand Filter
Wastewater treated in a RSF contains more oxygen that eliminates odors. “The final sand filter effluent also is of a higher quality. Recirculating sand filter media is somewhat coarser than other sand filter media and therefore is less prone to clogging. Hydraulic loading rates typically are two to three times higher than ISF's meaning that less land area is generally needed to treat the same amount of wastewater.
The RSF treats wastewater with the same processes as the intermittent sand filter. The three basic components of an RSF are a pretreatment unit, a recirculating tank and open sand filter.
Recirculating Sand Filter System
Wastewater first flows into a septic tank for primary treatment. The partially clarified effluent then flows into a recirculation tank, which is equipped with a pump, alarm, a timing mechanism, and float switches.
The volume of the recirculation tank should be equivalent to at least one day’s design flows. In the recirculation tank, effluent from the septic tank and the sand filter filtrate are mixed and pumped back to the sand filter bed. The dosing frequency is controlled by a programmable timer in the control panel.
After receiving treatment in the sand filter, the wastewater collects in the under-drains and a portion of it is directed back to the recirculation tank, where it mixes with the septic tank effluent and is recirculated to the sand filter. The remaining sand filter effluent bypasses the recirculation tank and goes to the leach field.
The ratio of sand filter effluent recirculation ranges from 3:1 to 5:1. How the amount is controlled varies with individual system designs.
RSFs require simple routine maintenance. Primary O&M tasks include inspecting the dosing equipment, maintaining the filter surface, checking the discharge head on orifices, and flushing the distribution manifold annually. The surface of the sand bed should be kept weed free.
The septic tank should be checked for sludge and scum buildup and pumped as needed. The recirculation tank should also be inspected and maintained as necessary.
The pump and floats should be checked annually and replaced or repaired as necessary. All electrical parts and conduits should be checked for proper operation and corrosion. Follow all of the manufacturer’s operation and maintenance instructions. All equipment must be tested and calibrated according to the manufacturers specifications.
Some Advantages and Disadvantages are listed below.
Advantages:
· RSFs provide a very good effluent quality with over 95% removal of biochemical oxygen demand (BOD) and total suspended solids (TSS).
· The treatment capacity can be expanded through modular design.
· RSFs are effective in applications with high strength wastewater.
· A significant reduction in nitrogen is achieved.
· Less land area is required than for single-pass filters.
Disadvantages:
· If appropriate filter media is not available locally, cost can be high.
· Weekly maintenance is required for the media, pumps and controls.