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Sand Filter Systems

Sand Filter Systems

Drains In a sand filter system, sewage flows from the house into one or several septic tanks, depending upon the size of the house and local requirements. Effluent from the septic tank(s) flows into a pump or lift tank. A pump introduces the effluent at the top of the watertight sand filter, using pressure distribution to apply the wastewater evenly to the filter surface to maximize treatment. A timer is used to dose the entire surface of the filter intermittently with wastewater. This draws oxygen from the atmosphere through the sand medium and its attached microbial community. The effluent is treated by physical, chemical, and biological processes. Suspended solids are removed by mechanical straining due to enhanced contact and sedimentation. Treatment occurs through the bacteria that colonize in the sand grains. Microorganisms use the organic matter and nutrients in the effluent for growth and reproduction.

The two main types of sand filters differ in the rate at which wastewater is introduced into the system. Loading rates determine the amount of maintenance needed and how long the system will last. A single-pass filter with a high loading rate needs regular cleaning (every six months to a year) of the sand surface to prevent clogging.

In high-rate sand filters, effluent is applied at rates of 1.6 to 5 gallons per day per square foot. This application rate means the surface of the filter must be easy to access. That is why high-rate sand filters are more common in warmer climates where they can be left open or have a lid that is easily removed.

Low-rate sand filters are the most common designs in Oregon. Effluent from the pump tank is applied at rates of 0.8-1.5 gallons per day per square foot. Sizing criteria used for low-rate sand filters are similar to those for rock beds in mound soil treatment systems. These systems are covered with 6 inches of loamy topsoil and vegetation to provide insulation during the winter.

Single-pass sand filters are an effective way to treat wastewater in an onsite application. The sand filter system has been used for more than 30 years across the United States and there is significant design, treatment, and maintenance experience with these systems. Sand filter systems are very reliable in treatment of BOD, TSS, and fecal coliform. The system protects the final soil treatment area because failure will occur in the sand filter before the soil treatment system is significantly affected. Single-pass sand filters require more area that recirculating filters and are not a good choice for small lots.

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