| OWDP Home > Online Resources > Treatment Systems > Intermittent Sand Filters | ||||
Sand filter systems have been used for wastewater treatment in the U.S. since the late 1800s. In 1876, the community of Lenox, Massachusetts, built a sand filter system and by 1893 six other communities were using sand filters for wastewater treatment. During the same period, sir Edward Frankland of England was documenting work with natural sand beds.
Why Use an Intermittent Sand Filter
ISFs have proven to be a reliable technology when they are properly designed, constructed, and maintained. Their performance is consistent and they have low operation and maintenance requirements.
ISFs may enable development or use of difficult sites. They can remedy existing malfunctioning systems and they can be a good option for homes in environmentally sensitive areas.
ISFs are used as the second step in wastewater treatment after solids in raw wastewater have been separated in a septic tank, aerobic unit or other sedimentation tank. Wastewater treated by sand filtration is usually colorless and odorless.
ISFs have 24-inch deep filter beds of carefully graded media. Sand is a commonly used media, but anthracite, mineral tailings and bottom ash have also been used. The surface of the bed is intermittently dosed with effluent that percolates in a single pass through the sand to the bottom of the filter. The dosing frequency is controlled by a programmable timer, much the same as the LPDS and mound. After a set volume is collected in the under-drain a float switch activates a pump and the treated effluent is pumped to a leach field. The two basic components of ISF systems are a primary treatment unit, such as a septic tank or other sedimentation system, and a sand filter.
ISFs remove contaminants in wastewater through physical, chemical, and biological treatment processes. Most treatment occurs in the first six to twelve inches of the filter surface. The organic matter breaks down in the filter. Particles stick to grain surfaces or get caught in crevices or voids on grains or in spaces between grains. In addition, negatively charged grain surfaces can attract positively charged waste particles and bond with them through a process called adsorption. Chemical bonding also takes place as certain particles in the wastewater come in contact with and react with the media.
Although the physical and chemical processes play and important role in the removal of many particles, the biological processes play the most important role in sand filters. Like the soil in every backyard, sand filters are home to a variety of organisms, many of which contribute to treatment by consuming organic matter in the wastewater.
Bacteria are the most abundant organisms in the filters, and they do most of the work. There are other beneficial life forms found in the filters such as protozoa and worms, which contribute to treatment.
After the filter has had a chance to mature, a complete enclosed ecological system develops as the organisms multiply and rely on each other to survive.
The most significant part of the filter ecosystem is a thick layer called the biomat that eventually forms near the surface of the filter. This layer contains bacteria that consume particles in the wastewater. In turn, protozoa feed on the bacteria and help prevent the biomat from becoming so dense that it clogs the filter. This balance between the various life forms and the physical and chemical processes that take place in the sand filter results in extremely efficient wastewater treatment.
ISFs are typically built below grade in excavations three to four feet deep and lined with an impermeable membrane. The under-drain is surrounded by a layer of graded gravel and crushed rock. Pea gravel is then placed on top of the graded gravel and then sand is laid over the top of the pea gravel. Another layer of graded gravel is laid down, with the distribution pipes running through it. A flushing valve is located at the end of each distribution lateral. Lightweight filter fabric is placed over the final course of rocks to keep silt from moving into the sand while allowing air and water to pass through. The top of the filter is then backfilled with topsoil that may be planted with grass.
One variety of buried ISFs is the gravity discharge ISF. It is usually located on a hillside with the long axis perpendicular to the slope to minimize the excavation required. Because the effluent leaving the sand filter flows out by gravity, the bottom of the sand filter must be several feet higher that the leach field. To achieve that difference in elevation a sand filter may be constructed partially above ground.
Another type of buried sand filter is the pumped discharge sand filter. This system is usually constructed on level ground, but its location in relation to the leach field is not critical since a pump located within the sand filter bed allows effluent to be pumped to a leach field at any location or elevation. Discharge piping goes over the sand filter liner, or through a sealed boot in the liner so that the integrity of the liner is protected.
Intermittent Sand Filter
A third type of buried sand filter has no impermeable liner and does not discharge to a leach field, but rather directly to the soil below the sand.
The operation and maintenance of a sand filter is minimal when properly sized. Buried sand filters used for residential applications can generally perform for extended periods of time.
Primary O&M tasks require minimal time and include inspecting the dosing equipment, maintaining the filter surface, checking the discharge head on orifices and flushing the distribution manifold annually. 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. The septic tank should be checked for sludge and scum buildup and pumped as needed.
In extremely cold temperatures, adequate precautions must be taken to prevent freezing of the filter system by using removable covers.
Some Advantages and Disadvantages are listed below.
Advantages:
· ISFs produce a high quality effluent.
· Leach fields can be small and shallow.
· ISFs have low energy requirements.
· The treatment capacity can be expanded through modular design.
· ISFs can be installed to blend into the surrounding landscape.
· The soil cover prevents odors.
· ISFs are stable and work well for intermittent usage.
Disadvantages:
· The land area required may be a limiting factor.
· If appropriate filter media is not available locally, cost can be high.
· Clogging of the filter media is possible.