Sand filters have been used in the United States for over 100 years. Design has improved significantly in the 1990's. They are a type of aerobic system, meaning they are oxygen rich. In addition to physically filtering the water, they perform as a biological filter. Microorganisms, highly adapted to decomposing wastewater, live on the sand grains. These organisms convert much of the carbon or organic matter in the wastewater to carbon dioxide (CO2). They also convert ammonia (NH3) and organic N to nitrate (NO3-). If the filter recirculates, then the nitrate will pass from the sand filter into an anaerobic or oxygen deplete environment in the recirculation tank. This allows denitrification, or the transformation from nitrate (NO3) to nitrogen gas (N2(g)). Nitrate may be reduced partially to nitrite or completely to nitrogen gas. Complete reduction requires an organic carbon source, which is usually abundant in the septic tank effluent. Recirculating sand filters have the advantage of being a technology capable of significant nitrogen reductions.
Sand filters have been used where conventional septic tank/absorption field systems have failed. They are a good option for sites with high groundwater, shallow bedrock, poor soils, or other restrictions. However, it is important to note that a absorption field is still required in Indiana, and therefore minimum site characteristics need to exist. Sand filters have also been used where centralized treatment is unavailable or too expensive for homes, businesses, institutions, and small residential developments/communities.
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