A single-pass sand filter system pretreats septic tank effluent by filtering it through sand before sending it to a soil treatment system. Various single pass sand filter types and designs have been extensively tested and used in the United States. Other wastewater treatment filters use peat, pea gravel, crushed glass, or other experimental media, but sand is the best understood and the most predictable.
Treatment mechanisms in a single pass sand filter include physical filtering of solids, ion exchange (alteration of compounds by binding and releasing their components), and decomposition of organic waste by soil-dwelling bacteria. A properly operating single pass sand filter should produce high-quality effluent with less than 10 mg/liter BOD (biological oxygen demand, a measure of organic material), less than 10 mg/liter TSS (total suspended solids), and less than 200 cfu/100 ml fecal coliform bacteria, an indicator of viruses and pathogens.
How it works?
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 single pass 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.
SEE ALSO: HAYWARD SAND FILTERS
Hayward Sand Filter is the oldest and most popular method of filtration. Hayward Sand filters share two things in common:
When in the filtration mode, water always flows from top to bottom
Hayward Sand Filters have some sort of lateral or underdrain with slots to hold back sand while allowing clean, filtered water to pass through.
High-rate Hayward sand filters use a special filter sand, normally .45 to .55 mm (also known as pool grade #20 silica sand), because it has sharp edges that separate particles, allowing filtration to take place. They operate on the basis of \"depth\" filtration where dirt is driven through the sand bed and trapped in the spaces between the sand particles. Initially, a clean sand bed removes larger particles, and then, as the bed starts to load up with dirt, the filter removes finer particles. The sand can be cleaned by backwashing which involves reversing water flow through the filter to the \"waste\" line.
Cleaning of the media, or sand bed, is accomplished through reversing the flow through the filter, to the \"waste\" line. This is known as backwashing.
