Sand is the oldest and most popular method of filtration. Sand
filters share two things in common:
- When in the filtration mode, water always flows from top to bottom
- They all have some sort of lateral or underdrain with slots to hold back sand while
allowing clean, filtered water to pass through.
High-rate
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 debris, the filter removes finer particles. The sand
can be cleaned by backwashing which involves reversing water flow through the filter to the "waste" line. Sand Filters
trap debris as small as 20 to 40 microns.
