Sand Filtration - Water Recycling Australia

Sand filters are probably the oldest type of filtration developed by Man. Sand filtration occurs in nature and has been filtering the worlds water since the beginning of time.

Sand filters come in many shapes and sizes and are mostly described by the speed or velocity at which they are designed to operate. The main varieties are, high-rate , medium-rate, rapid sand, gravity, up-flow, fluidised and continuous backwash.

All methods are used extensively in the water treatment industry throughout the world.

In general terms high-rate and to a lesser extent medium-rate sand filters tend to be used on recirculated systems of low load like swimming pools. Higher load single pass systems tend to operate better at slower velocity and therefore at the larger dia end of the range, this would normally be the territory of the slower medium-rate, rapid sand and gravity sand filters. produce very high quality water

Some designs work best with the addition of flocculants (direct filtration), these also tend to be in the slower end of the chart. Slow sand filters can produce very high quality water free from pathogens, taste and odour.

As a general guide a Sand filter system operating well without flocculants injection will typically filter water to around 20 microns.

Passing flocculants laden water through a slow sand filter can be an effective way of removing the floc particles, thereby reducing colloidal material including bacteria and other very fine solids. This style of system can filter water to less than 20 microns

Sand filters become blocked after a period in use, they are then reversed in flow direction to remove the collected dirt load. This process is called backwashing, a successful backwash will fluidise the bed and discharge the entrapped dirt load.

Sand filtration is a very reliable predictable method of filtering most types of water a relatively low costs. Sand filtration is an uncomplicated process and has proven its worth over many hundreds of years. However like all things it must be designed correctly in the first place if there is an expectation of being reliable and worthwhile.