SOil /sand filters?

Question

Whats the very best type of sands/soils to use for filtering water

I am thinking to use fine sand,and then a layer of soil under neither, and a compacted layer of some type of soil or clay..

wondering if the act of compacting it causes it to better filter it at micronis levels?

Answer 1

NO!

Sand used for filtering water is single sized, between about a half and one millimetre in size (it can be a little coarser; but no finer - beach sand is way too fine). Sand sieved between 14 and 28 mesh is ideal. The sand should be clean, clay free, and free from rock fragments; so crushed gravel is not suitable. Quartz river sand is the most likely source.

The sand is placed in a bed, about 450 mm deep (15 inches) over a grate (obviously; finer than the sand).  Many commercial swimming pool sand filters are constructed like this.  Or the sand could be placed over increasingly coarse beds of 2 - 4mm gravel; 4 - 8mm gravel, etc over a coarser grate.  Large gravity filters such as municipal water works use this type.

In a small filter; such as for a swimming pool, the water is forced through by pumping.  You need a large area and a lot of sand for a gravity filter.  Typically, a municipal filter may contain hundreds of tonnes of sand; but once set up, they last for years.

A layer of clay or soil would totally block the filter.  There are some commercial filters using very fine diatomaceous earth or cartridges; but these force the water through at high pressure.

You might wonder how such coarse sand could filter anything out of water at all; but a fifteen inch layer of sand can remove anything down to the micron size.  The average pore space is very small.

I have known some clever people who have made a pool filter out of an old trash can, using sand and gravel layers, but you need to know how to install a pump, as well.  The filter has to be periodically 'back flushed' to remove the particles trapped in the sand.

I used to make filter sand on an industrial scale.