Bob Hobden wrote:

Sand filters are not usually biological filters, or I should say
biological filters don't use sand as a growing medium for the bacteria.

They use it for algae, not bacteria - unless you're alling the old Essex
Water board wrong.

Sand filters are usually pressure filters for polishing the water.

You're talking about smallish scale ones in swimming pools, etc, where
the water is sterilised.

Proper biological sand filters both remove objects and living organisms,
as wel as absorbing many compounds such as nitrates.

Once a sand filter (and it doesn't need to be huge - a five gallon drum
will suffice) is populated with algae, you can pour in the foulest,
pea-soupiest, stinkiest water, and providing you don't want gallons per
minute, the outflow will be potable without the addition of milton or
similar.

Lava rock, bits of cut crinkly pipe, filter foam etc are used in
biological filters so there is little effect on flow rates and they have
a large surface area for the bacteria to grow on.
Dead bacteria and the resultant toxins are not what you want being
pushed into your pond in the spring when the fish are getting over a
long winter. Water quality is everything where fish keep is concerned
and a build up of Nitrites etc is bad news for fish health. Clear water
has nothing to do with the quality of the water in this respect.

So you want a sand filter column: large stones at the bottom, overlaid
with smaller ones, then gravel, coarse sand, finer sand, fine sand,
coarser sand, coarse sand, gravel, shingle, flat stones.

The grains of sand, gravel, and the stones all become coated with algae
- not bacteria - and the ingress of water is dispersed by the stones,
etc, so the fine sand isn't washed through, and the layers of increasing
sized partiles act as barriers to washing-through.

I have drunk water which has been through such a filter - water which
delighted the water-board boffins because of its stomach-churning odour,
its pea-green colour looking into it, and its orangey hue when held up
against the light, and with lots of little moving, bobbing and wriggling
things in it.

"Where did you get *THAT*? It's the best filthy water I've ever seen!"
said one, admiringly: "Just what we wanted!"

Two boffins, one District Venture Scout Leader (me) and two Venture
Scouts venured to try it. Quite unremarkable water.

Running a clean filter will gradually build up a bacteria base but it
will take some time to get to it's optimum working density which is why
a lot of people use a "starter" solution to seed the filter with the
"right" bacteria so it has a good start and to try to avoid toxin
build-up as feeding commences.

The algae will build up to a usable level in about a fortnight of
continuous trickle-running. From there on, it will gobble-up any other
algae, bacteria and dissolved chemicals such as nitrates and phosphates.

AFAIK it doesn't affect dissolved salts such as calcium carbonate,
calcium sulphate etc., but nearly everything else gets et.

--
Rusty