The math on the surface area of a straw seems correct. However you need to
remember straws are really tiny pipes, even when under pressure, the flow
rate of water through a straw is horrible. The water is much more likely to
go around the straws, compared to the water that will flow through the
straws.

I suppose that if you placed the straws vertically, and forced the water to
travel down a bed of straws, it would work. But if they were laid
horizontally, and the water just flowed around in a stock tank, very little
would actually flow through the straws.

Sameer


"Matt Rosing" wrote in message
...
Hi,

I want to change my filter media and was thinking about using drinking
straws and was curious as to what people thought. Characteristics of
good filter media a

1) High surface area per volume.

Assuming 12" long straws that are about 1/4" in diameter it takes about
16/in^2 * 144 in^2/ft^2 (=2304 straws) to fill 1 cubic foot. Each straw
has a surface area of 12" * 2 pi r in^2 on both the inside and outside
(=9.4 in^2). Since the straws are against each other say 25% of the
outside surface is lost, so the available surface area of each straw is
1.75 * 9.4 in^2 = 16.5 in^2. This * 2300 straws/ft^3 = 38000 in^2 = 263
ft^2/ft^3 (surface/volume). This is pretty good compared to something
like Springflow which appears to be between 60 and 120 ft^2/ft^3 space,
350 ft^2/ft^3 for lava rock, and 120-250 ft^2/ft^3 for open cell foam.

2) Displaces little water per volume.

Straws don't take much room. Rocks and pea gravel do.

3) Doesn't plug easily.

The inside of a straw can't plug. Possibly the outside could between
the straws but even that is nowhere close to what happens in lava rock.
I've been having problems with pluged filters (lots of algae) and I need
to have something that will go for a week with no problems at all. I
have pea gravel now and I'm having problems keeping the prefilter clean
so the pea gravel doesn't plug.

4) Light weight, easy to work with.

Straws are light. In a filter with straight walls it shouldn't be too
hard to keep them in place.

5) Cheap.

I went to a restaurant supply place on the net and found a case (25000
straws) of 7.75" straws for $42. That would mostly fill my 100 gallon
stock tank.

What do you think?