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Re: filter flow rates

On Wed, 29 Dec 1999, Tom Bates wrote:

> The filter flow rate as advertised by the manufacturer is a rating of
> maximum flow with no media installed. As restriction is applied (clean, new
> filter media) this flow rate is lowered. As the filter media becomes clogged
> to the point that the filter is in need of servicing, this flow is now
> severely compromised.

I wouldn't advise anyone to run a filter to the point where it's flow rate
is severly compromised.  There is no point to the expense of a large
filter if you're just going to let it get clogged.

Beside, I was refering to the *actual* flow rate, not the manufacturer's
spec.  You can use the specification as a standard, but if you need to
know the actual turnover rate for some odd reason, then you should measure
the flow rate under your operating conditions.

> The filter flow rate divided by the tank volume will only be a viable
> outcome if the filtered water was not returned to the unfiltered water. By
> mixing this water together, even if completely mixed, the filter is
> constantly taking in a combination of filtered and unfiltered water.

Actually, the ratio is accurate whether or not the filtered water is
returned and mixed with the original volume.  The case where the water
isn't returned is pretty simple; the tank volume/flow rate ratio gives
the amount of time it takes to run all the water from the tank through the
filter.  If the water is returned to the same volume then the ratio
measures the time it takes to run the average particle of water through
the filter once. In the same period of time, part of the water will not
have passed through the filter, part will have passed once, part will have
passed twice, and so on.  On average, the water will have passed through
the filter once.

> The purity coefficient takes into effect the gradual decrease of the flow
> due to the filters increasing restriction as the media becomes clogged. By
> inserting this factor into the formula we can arrive at an average turnover
> time (throughout the media's lifespan). Granted the times will be shorter
> when the media is new and longer when the media needs replaced (or
> serviced). This purity coefficient also leads to the implication that 99% of
> the water is now in a filtered state.

Everyone's filters will clog differently, so the coefficient is arbitrary,
and the result is only of qualitative value.  If it's going to be of
qualitative value, then just go ahead and use the flow rate/tank volume
ratio.  It's simpler and just as meaningful.

Then James Watford wrote:

> The formula Tom gave:
> snip<T(ave) = 12.75 x G divided by F
>  T(ave) is turnover average
>  12.75 is a purity coefficient (a constant factor [given])
>  G is the net gallons of the aquarium
>  F is the manufacturers rated flow of the filter>snip
> The formula is right on the money (The only difference in the one we used in
> testing in chemistry was the purity coefficient was rated at 12.755, but hey
> close enough)

What Tom's formula (due to Mr. Escobal) gives is an approximation to the
amount of time it takes for an ideal filter to remove an arbitrarily
selected proportion of some material added to the filtered water before
the filter is turned on.  The formula is fine for that application (of
course, one might choke on using 5 significant digits, but that's
something else entirely).  I have no problem with that.  Why do you want
to know that? It's completely unlike an aquarium.

In an aquarium filters are used mostly to control the buildup of materials
that are generated within the aquarium on a more-or-less continuous basis.
Some of those material consists of solids that are mechanically removed by
the filter or settled in the tank itself.  Some of the materials are
reactive chemicals that are acted on by bacteria in the filter, as well as
being used by plants and acted on by bacteria in the tank.

In either case, it's possible to build a formula that describes the
resulting concentration of the filtered material.  Under simple conditions
(those that generally neglect reality) the concentration is going to be
proportional to the turnover rate (rate, not turnover time).  But
different solutes, different particle size fractions and different
particle density fractions will all have different coefficients, and the
coefficients will depend on the rate at which the filterable material is
generated and consumed in the tank.  So what is the point to using some
arbitrarily selected value?

In reality, some of us have retired filters completely and experienced no
measurable increase in the concentration of filtered components like
ammonia.  What is the experimentally derived coefficient in that case?

Go back to the original context of this discussion; Daniel Greene
commented that his 1000 l/h (rated) pump was probably a little small for
his 400 l tank, based on it turning the water over only twice an hour.

Based on that turnover rate, I think Daniel's setup is fine, (particularly
in a planted tank) as long as the bioload isn't excessive and the filter
is cleaned regularly.  There's no reason to throw an arbitrary coefficient
into some formula to get that qualitative conclusion.

Roger Miller