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I have a 990l pond, and a Rockways 3-step formed waterfall that recommends a 4,000l/hour flow rate.

I need my pump to output 4,000l/hour. Do I need a filter that specifically supports such a flow rate? The pond is small, so do I need a 'big' filter that support the high flow rate of the pump, or do can I get away with a filter designed for smaller ponds (and thus built to expect a lower flow rate)?

Many thanks in advance!

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You will probably find that you cannot GET 4000l/hr through a smaller filter. Less area with more flow = higher pressure drop. Higher pressure drop moves your pump to somewhere else on its pump curve. If the filter is on the suction side, you may also get cavitation and move your pump off this mortal coil...(ie, self-destruct it.)

You could run the falls unfiltered, and run a smaller pump and filter for the filtering job.

You could try dividing flow from the falls pump, but I'm having difficulty imagining a way for that to be consistent under different filter loading conditions, without constant adjustments.

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I've moved this summary up from the bottom - this is where you arrive at after this longish answer:

  • The short summary is that you will be able to find pumps that provide both the flow rate and head that you want for almost any filter BUT, you will reduce your pump costs by using a large, low pressure drop filter. Filter costs should be low compared to pump costs and skimping on filter size is unlikely to be cost effective.

I do not see a pump specification. If that is an 'open' parameter you can notionally achieve almost any flow rate with any filter by using enough pressure BUT there are secondary affects which make it unwise to take this to extremes.

You should be able to buy a pump + filter combination that gives you the required "head" for your waterfall. The size of the bond has minimal bearing on the result. In your case a 4000 l/h flow will drain the pool fully in ~= 990/4000 x 60 minutes =~ 15 minutes - so minor variations in flow over periods of seconds to even tens of seconds will not make a lot of difference to pond depth or pumping head.

If the filter you us is rated at a lower flow rate than you wish to achieve you will get more flow rate with more pump power. There are sensible limits to how far you can take this.

As examples only:

If you have a filter where pressure drop is linearly related to flow rate then if it is rated for 3000 litres per hour at a given pressure drop, then it will handle 4000 litres per hour at a pressure that is 4000/3000 = 33% higher than its rated 3000l/hr pressure. This may be acceptable but would ideally be avoided. But -

If instead, pressure drop increased as the square of flow rate, then going from 3000l/hr to 4000 l/hr increases pressure by a factor of (4000/3000) squared = a 77% increase in pressure for an only 33% increase in flow rate. This is a substantial pressure increase for what you gain.

As a rough guide pump flow rate at a given head or head at a given flow rate relates directly to motor power and cost will increase very roughly accordingly. Whereas adding a second filter of the same size in parallel with the first doubles the flow rate you can achieve at the same pressure, and as filters and housing are liable to be much lower cost than pumps, it makes much more sense to use two filters, or to use a larger filter rather than substantially increasing motor size.

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