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I'm just about finished building my Aquascape style bog filter—snorkel/centipede design with water matrix settling chamber and 3 different sizes of gravel.
I do think that the snorkel/centipede and settling chamber aspects of the design make it superior to the basic slotted pvc + pea gravel design because it allows backflushing that would be impossible otherwise.
But after finding this in some Aquascape literature from way back when (excerpt from The Pond Builder's Bible), I think that using graduated gravel sizes may be unnecessary:
The whole idea behind the graduated gravel sizes and putting large cobble at the bottom and small gravel at the top is to allow the water start out moving slow at the bottom of the filter and speed up as it rises due to the reduction in void space as gravel size gets smaller. Theoretically, it should also make backflushing more effective because the increasingly smaller gavel should trap increasingly smaller bits of sediment that can then be backflushed into a larger void space below it, allowing it to fall down into the centipede and be pumped out.
But looking at the chart above and the explanation that goes with it, the water storage volume of each type of gravel is very consistent across 3 of the 4 types measured. That would indicate that water velocity (and space between rocks) would remain basically the same all the way up the water column.
This defies common sense—bigger rocks should have bigger spaces between them. That's obviously still true, but perhaps it doesn't matter in this case because the sizes are not different enough and the total volume of gravel used is not great enough for a real difference in water storage volume to play out. Maybe if you built and enormous filter and used bigger rocks on the bottom and pea gravel on the top you'd get closer to the expected results.
All that to say: If you're building an Aquascape style filter, you may be able to simplify the design by using a single size of gravel (maybe 1 - 2") without losing any functionality.
I do think that the snorkel/centipede and settling chamber aspects of the design make it superior to the basic slotted pvc + pea gravel design because it allows backflushing that would be impossible otherwise.
But after finding this in some Aquascape literature from way back when (excerpt from The Pond Builder's Bible), I think that using graduated gravel sizes may be unnecessary:
The whole idea behind the graduated gravel sizes and putting large cobble at the bottom and small gravel at the top is to allow the water start out moving slow at the bottom of the filter and speed up as it rises due to the reduction in void space as gravel size gets smaller. Theoretically, it should also make backflushing more effective because the increasingly smaller gavel should trap increasingly smaller bits of sediment that can then be backflushed into a larger void space below it, allowing it to fall down into the centipede and be pumped out.
But looking at the chart above and the explanation that goes with it, the water storage volume of each type of gravel is very consistent across 3 of the 4 types measured. That would indicate that water velocity (and space between rocks) would remain basically the same all the way up the water column.
This defies common sense—bigger rocks should have bigger spaces between them. That's obviously still true, but perhaps it doesn't matter in this case because the sizes are not different enough and the total volume of gravel used is not great enough for a real difference in water storage volume to play out. Maybe if you built and enormous filter and used bigger rocks on the bottom and pea gravel on the top you'd get closer to the expected results.
All that to say: If you're building an Aquascape style filter, you may be able to simplify the design by using a single size of gravel (maybe 1 - 2") without losing any functionality.