Where ever you are draining your pond bottom drain to will be at the same water level as your pond and by removing water from this pre filter the law of gravity is pulling water from the pond through the bottom drain line to fill the pre filter back up.
Bottom drain gravity fed pond filter setup.
All chambers except ch4 have 3in.
They should be more aptly referred to as a bottom filter feed or supply while a pump in the pond is aesthetically unattractive and can t remove large debris a 4 inch bottom drain can take large leaves as it continually vacuums your pond.
The fewer bends the better.
If a straight shot is not possible use 45 degree elbows to raise or turn the pipe rather than 90 s.
There is a waterfall 2 venturis v 2 bottom pushes bp a surface skimmer sk and 2 bottom drains bd in the pond.
There is an additional chamber the dry sumpattached to the filter.
Dig out the trench for the 4 bottom drain pipe and run it all the way to where the rest of the filter system will go.
You then have a pipe from the ponds bottom drain sloping downwards underneath the filter then up and into the top of the filter.
The term bottom drain is really a misnomer.
The main filter consists of 5 chambers ch1 to ch5.
Put the bottom drain and all the pipework in place to check all the measurements before gluing.