To explain the secondary bio chamber better -
Buried Under the rubble at one end is a small flow pump, this creates turbulance and flow through the rock as well as keeping it free of gunk.
The other end where flow is limited is the DSB.
The carbon and GFO returns into this secondary stage which is the water feed for this area at a low flow rate, so the water is circulated many times before re-entering the system.
There is a small weir to the rear of the sump where the water is returned to the main sump
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