Settling tank is a 55 gallon food grade drum I buried.Where and how big is your settling tank?
The 4" PVC line from the bottom drain goes under the pond and enters the tank about 8" above the bottom.
There is a sludge pump in the center of the tank for when I want to drain it. There is a rope I draped over the green foam covered inlet pipe to pull the pump out in the winter.
The external pump is obviously installed with unions so I can pull it out for winter.
The BD has an air bladder to push an air column up to promote flow towards the drain and it helps oxygenate the water.
The biological filter in the Helix also has a column of air to really promote bacterial growth.