What could be causing DO yo drop off in the effluent
DO dropping can be caused by many things, are you seeing high algae growth in any of your tanks? Algae rapidly depletes oxygen.
Also so high temperatures above 86 F do not allow oxygen to be as readily dissolved.
Do you have and aeration and/or mixing basin? Is it properly aerating the water, and when sampling are there dead zones (areas of little to no oxygen).
Your bugs may be using up all the oxygen to digest your organic and inorganic wastes. I would recommend spot sampling your aeration basin. Look to see and track your DO dead zones, DO should be low in the front of the system and highest in the effluent. If it is not that way you have a major issue and your bugs may quickly experience a massive die off.