tathraman wrote:
I have a SuperTreat septic system which waters my garden. Some plants
thrive, others don't. I'm wondering if this could be due to
phosphorous in dishwashing and laundry detergents?

I suppose it is possible but I wouldn't jump straight to that conclusion.
Another consideration is that cleansers (especially dishwasher detergent)
contain quite a lot of sodium carbonate (washing soda), this will raise the
pH and add sodium to the soil.

What area does the effluent get spread over?

How often do you do laundry and dishes?

Which plants do well and which do not?

Is there any sign that plants near drippers, emitters etc do better or
worse?

David