Septic system failed - should I try installing an aeration unit?
I just had a breakthrough in my situation in the last couple of weeks.    My leach field was about 95% clogged, very little if any movement.    I had already diverted the graywater to another solution, which helped for awhile, but slowly the field failed to the point of essentially total failure.  I have a single septic tank and wanted to try aerating it without the cost of a second tank.    I learned a number of things that I haven't seen explicitly explained on any of the forums so I wanted to put it here to hopefully help someone.

I installed my aerator a few months back.    Too much air volume and placing it in the center of the tank caused to much churning and cause solids to exit the tank.    I did notice the smell though went from putrid to a smell like freshly turned soil (not coincidentally; the same aerobic bacteria live in airy soil).   I spent a couple months thinking it didn't work.    Water still not going out.   But in the last couple of weeks I'm actually seeing the water levels fall to STAY at the edge of the outlet pipe!!!!     First time ever!!     I used the techniques below to avoid replacing my field without buying a $1000 solution.

You CAN aerate a single tank, but it's imperative that the aeration process doesn't agitate the incoming solids.    They have to come into the first baffle and separate so the lighter material can rise and heavier solids fall.    To accomplish this you have to use very fine bubbles and they have to be generated as far from the incoming line as possible, or they have to act inside an isolation vessel (there are videos where people use 12" pipe to contain the aerator).    You can get diaphragm diffusers for as cheap as $35 if you search around, connected to PVC pipe, and use an air valve to supply JUST ENOUGH air pressure/volume to activate the diaphragm (air RELIEF valve, not cutoff.   You'll damage your pump if you restrict airflow inappropriately).    Too much and you get a rolling torrent (which you don't want).    Place this diaphragm(s) near the output baffle or let it work IN the output baffle.    This allows all the agitation to happen away from the settling part of the tank.    THIS IS THE KEY, and is the reason why a separate aeration tank is ideal, so there's no chance of agitating the incoming.    Also I had a 4" TEE on the outlet side which served as the separator.  I put the two diaphragm diffusers on EITHER SIDE of the outlet TEE.  You don't want bubbles going up INTO the TEE which could siphon some debris up in there.    I got a septic filter brush and put down in the pipe to help filter the effluent, though as clear as the water is going out I don't think it was necessary.

One suggestion I received from a local contractor for my single concrete tank was to empty it, get inside and build a cinderblock wall giving 2/3 incoming/settling, and 1/3 aeration, but the tank has to be pretty good size to do that.   You need several hundred gallons of aeration space.    I put this on the burner as a next option, but turns out it wasn't necessary.

If you have a way of opening the other end of your leach lines to extend your field, at least temporarily, you get this freshly energized aerated bacteria through there quicker to let it go to work.   My temporary relief is now closed off and the field appears to be working!!

Note I did NOT need to buy super duper amped up bacteria to add.    From everything I've read this is hogwash and money makers for people.    That's like buying weed seeds;  give dirt some rain and you'll get the weeds.   The bacteria will fill the space if they have oxygen.

My findings confirm for me that the low agitation solutions available out there for $500-$1000 would certainly work.    I just chose to play around with a DIY solution.

Hope this helps someone.