Hello. I'd like to detect the liquid level and solid level in a septic tank. Is there a sensor that would do well for this? A bit of a corrosive environment.
Thanks
Do the solids float or sink 
I doubt if it's possible since "solid" is only a verbiage distinction from liquid. It's a sludge with a consistency slightly more dense than liquid.
Besides that, the liquid level in a properly functioning septic tank is always full.
Yes, ultrasonics - the obvious approach - are not going to "cut it" so to speak with no clear boundary.
Well that depends, if it floats, it has a less dense consistency. 
But I'm not an expert in this area ...
The poop settles at the bottom where the fat will settle at the top and a more watery liquid in the middle. I need to tell where the more solid stuff at the bottom begins
Not as though it needs to be a constant measurement.
Septic tanks usually run for years without needing the solids removed.
So with that in mind, check once a year with a dipstick.
If you follow smoke alarm battery replacement recommendations, do it on your birthday. 
a camera and a trained AI?
Watching my septic being pumped, there is a SOLID covering of about 8 inches, then liquid down to a sludge layer in the bottom.
Paul
it is recomended that a sepitc tank be pumped yearly.
the use makes the most important change from a mostly liquid to a solids filling up.
there are at least 2 styles. one have an overflow where the liquids maintain a constant level for years.
the other has an auto-siphon so that the field alternates from wet and dry.
a constant wet can result if a bio film blocking drainage.
the auto-siphon floods, then drys the field, supposedly reducing or eleminating the bio film.
as was stated, the stuff we would call the 'solids' do drop, but has a density not much different than the water.
your best best is a fish finder with a LOT of ability to adjust sensitivity to the bottom.
but, I have never heard of this application.
I think we get requests for sonar and the result is always, that there is not sensor for Arduino's.
Well, for the liquid height. just plumb a pressure sensor at the bottom. Pressure is linear to level.
For the sludge level, a marine depth transducer.
-jim lee
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