Lots of varied suggestions...thanks for chiming in gents!
Roots?
Could be. The 32yr. old drain field is surrounded by large live oak trees and covered with grass. The laterals are down about 24" from the surface according to my previous excavation.
Not enough water?
I don't think so. This parameter hasn't changed. About a thousand gallons/month.
Last Pumped?
About 6 years ago..for the first time when I needed to replace the line between the tank & field. With only me here, there's not much in the way of solids, nor waste. Nothing floating on top that I recall like a visible scum layer. Not that many solids in the tank either. The guy doing the work said it all looked fine. All vegetable/meat waste/scraps go in the trash. I never pour fat/grease/oil down the drain.
Filter?
No. It's never had one. But I think they're a great idea actually. If I had wife + kids, I'd definitely have one. I didn't think it necessary with just me here. I'm still puzzled why black water + waste was in the drain line. It should only be clear.
Chemical Treatments?
Never used any on a regular basis. I did pour a gallon of HCL (Muriatic) acid into the access port downstream of the tank late last night. A neighbor with considerable construction experience came over and suggested this as a first step. He also said he uses rock salt to discourage tree roots in the laterals. Said a 'bio-mat' can form over time around the holes, inhibiting draining.
One problem is I'm not sure of the molarity of the HCL acid. Another is I don't know the size of the drain field to determine an appropriate dose. I also don't know if there is a distribution "box" or a series of T's.
With 4" DWV, then the ID area per foot is ~ 150 cubic in. If the DF has two 16' laterals then that's 4,825 cubic in. or about 3 cubic ft. If it has 4, then it would be 6 cf. A gallon of water occupies 231 cubic inches. So that comes out to about 21ga. of water IF the entire field is accessible to water AND clear. So one ga. of acid may not be enough. If I remember correctly, acid is heavier than water so it will slide down underneath it along the 24' long drain line, pitched at 1/8" per foot. Once at the bottom of this line all the lateralls should be level so I don't see it going much further on its own.
He suggested buying a balloon fitting for a 4" pipe and using a garden hose, pressurizing the drain field with water, forcing the acid through the holes. Makes sense. Due to a temporary water utility issue, my current house pressure is 98psi. It's usually around 80psi. This could work. These balloon things are inexpensive at the box stores. Plus I could monitor the water flow rate into the field at the water meter. I'm liking this idea!
I'm also thinking about sodium hydroxide (lye). As I mentioned originally, strong alkalines liquify proteins + are very, very slippery. Since one ga. of acid is already down the hole, I'll stick with acid for the time being.
I'm liking copper sulfate too. Thanks!
Laundry detergent?
Always used powdered. It always dissolves too as I have soft water. This brings up another thing though...if I recall most washers filter the lint, then flush it down the line with the rinse water. Not sure how much this amounts to, but there sure is plenty of lint collected by the dryer screen.
I'm going to try the water balloon trick and see how it works.