My house was built in 1978. I don’t know if you would consider it “modern”. It is built on a concrete slab with brick veneer and is air conditioned. I noticed in your comments about things going into the septic tank, especially water from the washing machine. This a source of major problems for septic systems. When my plumber stubbed out the sewer drain lines there were three. He offered me some good advice, which I followed. The larger 4-inch line I ran to my septic tank, which was the waste from my toilets, bath tub/shower, and lavatories. The two remaining were 2-inch lines. One from the kitchen sink I connected to a grease trap and then a 60-foot “field line” buried in washed gravel. The second 2-inch line came from the washing machine. I connected it to the field line after the grease trap. This keeps all the bleach, strong soap, and clothing fibers out of the septic tank and the grease from the kitchen sink. During the time I’ve lived here with my family, I have only had to clean the tank two times. I did it myself with a bucket and shovel handle I improvised. As you said, there was about a two-foot layer of sludge that was soft. The rest was water. We are careful with the type of toilet paper we buy and make sure that no feminine products are flushed. Every few months we pour a package of yeast down the drain to the septic tank. Like you said, though, the natural chemicals are already there that cause the digestion process.