Hello vawtman,
Like jmk said garbage disposal.
The solids has to come from some where, chemicals may stop the decomposition in the tank adding to the solids, but think of this, mix those chemicals in 1500 gallons... (my tank size) with 50 gallons per person per day of water flow (mine) takes a huge amount of chems to realty cause a problem.
The chems will not stay in the tank, as new water comes in chems gets flushed out. Unless you keep adding the chems. Tanks is sized to take a weeks water for your home. (most codes) Reasonable use of chems will not cause you a problem. (septic safe ones). If you have softened water, use MUCH less soap, the water is already soapy! Had to cut down the dishwasher soap, it was eating the glassware.
Solids: my wife would grind the whole chicken after dinner. I told her to stop using the garbage disposal like that (she did but the damage was done, little did we know), Scraps are fine... all things relative. Think how much bulk is in your garbage, will be near the same in the tank, much of the solids do NOT decompose. Do this for a year.
Our field did last 20 years, cost $4800 to add another septic field. Fat does not break down, ends up floating. Lint is a killer, it does not always float or sink (floating or sinking is great, it stays in the tank), lint can plug up the septic field. Lint can decompose, so if limited it will not kill your field. We pumped the tank every 3 years, the solids were not bad, soil was not great. Should not have passed inspection. City inspector was fired later after I bought my house. (he approved many bad fields, In my sub, 1 of 3 fields failed within 5 years, ours was only marginally better)
Check out the library, I found couple books and read them before I started using septic systems.
Many urban legends about septic systems, be careful. The pumper guy is not always a septic expert, check before you ask. (Local co. use collage kids to pump the tanks, I think they create the urban legends:-)
Have fun,
Scott.