Following is a quote from https://brewerywastewater.com/brewery-wastewater-101/brewery-wastewater-101-septic/
(keeping in mind this is written for breweries, but i suppose some homebrewers rival nano breweries)
"Some of the issues of a brewery on septic are:
First there is the pleasant mental image of the septic tank turning into a fermenter. It will create foam, fill with spent yeast, and pH will drop. There is potential for back pressure issues due to CO2 generation, and due to the low pH and lack of oxygen the proper bacteria will not grow well so there will very little actual treatment going on in that tank. The tank will have to pumped quite often to remove the spent yeast, and this gets expensive.
Suspended solids (yeast, grain, hops) and dissolved solids (sugar) will pass through the septic tank and will plug the drain field over time. The solution here is usually to dig up the drain field and start over, probably on a new patch of ground. It takes time for this to happen, probably years, but it has happened over and over again and it’s expensive. Several projects have had to replace their drain field in under 2 years!"