Here are three ways to treat it https://www.femoran.com/hvac-plumbing-construction-blog/home-plumbing-how-to-fix-a-slow-drain
If the solutions above don't work, try getting a plumber's snake. It's safe for a septic system and useful for a lot of draining/plumbing problems!
You may need to clean out your traps so add some baking soda, about 1 cup to the drain, add enough vinegar to get to the trap and then let it fizz and work for an hour or so. It might be good to do this before going to bed so the mix can work longer. After, rinse with hot water. If you add a bit of baking soda when doing your dishes you will not have a problem with drains since it helps to cut the grease and keep your lines clean. .
Remove as much as possible with a snake or drain cleaning tool. Pour ½ cup baking soda then ½ cup white vinegar in the drain and cover, letting sit for 20 minutes. Boil water, remove cover, and pour water down the drain. You may have to repeat the process.
Also, green gobbler products are septic safe.
have you emptied septic tank recently? Is it full? https://bfplumbingbayarea.com/blog/clogged-drain-vs-septic-backup/ One of the safest things to do is boil a huge pot of water at least 8 qt. and dump into each of the drains repeat. Are the areas where the lines/pipes run free of any plants/trees that can cause drain pipe problems,like breaking of the pipes? These household chemicals kill much needed bacteria in septic systems http://paradisevalleyseptic.com/keep-your-septic-system-healthy-by-avoiding-the-dirty-dozen/ "People have also believed that putting lime or baking soda in their tanks is helpful. This is true in certain situations because most enzymes and bacteria grow best in a non-acidic environment. By adding lime or baking soda, in the proper amounts, the pH level is brought to a neutral condition. On the other hand, too much lime and baking soda can also be harmful. Acids should never be added to a septic tank." more info. http://www.kuettelseptic.com/media/pdfs/Info_About_Septic_Sys.pdf Do you use any type of enzyme maintenance? No septic tank additive will work better than a septic tank's natural function and useof healthy bacteria. No product will allow a homeowner to escape regular septic tankpumping and inspections. In fact, some additives can harm the system and causeseptic system failure, which will then require replacement.
Probably best to get the drain and septic system evaluated by a plumber. We do not recommend chemicals. We often write about these such things. Check us out https://www.santarosaplumbingservice.com/santa-rosa-plumber-blog for more information.
Feel free to visit http://santarosaplumbingservice.com as well.