On Tue, 19 Mar 2019 09:54:20 -0700 (PDT),
wrote:

I was battling drain flies for a couple of years, not knowing what was going on for many months. A few months ago, with a stench coming from beneath the kitchen sink, I took apart the drain piping and found the worms that beget the flies. I tried putting bleach down the drains. I tried vinegar. I tried vinegar and baking soda. I tried overnight soaks, blocking off the drains so I could let the various chemicals sit for several hours. Finally I found a web site that said to use an "enzyme" drain cleaner. From Home Depot, I purchased "Instant Power Commercial Drain Maintainer" advertised on the label as "bio-enzymatic with natural enzymes," made by Scotch Corporation, Product #1510. It has a "100% money back if I fail 'but I won't' " sticker on the gallon jug. It is also called "Instant Power Commercial Drain Cleaner," with the same product number (1510). I stopper'd the drain pipe under the kichen sink and let the pipes soak for 1.5 hours, mixing 1 part of the chemical to 2.5 parts
water until I filled the drain pipes completely.

Usually I see half a dozen drain flies by noon. So far I have seen only one.

The staff at Home Depot also recommend the "Green Gobbler" products they have. The above products are among the drain cleaners in the plumbing section.

I paid $14 for the gallon jug at Home Depot.
Septic or city sewer?