We have gnats (or bity flying bugs) in our black tank.I need ideas.

We keep the black tank closed except for dumping. Gray tank open most of the time (99%). We: rinse / swirl every dump, I've done the  - down the tank swirl cleaner sprayer, added clorox to the cleaning induction mixer , tried chems, filled the tank with clorox water and let stand, 1/2 filled the tank with clorox and gone for a drive, sprayed barn strength flying insect pyrethrins into the toilet.

Now the good side. This only happens at one spot - southern Texas. RGV. We leave here and in a week they are gone. Did not have a continual invasion in Arkansas, Memphis, Michigan, Kansas, Fredericksburg, TX, until 1 month after getting to the RGV.

Any ideas folks? I'm out of ideas.

Sounds like sewer flies to me. Try putting a dip in the waste hose to act as a trap.

OK - Black tank is closed except for dumping. I thought that would stop them.

Nothing in any of the "trapped" drains and with the flood of black (bi-weekly) water / waste the outflow line is full for 3-5 minutes. (Clear 90 on the dump line) so that was one thought I had also.

Thanx for the speedy reply.

Edited December 8, 2018 by Bill B

The roof vent connection pipe is likely connected to both tanks, giving the flies a path to the black tank from the open grey tank. The traps at the sinks, shower, etc., would keep them from showing up inside. I've often seen the flies when uncapping the sewer connection at various parks to hook up the waste hose. Adding a trap in the waste hose will keep them out of both tanks, yet still allow the grey valve to be left open.

Off to HD and get some parts to try your suggestion Dutch. Never Mind - just thought of a way to try it with a couple of 45's to get the elevation I need to make a hose trap. Sewer is at ground level and I need some elevation to make the trap.

Oh the fun, playing with sewer hose in the rain. LOL

2 Tanks, 2 roof vents, 2 different locations and I can see the below decks vent pipes going up into the walls.

OK Dutch - U trap made with a 3" standing water on both sides. (we park within 2' of the sewer connection) Changed the 3' length hose for a 5' turned the 90 to right angles and 'pointed a 45 up 

Edited December 8, 2018 by Bill B

follow up

Fecal flies can move through water, so I would suggest closing both tanks, then REMOVE HOSE AND THOROUGHLY FLUSH AND LET DRY.  Next time you dump, after you have run black, then grey, close both and remove end of hose at sewer connection and cap off, slip under rig.   Yes, it will make more work for yourself, but.....

I discovered drain flies in the shower and found this interesting website:   https://www.doyourownpestcontrol.com .

The home page has tons of links by pest category which lead to further help to identify what specifically you are dealing with.

Here is one of the links on fly categories:   https://www.doyourownpestcontrol.com/fly-table-summary.html

You can also search midges, fruit flies, biting flies, gnats and more.

This is a product they recommend for drain flies, safe for RVs and septic.  

"InVade Bio Drain Gel combines citrus oil and premium microbes in formulation thick enough to coat the insides of drains allowing it to eliminate odors and eat through scum. When first using InVade Bio Drain apply every other day for the first week and then continue to use once a week afterwards."   https://www.domyown.com/invade-bio-drain-p-352.html  

The 4th question in Q & A specifically is about RV use, with a tip to follow up with HOT water after it sits overnight.  Also "pet safe if used as directed."

Many reviews on amazon said it worked with only one or two applications.  I know this discussion is about the black tank system, and I'm working with the grey, but I'd call the people at Do Your Own Pest Control to see what they recommend for that specific application.

The price seems to vary even on the website and in their store, where it was cheaper than the link I gave you above.  Check elsewhere, too.  Good luck and keep us posted. I'd like to know how you resolve the issue.

Boiling water, bleach, vinegar and soda...nothing's worked with these stubborn things living in the shower drain. I'll be ordering some Invade Bio after we're settled at our next stop.

One more link, same source:

How To Get Rid Of Drain Flies:  "Drain Flies are also called moth flies, sewer flies or filter flies."

https://www.doyourownpestcontrol.com/drainflies.htm  

try putting a cup of any cheap cooking oil in the tanks. then dumping in very hot water to spread it out over the "water" bugs will drown, or just get stuck to it and not be able to fly. thus no breeding.

Thanx pack - same.this year - left, gone a week - so are they - back a week and so are the bugs.

The cycle continues 

I guess on solution would be stay out of TX as you say you only have this problem when visiting there. 

LOL @ Toes - I saw the weather / roads  north of here - gnats are fine in comparison to thawing out the tanks

  On 11/14/2019 at 9:47 PM, Bill B said:

- left, gone a week - so are they - back a week and so are the bugs.

Do you leave your gray tank dump valve open? Sewer flies live in most sewer systems and given time they usually do lay eggs in the RV waste tanks. I have found that when I leave the gray valve open I also make a P-trap in the sewer hose by forming a loop that will stay completely full to prevent the little buggers from visiting.