Get Rid Of Sewer Files in Your RV

how to get rid of drain flies in RV

The best way to get rid of sewer flies in an RV is to eliminate their breeding ground. This means discovering if they are coming from black tanks, p traps or plumbing lines. Once you destroy the breeding site, the adult flies will have no place to lay eggs and their lifecycle will cease.

Remaining adult flies can be sprayed with an aerosol. You can prevent future infestations by maintaining adequate sanitation and keeping your hose dump valves closed when hooked up.

get rid of flies in rv tanks

Keep reading to find out more on how to keep your motor home sewer fly free!

What are sewer flies?

Sewer flies, also known as drain flies, are small black flies that spend most of their time perched on walls or ceilings. They prefer damp habitats and live in drains, sewers, septic systems, and shallow water with scum build-up.

getting rid of sewer flies in an rv image

The larvae, which are aqueous, primarily live and remain in the water, grow, and feed on organic matter.

Needless to say, nothing brings down a vacation like a drain fly infestation. While they are not known to transmit diseases, they look nasty, and leave residue when crushed. 

How to identify drain flies:

  • The adult flies are small, about ⅕ to ⅙ inch long.
  • They tend to be black, gray, or brown.
  • Adult flies may have wings covered in fuzzy hair wider than their bodies, which appear heart or roof-shaped when at rest.
  • Drain flies look like moths, and they fly in short, jerky lines.
  • They have long segmented antennae.
  • They are commonly found near their breeding site or point of entry. 

While they may have a lifespan of between 8-24 days, drain flies can lay and hatch up to 300 eggs in just 48 hours. This is why spraying the adults without getting rid of the larva will not help get rid of them!

Signs of a Fly Infestation 

Recurring presence of adult flies are generally a sign of a real problem. In a motor home, you should take action immediately upon seeing any adult drain flies. Female flies lay anywhere from 30 to 100 eggs, which hatch in about 32 to 72 hours!

What Causes Drain Flies in an RV?

RVs have a unique water holding system. Gray and black (sewer) water is stored in holding tanks until emptied. The raw sewage in the black water tank is prime real estate for drain flies.

However, the gray water tank can also be a home to flies.

So how do they get in?

A common entry point into your RV is through open dump hose valves connected to the sewer hookup. When you leave the gray or black tank valves open, flies and gnats can make their way inside from below. Once inside, it doesn’t take long for them to make a new home.

RV sewer line hookup for black holding tank

Drain flies thrive in moist organic waste, especially solid waste. Black holding tanks provide an ideal environment, although buildup in p traps (if your RV has those) can also become breeding grounds if there is enough buildup. 

Once they begin breeding in holding tanks, a focused effort will be required to get rid of them once and for all. If you do not execute properly, they will return. 

How to Get Rid of Sewer Flies

You can rid of RV sewer flies by using the following these steps:

Step 1) Flushing Black and Gray Holding Tanks

Traditionally chemicals like bleach are not very effective on drain flies or larvae since the liquid only momentarily makes contact with them as it passes through the drain. A holding tank is a different story. 

Tools to clean the RV’s holding tanks include:

  • Built-in Rinsing systems: Some RV’s have their own built-in rinsing systems. The video below may help you identify if yours has one.
  • RV Dual Flush: Connect a hose line, sprays water into RV sewer lines to clear out clogged pipes and rinse tanks. Creates a powerful spray that helps reduce odors and malfunctions in RV holding tanks and hoses.
  • Tornado Rotary Tank Rinser: Dislodge and flush stubborn waste deposits and odor-causing particles left after holding tank is emptied. Has a 360-degree rotating head and 10 high-pressure nozzles. Clean all surfaces of your holding tanks.
Tornado Rotary Tank Rinser with Hose
  • Tank wands: This allows you to flush the black water tank by connecting it to a waterline. You can connect it to a hot water line to give the tank a power rinse.
RV tank wand cleaner

You can also have them hydro-cleaned to get rid of any dried out waste that might be stuck at the bottom or on the outlet. There area number of companies that provide this service.

Step 2) Eliminate the Breeding Ground 

Once the tanks are clean, you must kill off any remaining eggs or larva. This can be done by using:


Take caution when using bleach in your system or tanks. Some do not recommend it, as it can interact with the materials and coatings. Many other RVers use bleach without issue (or so they think?). 

red bleach bottle for RV tanks

Unlike treating drain flies in pipes, an RV has a closed black tank, which allows the chemicals to sit in place. You can use bleach to kill larvae. Empty the contents of the black tank, fill it with fresh water, and add bleach. Let it sit for a while, then drain and rinse the tank again. 

Take caution to avoid mixing bleach with ammonia or any other chemicals. The combination of bleach with other liquids can be potentially fatal. For instance:

  • Ammonia create toxic chloramine vapors. 
  • Rubbing alcohol creates chloroform.
  • Mixing bleach with vinegar results in toxic chlorine gas.

Enzyme Cleaners

These attack the layer of film of organic materials where sewer flies lay eggs, and larvae develop into adult flies. By getting rid of this layer, you take away the flies’ food source.

  • Drain the black tank of its contents
  • Fill it with water and add the enzyme drain cleaner.
  • Allow it to site overnight.
  • Drain and rinse the tank.

These cleaners have the additional benefit of getting rid of the odor.

Drain Fly Gels

invade bio gel for drain flies

These come in the enzymatic or chemical form and remove organic buildup in pipes or blockages. They digest the solid wastes, eliminating the flies’ breeding grounds. Examples of  common drain fly gels are:

Drain gels are usually expensive, and are best used in pipes.

Step 3) Kill Adult Flies

electric fly swatter

After breeding grounds have been removed, you need to get rid of the adult flies as well. Common methods include using:

  • Flytraps: Use apple cider vinegar to create a fly trap and set it near the drain to attract and kill the adult flies. Other different versions of fly traps include the disposable bag traps and the sticky yellow ribbons.
  • Electric bug zappers/Fly light traps: flies are attracted to light. A zapper/fly light trappers can get rid of adult flies. The main disadvantage is that they can’t eliminate the breeding grounds.
  • Insecticides: The use of contact aerosols (Pyrethrins) can help kill adult flies. Read and follow the instructions and safety guidelines given. 

Prevent RV Sewer Flies from Occurring

RV sewer hookups are often the culprit for fly invasions. An easy way to prevent drain flies is to close the black and gray RV tank dump valve. Only open is when you are actively dumping. This will help prevent unwanted sewer-dwelling guests from drifting into your RV.

Check out the HepvO Sanitary Waste Valves as an option instead of p-traps (which can also become breeding grounds when not maintained. 

Standing water deposits – other than those in tanks – are also a good idea to remove.

Final Thoughts

Identifying and treating sewer flies with the proper methods is the only way to get rid of them. It may take several methods and attempts to get rid of the flies completely. Therefore, It is crucial to focus on prevention. If you ever see a stink bug in your RV, learn what you can do to prevent those as well!

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