How to Get Rid of Drain Flies for Good (Sink & Sewer Flies)

Short answer: Drain flies breed in biofilm — the slimy layer of grease, soap, and food residue coating the inside of your drains. Sprays, bleach, and boiling water hit the flies you can see but leave that film in place, so new flies hatch within days. To get rid of drain flies for good, find the drain they are coming from and remove the buildup they breed in. One ounce of Pure B.S. Bio Surfactant Concentrate poured into each affected drain overnight, once a week for four weeks, removes the breeding site.

What are drain flies?

Drain flies (Psychodidae) go by a lot of names: moth flies, filter flies, sink flies, sewer flies, drain gnats, sewer gnats, or just “those little bugs in the bathroom.” They are all the same insect. If you are searching for how to get rid of sink flies or sewer flies, this is the guide.

They are weak fliers and usually stay in the same room where they hatched — most often kitchens, bathrooms, basements, utility sinks, floor drains, or commercial food-prep and restroom drains.

A female drain fly can lay up to 300 eggs in 48 hours. Larvae hatch in 32–48 hours and develop into adults within 8–24 days. Because the generation time is so short, a single unnoticed drain can produce a visible swarm within two weeks.

What do drain flies look like?

Drain flies are 1.5–5 mm long, gray to dark brown, with fuzzy, hair-covered bodies and wings that fold flat over the back like a roof. At rest they look like a tiny moth stuck to the wall near a sink, tub, or floor drain. They make short, hopping flights rather than sustained flying.

Are drain flies the same as gnats or fruit flies?

No. Fruit flies are tan with red eyes and gather around ripe fruit and trash. Fungus gnats are slender, long-legged, and hover around houseplant soil. Drain flies are fuzzy, moth-shaped, and cluster on walls near drains. If the flies are near a drain and look like miniature moths, they are drain flies, and the drain is the source.

Why are drain flies in my house?

Drain flies come for one reason: a food source. They feed on organic matter in standing water and damp buildup inside drains. The most common places drain flies come from:

• Kitchen sink drains (grease, soap scum, food residue)
• Bathroom sink and shower drains (hair, soap, skin cells)
• Floor drains in basements and garages
• Rarely-used guest-bath drains where the P-trap has dried out
• Commercial floor drains and mop sinks
• Garbage disposals
• Septic vents and sump pits

If you only see flies in one room, the source is almost certainly a drain in that room — not an outdoor entry point.

How do I find which drain the flies are coming from?

Dry each suspect drain, then tape a piece of clear packing tape across the opening overnight, leaving a small gap for airflow. In the morning, the drain producing the flies will have them stuck to the tape. Treat that drain first, and treat the others in the room while you are at it.

Are drain flies harmful?

Drain flies are not vectors for human disease and do not bite. They are primarily a nuisance and sanitation signal. Because they live in biofilm — which contains mixed bacterial populations — they can mechanically transfer bacteria onto surfaces. In a food-service or healthcare environment, that is a failed health inspection waiting to happen. In a home, the reputational damage is the bigger issue: guests notice.

Do home remedies work?

The common advice — boiling water, bleach, baking-soda-and-vinegar, or insecticidal drain sprays — works on the symptom (adult flies) but not the cause (biofilm). Drain fly home remedies fail for the same reason: the breeding site survives.

Will bleach kill drain flies?

Bleach can kill some adult flies and larvae it touches directly, but it does not get rid of drain flies. It rinses through the pipe before it gets into the biofilm where eggs and larvae live, so the next generation hatches on schedule. It can also corrode metal drains, and if you are on a septic system it kills the biology your tank runs on — your tank depends on the anaerobic bacteria bleach destroys, so you trade a fly problem for a tank problem.

Does boiling water kill drain flies?

Boiling water kills whatever it touches for a moment, then cools quickly once it hits cold pipe and runs past the biofilm stuck to the pipe wall. It flushes some loose debris but leaves the breeding site in place, so flies are back within days.

Does vinegar kill drain flies?

Baking soda and vinegar produce a short foaming reaction with almost no scrubbing effect and almost no dwell time. A vinegar trap (a cup of vinegar with a drop of dish soap) will catch some adults, which is useful for confirming you have drain flies, but it does nothing to the eggs and larvae in the drain.

Do drain fly sprays work?

Drain-fly sprays and foggers kill the adults you can see but do nothing to the 300 eggs laid in the last 48 hours. Adults re-emerge on a 2–3 week cycle, which is why it feels like the spray “stopped working.”

The only durable fix is to remove the biofilm itself.

What actually works: remove the biofilm

Biofilm is the glue that holds a drain-fly infestation in place. Larvae eat it. Adults lay eggs in it. Remove the biofilm and there is nothing left to feed the next generation.

The most effective way to remove biofilm without corroding pipes or harming septic biology is a bio surfactant — a surface-active compound produced by Bacillus bacteria that loosens the film from the pipe wall so it washes away with normal water flow.

How Pure B.S. works

Pure B.S. Bio Surfactant Concentrate is made with surfactin produced by Bacillus subtilis. It helps wet, lift, and release the organic film on the pipe wall so it rinses away, which removes the buildup where drain-fly larvae feed and adults lay eggs. It is septic-safe, non-hazardous, non-corrosive, and safe on PVC. Unlike bleach, it won’t wipe out the biology in your septic tank downstream.

Pure B.S. does not kill flies. It removes what they breed in. It is a cleaner for organic buildup, not a disinfectant or sanitizer.

How to use it for drain flies

• Dose: 1 oz per affected drain
• Method: Pour into the drain at night, let it stand undisturbed 8+ hours, rinse with hot water in the morning
• Frequency: Once a week for 4 weeks, then once a month for maintenance
• Why four weeks: Eggs are already in the pipe when you start, and the drain fly life cycle runs 8–24 days. Eggs laid before your first treatment can still hatch, so a single treatment is not enough. Four weekly treatments cover the full cycle.

For commercial kitchens and floor drains, increase the dose to 2–4 oz per drain weekly. Bulk packaging is available for facility use.

Do I need a foaming applicator?

For a normal household drain, no. Here’s why the overnight pour works, and where its limit is.

Your sink has a P-trap — the U-bend under the basin that holds a permanent plug of water to block sewer gas. When you pour an ounce of bio surfactant in at night, it settles into that trap and stays there. The trap and low points stay wet continuously, which is why treatment placed there keeps working overnight. Eight hours of undisturbed contact is the whole reason the protocol says “overnight” instead of “pour and run the tap.”

What a pour cannot do is keep the vertical pipe above the trap wet. The tailpiece — the straight section between the drain basket and the U-bend — gets rinsed as the liquid passes through, then drains dry. Same for the sink overflow, that small hole near the rim with a channel behind it. If your buildup is concentrated in those above-water surfaces, an overnight soak reaches them for seconds, not hours.

That’s the gap a foaming applicator fills. Foam is gas dispersed in liquid, so it expands to fill a space and clings to vertical wall instead of running past it. Virginia Tech Extension recommends a microbial foam for drain fly larvae for exactly this reason, noting that foam “can fill fly breeding spaces that brushes and gels cannot access,” left in place several hours before rinsing.

We don’t sell a foaming applicator and we won’t point you at a random one. Useful foam depends on both the chemistry and the equipment generating it — shaking the bottle, spraying it, or improvising with a squeeze applicator does not produce foam that holds long enough to matter. A device that makes weak foam is worse than a good pour, because it feels like you did more.

Here’s what actually closes that gap, and it’s free: after you pour, brush what you can reach. Pull the strainer or stopper, run a stiff bottle brush down the tailpiece as far as it goes, and scrub the overflow opening. Virginia Tech recommends the same follow-up — a stiff brush and hot water to remove remaining biofilm after treatment. The trap gets the overnight soak, the tailpiece gets the brush, and between them you’ve covered both zones without buying anything.

If you manage a commercial kitchen, a multi-unit building, or long vertical stacks where brushing isn’t practical, foam delivery is a legitimate professional method and worth asking a pest or plumbing contractor about. That’s the setting where the apartment-complex project below used it.

Does it work at scale?

An apartment building came to us with chronic drain and odor problems across the whole stack — the kind of biofilm load a single-family drain never reaches. Regular treatment broke down the accumulated solids and stripped the biofilm out of the lines, which resolved the plumbing issue and the smell together. The full write-up is here: Apartment Building Plumbing Problems: A Case Study.

How do I prevent drain flies from coming back?

Biofilm re-forms anywhere organic matter and moisture meet. Prevention means routine maintenance, not one-time treatment:

1. Treat drains monthly with a bio surfactant in high-humidity or high-use areas.
2. Fix slow drains within a week. A slow drain means organic material is accumulating — perfect biofilm conditions.
3. Run water in unused drains weekly. Dry P-traps let sewer gases and flies into the building.
4. Clean garbage disposals monthly with hot water and a bio surfactant.
5. Keep standing water out of basement floor drains and mop buckets overnight.

Do I need to call an exterminator?

For a residential infestation sourced from indoor drains, no — commercial pest-control sprays target the adults, not the breeding site, and cost far more than a bottle of bio surfactant. Call a professional only when:

• The source is outside your building (broken sewer line, cracked septic tank, standing water on the property)
• Infestation persists after 4 weeks of weekly treatment of every drain
• You are in a regulated facility (hospital, food plant, day care) with compliance documentation requirements
• The problem is a physical blockage, not a film — hair mass, wipes, mineral scale, roots, or a sagged or collapsed pipe. A bio surfactant breaks down organic buildup; it is not a drain opener and will not clear an obstruction.

Drain fly questions

How do I get rid of drain flies permanently?

Remove the biofilm they breed in and keep it from rebuilding. Find the source drain with the overnight tape test, treat every affected drain weekly for four weeks so you outlast the 8–24 day life cycle, brush the tailpiece and overflow, then treat monthly. Flies come back when the buildup comes back.

How do I get rid of drain flies in the bathroom?

Treat every bathroom drain, not just the sink: sink, shower, tub, and floor drain. Pull the stopper and brush the tailpiece and the sink overflow, where buildup sits above the water line. If the bathroom is rarely used, run water for 30 seconds first — a dried-out P-trap is a common cause.

How do I get rid of sewer flies or sewer gnats?

Sewer flies and sewer gnats are drain flies. Treat them the same way: find the drain they are coming from and remove the biofilm inside it. If flies persist after four weeks of treating every indoor drain, look for a source outside the drains — a cracked sewer line, septic vent, sump pit, or standing water.

How do I kill drain fly larvae?

Drain fly larvae live inside the biofilm on the pipe wall, so the way to get rid of them is to remove that film. Pouring a bio surfactant overnight and brushing what you can reach removes the larvae’s food and habitat. Weekly treatment for four weeks covers the eggs that hatch after the first treatment.

What’s the fastest way to get rid of drain flies?

Do three things the same night: tape-test to find the source drain, pour 1 oz of bio surfactant into every affected drain and leave it overnight, and catch the remaining adults with a vinegar-and-dish-soap trap. Then repeat the drain treatment weekly — the adults you see are already out of the pipe, but the next generation is still in it.

Quick troubleshooting

I treated all my drains and flies are still coming. Check less-obvious sources: washing machine drain pan, dishwasher air gap, refrigerator drip tray, HVAC condensate line, basement sump pit.

Flies in a rarely-used bathroom. The P-trap has dried out. Run water for 30 seconds and pour 1 oz of bio surfactant into the drain.

Flies in an apartment — treatment not holding. The source may be in a shared vent stack or a neighbor’s unit. Treat your drains, then notify building maintenance.

Flies persist after a plumbing repair or waste line replacement. Replacing a section of pipe removes the biofilm in that section only. The breeding site is frequently in a part that was retained: the strainer body, tailpiece, P-trap, sink overflow, disposal connection, or the branch line upstream or downstream of the new work. Start by finding which drain is actually producing flies — tape clear packing tape over each suspect drain overnight, leaving a small air gap, and check in the morning. Then inspect and scrub the parts that weren’t replaced. If nothing turns up, widen the search to sources outside the line that was worked on: HVAC condensate pan and line, sump or ejector pit, floor drains, refrigerator drip tray, and any leak that has kept an area damp. A new pipe can also re-colonize if a fixture upstream keeps feeding organic material into it.

Commercial kitchen — health inspector flagged flies. Treat every floor drain and mop sink with 2–4 oz of bio surfactant overnight. Document weekly applications for the re-inspection.

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Questions or bulk-order interest? Email success@southlandorganics.com or call 800-608-3755. We help commercial facilities and multi-site operations build recurring bio surfactant programs.