These flies sometimes breed in huge numbers around sewage treatment plants where there are settling ponds filled with organic matter. They can be a problem in water cooling towers where algae builds up and provides the wet habitat needed for the larvae. Within a structure the most common place to find their needs of wet surfaces with a buildup of organic crud will be within drains, and thus their common name. But, they could breed in many other places, so in a restaurant or food plant where floors are washed regularly there may be a buildup of wet sludge in crevices and gaps around the floors. They may find a constant wet area and the resulting presence of slime under equipment that leaks water, under A/C units, under the fridge, or even outside in the landscape or within the catch pans under potted plants. Any micro-habitat that retains water and grows organic sludge is a potential breeding site for their larvae, and this is the essence of control of these flies. You must find the source and eliminate it, or more adult flies will continue to come from it. 

These kinds of sources should be viewed as sanitation problems, so in a sense the presence of flies such as drain or phorid flies is a signal that some unsanitary condition exists, and this presents an opportunity for the PMP to show his expertise. We know what kinds of places may offer this micro-habitat and we inspect until we find it. Then we can eliminate that problem and fortunately have many excellent products to use. Our industry has taken on this role in the past 15+ years and Univar carries many different cleaning materials, many for drains and others for equipment or surfaces. Most of these can be called "bio-cleaners" because the active ingredients are natural, as bacteria that feed on the organic crud or enzymes that dissolve it. One of our major manufacturers of them is Rockwell Labs, for instance, with their Invade product line. This includes large blocks that can be placed into grease traps of restaurants to feed on the buildup of grease that also produces phorids and drain flies. 

So, start with a flashlight and a very careful inspection of the structure, perhaps beginning in the area where the flies are most often seen, and look for any place where water drains or sits to see if it remains wet and offers the buildup of organic slime. Then use the appropriate cleaning product to remove that buildup and offer this as a regular service to this account. Removing it once will not keep it clean if the other conditions don't change and water continues to flow there. Of course, if it's a leak that should be repaired that's another issue, but regular service and cleaning with biological cleaners is a very good add-on service. Don't focus on the adult flies. You can easily kill these with vacuuming or a light mist of pyrethrum once the source is dealt with.