Pull a camera through any drain that has been in service for more than a few months and the wall is not clean. It is coated in a thin, dark, slippery film of bacteria and the material they live in, and that film is biofilm. It is normal, it is present in every sewer on earth, and a camera operator who reports it as a defect is reporting the weather. But biofilm has a few things to say. Its thickness tracks how slowly the water moves, its colour tracks what is in the water, and in some lines it is the direct cause of odour, drain flies and the slimy clogs that collect hair in a bathroom drain.
Quick answer: biofilm shows on camera as a thin, dark, glossy or matte coating on the pipe wall that does not noticeably narrow the bore, often with a slightly furry or streaked texture, in every material. It is not a defect. A thick, ropey or hanging growth means slow flow or a flat run; a pink or orange film means a specific bacterial type common in bathroom drains; a heavy growth in a small line with odour and flies is a cleaning item. Biofilm is never a reason on its own to jet a lateral.
What it looks like
A dark film over the wall colour, grey-black in most sewer lines, sometimes brown, occasionally pink or orange in bathroom branches. It is thin enough that the pipe’s texture, joints and defects show through it. At joints and rough spots it is thicker. In slow water it grows into streamers that wave in the flow. Under full LEDs it reads as a dark, slightly reflective wall; dimmed, the texture shows. It is on everything: clay, iron, plastic, concrete. The guide to interpreting sewer camera footage treats it as the baseline, not a finding.
Biofilm, sludge or grease?
Thickness and position. Biofilm is thin, on all surfaces, and does not reduce the bore. Sludge is thick, on the invert, and is settled solids rather than growth. Grease is thick, on the crown and sides, and is waxy rather than slimy. A camera that shows a dark wall with the joints still visible is seeing biofilm; one that shows a dark bed on the bottom is seeing sludge; one that shows a pale coating narrowing the top is seeing grease. The sediment guide and the grease guide cover the other two.
When it is a sign of something
Heavy, ropey growth means the water is moving slowly enough for the film to grow thick, which points at a flat run, a belly or a line that is rarely used. Growth that starts at a specific distance often marks where the slope changes. Pink or orange film in a bathroom line is a common airborne bacterium that thrives on soap residue and is harmless to the pipe, though it is what shows up around shower drains. A sudden change in colour or thickness along the line can mark where a different discharge enters, such as a kitchen or a laundry branch. None of these is a repair finding; they are context for the real findings.
When it is the problem
In small drains: the slimy mass that collects hair in a bathroom sink or tub trap is biofilm bound with hair and soap, and it is the usual small-drain clog. In lines with odour: biofilm in a trap arm or a rarely used floor drain is a source of sewer smell and of drain flies, which breed in it. In those cases cleaning the biofilm is the fix, usually with a brush, hot water and an enzyme product rather than a jetter. The drain fly guide covers using a small camera to find the growth.
Why not to jet a lateral for it
Because it grows back within weeks, because it does no harm, and because a jetting job sold on the basis of a dark wall is a job sold on nothing. A homeowner shown a camera picture of biofilm and told the line is dirty is being shown a normal sewer. The question to ask is what defect the operator found, not how dark the wall was. Inspection upsells to watch for lists this one.
Recording it
Mention it once, as the general condition of the wall, and move on. Note heavy growth with a distance if it marks a slope change. Note pink or orange film in a bathroom branch if odour or flies are the complaint. Do not grade it as a defect, and do not let it obscure the real findings in the report.
Showing it to a customer
If a customer asks about the dark wall, explain it plainly: every working drain has a living film on it, this one is no different, and the inspection found the following actual defects. A homeowner who understands that the slime is normal is less likely to be sold an unnecessary jetting job by the next person with a camera, and more likely to act on the real finding.
Common questions
Is the black slime in my drain pipe a problem?
In a sewer line, no, it is normal. In a bathroom sink or tub trap, it is what clogs the drain with hair and is cleaned at the fixture.
Does biofilm cause sewer smell?
In a trap arm or a dry floor drain, it can. In a working lateral, the smell has another cause.
Should I have my lateral jetted because of slime?
No. Jet it for grease, sludge or roots. Slime returns in weeks and harms nothing.
Related guides
See finding a sewer smell with a camera, enzyme and bacterial drain treatments, and what a sewer camera can detect.