Can you see legionella in water?

No, you cannot see legionella in water. The bacteria are microscopic rods, and the counts that matter in a water system, measured in hundreds or thousands of colony forming units per litre, are far below anything the eye could detect. A glass of water from a badly colonised system looks exactly like a glass of water from a clean one.

This is worth dwelling on, because it overturns the most common instinct people have about water safety. The trigger for concern is usually visible: brown water, cloudiness, bits. But legionella risk has no visible signature. The action levels used in UK guidance are 100 and 1,000 colony forming units per litre, and both are invisible quantities. Water can pass every visual check and still be well above the level that triggers urgent action after sampling.

What you sometimes can see: the habitat, not the organism

You will never see legionella, but you can often see the conditions that let it thrive. Limescale crusted on taps and shower heads, a slime layer inside a cold water storage tank, sediment in the base of a calorifier, orange or brown discolouration from corroding pipework: each of these is part of the environment legionella grows in. Scale and sediment feed and shelter the biofilm in which the bacteria live, and biofilm is where most of the legionella in a building actually resides, not floating free in the water.

So the right response to something visible is not "is this legionella?" but "this system has scale, sediment or slime, and those need dealing with". Recording what you find during tank and calorifier inspections, and cleaning or descaling on a schedule, removes the habitat. The guides on cold water storage tank inspection and calorifier inspection set out what to look for and how often.

Cloudy water, milky water, brown water

None of these indicates legionella. Milky water that clears from the bottom up is dissolved air. Cloudiness or particles are usually sediment disturbed in the mains or the storage. Brown or orange water is typically rust from corroding iron pipework, and it matters because corrosion products and sediment feed biofilm, not because the colour itself is legionella. Persistent discolouration deserves investigation as a system-condition problem, which is a different question from bacterial contamination.

How legionella is actually found

Detection means a laboratory. A water sample is taken, cultured, and the result reported as colony forming units per litre, read against the 100 and 1,000 CFU/l action levels. Legionella sampling and action levels explains when sampling is required and what to do with the results. For most buildings, though, sampling is not the routine control. The routine control is keeping hot water at 50°C or above at outlets, cold water below 20°C, flushing little-used outlets weekly, and keeping the system clean, all of it recorded in a log book.

Your risk assessment and written scheme should define the checks that replace eyesight: outlet temperatures, tank and calorifier inspections, flushing, and descaling. Because you cannot see the hazard, the schedule and the record are the control. If you want a structured way to take the temperature readings, the legionella temperature checker walks through it.