A well maintained sewage treatment plant should not smell of ammonia. Ammonia is present in all incoming sewage, but a healthy aerobic plant converts it during treatment. A persistent ammonia or sour smell usually points to a problem with aeration, loading or servicing rather than to normal operation.
Few things unsettle a homeowner off mains drainage faster than catching something unpleasant on the breeze in their own garden. A sewage treatment plant smell has a habit of arriving at the worst possible moment, usually the afternoon the neighbours are round. It also arrives wrapped in a great deal of confident bad advice, most of it about ammonia. Some owners are told a smell is simply part of the deal. Others are told an ammonia smell means the bacteria have died and the system needs replacing. Neither is right, and the difference matters when you are deciding whether to call a service engineer or check a single component. Here is what ammonia really does inside a small sewage treatment plant, a packaged system that treats wastewater from properties not connected to the public sewer, and which odour myths deserve to be retired.
What causes a sewage treatment plant smell in the first place?
Every litre of sewage arriving at your plant already contains ammonia. It forms as urea and proteins break down, which begins in the drain long before anything reaches the tank. Ammonia has a sharp, pungent character that most people recognise from household cleaning products or a cat litter tray.
The second culprit is hydrogen sulphide, the rotten egg smell people associate with drains. That one forms when organic matter breaks down without oxygen, which is to say under anaerobic conditions. Both gases are detectable by the human nose at very low concentrations, which is why a small problem can produce a smell out of all proportion to its size.
Understanding this split matters, because the two smells tell you different things. Rotten eggs suggests something has gone anaerobic. A sharp, ammonia-like smell suggests the biological stage is not finishing the job.
Is ammonia really the cause of a sewage treatment plant smell?
Partly, but it is a symptom rather than a cause. Inside a working plant, nitrifying bacteria convert ammonia first to nitrite and then to nitrate, a process called nitrification. Those bacteria need a steady supply of dissolved oxygen to do it, which is exactly what the aeration system provides.
When oxygen runs short, nitrification slows or stops. Ammonia then stays in the liquid, builds up, and some of it escapes into the air as gas. So the ammonia you can smell is not the problem in itself. It is evidence that oxygen, loading or servicing has slipped somewhere upstream of it.
That reframes the question usefully. Rather than asking how to mask a sewage treatment plant smell, ask what is stopping the biology from working.
Myth one: is a sewage treatment plant smell just normal?
This is the myth that costs owners the most, because it persuades them to live with a fault for years. A correctly sized, correctly installed and properly serviced plant should be close to undetectable in normal use. Standing directly over an open lid during the aeration phase, you may notice a faint earthy or musty smell, and that is reasonable. Smelling your plant from the patio, the kitchen window or the far end of the garden is not.
Environment Agency are clear that badly maintained or incorrectly installed systems can cause odour as well as pollution. Your plant should be serviced and desludged in line with the manufacturer’s guidelines, which is the only schedule that reflects how your particular system is built and how it is being used. A persistent smell is often the first outward sign that this has lapsed. Separately, if your plant discharges to a watercourse or to ground, it needs to meet the general binding rules for small sewage discharges or hold a permit.
Myth two: does an ammonia smell mean the bacteria have died?
Rarely. The activated sludge in a domestic plant is more resilient than most owners expect. After a power cut of a day or two, the biomass generally recovers on its own once aeration resumes, although full nitrification can take a little longer to re-establish because nitrifying bacteria multiply slowly.
What usually happens is not death but suppression. The bacteria are alive and hungry, yet starved of oxygen, swamped by more wastewater than the plant was sized for, or knocked back by a heavy dose of bleach or disinfectant poured down the drain. Nitrifiers are the most sensitive population in the tank and the slowest to bounce back, which is why an ammonia smell tends to appear before any other symptom.
Replacing the plant is almost never the answer. Restoring the conditions the bacteria need almost always is.
Myth three: will additives cure a sewage treatment plant smell?
Sachets of bacteria, yoghurt, yeast and various internet remedies all get recommended for this, and none of them will fix a mechanical fault. If the air pump diaphragm has worn out, no additive will put oxygen into the tank. If a diffuser is blinded or an air line is kinked, the biology cannot recover whatever you feed it.
Bleach deserves a special mention, because it is the most common attempt at a cure and reliably makes things worse. Bleach and other disinfectants should never be poured into a sewage treatment plant. Even a single heavy dose suppresses the very bacteria that remove the ammonia you are trying to get rid of.
Additives have a limited role in a genuinely starved system, for example after a long empty period. They are not a substitute for finding the fault.
What really makes ammonia build up in a treatment plant?
In most cases the cause sits in a short list. The air pump may have failed, tripped or been switched off at the isolator, sometimes months earlier during other work. Diaphragms wear and lose output gradually, so aeration can decline long before the pump stops entirely. Diffusers blind over time, and air lines can be crushed, kinked or blocked.
Loading matters just as much. A plant sized for four people and serving seven will struggle, and so will one taking rainwater from a misconnected downpipe, because hydraulic overload flushes solids and biomass through before treatment finishes. The opposite problem is a holiday home standing empty for weeks, where the biology starves and takes time to recover on your return.
Housekeeping accounts for the rest. Wet wipes, fats and oils should never be flushed or poured into a sewage treatment plant, because they do not break down in treatment. Alongside overdue desludging, they reduce the working volume of the tank and push conditions towards anaerobic. Ventilation plays a part too, since gases need a route to escape above roof level through the soil vent pipe rather than seeping out at ground level.
How do you stop a sewage treatment plant smell coming back?
Start with the simple checks. Confirm the air pump is running and pushing air, listen for a change in note, and look for water in the pump housing or a tripped breaker. Check whether the smell is rotten eggs or ammonia, since that alone narrows the diagnosis. Note when it started and what changed around then, whether that was a new bathroom, extra occupants, a power cut or a deep clean.
Then get the plant serviced properly. A service by a competent engineer, carried out at the interval set in the manufacturer’s guidelines, covers pump output, diffusers, air lift function, sludge levels and settlement quality, and it catches the slow declines an owner cannot see. Servicing carried out by a provider working to the British Water code of practice gives you a documented record, which also helps when you come to sell. Keep desludging on schedule, and make sure everyone in the house knows that wipes, fats, oils, bleach and disinfectants must never go into the system. Get those two things right and most odour problems never start.
Does plant design affect how much a sewage treatment plant smells?
It does, and the difference shows up most in how consistently the tank stays aerobic. GRAF’s treatment plants use the SBR process, or sequencing batch reactor, which treats wastewater in timed batches of aeration and settlement rather than in continuous flow. Aeration is controlled and repeated, so the plant is far less likely to drift into the oxygen-starved conditions that release ammonia and hydrogen sulphide.
Sizing is the other half. Plants tested to BS EN 12566-3 are certified against a stated population equivalent, and that figure needs to reflect who will actually live in the property, including the loft conversion planned for two years’ time. An undersized plant with a smell is a design problem, not a maintenance one, and it is much cheaper to solve on the drawing board than after commissioning.
Getting it right from the start
Ammonia is not something to fear in a treatment plant. It arrives with every flush, and a healthy aerobic system deals with it quietly. What you should pay attention to is a smell that persists, because that is your plant telling you something has changed, and almost always something fixable.
If you are specifying a system for a new build or replacing one that has never quite behaved itself, the right starting point is a plant sized properly for the property and built around reliable aeration. Use our sewage treatment plant sizer to work out the right size for your project or talk to our team for further guidance.
Posted by Callum Vallance-Poole, on September 4, 2026.