Why Does a Sewage Pump Smell?

Someone opens the equipment-room door and says, "The sewage pump smells."

That sounds reasonable. The smell is strongest near the pump, so the pump gets blamed first. But replacing it may change nothing. In many cases, the motor and impeller are doing their job. The odor is escaping somewhere else-around the tank cover, through a cable opening, from the vent connection or even from a dry floor drain nearby.

We normally start by asking when the smell appears. Is it there all the time? Does it become stronger just after the pump runs? Did it show up after the bathroom had not been used for several weeks? The answer usually tells us more than the strength of the smell.

A closed sewage lifting station should keep wastewater and gas inside the tank. That sounds simple, but there are several openings to deal with: the inlet pipe, discharge pipe, vent, electrical cable entries and inspection cover. If one connection is not properly sealed, odor finds its way into the room.

The tank cover is often the first place to check. A gasket may look acceptable from the outside but still be folded, compressed unevenly or damaged around one corner. We have also seen installations where the bolts were tightened on one side first, leaving a slight gap on the other.

Cable entries are easy to overlook. Installers sometimes use a larger opening than the cable requires, especially when several float-switch cables have to pass through the same area. The equipment works, so the opening is forgotten. Later, the customer notices a sewage pump smell and assumes something has failed inside the tank.

The vent deserves the same attention. As wastewater enters, air must leave the tank. When the pump lowers the level, air needs to return. If the vent is blocked or incorrectly connected, pressure changes can push odor through whichever seal is weakest. A short vent pipe that ends inside the plant room is not really a venting solution-it has simply moved the smell from the tank into the room.

There is another possibility that has nothing to do with the lifting station. A nearby floor drain may have a dry trap, particularly in a warm room that is rarely washed down. Sewer gas then enters through the drain, but because the pump is the most obvious sewage-related equipment in the room, it gets the blame.

A smell that appears after a long period without use is usually different from one that appears after every pumping cycle.

Some wastewater always remains below the pump's stop level. If a basement bathroom is unused for weeks, that water becomes stale. Soap, organic matter and toilet residue stay on the tank wall. The next time the toilet is flushed, the incoming water disturbs the deposits and the odor becomes noticeable again.

Laundry wastewater creates its own pattern. It may look relatively clean, but it carries detergent, lint and material released from clothing. Over time, a grey coating can build up around the tank wall, sensor tube or float. The pump may continue operating normally while the tank becomes harder to keep clean.

Kitchen wastewater is more demanding. Grease and food residue can stick to internal surfaces and may also affect the level control. A standard sewage lifting station should not be selected for hot, greasy kitchen discharge without checking the permitted liquid temperature and the type of solids entering the system.

If the smell becomes stronger shortly after the pump stops, listen for water returning through the discharge pipe. A check valve held partly open by debris may allow wastewater to flow back into the tank. Sometimes there is a clear gurgling sound. In other cases, the only clue is that the liquid level slowly rises even though no toilet, shower or washing machine is being used.

The leaking check valve may not be the direct route through which gas enters the room. It does, however, return dirty water to the tank and can make the pump start more often. If the cover is already poorly sealed, the odor becomes much more noticeable.

A burnt smell should be treated separately. Hot insulation, a damaged cable or an overloaded motor does not smell like ordinary sewage. If the odor resembles hot plastic or electrical wiring, stop using the connected fixtures and have the equipment checked. Cleaning the tank will not solve an electrical problem.