Three kinds of pump appear around septic systems, and they are not interchangeable.
Effluent pump
Sits after the septic tank, in a pump chamber, moving the clarified liquid to the drainfield.
It handles liquid with only small suspended solids, because the tank has already settled the rest. Passages are modest and it is not built to pass rags or solids.
Used when the field is uphill from the tank, or where pressure distribution is required to dose the field evenly.
This is the pump on most conventional pumped systems, and the effluent filter ahead of it is what keeps it alive.
Sewage ejector pump
Sits before the tank, moving raw wastewater from a basement bathroom or a low fixture up to the level of the gravity drain.
It is built to pass solids of a stated size, and it does not grind them.
Used when part of the house is below the level of the main drain.
Grinder pump
Also before the tank, and it macerates solids before pumping.
It discharges through a much smaller pipe than a sewage ejector, which is why it is used where a long or small-diameter run is unavoidable — and on municipal low-pressure sewer systems.
More expensive, more powerful, and more tolerant of what arrives — which does not mean things that should not be flushed become acceptable, because they still accumulate.
Fitting the wrong one
An effluent pump ahead of the tank will jam on the first solids it meets.
A sewage ejector after the tank works and is oversized for the job, using more energy than needed.
A grinder where an effluent pump belongs is expensive and unnecessary.
Each of these appears in the field, usually because a failed pump was replaced with whatever was available.
What kills them
Wipes, including flushable ones. They wrap around the impeller and they are the single most common cause of pump failure in residential systems.
Dental floss and hair, which bind the shaft.
Grease, which coats the float switches so they stop moving freely.
Running dry, from a float set wrong.
Short cycling, from a chamber too small or floats set too close together.
Age. Pumps are consumables. Five to ten years is a reasonable expectation.
The alarm
Every pumped system should have a high-water alarm on a separate circuit from the pump, so a tripped pump breaker does not also silence the warning.
An alarm sounding means the level is rising and there is a limited reserve before it backs up. It is not an emergency in the first hour and it is not something to silence and forget.
Test it periodically, and know where the reset is.
Sizing and spares
A pump is sized on the flow required and the total head — vertical lift plus friction in the pipe. A pump that is too small will not deliver; one that is too large short cycles.
For a household on a pumped system, keeping a spare pump of the correct model is worth considering. A failure is not something to wait a week for, and prices are better when nobody is under pressure.
Power
A pumped system stops working in a power cut, and the chamber holds a limited reserve.
That makes the pump one of the loads worth backing up in a household with a generator, and worth knowing about even without one — during an extended outage, water use has to be reduced accordingly.
Access
The pump chamber needs a riser to grade with a secure lid, and the electrical connection needs to be reachable without excavation.
A pump buried under a foot of soil is a pump replaced with a shovel, in whatever weather the failure happens in.