One of the most important numbers on a pump's specification is also one of the most overlooked: its solids handling, often listed as free passage. It tells you the largest solid the pump can pass without clogging, and getting it wrong is the difference between a pump that clears the job and one that jams solid on the first attempt. Whether you are pumping clean water, gritty site water or foul water full of soft solids, the solids handling figure is what you check first.
What free passage actually means
Free passage, sometimes called solids handling or solids passage, is the diameter of the largest sphere that can travel through the pump without blocking it. If a pump is rated for 10mm solids, anything up to 10mm across should pass through; anything larger risks lodging in the impeller or the volute and stopping the pump. It is a simple, physical number, and it is set by the size of the internal passages and the design of the impeller. The bigger the free passage, the dirtier the water the pump can handle, but usually at the cost of the fine, high-pressure performance a tighter clean-water pump can offer.
What the numbers mean in practice
Solids handling figures sort pumps into broad classes, and matching the class to your water is the whole game.
You can see these classes across our ranges: the dirty water pumps for muddy and gritty water, and the sewage pumps for foul water with large soft solids, such as the vortex APP BCV.
Solids handling is not the only thing to check
Two more points sit alongside the free passage number. The first is the nature of the solids, not just their size. Abrasive solids like sand and grit wear a pump out even when they pass through easily, so for gritty site water you want a pump built for abrasion, such as an agitator pump like the EVAK Trenchman, which stirs settled solids into suspension so they can be pumped away. The second is fibrous material, which can wrap around an impeller even when it would nominally pass on size, and that is where a cutter pump earns its place. We cover how impellers move solids in our guide to how submersible pumps work.
Matching the pump to your water
Decide what is in your water before you choose. Relatively clean water with fine grit suits a clean-water or puddle pump. Muddy, silty or leafy site water needs a dirty-water pump with around 10mm passage, and abrasive grit on top of that needs a pump built to resist wear. Foul water with soft solids needs a sewage pump with a large free passage. If you are not sure, our guide to sewage pump vs sump pump helps you route the choice. Tell us what you are pumping and we will match the right solids handling. Browse our site drainage pumps, use the Pump Finder, or call 0115 987 0358.
