Quick answer: An agitator pump has a rotating agitator at its intake that stirs settled silt and fines back into the water, so the pump removes the material rather than just the clearer water on top. Choose the EVAK Trenchman for automatic dewatering of silty trenches and excavations (15mm free passage, 110V or 230V). Choose the EVAK Agivort for genuine slurry and sludge, where an agitator and vortex impeller are needed together. If the water is reasonably clear and the silt is best left alone, you don’t need an agitator.
Why ordinary sub pumps struggle in silt
A pump can only move what is in suspension. In an excavation, silt, sand and fines settle quickly. A standard sub pump sitting on that layer draws the clearer water above it and leaves the sludge behind. The intake chokes, flow drops, the pump trips, and the team spends more time clearing strainers than pumping.
We saw exactly this in our foundation dewatering case study, where a contractor’s pumps kept clogging until they switched to a Trenchman.
How an agitator works
The agitator is a hard-wearing rotating element at the pump inlet. As it turns, it disturbs the settled bed and lifts the fines back into the water, so the pump carries them away. Combine that with a large free passage (see pump solids handling explained) and the material has somewhere to go instead of blocking.
The result: you remove the silt, not just the water sitting on top of it, which is usually the point of dewatering a trench.
Trenchman, Agivort or Drainox?
All three are EVAK contractor pumps. They look similar but do different jobs.
| Pump | Built for | Key features |
|---|---|---|
| EVAK Drainox | Reasonably clear water where sediment is best left in place | Stainless steel vortex, 5mm free passage, no agitator |
| EVAK Trenchman | Silty trenches, footings and excavations | Agitator, 15mm free passage, automatic float, 400A and 750A in 110V or 230V, 1500A 230V |
| EVAK Agivort | Slurry, sludge, settlement tanks and lagoons | 316 stainless agitator plus vortex impeller, up to 1,000 L/min and 26m head |
Choosing a Trenchman
- Trenchman 400A: up to 220 L/min and 12m head. Ideal for extensions, footings and service trenches. Also sold as the Silty Trench Dewatering Pump Kit with 20m of 50mm layflat, which still delivers around 189 L/min at a 2m lift.
- Trenchman 750A: up to 350 L/min and 18m head, for deeper digs and sustained groundwater.
- Trenchman 1500A: 230V hard-wired, for the heaviest Trenchman duties.
On most UK sites you’ll want 110V; see 110V vs 230V pumps on site.
Choosing an Agivort
The Agivort is a true slurry pump. Its agitator stirs the solids and its vortex impeller passes them without clogging. Four models (5.05, 5.10, 5.20 and 5.50) run from 0.37kW to 3.7kW with 2, 3 and 4 inch outlets. The smaller models are 230V automatic; the larger ones are 400V three phase. Use it for cleaning out settlement tanks, lagoons, sediment traps and heavily laden water in civils, utilities and tunnelling.
When not to use an agitator pump
An agitator is a specialist tool. In a permanent sump, a clean water chamber or a tank you’re emptying for reuse, stirring up sediment means pumping abrasive material through the pump and depositing it wherever the hose ends up. If the silt can stay where it is, use a Drainox and keep the intake off the bottom.
Using it on site
- Stand the pump on the firmest part of the base so the agitator works the material rather than burying itself.
- Keep the float free of the trench wall and debris, and check it regularly in silty water.
- Size the hose properly. Remember that published flows are for clean water: silty water is denser and will flow more slowly. See why max flow is not your real pump flow.
- Flush the pump and hose with clean water after every job.
- Plan where the water goes. An agitator pump deliberately sends silt out of the hose, so it must not go to a drain or watercourse untreated. Our guide to sediment filters for site water covers this.
For a side-by-side of clean-water and silty-water options, see water in an excavation: which pump do you need?
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