A two-phase decanter centrifuge splits a slurry into solids and liquid. A three-phase decanter centrifuge — often called a tricanter — splits it into solids, oil and water in a single continuous pass. The two machines look almost identical on a factory floor, and both are horizontal screw decanters, yet the difference decides whether you get one clean stream or two contaminated ones. This guide explains how each machine separates, which design details actually matter, where each type belongs in a plant, and how to tell a three-phase decanter from a three-phase disc separator before you specify the wrong one.
Two-phase and three-phase separation in one sentence each
- Two-phase (solid–liquid). The centrifuge separates a suspension into a dewatered solid (cake) and a clarified liquid (centrate). Typical duty: sludge dewatering, mineral and chemical slurry dewatering, clarification of process liquids.
- Three-phase (solid–liquid–liquid). The centrifuge separates a mixture into solid, a heavier liquid phase and a lighter liquid phase at the same time. Typical duty: animal fat and rendering, edible oil, biodiesel washing and glycerine separation, waste cooking oil and kitchen waste, fish oil, and industrial oil recovery.
Both are continuous machines that feed, separate, wash and discharge at full speed, so neither needs a batch cycle. What changes between them is the geometry of the discharge, not the basic principle.
How a horizontal screw decanter separates three phases
Inside the bowl, the construction is a column-cone bowl with a scroll (spiral stoker) turning inside it at a slightly different speed, driven through a planetary gear differential. Feed enters through the centre of the scroll and is accelerated up to bowl speed.
- The heavier solid phase settles against the bowl wall and is conveyed by the scroll to the cake discharge ports at the conical end.
- The two liquid phases settle as concentric rings: the heavier liquid (usually water) occupies the outer ring, the lighter liquid (usually oil or fat) the inner ring.
- Each liquid leaves through its own adjustable discharge — a weir or a pump — so the position of the interface between the two liquid layers can be tuned for the actual oil-to-water ratio of the feed.
The practical consequence: on a three-phase machine, the light-phase and heavy-phase outlets are separate and the interface level is a controlled operating parameter. On a two-phase machine there is only one liquid outlet, so any oil or fat present leaves with the water, and any fine solids leave with the liquid.
Design details that decide whether the oil stays clean
- Interface control. Adjustable weir plates or a light-phase pump with controlled flow let the operator set where the oil/water interface sits relative to the discharge. Too low and oil leaves with the water; too high and water contaminates the oil.
- Dual-motor, dual-inverter drive with energy recovery. Two variable-frequency drives give stepless control of the differential speed between bowl and scroll, and the back-drive returns energy to the system instead of dissipating it as heat in the gearbox.
- Scroll flight protection. The flights are hardfaced with tungsten carbide. This matters more on three-phase duty than most buyers expect: fat, protein, bone grit and nutshell fines are abrasive, and a worn scroll stops conveying correctly long before it fails visually.
- Wetted-part material and per-language certification. Contact parts in stainless steel — with the grade selected against the acidity and chloride content of the feed — plus seal selection for hot or volatile phases.
- Temperature handling. Animal fats and edible oils separate best above their melting point, so these machines are normally specified with a hot-feed arrangement and insulation.
Which one do you actually need?
| Feed | Three-phase decanter | Two-phase decanter | Note |
|---|---|---|---|
| Municipal or industrial sludge | Not needed | Preferred | Polymer conditioning plus high G for cake dryness; see our SS series dewatering filter centrifuge for filter-type duties |
| Animal fat, rendering, slaughterhouse waste | Preferred | Insufficient | Solid / fat / water must all leave separately |
| Edible oil, olive and palm oil, fish oil | Preferred | Insufficient | Requires interface control and hot feed |
| Biodiesel washing and glycerine separation | Preferred | Insufficient | Density difference between phases is small — G-force and interface control both matter |
| Kitchen waste and waste cooking oil recovery | Preferred | Sometimes used upstream | Three-phase recovers a saleable oil stream and an organic solids stream |
| Mineral, chemical or crystalline slurry dewatering | Not applicable | Preferred | One liquid phase only |
| Dairy, beverage and juice clarification | Disc separator usually better | Possible | See the disc centrifuge separator for low-solids, fine-particle polishing |
Three-phase decanter or three-phase disc separator?
Both can split solid, oil and water, and buyers frequently compare them. The deciding factor is the solids load and particle size, not the number of phases:
- Choose a three-phase decanter when the feed carries a substantial, variable solids load — rendering material, kitchen waste, high-fat wastewater, slurries with fibre or grit — and you need continuous cake discharge without stopping to clean the bowl.
- Choose a disc centrifuge separator when the feed is a liquid with a low solids content and very fine particles, and the objective is high-clarity polishing of two immiscible liquids at high G. Disc machines shorten the settling distance and multiply the effective settling area with a stack of discs, which is why they reach high clarification efficiency — but they do not carry a heavy or abrasive solids load as well as a decanter, and the separated solids accumulate to be removed either manually after shutdown or by a discharge mechanism.
Selection data to agree before ordering
The separation limits of our horizontal screw decanters are published and worth checking against your duty: suspension with a solid particle size greater than 0.005 mm and a concentration in the range 2–40% is the working window, and on machines with a large adaptability range the separable particle size spans 0.005–2 mm — including material whose particle size is uneven. On top of that window, five figures decide the order specification:
- Feed composition: percentage of solids, oil/fat and water, and how each varies over a shift.
- Throughput: m³/h of feed, and operating hours per day.
- Feed temperature and whether the fat phase must stay above its melting point.
- Target results: residual oil in the water phase, residual water in the oil phase, and the required cake dryness.
- Discharge arrangement: gravity weirs, light-phase pump, solids conveying, and whether a wash-water injection point is required for salt or soap removal.
Five specifying mistakes that cost a rebuild
- Buying three-phase for a two-phase job. If there is no second liquid phase, a three-phase machine only adds an extra outlet and a more delicate interface to control.
- Buying two-phase and hoping the oil leaves with the water. It will — into your effluent charge and your polymer bill.
- Ignoring the solids load. Interface control cannot compensate for a scroll that is already overloaded with solids.
- Forgetting heat. Fat that solidifies in the bowl at shutdown turns the next start into a manual cleaning job.
- No material check. Chlorides, acidity and hot saline feeds decide whether standard stainless steel is enough or a duplex grade is required — this is quoted, not assumed.
Frequently asked questions
What is the working principle of a three-phase decanter centrifuge?
The bowl and the scroll inside it rotate in the same direction at a slightly different speed — the differential speed. Feed is accelerated into the bowl, where the centrifugal field separates it into three layers: solids against the bowl wall, the heavier liquid in the outer ring, the lighter liquid in the inner ring. The scroll continuously conveys the solids to the conical end for discharge, while each liquid phase leaves through its own weir or pump at the cylindrical end. The separation never stops, so the machine runs continuously at full speed.
Is a tricanter the same as a three-phase decanter centrifuge?
Yes. “Tricanter” is the common trade name for a horizontal screw decanter configured to discharge three streams — solid, oil and water — from one machine. If you are asked for a tricanter, you are being asked for a three-phase decanter centrifuge.
Can a two-phase decanter be converted into a three-phase machine?
Only in limited cases. The conversion needs an additional light-phase outlet, a different or adjustable liquid-level arrangement, and in most designs a different scroll geometry — which usually means a new machine rather than a field modification. It is cheaper to specify correctly the first time, which is why we ask for the composition of the feed before quoting.
What differential speed should a three-phase decanter run at?
As low as the solids load allows. A lower differential speed gives the liquids longer to separate and produces a drier solid, but it also reduces the solids throughput and raises scroll torque. On the machines we build, two variable-frequency drives make the differential speed steplessly adjustable, so the setting can be optimised on site against the actual feed rather than fixed at the factory.
Does a three-phase decanter remove dissolved contaminants?
No. A decanter separates phases by density and centrifugal force: it removes free oil, solids and fine suspended matter, not dissolved salts or dissolved organics. Where dissolved load must be reduced, the centrifuge is a pre-treatment stage ahead of biological or membrane treatment — see the screw press machine for the solid–liquid pre-dewatering alternative on fibrous material.
Tell us your feed, and we will size the machine
Two-phase or three-phase, the sizing question is the same: what is in the feed, and how much of it per hour. Send us the composition of your stream (solids, oil, water), the throughput in m³/h, the operating hours, the feed temperature, the target quality of each stream, and the power supply available on site. With those five answers we will tell you whether a two-phase or three-phase decanter is the right machine, which bowl diameter and speed fits, and what the drive and wear-protection package should be — and if a disc separator or a plate and frame filter press is the better answer for your duty, we will say so.