Sludge Dewatering Comparison: Centrifuge vs. VOLUTE™ Screw Press
CAPEX Is Not the Whole Story. TOTEX Is.
When choosing between a centrifuge and a screw press, the purchase price is the least important number on the table. What matters is what the machine costs you every single day — in electricity, maintenance, water, polymer and operator time.
Why Do We Dewater Sludge in the First Place?
On the surface, the answer is simple: remove water to reduce sludge volume and mass, lower transport costs, and enable further utilisation or disposal. The key metric looks obvious — the percentage of dry solids (DS) at the output.
In practice, however, that number is just one piece of the puzzle. What really determines the success of a dewatering installation is whether operations are stable, total running costs are as low as possible, and the system works long-term without constant operator attention. Dewatering should not become an endless daily project.
“DSC is not everything. TOTEX is.”
The Centrifuge: High Throughput, High Demands
A centrifuge (or decanter) is the right choice when you need to dewater very large volumes of sludge. It handles high flow rates well and can scale to treatment plants processing hundreds of thousands of population equivalents. For these extreme sizes, the benefits of a compact footprint and high throughput can outweigh the operational challenges.
But there is another side of the coin. A centrifuge is a high-speed machine — it typically runs at around 3,000 RPM. That speed translates directly into:
- Specific energy consumption of approximately 31 kWh/tDS — up to 10× more than a screw press
- Larger electrical grid connections required, which are costly and not always feasible
- Special reinforced concrete foundations needed to absorb vibrations
- Very high noise levels — hearing protection and sound enclosures are typically mandatory
- Cannot operate at night in noise-sensitive locations
- Foreign objects at high rotational speed cause major damage
The VOLUTE™ Screw Press: Low Speed, Low Cost, High Stability
A screw press is the right technology when you want peace of mind, operational stability, and predictable costs. The VOLUTE™ disc screw press operates at just 2–5 RPM. The near-zero operating speed explains virtually all of its advantages: no vibrations worth mentioning, no sound enclosures required, and the ability to run fully unmanned.
- Specific energy: 3.5–3.8 kWh/tDS — approximately 90% less than a centrifuge.
- No need for large electrical connections or reinforced foundations.
- Suitable for fully automated, unmanned operation.
- Compact footprint from 1.5 m²; can be delivered as a containerised plant.
Head-to-Head Comparison
Real-World Evidence: Davyhulme WwTW, Manchester — VOLUTE™ vs. Centrifuge
Evergreen Water Solutions partnered with United Utilities to run a full-scale, side-by-side comparison using the same sludge feed to both existing centrifuges and the VOLUTE™ FS-402. The trial was funded under the UK government’s Industrial Energy Efficiency Accelerator (IEEA) programme and independently monitored.
- Centrifuge (site average): 31.1 kWh/tDS
- VOLUTE™ at 800 kg DS/h: 3.51 kWh/tDS
- Measured energy saving: 84% (target was 70%)
| Factor | Centrifuge | VOLUTE™ Screw Press |
|---|---|---|
| Operating speed | ~3,000 RPM | 2–5 RPM |
| Energy consumption | ~31 kWh/tDS | 3.5–3.8 kWh/tDS |
| Vibrations | Significant — special foundations required | Negligible — no civil works needed |
| Noise level | Very high — enclosures & hearing protection required | Quiet — no protective measures |
| Maintenance | High — bearings typically replaced annually | Low — 10,000 h rings / 30,000 h screw |
| Polymer consumption | ~5.7 kg/tDS | 3.0–5.0 kg/tDS |
| Operator attention | Requires regular monitoring & adjustment | Can run fully unmanned |
| Sludge cake (DS%) | 20–30%+ DS (highly sludge dependent) | Similar (highly sludge dependent) |
Note: Both technologies often achieve very similar dryness. VOLUTE™ can achieve >30% DS, but results always depend on the specific sludge type and conditions.
Cumulative Cost: The Numbers That Matter
The screw press consumes roughly one-tenth the electricity of the centrifuge for the same tonnage of dry solids processed. Over years of continuous operation, that gap is transformative.
Energy Usage (10-Year Cumulative) [MWh]
CO₂ Emissions (10-Year Cumulative) [t]
The 24/7 Advantage for CAPEX and Capacity:
When evaluating CAPEX, centrifuges appear cheaper at first glance for high-volume applications. However, the VOLUTE™ changes the equation through its ability to run 24/7 without supervision:
- Capacity Boost: If a plant needs more capacity, switching from 8-hour monitored shifts to 24-hour unmanned operation with VOLUTE™ acts as an immediate plant upgrade.
- Sizing Efficiency: Because it runs safely at night, a smaller VOLUTE™ can often do the work of a much larger centrifuge, bringing the CAPEX into a highly competitive position.
Cumulative Electricity Costs
While a centrifuge might seem viable initially, the high power draw creates an enormous ongoing financial burden. As shown in the comparison graph, the gap in electricity cost over a 10-year span heavily favors the VOLUTE™ Screw Press, easily returning the initial investment several times over.
Electricity Costs (10-Year Cumulative) [€]
What to Watch Out for When Choosing
| CONSIDER A SCREW PRESS (VOLUTE™) WHEN: | A CENTRIFUGE MAY STILL MAKE SENSE WHEN: |
|---|---|
| Minimising electricity costs is a priority | You process very large volumes (>100 m3/h) |
| Site is noise-sensitive or has limited power | High power infrastructure is already in place |
| You want automated, unmanned operation | Footprint is the primary constraint at extreme scale |
| Civil works budget is constrained | Plant already has sound/vibration isolation |
Why Stability is Everything
When designing a complex sludge treatment line that includes a thermal dryer, the operational differences between technologies become critical. A dryer requires a constant, predictable feed to maintain thermal balance and prevent “clogging” or safety trips.
Why the centrifuge needs a buffer: A centrifuge’s high-speed stabilization period (15–30 mins) and sensitivity to feed fluctuations mean it cannot safely “talk” to a dryer directly. This requires a sludge buffer bunker with dosing screws, which inflates CAPEX, increases the footprint, and introduces a new source of odor and maintenance.
The VOLUTE™ Advantage: VOLUTE™ operates as a “plug-and-play” feeder, achieving a leaner, significantly cheaper process line. In other words, sludge dryers equipped with VOLUTE™ dewatering presses share one key characteristic: both technologies serve their operators best when they run continuously for as long as possible. This combination of a centrifuge and a dryer is simply not possible.
See the Numbers for Your Site
One word of caution: every manufacturer — including us — can select reference conditions where their technology appears superior. The only way to be certain for your specific sludge type and operating conditions is a pilot test on site. We always recommend this before a final investment decision.
Every installation is different. Send us your sludge data and we will calculate an indicative TOTEX comparison.
Sources & References:
- [1] Evergreen Water Solutions. Case Study – VOLUTE vs Centrifuge (Davyhulme WwTW, UK) – evergreenwatersolutions.co.uk
- [2] AMCON Europe s.r.o. Internal Data. Comparison of CAPEX/TOTEX configurations and thermal drying process synergies.