Every dewatering project starts with the same question - which machine? And it is usually answered with the wrong number, the achievable dry solids content. That number matters, but a machine that reaches two points more dry solids while consuming twenty times the electricity and requiring an operator on site does not necessarily win.
How to read this comparison
The ranges below are typical operating figures for municipal and industrial sludge. Your own sludge decides: the same machine can behave very differently on primary, secondary, digested or flotation sludge.
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Describe your goal, effluent limit or sludge volume and receive a technology shortlist plus a sizing proposal – by email, no phone call required.
The five criteria that decide the case
| Criterion | Screw press | Decanter centrifuge | Belt press |
|---|---|---|---|
| Specific energy | very low | high | low to medium |
| Dry solids, municipal | 18-25 % | 22-30 % | 16-22 % |
| Noise level | low | high | low |
| Staffing | unattended operation feasible | supervision usually required | supervision usually required |
| Polymer demand | moderate | moderate to high | moderate |
| Wash water | low | none | high |
0.03-0.1 kWh/m3
typical screw press energy demand
1-4 kWh/m3
typical decanter centrifuge energy demand
80 %
volume reduction achievable in good conditions
When the screw press is the right answer
The screw press turns slowly, has very few wear parts and tolerates being left alone. That combination is decisive whenever:
- there is no operator on site around the clock,
- electricity price or carbon footprint is part of the evaluation,
- the plant is close to residential buildings and noise matters,
- sludge volumes are small to medium and continuous operation is preferred over batch campaigns.
When another technology wins
Do not force the technology
A decanter centrifuge is the better machine when very high throughput has to be handled in a short campaign window, or when the last two percent of dry solids genuinely change the disposal route and price. A chamber filter press remains unbeaten for the highest dry solids on difficult mineral sludge.
Filtrate quality: the criterion everybody forgets
Dewatering returns filtrate to the head of the plant. If the filtrate carries solids, the internal load rises and part of your dewatering benefit is cancelled out. Ask every supplier for expected filtrate TSS, not only for cake dry solids - and check whether the filtrate needs its own treatment step.
Making the business case honestly
Cost items to include
- Disposal cost
- EUR per tonne of wet cake, by route
- Electricity
- kWh/m3 x price x operating hours
- Polymer
- kg active polymer per tonne of dry solids
- Labour
- attended hours per week
- Wash water
- m3 per operating day, plus its own treatment
Disposal dominates almost every calculation. Each additional percentage point of dry solids removes water that would otherwise be transported and paid for by the tonne - which is why an apparently small performance difference can be worth more than the machine price difference.
How to run the selection
- 1Characterise the sludge: origin, dry solids, organic share, seasonality
- 2Fix the operating window: hours per day, days per week, attended or not
- 3Request performance data on your sludge, ideally from a trial
- 4Build the annual cost model with disposal, energy, polymer and labour
- 5Decide - and keep the trial data as your acceptance criterion
Prove it before you buy
Sludge is individual. A trial with a mobile, skid-mounted unit on your own feed removes most of the risk from the decision, and the trial data becomes the performance guarantee in the contract.
Equipment we supply for this
How much dry solids can I expect from a screw press?+
18-22 % on municipal surplus sludge, 22-28 % on mixed sludge and 25-35 % on many industrial sludges - always subject to conditioning quality.
Does a screw press really run unattended?+
Yes, provided sludge feed, polymer dosing and cake discharge are automated and monitored. That is a design requirement, not a property of the machine.
How long should a dewatering trial run?+
Long enough to cover the load variation you actually have - typically one to four weeks, including at least one production peak.
Engineering Hub: get this sized for your plant
Describe your goal, effluent limit or sludge volume and receive a technology shortlist plus a sizing proposal – by email, no phone call required.