Dredged Sediment Dewatering: Polymer Selection for Geotubes and Presses

Short answer: dredged sediment is not municipal sludge, and polymer selection follows the sediment, not the equipment brochure. Characterise the solids first — particle size distribution, clay and organic content, salinity and pH — then screen charge and molecular weight on the actual slurry, then confirm on the dewatering route you will actually use (geotube, plate-and-frame, belt press or centrifuge). Polymer reduces water and therefore volume and haulage. It does not treat contamination.

Illustrative dredging context. This image does not depict a project described below and is not evidence of a ChinaPAM project result.

Characterise the sediment and the water, in that order

Two dredging projects a few kilometres apart can need different chemistry. The variables that actually decide the answer:

• Particle size distribution. Sand fractions settle without help and can often be separated mechanically before any polymer is considered. The fines — silt and clay — are what polymer is for, so the useful number is the fraction below roughly 63 µm, not an average.
• Clay mineralogy. Swelling clays hold water in a way that resists mechanical dewatering and tend to push you toward a different charge and molecular weight position than a silt-dominated sediment.
• Organic content. Organic-rich sediment behaves more like a sludge, usually shifting the answer cationic, while mineral-dominated sediment often responds to anionic chemistry.
• Salinity and ionic strength. Marine and estuarine sediment carries high ionic strength, which compresses polymer chains and normally reduces the effective reach of a high molecular weight anionic product. A grade selected on freshwater sediment can underperform in a harbour without anything being wrong with the product.
• pH and hardness of the carrier water, measured on the slurry you will actually pump.
• Solids concentration as dredged, and how much it varies. A hydraulic dredge output can swing widely within an hour, and a polymer system dosed for the average will be wrong most of the time.

Take samples that represent the depth and the area being dredged, not a single grab. Sediment stratifies, and a polymer programme designed on the top layer will be re-designed in week two.

Volume reduction is not contaminant treatment

This distinction decides project scope, budget and permitting, and conflating the two is the most expensive mistake in this application.

What dewatering does: separates water from solids, so the solids can be handled, stacked, hauled or placed at a much lower tonnage and volume than the dredged slurry. Since haulage and disposal are usually charged by tonne or by volume, this is where the money is.

What dewatering does not do: destroy or remove contaminants bound to the sediment. If the fines carry metals, PAHs or other contaminants, those contaminants stay with the fines. A dewatered cake from contaminated sediment is contaminated cake at lower moisture. Polymer does not render material non-hazardous, does not certify leachability, and does not replace site-specific characterisation and compliance testing.

There is a second-order effect that does matter for compliance: because contaminants are often associated with the fine fraction, capturing fines efficiently keeps them out of the return water. That can make the filtrate or effluent easier to meet a discharge limit with — but the limit itself still has to be demonstrated by analysis of your water, not inferred from good floc formation.

Choosing the dewatering route

On geotube performance, USACE project literature reports roughly 25–50% solids for geotube dewatering within its own project context. That range is from USACE project documentation for that project, and achievable solids in your case depend on sediment character, conditioning and how long the tube is allowed to consolidate — it is a reference point for feasibility discussion, not a specification you can order against.

Screening charge and molecular weight without promising a grade

Anyone who names a grade before seeing your sediment is guessing. What can be said honestly is how the screening space is bounded, so you test a short candidate list instead of a catalogue.

• Charge type. Sediment fines with organic content generally respond to cationic polymer; low-organic, mineral-dominated fines often respond to anionic or nonionic chemistry, sometimes with a coagulant ahead of it. Test both directions rather than assuming.
• Charge density. Higher organic content and higher surface charge usually push toward higher charge density. Too high, and you overshoot into charge reversal and lose the floc.
• Molecular weight. Higher molecular weight builds larger, faster-settling, more free-draining flocs, which suits geotubes and gravity drainage. It is also more shear-sensitive, which penalises centrifuges and long pumped fill lines.
• Salinity. Brackish or marine pore water compresses the polymer coil and suppresses the effective reach of high-charge anionic chemistry. Screening must be done in the actual water, not in tap water.
• Coagulant first. Where fines are very fine and colloidally stable, an inorganic coagulant or a low-molecular-weight cationic coagulant ahead of the flocculant often does more than changing flocculant grade.

Screening narrows candidates. Field conditions — actual shear, actual line length, actual feed variability, actual weather — decide the working grade and dose. See the jar test procedure for the bench method and dosage calculation for converting a bench dose into a plant dose rate.

Cylinder and pillow tests

Two bench tests carry most of the decision weight here, and they answer different questions.

Graduated cylinder settling test. Condition a measured slurry volume, mix to a defined and repeatable energy, then record interface height against time plus final supernatant clarity. This ranks candidates for settling rate and decant quality, which is what matters for lagoons, drying beds and the clarity of geotube filtrate.

Pillow test (hanging bag or filtration cone). Condition the slurry and pour it into a small sample of the actual geotextile fabric, then record drained volume against time, filtrate turbidity and the solids content of the retained material. This is the closest bench proxy for a geotube, because it tests the polymer, the sediment and the fabric together.

Run both with the same conditioning, and record mixing energy and mixing time explicitly. A pillow test result without its mixing description is not reproducible and cannot be compared against anyone else's number.

The five numbers to record on every test

Record polymer consumption on active polymer per dry tonne, not litres of solution per hour. Solution concentration and slurry solids both move, so the volumetric number is not comparable between trials or between suppliers.

Cost per dry tonne, with the variables visible

The comparison that decides the project is cost per dry tonne of solids removed from the water, not the price per kilogram of polymer. Build it from variables you measure on site:

• Polymer cost = (kg active polymer per dry tonne) × (delivered cost per kg active polymer)
• Disposal cost = (wet tonnes per dry tonne, set by cake solids) × (haulage + tipping rate per wet tonne)
• Equipment cost = (rental or amortised capital + energy + consumables such as fabric or cloth) per dry tonne
• Labour and schedule cost = crew hours per dry tonne, including time lost to weather or cycle turnaround
• Return-water cost = any additional treatment needed to bring filtrate to a dischargeable standard

Cake solids can dominate this model where haulage and disposal are material costs on your project. As a transparent example: moving cake from 20% to 30% solids takes wet tonnage per dry tonne from 5.0 to 3.33, a reduction of a third. Whether that outweighs a difference in polymer unit price depends on your own haulage and tipping rates, so run the arithmetic with your numbers rather than assuming the ranking — but run it on this model rather than on unit price alone, because a cheaper polymer producing wetter cake can be the more expensive choice.

Failure modes and what they look like on site

What bench testing proves, and what it cannot

A cylinder and pillow programme can prove: that a charge direction and molecular-weight range works on your sediment in your water; the approximate dose window; the relative ranking of candidate products on drainage rate, filtrate clarity and retained solids; and whether your chosen fabric is compatible with the conditioned slurry.

It cannot prove: the final cake solids you will reach after weeks of consolidation in a full-size tube; the dose you will actually run once the slurry passes through a real pump and a real fill line; how the programme behaves when the dredge cuts into a different stratum; whether your filtrate meets a permit limit, which requires laboratory analysis of the actual water; or anything about contaminant mobility, leachability or waste classification.

Scale-up also has hard limits worth stating plainly. Bench mixing is gentle and uniform; field conditioning is neither. Bench drainage is at low head over a small fabric area; a tube drains under increasing internal pressure over a huge area with a developing filter cake at the wall. Expect field dose to sit above bench dose, and expect the first days of a fill to behave differently from the last.

Buyer checklist before you request quotes

1. Sediment characterisation: particle size distribution, fines fraction, organic content, and pore-water salinity, pH and hardness.
2. Contamination status and the applicable regulatory framework, including which tests the receiving site requires. Note that this is a compliance question, separate from dewatering.
3. Slurry solids as dredged, and expected variability across the dredge area and depth.
4. Volume to be handled and the schedule, including any weather window.
5. Intended dewatering route, or the constraint that decides it — available area, required cake dryness, enclosure requirement.
6. Fabric or equipment specification, so screening uses the right filtration medium.
7. Return-water destination and the discharge limit that applies to it.
8. Make-down capability on site: water quality, mixing equipment, and whether powder, emulsion or solution feed is practical.
9. Acceptance criteria you will judge on: kg active polymer per dry tonne, filtrate turbidity, cake solids, and drainage rate.

With those nine items, a supplier can send a sensible candidate shortlist and a test plan. Without them, any grade recommendation is marketing. If contamination is in scope, also read the safety and SDS handling guide, and municipal sludge dewatering for the contrast with a stable, organic, well-characterised feed. Commercial terms — grade availability, documentation and order-specific conditions — are covered in the supplier, MOQ and pricing guide.

Sources

• SNF, Dredging — supplier overview of polymer-assisted dredging and dewatering routes. snf.com/industry/dredging
• SNF, Dredging — Filtration — supplier description of filtration and geotextile dewatering practice. snf.com/industry/dredging/filtration
• US Army Corps of Engineers, New England District, Final Focused Feasibility Study — source of the roughly 25–50% solids geotube dewatering figure cited above, within that project's context. nae.usace.army.mil (PDF)
• US Army Corps of Engineers, Long Island Sound Dredged Material Management Plan — Final Programmatic EIS — dredged material management and placement framework. nae.usace.army.mil (PDF)

Third-party names and documents above are cited for reference and attribution only. They do not indicate any affiliation, partnership or endorsement, and no figure from them describes a ChinaPAM project or result. Performance in your application depends on your sediment, water, equipment and operating conditions, and must be established by testing on your own material.