Application · Ports, waterways, dredging

Dewatering dredged material with flocculation

Dewatering dredged material means removing enough water from dredged harbour silt and sediment for it to be transported, reused or deposited. A suitably chosen polymer binds the fines into flocs that stay behind in the geotextile tube, decanter or press.

Updated

  • Polymer matched to salinity and organic content
  • Ahead of your geotextile tube, decanter or press
  • Rental containers for maintenance dredging campaigns
Request a free sludge test  Call us +49 172 7009709

Why harbour silt holds on to its water

What is dredged from harbour basins, approach channels and lock approaches is rarely clean sand. Harbour silt consists mainly of silt and clay mixed with organic matter, and it arrives with a lot of water from the suction line or hopper barge. Organic matter binds additional water and keeps fine particles in suspension.

In a confined disposal site such material settles slowly and takes a long time before it can be driven on or loaded. At the same time, suitable land is getting scarce. Without flocculation, geotextile tubes blind quickly, and decanters or presses cannot run stably.

Typical sites

  • Harbour basins and berths
  • Approach and navigation channels
  • Locks and canals
  • Marinas and yacht harbours
  • Disposal sites and settling polders

Process chain from dredger to dewatered material

Flocculation sits between conveying and separation. This is where it is decided whether the fines stay in the tube or machine or leave with the return water.

  1. Existing or partner

    Dredged material feed

    Suction dredger, grab with hopper barge or pipeline brings the mixture ashore.

  2. Existing or partner

    Sand pre-separation

    A hydrocyclone or screen takes out the sand; only the silt goes on.

  3. FMfloc supplies

    Polymer make-down and dosing

    Make-down with fresh water where possible, ageing, then dosing in proportion to sludge flow.

  4. FMfloc supplies

    Mixing and flocculation

    Gentle mixing so the floc survives the way into the tube or machine.

  5. Existing or partner

    Separation and dewatering

    Geotextile tube, decanter or press retains the flocs while the water drains off.

Return water goes back to the water body as set out in the permit, or serves as transport water for the pipeline.

FMfloc supplies make-down, dosing and mixing. Dredger, sand separation and dewatering machine are provided by you, your marine contractor or a partner.

What FMfloc supplies at the port

We do not build dredgers, tubes or decanters. We supply the polymer technology ahead of them and adapt it to the dredged material and the water on site.

Make-down with the right water

Make-down unit for powder or liquid polymer, 0.1–1.0 % make-down concentration, with a fresh water connection. Saline make-down water weakens the polymer.

Dosing for changing material

Dosing pumps and control based on sludge flow. When the material changes from sandy to silty, the dose is adjusted rather than left at a fixed setting.

Mixing ahead of tube or machine

Mixer or flocculation reactor at the right point in the line, so the floc forms and does not break up again on its way into the geotextile tube.

Rental containers and remote monitoring

Container unit for the length of the campaign. Flow, dosing rate, tank levels and alarms go to the cloud, turbidity is available as an option.

More on the components Polymer make-down units Mixers Mobile units Remote monitoring

Disposal site or flocculated dewatering

A disposal site remains an option where land and time are available. The comparison shows what changes with flocculation ahead of a tube or machine.

Disposal site without flocculation Flocculation ahead of tube or machine
Land Large polder, occupied for a long time Tube area or machine pad, for a limited period
Duration Settling and drying over months to years Machine in continuous flow, tube after weeks depending on material
Return water Fines settle slowly, discharge hard to control Flocs retain the fines, discharge can be measured
Control Little scope for intervention once pumped in Dose follows the changing dredged material
Weather Rain and frost extend the drying time Containerised unit, usable outside the dry season

How we test your sludge

Only a test with your own sludge shows which polymer works and how much of it you need. There are three steps.

Send 2 litres of typical dredged material, ideally with some water from the site and a note on whether it is fresh, brackish or salt water.

Request a free sludge test

  1. Send a sample

    Two litres of sludge or wash water are enough. The test is free of charge. We agree with you beforehand how to fill and ship the sample.

  2. Laboratory test

    We test several polymers on your sample and compare floc formation, settling behaviour and clarity of the water. You receive a report with a product recommendation and dosing range.

  3. Trial unit on site

    A rental unit runs at your site under real conditions. Remote monitoring records flow and dosing rate, so the results are documented afterwards.

Test, rent or buy

The right route depends on how long you need the unit and how predictable the job is. All units share the same controls and the same cloud connection.

Test

Prove it first, then decide

A laboratory test and an on-site trial show which polymer and which dose your sludge needs. You decide on any investment based on your own measured values.

Rent

For projects with an end date

Container and trailer units, rented by the week, ready to run with a mobile data connection. Suited to construction sites, pond clearance, campaigns and interim periods.

Buy

For continuous operation

A stationary make-down and dosing unit, sized to your sludge volume and integrated into your existing controls. The separation or dewatering unit remains existing equipment or comes from a partner.

For maintenance dredging you rent the unit by the week for the campaign; the separation machine is provided by you or your partner. Rental units

In depth

Background for planning, operation and permitting. Each section can be expanded on its own.

Choosing a polymer for harbour silt

For predominantly mineral dredged material, a high molecular weight anionic polyacrylamide often works. The more organic matter the silt contains, the more likely cationic products are to form robust flocs. Which product fits is decided by the lab test with your material.

The separation machine also shapes the choice. In a geotextile tube the floc must be large and stable enough to stay inside the fabric while letting water through. In a decanter, shear resistance counts. Powder polymer pays off on long campaigns with steady consumption, liquid polymer on short jobs and trials.

Dewatered dredged material and reuse

How dry the material gets depends on its organic content and on the machine. Sandy, mineral dredged material releases water far more readily than silt rich in organic matter. Reliable dry solids figures only come from testing.

For the route onwards, the German HTG paper on the reuse of dredged material and, for relocation in coastal waters, the GÜBAK guidelines offer orientation; elsewhere, local guidance applies. Separated sand is often easier to reuse than the fine fraction. Whether dredged material is contaminated and where it may go is decided case by case after analysis. Flocculation does not change that classification; it only makes the material manageable.

Permits and records

Dredging, interim storage and returning the water all require permits. Which authority is responsible and which limits apply depends on the water body and the region, and is set out in the local permit. We cannot promise you an approval.

What we supply is data. The unit records flow, dosing rate, tank levels and alarms continuously in the SmartFloc Cloud, with turbidity measurement in the discharge as an option. This lets you show when how much material was treated and that dosing was running. If a fault occurs, the alarm goes to your staff, even when nobody is at the unit at night.

Salinity and organic content: what they mean for polymer and make-down

In brackish and salt water the molecular chains of many polymers coil up, the floc becomes smaller and the demand rises. Flocculation still works there if product and make-down are right. The make-down water matters: polymer is made down with fresh water wherever possible. Whether it can then be diluted with harbour water is shown by testing.

Organic content pushes in the same direction. Organic matter binds water and has a large surface area, so silt rich in organics usually needs more polymer per tonne of dry solids than mineral material and reaches lower dry solids. Where salinity varies with tide or river flow, we test several water conditions in the lab.

Frequently asked questions (FAQ)

Disposal site, geotextile tube or mechanical dewatering: which fits when?

It depends on land, time frame and volume. A disposal site suits places where land is available for a long time and time is not critical. Geotextile tubes (Geotube) need less land and little equipment but dewater over weeks. Decanters or presses work in continuous flow and quickly deliver material that can be loaded. Tubes and machines both need flocculation upstream.

Does flocculation work in salt or brackish water?

Yes, provided the polymer and the make-down water suit the site. High salinity weakens many polymers, so we select the product in a lab test with your dredged material and your water. Make-down is done with fresh water wherever possible. Polymer demand in salt water is usually higher than for the same material in fresh water.

How is dewatered dredged material reused?

That depends on how the material is classified in each case. Separated sand can often be reused; for the fine fraction, reuse, placement or landfill may apply depending on properties and contamination. The German HTG paper on dredged material reuse and, for coastal waters, the GÜBAK guidelines offer orientation. Flocculation makes the material manageable but does not change its contamination.

What dry solids content can dredged material reach?

That depends mainly on the organic content and on the machine. Sandy, mineral material gets considerably drier in a decanter or press than organic harbour silt, and a geotextile tube only reaches its final value after weeks. A reliable figure for your material comes from the lab test with a 2 litre sample, followed by a trial unit on site.

Are rental units available for maintenance dredging campaigns?

Yes, you rent the make-down and dosing unit as a container by the week for the length of the campaign. It arrives wired and with a mobile data connection; sludge line, make-down water and power are connected on site. The separation machine is provided by you, your marine contractor or a partner. If dredging recurs every year, buying may pay off.

How does organic content affect polymer demand?

As organic content rises, polymer demand usually rises too. Organic matter binds water and offers a large surface on which polymer is consumed. Cationic products are then often more effective than anionic ones. Because organic content and salinity vary between dredging sections, we determine the dose in testing and adjust it during operation.

All applications

Have your dredged material tested

Send us 2 litres of your dredged material. We test polymer and make-down water in the lab and tell you how your material can be flocculated ahead of tube, decanter or press.

Request a free sludge test  +49 172 7009709

Phone +49 69 5000060 or email info@fmfloc.de. We reply on working days.