Application · Soil remediation and soil washing

Wash water treatment for soil washing

Wash water treatment in soil washing flocculates the contaminated fines out of the process water, so clear water returns to the wash and the sludge can be dewatered. FMfloc supplies the polymer make-up, dosing and remote monitoring, to buy or to hire.

Updated

  • Make fines with heavy metals and PAHs separable
  • Wash water recirculated instead of discharged
  • Hire unit for the duration of the remediation
Request a free sludge test  Call us +49 172 7009709

The contaminants sit in the fines

A soil washing plant separates contaminated excavated soil by grain size. Sand and gravel are wetted, scrubbed and screened. Silt and clay below 63 µm end up in the wash water, and with them most of the particle-bound contaminants.

These fines barely settle on their own. If they stay in circulation, they coat the washed sand again, slow the thickener down and reach the press as wet sludge that is hard to dewater. Then there is variation: every batch of soil brings a different grading and contamination.

Typical sites

  • Former gasworks and coking plants
  • Industrial brownfields and conversion sites
  • Shooting ranges with lead contamination
  • Permanent soil treatment centres
  • Mobile remediation sites

How process water moves through a soil washing plant

Flocculation starts after sand separation, once only fines are left in the water. It determines how clear the return water is and how well the sludge dewaters.

  1. Existing or partner

    Wash water from soil washing

    Wet washing and attrition detach the fines from the coarse grains. They leave with the wash water.

  2. Existing or partner

    Pre-separation

    Hydrocyclone and classifier recover the washed sand. The contaminated fines remain in the process water.

  3. FMfloc supplies

    Make up and dose

    Polymer is made up and dosed to flow, with a coagulant on a second line where needed.

  4. FMfloc supplies

    Mix and flocculate

    In the mixer, fines and attached contaminants bind into flocs that hold together until separation.

  5. Existing or partner

    Separate and dewater

    A thickener or lamella clarifier removes the flocs; the chamber filter press turns thickened sludge into cake.

Loop: clear water from the thickener and press filtrate return to the wash. Fresh water mainly replaces what leaves with the sand and the filter cake.

FMfloc supplies make-up, dosing and mixing including remote monitoring; the washing plant, hydrocyclone, thickener and press come from the operator or partners.

What FMfloc covers at a soil washing plant

We do not build washing plants or presses. We supply the flocculation stage in between and tune it to your soil.

Polymer make-up

Make-up unit for powder or emulsion with ageing time, concentration 0.1–1.0 %. For short campaigns, liquid polymer from an IBC is often enough.

Second dosing line for coagulant

If the lab test calls for it, a separate pump doses an iron or aluminium salt ahead of the polymer. Both rates follow the flow.

Mixing ahead of the thickener

Mixer and dosing points placed so that the floc reaches the thickener intact. Sized to the sludge volume of your plant.

Remote monitoring and records

Dosing rate, flow, tank levels and alarms are logged in the cloud, with turbidity and pH as options. The records stay available for the project file.

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

Fines in the loop: without and with controlled flocculation

The soil washing itself does not change. The difference is how reliably the fines are removed from the process water.

Without controlled flocculation With FMfloc flocculation
Return water to the wash Turbid, fines end up back on the sand Clarified, fines stay in the sludge stream
Thickener Slow settling, throughput at its limit Flocs settle quickly, throughput stays steady
Sludge to the press Thin and hard to dewater Conditioned, dewatering determined in trials
Changing soil batches Fixed pump setting, over- or underdosing Dosing follows flow, recipe can be checked per batch
Records Handwritten shift sheets Dosing rate and flow logged continuously in the cloud

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.

Sludge from the thickener feed gives the most meaningful result; please state the known contamination so that we can handle the sample safely.

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.

A remediation project ends with the last section of the site, so the hire unit runs exactly as long as your washing campaign. Rental units

In depth

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

Choosing a polymer for contaminated fines

The fines from soil washing are rarely purely mineral. Humus, tar residues and clay minerals change the surface charge, and with it the choice of flocculant. For sandy, silty material an anionic polyacrylamide is often sufficient. With a high clay or organic content, a cationic product or a coagulant ahead of the polymer gives more stable flocs.

The decision is made in a settling and filtration test with your sludge, not from a catalogue. Because batches vary, a second sample from different soil is worth it. Liquid polymer from an IBC suits short projects; powder polymer saves on active substance in long campaigns.

Filter cake, waste classification and disposal route

The filter cake collects the separated contaminants in concentrated form. Its route follows from the waste characterisation analysis: assignment to a landfill class on a case-by-case basis, or treatment before disposal.

Costs depend on mass as well as contamination. Every litre of water in the cake is hauled and paid for, so a higher dry solids content cuts tonnage and disposal costs. How dry the cake gets depends on grading, organic content and the press; flocculation lays the groundwork. The washed sands are a separate fraction, and the operator documents their reuse.

Permits and records

Soil washing plants usually operate under an industrial installation permit. Waste acceptance, landfilling and the reuse of washed sand as a secondary building material are governed by waste and landfill regulations. Limit values and monitoring duties are set out in your local permit. This is guidance, not legal advice.

What we contribute is a complete operating record. The unit logs dosing rate, flow, tank levels and alarms continuously in the cloud, with turbidity and pH on request. This shows how flocculation ran for each batch. It does not replace sampling and laboratory analysis.

Contaminants on fines or in solution

Flocculation separates particles from water. Whatever is attached to silt, clay or organic matter moves with the floc into the sludge and then into the filter cake. This mainly applies to heavy metals and PAHs, which in soil are largely particle-bound.

The floc does not retain dissolved substances. Volatile hydrocarbons such as BTEX or chlorinated solvents, dissolved cyanides and PFAS largely stay in the water. Whether a coagulant captures part of the dissolved metals depends on water chemistry and pH and is checked in the lab. The rest needs downstream water treatment designed by the operator or a specialist partner.

Frequently asked questions (FAQ)

Why do contaminants sit in the fines?

Because fine particles have a very large surface area relative to their mass. Clay, silt and organic matter bind heavy metals and PAHs on these surfaces, whereas sand and gravel offer little room for them. Soil washing exploits this: it scrubs the fines off the coarse grains and separates by grain size. What remains is a small, heavily contaminated fraction in the wash water.

How is the wash water kept in circulation?

Clarified water from the thickener returns to the wash, and so does the filtrate from the press. Fresh water mainly replaces the share that leaves the plant with washed sand and filter cake. Dissolved substances build up in the process, because flocculation does not remove them. How much water must therefore be bled off and treated is set by the operator based on their measurements.

Which disposal route applies to the filter cake?

That is decided by the waste characterisation analysis of the filter cake. It determines which landfill class the cake is assigned to in each case, or whether it needs treatment first. Mass also drives the cost: the higher the dry solids content, the less water is transported and paid for. Flocculation conditions the sludge so that the press can dewater it well.

What evidence and records are possible?

A continuous operating record of the flocculation stage in the cloud. It shows dosing rate, flow, tank levels and alarms for every shift, with turbidity and pH added on request. This lets you show consultants and authorities how the unit was operated. The record does not replace analysis: water samples, waste characterisation and evidence for the washed sands remain the job of accredited laboratories.

Can I hire the flocculation unit for a fixed-term remediation project?

Yes, the make-up and dosing unit is available as a container or trailer for hire over the project period. It arrives fully wired and with a mobile data connection for remote monitoring. It is connected to the washing plant, thickener and press supplied by you or your partners. After the last batch it goes back, so no capital outlay or decommissioning stays with you.

What happens to dissolved contaminants?

Dissolved contaminants largely remain in the water, because flocculation only separates particles. This applies to volatile hydrocarbons, dissolved cyanides and PFAS, for example. Unless water is bled off, they build up in the loop. They need downstream water treatment by the operator or a specialist partner. In the lab test we establish how much is particle-bound and what a coagulant captures in addition.

All applications

Have sludge from your soil washing tested

Send us 2 litres of sludge from the thickener feed. You receive a lab report with a polymer recommendation and, if needed, a proposal for the coagulant. If you wish, a trial unit at your washing plant follows.

Request a free sludge test  +49 172 7009709

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