Contaminant concentration in the sludge and recovery of valuable fractions
Soil Washing
Integrated tertiary treatment to support discharge compliance
Active research on PFAS with Politecnico di Milano
Soil-washing plants and process solutions for contaminated-soil remediation
From laboratory characterisation of the contaminated material to tertiary treatment on activated carbon filters, Tecnoidea designs and builds water and sludge treatment solutions for Soil Washing plants, drawing on 45 years of experience with complex feed materials in the quarrying and environmental sectors.
The complexity of water and sludge treatment for contaminated soil
In contaminated soil, pollutants — heavy metals, hydrocarbons, PAHs, PCBs, dioxins and PFAS — are not evenly distributed but tend to bind to the fine fraction (silt and clay, below 63 µm). Mechanical washing separates the valuable fractions — gravel and sand, recoverable as secondary raw materials — from the fine fraction, where the contamination is concentrated. The process can significantly reduce the volume of material requiring disposal by concentrating contamination in the fine fraction.
In practice, effective remediation depends on how well the different process stages work together: mechanical separation must be matched to the water-treatment chemistry, sludge conditioning must take account of the variable feed material, and tertiary treatment must be sized for the contaminants that remain dissolved.
To integrate these process stages, our soil washing solutions start in the laboratory, where the contaminated material is characterised: particle size distribution, type and concentration of contaminants, flocculation behaviour, sludge filterability and chemical conditioning requirements. The aim is to concentrate the pollutants in the solid residue and produce treated water suitable for closed-loop reuse or for environmental discharge in line with applicable requirements.
The Tecnoidea solution
Water and sludge treatment in a Soil Washing plant: from material characterisation to compliant discharge
Physical pre-treatment: removing light fractions
Wash water from contaminated soil carries light fractions such as polystyrene flakes, plastic, small pieces of wood and roots. If these floating particles entered the clarifier, they would accumulate on the surface, hindering the settling of the sludge flocs and potentially clogging the clarified-water recirculation pumps. A static curved screen installed upstream of the treatment process intercepts them before flocculation and clarification: the water passes through the curved screening surface, while the light materials are retained for separate handling.
Collection and chemical reaction
In soil washing water treatment, the turbid water does not simply flow through a transit sump: it requires an agitated reaction tank. This is where the reagents required by the feed material are dosed — coagulants to neutralise the surface charges of the finest particles, antifoam agents and, where needed, pH correctors. The tank provides the contact time required for the reagents to act before clariflocculation. The number of chemical dosing units depends on the complexity of the feed material.
Flocculation and clarification
From the reaction tank, the water is pumped to the static clarifier. Flocculant is dosed along the feed line, promoting the aggregation of the contaminant-laden fine particles into larger, denser flocs. Physical separation takes place inside the clarifier: the flocs settle and accumulate in the clarifier cone, while the clarified water overflows at the surface. Two proprietary technologies continuously optimise the process. The SADF (automatic flocculant dosing system) samples water from the central feed well to measure the settling velocity and automatically adjusts the flocculant dosing pumps. At the same time, the LEM (sludge-level monitoring system) measures the sludge–water interface in the cone and triggers sludge discharge only when the sludge is sufficiently thickened. This improves process efficiency and reduces unnecessary empty cycles on the downstream filter press, helping reduce energy waste.
Chemical sludge conditioning: milk of lime
Milk of lime acts as a filtration aid, conditioning the sludge and improving its filterability even when the feed material is clay-rich or variable, as is the case at treatment facilities that accept soils from different sources. By increasing the pH, milk of lime can also promote the precipitation of certain dissolved metals, depending on the composition of the feed water, allowing them to be transferred to the sludge phase. This takes place in a tank equipped with a multi-stage agitator (with impellers at different heights), which provides the vigorous mixing required for lime-conditioned sludge.
Dewatering: where the contamination is concentrated
The conditioned sludge is pumped to the filter press, which separates the liquid fraction and produces dewatered, spadeable filter cakes with a residual moisture content typically around 30% under suitable operating conditions. These cakes contain the contaminants associated with the fine solid fraction separated during the washing process. This is the purpose of soil washing: what enters as a large volume of contaminated soil leaves as recoverable coarse fractions on one side and a reduced volume of contaminant-rich filter cake on the other, requiring disposal or further treatment depending on its classification. Less waste volume means lower disposal costs.
Tertiary treatment for discharge compliance
Clarification removes suspended solids but not dissolved contaminants. Dissolved contaminants, including certain hydrocarbons, solvents and PFAS, may remain in the water phase and can accumulate in a closed-loop system if they are not specifically removed. When the plant has excess process water to discharge, or when this build-up reaches critical levels, the tertiary treatment section comes into operation. Neutralisation lowers the pH, previously raised by the lime, using acid or CO₂. Sand filters remove residual suspended solids and protect the next stage. Activated-carbon filters adsorb dissolved organic contaminants and can also remove certain PFAS, depending on their characteristics and the treatment conditions. For contaminants requiring specific treatment conditions — including arsenic and other dissolved pollutants — dedicated treatment stages can be added.
Products, plants and technologies for the Soil Washing sector
Water and sludge treatment for recovery plants and site remediation.
Disidratazione fanghi
La disidratazione meccanica rappresenta la fase più importante nella gestione dei fanghi di processo. Attraverso una filtrazione ad alta pressione,...
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Sidebar filter press
Designed for the mechanical dewatering of sludge in standard and medium-output applications (with a capacity of up to around 3.3...
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Filtropressa a trave alta
È la macchina progettata per la disidratazione meccanica dei fanghi nel caso di grandi produzioni. Separa l’acqua dal fango e...
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Impianto dosaggio latte di calce
L’impianto per la preparazione e il dosaggio del latte di calce è l’unità fondamentale per ilcondizionamento chimico dei fanghi progettata...
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Vaglio statico a griglia curva (Vaglio a Banana)
Unità indispensabile negli impianti di soil washing, spazzamento stradale e trattamento rifiuti da costruzione e demolizione, il vaglio statico a...
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Chiarificatore statico verticale
Il chiarificatore statico verticale è la tecnologia di riferimento privilegiata da Tecnoidea per separare i solidi sospesi dall’acqua di lavaggio....
Discover the productStatic curved screen
The static curved screen is a preliminary physical-filtration unit installed upstream of the water-treatment stage, used in Soil Washing, road-sweeping...
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Tertiary processes and ancillary systems
Tertiary treatment and supporting infrastructure play an important role in closing the water loop and supporting overall plant performance. From...
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pH neutralisation plant
The pH neutralisation plant is used as a tertiary treatment stage when clarified water has an excessively high pH. By...
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ContactsSpecialist expertise
In Soil Washing, process performance depends not only on mechanical washing but also on water-treatment chemistry. Tecnoidea brings 45 years of water and sludge treatment experience to this sector, developed in the aggregates industry and extended to more complex environmental applications. We characterise the feed material in the laboratory, define chemical conditioning (lime, coagulant and pH-corrector dosing), design the tertiary treatment section and engineer the process to concentrate contaminants associated with the fine fraction in the dewatered filter cake while treating contaminants that remain in the water phase. We offer an in-house analysis laboratory, process automation systems (LEM, SADF) and a full range of units for treating contaminated water and sludge.
Applied research on PFAS with Politecnico di Milano
PFAS are synthetic contaminants characterised by an extremely stable carbon–fluorine bond; many PFAS are persistent and mobile in water and difficult to remove. Activated-carbon adsorption is one of the most established treatment technologies for PFAS-contaminated water, but it involves significant operating and regeneration costs, because the carbon progressively becomes saturated and requires replacement or regeneration. Tecnoidea is working with Politecnico di Milano to develop combined solutions that reduce activated carbon consumption, specifically for wash water from contaminated soil.
From the laboratory to the pilot plant
Treating contaminated material effectively starts with characterising it. Tecnoidea is expanding its in-house laboratory with a dedicated testing area, where a pilot-scale Soil Washing plant is planned to reproduce the process on a small scale before the full-scale plant is sized. The objective is to support full-scale design using data generated on the customer’s specific material — a level of rigour that is particularly important when treating variable, contaminated materials.
45
Years of experiencein water and sludge treatment
17
Water treatment sectionsdesigned and installed for Soil Washing and remediation projects
200
t/h of material treatedat the Milano Santa Giulia soil-washing plant
References
Integrated dual-line water and sludge treatment plant
SwissSector Road Sweeping
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Amrein AG
A complete plant developed in response to new regulatory restrictions
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Germany
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Swiss
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ViTo Recycling AG
Water and sludge treatment plant for washing contaminated aggregates and treating drilling muds.
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Italy
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ATI SEMP-VEOLIA
Soil-washing plant for environmental remediation ahead of the Milano Cortina 2026 Winter Olympics.
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