Physico-Chemical Processing


Physico-chemical processing

process scheme

Dredged sludge often contains pollutant which limit its reuse as soil. Chemical leaching is a technology that aims to remove the pollutants from the sludge by relatively simple  and robust mechanical and chemical treatment steps. Based on their physical and chemical behaviour, the pollutants are concentrated in the finest clay fraction, while the purified coarser fractions can be reapplied as soil.


This technology removes the pollutants from dredged sludge. The resulting purified fraction can be reapplied as soil instead of buried in a special landfill. 


By applying mineralogical techniques on dredging sludge 5 different fractions can be obtained. During this separation process, the pollutants are removed from the four coarsest fractions and concentrated into the finest fraction . The resulting fractions have stable characteristics which allows using them as secondary raw material.

Difference to BAU technology/approach:

Currently, dredged sludge is treated as a waste stream without valorisation possibilities. Due to contamination, most dredging sludge is buried in landfills.

dredging sludge

Input stream requirements:

Any kind of sludge with a solid content between 40 and 60 % and a particle size distribution between 0 and 40 mm. The installation requires a minimum volume of 12 m³ sludge per trial.

Dredging of sludge

Available quantity:

Type A: non polluted sludge: 35%
Type B: polluted sludge: 65% (polluted by heavy metals, PCB, PAH or a mixture of various pollutants.

Drivers for treating this stream:

Dredged sludge from surface waters is treated as a waste stream.
Drivers: (1) valorization of non polluted fractions from sludge, (2) volume reduction of polluted fractions by concentration and dehydratation and (3) stabilization or valorization of polluted sediments

Potential environmental risks related to this waste stream:

Due to the potential presence of contaminants, this stream is treated as waste.

Economic/technical barriers to collect this stream:

Need for sufficient valorization options for the different fractions in order to be economic viable.

Output stream:

The treatment results in 5 different fractions: gravel, coarse sand, fine sand, silt and clay (which also contains the pollutants).

F1: gravel

F2: coarse sand

F3: fine sand

F4: silt

F5: clay

Potential uses of this output stream:

The output fractions can be used as alternative fuel due to high organic content (silt); use as secondary raw material (fine sand), use by incorporation into soils and in ceramic materials for dense or light materials (coarse sand and gravel).

Reuse of fine sand in building blocks

Reuse of silt in gardening soil


Potential environmental risks/benefits related to this output stream:

A complete purification, i.e. the direct separation between pollutants and particles is quite difficult.

Economic barriers/drivers for market introduction of this output stream:

Increased valorization of (polluted) output stream. 

Technical barriers for market introduction of this output stream:


Legislative barriers for market introduction of this output stream:

Dredging sludge from surface water is regarded as waste from a legislative point of view.

Pilot description:

The pilot installation serves as a research tool (versatile set-up, upgradable,…) but also as a tool for technological validation and a demonstrator for industrial feasibility studies. It can produce material batches of sufficient quantity to perform application tests with the resulting materials (e.g. for road construction, in cement production, in ceramics,…). It can also be used to extrapolate operating costs.


The pilot installation consists of 8 main treatment steps: 5 granulometric separations (at 20mm, 2mm, 250 µm, 70 µm, 15 µm), a froth flotation cell, a filter press and a final water treatment. 4 additional treatments steps are optional: attrition, gravimetric separation using spirals, magnetic separation, and wet sieving ( to remove organic matter from mineral matter).

rotating trommel

curved screen




Typically 1.0 - 1.5 m³/h of sludge with 40 - 60% of dry content and a granulometry of 0-40mm.

Scale of the equipment:

Large pilot scale

Physico-chemical treatment installation

Main technological barriers for market introduction:

It is impossible to have a single set of operating conditions suitable for all kind of sludge. treatment results are very dependent on the origin of the sludge, which makes initial on-site characterization crucial, but difficult (need for more efficient and quick analysis tools).

Main economic barriers/drivers for market introduction:

The valorization of dredged sludge is uncertain because of incertainties in legislation. Tthe rules to determine if treated sludge is to be regarded as a waste or a product are unclear. This uncertainty decreases the willingness to invest in this technology.

Lack of dedicated budget at government level to maintain the channels and waterways

Research steps:

Research will be completed by 2015. Validation trials at large scale are still to be performed to estimate the efficiency of the technology for different origins of dredging sludge. The necessary expertise and equipment to perform such trials are available in research institutes (CTP), but industrial partnerships are needed to provide large volumes of sludge, in order to be able to do experiments close to real working conditions (minimum 50 m³ per trial).

Economic steps:

A suitable economical scheme  is needed in order to make the treatment of dredging sludge economically viable. In order to make the treatment cost effective, both process costs need to reduce, while the costs of disposal (landfilling) need to increase.

Legislative steps:

The main barrier to this technology is the lack of uniform legislation concerning the treatment of sediments and sludge. Indeed, sediments are mentioned in various legal texts, such as water regulation, soil regulation and waste regulation. At European level there has been a discussion on this topic for several years, but no harmonisation of legislation has been achieved so far.


Expected time to market: short implementation time

Contact details:
Organisation: CTP

Department: Research Department

Leading researcher: Hervé Bréquel

Phone number: +32 69 88 42 51




Presentation: Pilot facility dedicated to the treatment of dredged sludge and their potential ways of valorization, Hervé Bréquel, RENEW Technology Foresight Conference 23/04/2013