Particularly for smaller cases of redevelopment of polluted land sites, on-site decontamination often represents a more logistically and economically favourable solution. In the following, a highly mobile, small soil washing plant is presented which can be transported complete in 13 containers to the polluted site and reassembled and disassembled within a few days.
The solution to the problem of contaminated sites is still one of the great challenges of our society. Of the many feasible decontamination technologies, particularly soil washing processes have proved their excellent suitability in the past years. These technologies are considered to be of particular environmental benefit since they are based on well-known wet processing steps and use water as "solvent".
Besides stationary soil processing-plants, small washing plants of high mobility have also been very successful in the redevelopment and decontamination market. Both types of plant are developed and built by AKW Apparate + Verfahren GmbH. The special advantage of these highly mobile washing plants is their short setup times which also enables an on-site decontamination of smaller cases of site pollution to be economically justified. After the soil has been treated, it is returned directly to its original location thus eliminating unnecessary road haulage.
First application:
Based on the problem area described and the experience gained on soil washing plants, AKW Apparate + Verfahren was commissioned by a major Duisburg construction company, to build a highly mobile soil washing plant with a throughput of 10 to 20 tph. Its first operation site is a formerrailway repair shop in Kleve (Germany). Using wet separation techniques, the contaminated soil is separated and cleaned into various fractions (< 50 µm, 50 µm - 2 mm, 2 - 32 mm, 32 - 100 mm). Simultaneously, contaminants are separated and concentrated (heavy metals, mineral oils, PAC and light materials) in the fine fraction < 50 µm.
The processing-plant has been designed for the contaminant fractions to be obtained separately and these can thus be disposed accordingly. The water, containing particles and suspended material, is led to a water treatment plant and cleansed to process water quality standards. Due to the novel plant concept, a waste water discharge is generally not necessary. To improve process and waste water quality, a water treatment unit consisting of sand filters and activated carbon adsorbers has been integrated.
All machines and units were housed in 13 transport containers, to achieve a high mobility of the plant.
The time for reassembly and disassembly can be limited to just a few days.
Processing units
- The wet part of the soil washing plant comprises following processing "unit-operations":
- Feed unit with box feeder
- Soil washing and cleansing in a washing drum
- Sizing with screening machines and hydrocyclones
- Density separation with spiral concentrators and jigs
- Process and waste water treatment
- Dewatering with thickeners and screen belt press.
In order to prevent concentration phenomena of contaminants in process water, a process water side-stream of about 10 % is discharged and to a great extent freed of particles in a sand/gravel filter and subsequently purified in activated carbon filters. The water treated according to this procedure can be led back to the process water circuit or discharged into the sewerage system. The activated carbonfilters have a simultaneous function as solid bed reactors.
To provide an oxygen supply for the microorganisms settling into the activated carbon surface, a membrane oxygen transfer unit is connected in parallel. The adsorbed organic contaminants are degraded into carbon dioxide, water and other inorganic materials and the activated carbon is simultaneously regenerated. This enables waste and/or process water quality to be improved and the operating times of the activated carbon to be extended by means of biological regeneration.This in turn enables the corresponding disposal costs for exhausted activated carbon to be considerably reduced.
About 5000 t contaminated soil, which has subsequently been put back on the site, have meanwhile been freated in this plant in the first half year of operation. This represents an important contribution to the protection of our environment and our natural resources
