Additional tents

Rigorous monitoring system

Varying degrees of contamination in the material

Multi-stage excavation procedure

Rhine used as a transport route

a temporary structure used by Marti Infra AG for remediating contaminated sites

Science

An airtight remediation setup

The new remediation company reviewed the remediation site, reinforced the containment measures and implemented new work procedures, including ensuring that no dust from the remediation site was impacting the neighboring communities. Additional containment tents were erected, and the loading system to the barge was improved. Air monitoring was conducted by the Basel Air Hygiene Office, which informed the public of the results via the office’s web page.

Text by Linda Bergsten, illustration by Ikonaut, photos by Gregory Collavini

Setting up the soil washing plant.

Table of contents

Additional tents

Rigorous monitoring system

Varying degrees of contamination in the material

Multi-stage excavation procedure

Rhine used as a transport route

Published on 01/07/2021

The new remediation company, ARGE MZ Hueningen (comprising Marti Infra AG, Marti AG, Züblin Umwelttechnik GmbH and Strabag Umwelttechnik GmbH), conducted a review of the remediation site and what caused the shutdown, before the work was commissioned.

The review found that during the initial remediation phase, soil had been stored outside the tents, the barge had no containment system, the soil-washing plant operated in the open air and the existing tents were not sufficiently airtight. The issue with the tents had contributed to the odor development and issues in 2013. Note that the initial site setup and approach was aligned with the prefectural order of 2012.

To ensure that work could be carried out without generating dust, each work step was validated by air monitoring before the step was carried out. It took about a year to validate the revised remedial measures to ensure that all the lessons learned had been addressed.

Hydraulic motor for moving the marquees in action.

Additional tents

The main containment measure – the five tents erected over the contaminated areas – needed improvement. This was done by erecting additional tents inside the existing tent structure. More tents were added in the less contaminated areas.

All tents were under negative pressure (set to minus 4 pascal, compared to the previous setting of minus 1 pascal), ensuring that they were airtight. Air was extracted to an improved and expanded air treatment system. The air extraction system was set up across the site in a rigid extraction network with a maximum capacity of 175,000 m3/hour for all tents, which focused on the heavily contaminated areas. In addition, the air was treated in a series of carbon filters. To minimize dust generation, the outdoor surfaces surrounding the tents were paved with concrete and cleaned regularly, while water sprinklers were used inside the tents.

To ensure that dust and air emission were contained inside the tents when people, machinery, containers and drums moved from inside to the outside zone, air and decontamination locks were installed.

Earthmoving equipment remained inside the containment tents and was stored in an area dedicated to the machines. The air from the wastewater treatment system was also directed to the carbon filters for treatment, ensuring that all emissions were controlled and treated.

Airlock between the outside area and the inside of the marquee to safely let the driver of the excavator in and out.

Rigorous monitoring system

A rigorous continuous air monitoring system was put in place, both on-site and off-site, which included ambient air monitoring. The compounds monitored were HCH, benzene, toluene, ethylbenzene, xylene, chlorobenzenes (in particular, TCBs), dioxins and furans. The monitoring points were placed at a height of 1 to 2 meters. The Basel Air Hygiene Office (LHA) also monitored the air and checked for odors (on the Swiss side). The results were presented bimonthly on their web page.

Some monitoring points were designed for continuous use, while others were used selectively for certain site activities. The sampling was facilitated by dust collectors, and every second week these collectors were exchanged and samples analyzed. The monitoring was conducted by an independent company, and the results were reported monthly to the authorities.

Dust collectors were also installed at peak operation at six places surrounding the site that were continuously analyzed for total HCH concentration and chlorobenzenes in the dust.

Further precautions were taken. Because benzene is highly volatile, air suction systems with carbon filters were installed directly on the excavators to capture benzene at source as soon as the soil was moved.

During the warmer summer periods, when the temperature inside the tents sometimes reached 60 °C, HCH and other compounds evaporated and overloaded the carbon filters. To solve the evaporation problem, the air inside the tents was cooled with groundwater to allow condensation of the HCH and TCB to liquid form, which was then discarded as hazardous waste. The outgoing air emissions from the carbon filters were continuously monitored with a gas chromatograph. In addition, a warning system was installed for early detection in case the treatment became less effective – for example, if the carbon filters were saturated – so that precautionary action could be taken.

Dividing the excavated soil into four categories according to the respective concentrations of lindane.

Varying degrees of contamination in the material

The remediation technology selected was ex situ, i.e. digging the affected soil out and transporting this material to approved waste and/or recycling facilities. Some of the soil and gravel was treated on-site to be reused as backfill. This soil was sieved and washed in an enclosed gravel washing plant. Analysis was done to ensure that the treated soil was below the site remediation target of 5 mg/kg. The remaining material to be treated and/or recycled off-site was categorized based on its HCH concentration: high, medium or low.

Soil with a concentration below 5 mg/kg was stockpiled on-site and later used as backfill. Waste with a low HCH concentration, between 5 and 11,000 mg/kg, was treated at a thermal remediation treatment facility in the Netherlands.

Waste containing a medium level of HCH concentration was treated through thermal desorption. This is a technology where heat increases the volatility of contaminants so they can be separated from the soil. The contaminants are subsequently collected in an off-gas treatment system.

Waste containing high HCH concentrations, i.e. over 80 % of HCH in the soil, was incinerated at a temperature over 850 °C, followed by a second incineration at 1000 °C, at a waste facility. The different temperatures are optimized to volatilize and combust organic compounds in hazardous wastes.

Cross-section of the remediation site.

Multi-stage excavation procedure

The principle of the excavation work was as follows:

Site preparation: clearing, demolition, stripping of topsoil, removal of asphalt mixes
Systematic excavation of soils with pollutant content above the set concentration threshold; earthworks and armoring to prevent collapse of trench walls
On-site pretreatment of selected materials (screening/washing) to optimize the volume of materials sent for treatment
Off-site treatment for soils with an HCH content over 5 mg/kg;
Filling with sampled and approved clean materials

During the excavation, samples were taken from the affected area (one sample per 300 m3 of soil) to determine which fraction the waste belonged to and what treatment it required. Sampling of the heaps or batches of soil material and crushed concrete was carried out in accordance with the requirements for recovery/disposal of waste of Germany’s Federal/State Waste Committee (Bund/Laender-Arbeitsgemeinschaft Abfall, LAGA), as no equivalent guidance existed in France. The soil was stored in piles inside the tents awaiting the sampling result. If the remediation target of < 5 mg of HCH/kg was not achieved, the excavation dug deeper into the soil until the target was reached. Excavation was carried out with space limitations inside the tents in mind, since none of this material could be stored outside the containment area.

Material with a high concentration (above 80 %) of HCH, which was found in the HCH pit, was stored in 100 kg drums and transported via the air locks onto trucks for onward shipment to a waste treatment facility. Some of this waste material was reused by a chemical company as a raw material in producing hydrochloric acid.

Material with a medium concentration of HCH was placed in specially designed containers equipped with seals for air tightness and an aeration system with activated carbon for passive gas treatment. The containers were top-loaded inside the tents, and the container surfaces were washed off to remove contamination. The containers could then easily be transported by any means, in this case, from the site in France to the railway hub in neighboring Weil am Rhein in Germany. From there, they were shipped by rail to the waste and recycling facilities.

Transporting the contaminated soil from inside the marquee to the ship.

Rhine used as a transport route

Almost 90 % of the least polluted material was transported by barge to a facility in the Netherlands. On arrival, the waste was sampled to confirm its properties. The soil was heated to a high temperature, which causes organic contaminants such as tar, oil, fat and petrol to combust and to be converted to gas. The gas was subsequently routed to a flue gas scrubber for further treatment. The treated material was used as recycled filler, for example, in road construction.

Two purpose-built barges shipped the material to the Netherlands. The barges were loaded via a hopper located inside the tent and an enclosed conveyor belt equipped with a counterflow air intake. Before 2013, the loading was confined but not conducted under negative air pressure; this was one of the improvements implemented by the new service provider. The vessels were fitted with sprinklers to minimize dust generation and a special protective cover during loading that prevented emissions from accumulating. The vessels were also equipped with carbon filters to reduce any potential emissions during transportation and were double-hulled to prevent fuel leakage. A mooring system was installed in the river to accommodate barges with a length of 100 meters. To further enhance transportation, a “twin barge” system was designed with a total length of 183 meters. This increased the capacity of the barge transport from 3,200 tonnes using one barge to 5,500 tonnes using two interconnected barges.

By moving a significant proportion of the material by river and rail, the project minimized energy consumption and its carbon footprint, and increased road safety. For example, transporting 2,000 tonnes of contaminated soil by barge is equivalent to taking 100 trucks off the road. This had a major impact on transport-related emissions, such as carbon dioxide (CO2), particulates, nitrogen oxide (NOx), sulfur oxide (SOx) and noise.

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