1.1.04 Reliable long-term effectiveness – in-situ treatment wall at the Rheine site

Treatment (reactive) walls are an extremely promising approach to sanitising or safeguarding contaminated aquifers. Germany’s first all-over permeable reaction wall was installed as a pilot scheme at Rheine in North Rhine-Westphalia in 1998, in a quaternary aquiferIn the field of hydrogeology, an aquifer (from the Latin words aqua = water, and ferre = to bear or carry) is an underground layer carrying groundwater. Aquifers are also referred to as water-bearing strata. Different types of aquifer are distinguished depending on the individual rock type (and thus the rock’s porosity). polluted with chlorinated hydrocarbonsCollective name for organic compounds containing at least one chlorine atom bound directly to a carbon atom. CHCs are contained in many products and are used as base materials in the chemical industry, as solvents and as pesticides. As a result of their wide-spread use and high stability, CHCs are now ubiquitous in the environment. (DBU project). In a subsequent, further-reaching research and development project funded by the BMBF, partners from science and economics worked together to investigate the durability of the treatment wall and the degree to which iron can be used as a reactive material at that particular site.

In June 1998, Hanover-based Mull und Partner Ingenieurgesellschaft mbH used funding from the Deutsche Bundesstiftung Umwelt (DBU, German environmental foundation) to erect a groundwater treatment wall as a field-scale project in order to remove volatile halogenated hydrocarbons (VHHC) such as tetrachloroethylene (PCE) or trichloroethylene (TCE) from contaminated groundwater. This involved using a new porous zero valent iron (Fe⁰), “iron sponge”, as the reactive material.

Pollutants from a laundry

The reaction wall was installed approximately 700 metres downstream of a massive source of underground PCE contamination. The cause of this pollution is a laundry that once operated on the premises. The treatment wall is a structure enabling all-over permeabilityMeasure of the ability of a porous material, such as rock or soil, to allow gases or fluids to pass through it. of contaminated groundwater and measures around 6 metres deep, 22.5 metres long and 88 centimetres thick. It is filled with two reactive materials up to a height of around 3.5 metres, i.e. above the maximum anticipated level of the groundwater. The materials used are an iron sponge supplied by MITTAL Steel Hamburg GmbH (formerly ISPAT Hamburger Stahlwerke GmbH) and a 70% pea gravel and 30% grey cast iron granulate blend from Gotthart Meier AG in Rheinfelden. This twin-layered effect allows the performance of the materials to be compared. This procedure has seen the amount of PCE reliably drop from its initial concentration of several thousand micrograms per litre by over 99% since 1998.

Studies on long-term effectiveness

As well as the activity mentioned, the plant was also used for various long-term investigations. The BMBF funded the following projects: Pre-investigation, monitoring and quality management regarding Reactive Walls (Christian Albrecht University of Kiel), Evaluation of the durability of an iron-reactive wall with the example of the site at Rheine (Mull und Partner Ingenieurgesellschaft mbH) and Biological processes in a reactive iron wall (TU Berlin). The objective of the research activities was to observe the long-term behaviour of the treatment wall and to develop a monitoring programme to investigate the geochemical, hydro-geological and biological processes in and around the Fe⁰ treatment wall.

Reliable for many years now

The reaction wall at Rheine has been providing assured constant remediation of the groundwater for over ten years now, with the two different materials producing different cleaning performances. The groundwater investigations have shown that the iron sponge achieved a cleaning performance of around 70 to 80% in the first 6 to 12 months following installation, a figure that then rose to more than 99%. For several years now the concentrations of VHHC measured downstream have been under 10 micrograms per litre.

The concentrations of VHHC downstream of the section of wall filled with a mixture of grey cast iron granulate and pea gravel were somewhat different: an excellent cleaning performance of 99% was recorded at the start, but once the reaction wall had been in operation for around 8 to 12 months only around 80% of the inflowing VHHC content was still being broken down. This level of cleaning performance has remained virtually constant up to the present day. The scientists were able to determine from core drilling that a partial separation of gravel and iron was the cause of the reduced degradation in this section of the treatment wall.

Both flow modelling and pump-and-trace attempts clearly demonstrated that the permeabilityMeasure of the ability of a porous material, such as rock or soil, to allow gases or fluids to pass through it. of the treatment wall is assured and nothing is flowing over or around it. The experts were also able to substantiate that hydraulic changes had taken place during the time of operation through precipitates or gas formation.

Biological aspects

The scientists at TU Berlin also demonstrated during the project the appearance of bacteria in the two iron materials used after a few years. The description of the biological activities of all relevant physiological bacteria groups offered fundamental insights into the microbial colonisation. As it was discovered, there are no anticipated negative effects within a foreseeable timeframe on the long-term stability of the dechlorination performance of iron treatment walls as a result of the micro-organisms present.

Applying the results

The synopsis of all the research work clearly demonstrates that the pilot and demonstration treatment wall at Rheine can be successfully applied for long-term use. The project results also included Mull und Partner Ingenieurgesellschaft mbH registering a trade name for the iron sponge. It is marketed under the name “ReSponge” and has been registered with the patent and trademark offices in Europe (07/2005) and the USA (12/2005). The company also entered a contractual agreement with MITTAL Steel GmbH in Hamburg in 2003 for the marketing of iron sponge for remediation purposes.

Project website www.rubin-online.de

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1.1.05 Funnel and gate

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1.1.03 Permeable treatment walls