1.1.05 Funnel and gate – an innovative reactor concept successfully combating contamination

The Lang tar works in Offenbach prepared and processed tar from 1915 to 1929. After most of the buildings were torn down in 1930 and a period of various intermittent uses, the premises are now predominantly wasteland. However, the soil and groundwater are just as polluted with tar oil and tar-oil-related substances as they were before. What is therefore required is a straightforward, economically feasible and safe remediation procedure that also affects the surrounding office locations as little as possible. Scientists working under the RUBIN research programme used an innovative “funnel-and-gate system” with three built-in bioreactors to develop a procedure to meet these requirements.

The impact of 14 years of tar production at the Offenbach site is still clearly measurable today: the contaminating tar oil and tar-oil-related substances have penetrated right through to the base of the quaternary, sandy-gravelly aquifer. Underneath this lies tertiaryThe Tertiary is the geological period of the Cenozonic era, i.e. from the start of the Cretaceous period (65 million years ago) to the start of the Quaternary (approx. 2.6 million years ago). clay (Rupelian clay), which has a blocking effect to prevent further penetration of the contaminants. Several measuring points indicate this pollution to be a 20 to 80 centimetre tar oil phaseA phase is a spatial area in which the physical parameters and chemical composition of the relevant matter are homogeneous. at the base of the quaternary aquifer. Polycyclic aromatic hydrocarbonsThe aromatic hydrocarbons benzene, toluene, ethyl benzene, xylene (►BTEX aromatics) are contained in coal tar but usually obtained from crude oil. They increase the octane number of petrol and are also used as solvents and degreasing agents or as raw materials in the chemical industry. Aromatic hydrocarbons are generally highly toxic and – at least in the case of benzene – carcinogenic. (PAH) are the dominant pollutants present; analyses indicated over 150 milligrams per kilogram of soil, and up to several grams in places. The researchers found BTEXAromatic hydrocarbons (benzene, toluene, ethyl benzene, xylene) concentrations of up to 30 milligrams per litre at the source of the contamination.

Low maintenance and control requirements

After a detailed inspection of the site, experts estimated that it would cost around EUR 18 million to perform a conventional clean-up by means of digging up the soil. That led to a search for more cost-effective alternatives. In a study of the various options, scientists weighed up whether to clean up just some areas or to seal off the contaminated soil with a barrier and a surface seal. A funnel-and- gate system with bioreactorA bioreactor is a vessel in which microorganisms, cells or small plants can be cultivated or fermented under optimum conditions. The breakdown of chemical compounds using bacteria can also be performed in bioreactors. In this context, they play in important part in the biological cleaning of wastewater, for example. was discussed as an additional option. This solution should be simple to implement with low maintenance and control requirements, and would have minimal impact on the existing land usage. The estimated costs were comparatively low, coming in at EUR 1.5 million. However, there was a problem: such a reactor had never been built before; the feasibility of this proposal needed to be proven first.

Funding from the BMBF smoothed the way from the initial idea through to the now obtained proof of functionality and effectiveness. Working under the RUBIN research programme (see project 1.1.03), a team of scientists performed the required experiments (in the lab and on site), produced the necessary models and implemented the construction and test run of the reactor on a pilot scale. Other key requirements for the success of the project were the approval of Darmstadt council, Department for the environment in Frankfurt, and the equipment provided by the state of Hessen.

Planning, construction and operation

The concept implemented on a pilot scale involved a heavily structured reactor: it comprised a baffle plate thickener to remove iron and other solids from the water, three inseries bioreactors and an activated carbonCollective name for a group of artificially produced, highly porous carbons with a sponge-like structure. This pure material is characterised by its large specific surface area (up to 300 m2/g). Activated carbon is manufactured from peat, wood, lignite, black coal or nutshells. The materials are first carbonised, during which tiny pores are created. Activated carbon adsorbs organic substances from water and the air and is thus able to clean contaminated water or polluted air. phase. Upstream of the baffle plate thickener and before each of the three bioreactors was an open water area (free water area) to distribute the groundwater over the entire flow cross-section of the bioreactors. Oxygen (as H₂O₂) and nutrients were also fed into the groundwater at several points within these free water areas to stimulate the biological degradation of the pollution. The reactor concept therefore took the general development away from passive, difficult-to-control systems to ones that enable intervention and control.

The pilot funnel-and-gate system was constructed between October 2006 and March 2007. The 30 metre long guiding walls (funnel) connect to the permeable reaction zone (gate) on the east and west flanks. They were designed as 550 millimetre thick walls in the mixed-inplace (MIP) procedureThe mixed-in-place procedure (MIP) is used to create vertical concrete walls in the soil. The ground is mixed with a cement suspension thus forming a concrete-soil structure. The main advantage of this process is that no other materials are required. The procedure is used to seal off landfills and contaminated sites. and extend at least one metre into the Rupelian clay. The actual reaction areas between the free water areas were filled with a gravel with a grain size of 2 to 8 millimetres to serve as a growth body (carrier) for the pollution-degrading micro-organisms.

The reactor is operating during the pilot with a flow rate of 230 to 500 litres per hour, which cannot be achieved by the natural gradient of the groundwater alone. A pump is therefore necessary to achieve this. This active intervention ensures a constant throughflow and keeps feeding quantities and degradation conditions as constant as possible, leading to a clear reduction in operating and monitoring requirements compared with a passive approach. The flow rate control also enables modification to the hydraulic framework conditions at any time (e.g. groundwater sampling from the area).

Oxygen and nitrate are added to the water in order to stimulateaerobicState involving the presence of oxygen (O2). Also used in the context of organisms that require oxygen to live, or to describe chemical reactions that require the presence of oxygen. and aerobicState involving the presence of oxygen (O2). Also used in the context of organisms that require oxygen to live, or to describe chemical reactions that require the presence of oxygen.-denitrifyingProcess in which bacteria break down nitrate and release nitrogen (N₂) as it occurs in the air. Byproducts of denitrification are nitrous oxide (N₂O) and nitrogen oxide (NO_x). degradation of the pollutants. O-phosphateOxygen-phosphate bond (O is the symbol for the chemical element oxygen). also ensures that there is sufficient phosphorus present in the water. Nitrogen – another essential nutrient – is available to the microorganisms in the form of naturally occurring ammonium within the groundwater. Hydrogen peroxide (H₂O₂) solutions, sodium nitrate and a mixture of monopotassium phosphate (KH₂PO₄) and buffer solution Na₂HPO₄ are buried to serve as working chemicals.

The microbial colonisation of the four gate modules was controlled by feeding in additives and monitored over 800 days across the entire bioreactorA bioreactor is a vessel in which microorganisms, cells or small plants can be cultivated or fermented under optimum conditions. The breakdown of chemical compounds using bacteria can also be performed in bioreactors. In this context, they play in important part in the biological cleaning of wastewater, for example. system using a microbiological monitoring program.

Effectiveness proven

In the baffle plate thickener, the addition of H₂O₂ converts the iron, which is then deposited as sludge through sedimentation. A large proportion of the pollutants is already broken down at this stage through the aerobicState involving the presence of oxygen (O2). Also used in the context of organisms that require oxygen to live, or to describe chemical reactions that require the presence of oxygen. stimulation in the baffle plate thickener. The main reduction is that of PAH and BTEXAromatic hydrocarbons (benzene, toluene, ethyl benzene, xylene) aromatics (benzene, toluene, ethyl benzene, xylenes), to around 70%. The other contaminants relating to the tar oil (tar-oil-based pollutants) such as NSO-HETHeterocyclic hydrocarbons (See: Heterocylics). (NSO heterocyclic compounds) and the other aromatic hydrocarbonsThe aromatic hydrocarbons benzene, toluene, ethyl benzene, xylene (►BTEX aromatics) are contained in coal tar but usually obtained from crude oil. They increase the octane number of petrol and are also used as solvents and degreasing agents or as raw materials in the chemical industry. Aromatic hydrocarbons are generally highly toxic and – at least in the case of benzene – carcinogenic. are reduced by around half.

The remaining aromatic hydrocarbons and PAH 2-16 are reduced to around 40% each in bioreactorA bioreactor is a vessel in which microorganisms, cells or small plants can be cultivated or fermented under optimum conditions. The breakdown of chemical compounds using bacteria can also be performed in bioreactors. In this context, they play in important part in the biological cleaning of wastewater, for example. 1 by the aerobicState involving the presence of oxygen (O2). Also used in the context of organisms that require oxygen to live, or to describe chemical reactions that require the presence of oxygen.-denitrifyingProcess in which bacteria break down nitrate and release nitrogen (N₂) as it occurs in the air. Byproducts of denitrification are nitrous oxide (N₂O) and nitrogen oxide (NO_x). processes; the BTEX aromatics and NSO-HET to above 20%. The remaining pollutants are broken down in bioreactors 2 and 3.

The first time that no traces of tar-oil-based pollutants were recorded in the bioreactor process was in September 2009. The trialled funnel-and-gate system is removing the pollution from the groundwater by means of aerobicState involving the presence of oxygen (O2). Also used in the context of organisms that require oxygen to live, or to describe chemical reactions that require the presence of oxygen.-denitrifyingProcess in which bacteria break down nitrate and release nitrogen (N₂) as it occurs in the air. Byproducts of denitrification are nitrous oxide (N₂O) and nitrogen oxide (NO_x). degradation alone. Other potential elimination processes such as retardationThis term is taken from the Latin word “retardare” (= delay) and is used in the context of material transport. It describes the deceleration of a substance relative to the moving medium in which it is transported. and volatilisation are playing little or no role.

System expansion

The positive experiences with the test reactor are now leading the scientists to investigate the suitability of the system for treating the entire downstream contamination. They are currently comparing various versions with one or two gates and also passive and active components.

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1.1.06 Treatment walls combating TCE

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1.1.04 In-situ treatment wall at Rheine