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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 tertiary 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 phase at the base of the quaternary aquifer. Polycyclic aromatic hydrocarbons (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 BTEX 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 bioreactor 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 carbon 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) procedure 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).

Longitudinal cross-section of the innovative funnel-and-gate system

Longitudinal cross-section of the innovative funnel-and-gate system
Longitudinal cross-section of the innovative funnel-and-gate system

Oxygen and nitrate are added to the water in order to stimulateaerobic and aerobic-denitrifying degradation of the pollutants. O-phosphate 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 bioreactor 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 aerobic stimulation in the baffle plate thickener. The main reduction is that of PAH and BTEX aromatics (benzene, toluene, ethyl benzene, xylenes), to around 70%. The other contaminants relating to the tar oil (tar-oil-based pollutants) such as NSO-HET (NSO heterocyclic compounds) and the other aromatic hydrocarbons are reduced by around half.

The remaining aromatic hydrocarbons and PAH 2-16 are reduced to around 40% each in bioreactor 1 by the aerobic-denitrifying 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 aerobic-denitrifying degradation alone. Other potential elimination processes such as retardation 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.

HIM GmbH, Remediation of Contaminated Sites Division
Dipl.-Ing. Christian Weingran
64584 Biebesheim, Germany
Waldstraße 11
Tel.: +49(0) 64 28/92 35 11
Fax: +49(0) 64 28/92 35 35
E-mail: asg.weingran@t-online.de
Funding reference: 02WR0293

CDM Consult GmbH
Dipl.-Ing. Jörn Müller
64665 Alsbach, Germany
Neue Bergstraße 13
Tel.: +49(0) 62 57/50 43 15
Fax: +49(0) 62 57/50 43 60
E-mail: joern.mueller@cdm-ag.de

I.M.E.S. GmbH
Dr. Hermann Schad
88279 Amtzell, Germany
Martinstraße 1
Tel.: +49(0) 75 20/92 36 00
Fax: +49(0) 75 20/92 36 04
E-mail: hermann.schad@imes-gmbh.net

DVGW Technologiezentrum Wasser (TZW)
Dr. Andreas Tiehm
76139 Karlsruhe, Germany
Karlsruher Straße 84
Tel.: +49(0)7 21/9 67 81 37
Fax: +49(0)7 21/9 67 81 01
E-mail: andreas.tiehm@tzw.de
Ressource Wasser
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Funnel and gate – an innovative reactor concept successfully combating contamination

Remediation procedure at former tar works with construction of a funnel-and-gate system with bioreactor Groundwater polluted with tar
Low maintenance and control requirements
First implementation of this technique