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BIOTRAC Résumé de rapport

Project ID: 41896
Financé au titre de: FP6-MOBILITY
Pays: Netherlands

Final Activity and Management Report Summary - BIOTRAC (Bioavailability of Trace Metals in Anaerobic Granular Sludge Reactors)

Anaerobic treatment of wastewater is an example of stable, energy efficient and cheap bio-technology. It is well established within developed countries and in the last decades, it is often successfully applied in tropical and subtropical countries as well. Of the anaerobic bioreactors treating wastewater, reactors with granulated biomass, e.g. the anaerobic up-flow sludge blanket (UASB) reactor, are used most frequently. Certain types of wastewater (e.g. wastewaters from alcohol distilleries) contain low concentrations of the metals essential for growth and activity of methanogenic microorganisms. Lack of e.g. Co, Ni or Zn causes a decrease of methanogenic activity. Consequently, an undesirable evolution of microbial communities of anaerobic granular sludge (development of acidogenic organisms) takes place, leading to a complete break-down of the bio-reactor operation.

The aim of this EIF research was to elucidate the fate of essential metals in the granular sludge and to suggest the optimal strategy to dose these metals to anaerobic bioreactors that suffer from limitation of essential metals. Various forms of the essential metals (e.g. CoCl2, [CoEDTA]2- or vitamin B12 as cobalt sources) were investigated for their ability to be taken-up and utilised by methanogens. The chemical speciation of the essential metals in the methanogenic media was also studied, because it can explain the ability of essential metals to be bound by physical-chemical processes (precipitation, adsorption, etc.) in a non-bioavailable form. It was found that dosing weakly bound metals such as CoCl2 brings the danger of a poor metal distribution within the granular sludge matrix, because it is precipitated in the lower part of the reactor. In contrast, chelated metals were well distributed over the granular sludge bed, but were more rapidly washed-out from the reactor.

Another important aspect of the bioavailability of the essential metals is their transport within the methanogenic granules. Magnetic resonance imaging (MRI) presents a non-destructive, non-invasive method to study metal transport in porous matrixes (such as methanogenic granules) under in situ conditions. Transport of iron and cobalt within methanogenic granules was studied and kinetics of the process was investigated. Moreover, the occurrence of fine structures (such as channels, cracks, precipitates deposits etc.) in the granules and their influence on the metal transport was studied.

A new MRI method has been developed for transport studies of paramagnetic metals in methanogenic (granular) biofilms. The chemical form of a metal strongly influenced the transport of metals in methanogenic granules. It was shown that free metals tend to form a reactive barrier, which slows down the penetration of the given metal in the centre of the granule. Chelated metals penetrate the granular biofilm faster, but were repulsed by the negatively charged organic compounds present in the biofilm matrix.


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