Biocement by Denitrification, Investigation Experiments

After incubation they are mixed with substrates and flushed into a sand column to induce the biocementation process. The figure to the left shows a CT image of a treated sand column. In this figure the cracks are formed by gas that moves through the column along with the flow.

Research steps 1. Parameter investigation

Parameters that govern the reaction rate and phase distributions are studied by small scale laboratory experiments.

2. Physical modelling Effect of biocementation on the occurrence of piping or liquefaction will be tested in large scale experiments, simulating the actual phenomena.

3. Pilot test

The feasibility of the technique will be tested for full scale applications.

Current results A first series of experiments was carried out in order to test the influence of the particle size on the reaction rates and the transportation of substances. Different columns were packed with very coarse, medium and fine grained sand and were saturated with a liquid containing denitrifying bacteria and substrates (mainly calcium acetate and calcium nitrate). Gas formation resulted in water being expelled from the columns during the course of the experiment. In the first five days and the highest amount of water was expelled from the coarse sand. After five days the water flow in the coarse sand came to a stop while it continued to flow in the fine sand. The different observations can be explained by differences in permeability and capillary suction between the columns.

Introduction Denitrification is a microbial step in the nitrogen cycle where nitrate (NO3

-) is reduced to nitrogen gas (N2) and organic carbon is oxidized to carbonate (CO3

-2). Under the right conditions this reaction leads to the precipitation of calcium carbonate and some nitrogen gas and excess carbon dioxide. This process may lead to a low-cost and effective ground improvement method for preventing the occurrence of piping or liquefaction.

Bacteria from activated sludge are incubated in fine sand.

Pieces of sand-stone from the treated sand column.

• Permeability change: Gas pockets and biomass change the flow through the system

• Strength improvement: Particle bond is enhanced

Denitrification

Tranport in the porous media

Soil

C-source

N-source

Denitrifiers

Ca+2

Liquid Biomass CaCO3

Soil

C-source N-source

Denitrifiers Ca+2

Liquid

Gas

The denitrifying biogrout process

The figure shows microscopy images of the expelled liquid containing bacteria from the columns. Varying grain sizes show a difference in biomass concentration in the expelled liquid: A smaller grain size results in more adsorption of bacteria.

Very coarse sand Medium sand Fine sand

V. Pham1, L. Bergwerff1,2, L.A. van Paassen1, T.J. Heimovaara1 1. Department of Geoscience and Engineering, Delft University of Technology, The Netherlands 2. Department of Biotechnology, Delft University of Technology, The Netherlands