Comparaison de L'Activite Biologique de Differents Charbons Actifs en Grains Comparison of Biological Activity of Different Types of Granular Activated Carbons

Abstract

In drinking water production plants, two phenomena occur into the granular activated beds: physical adsorption of organic matter, especially of micropollutants and biodegradation of a part of the dissolved organic carbon. In order to get a better understanding of the development of biological activity a comparative test has been studied on four GAC from different origins and way of manufacturing, and with various characteristics (porosity, density, specific surface,…). For that purpose, preliminary experiments led to the choice of an organic compound, nitro-4-phenol, which presents an average biodegradability. Four minicolumns, each one filled with one GAC, plus another one filled with sand (as a non-porous reference material) were run in parallel at a flow rate of 0.5 l h−1 (i.e. a linear velocity of about 5 m h−1). In the first step of the test, GAC were saturated in batch with an aqueous solution of nitro-4-phenol at a standard temperature of 25°C. In the second step, columns of GAC were fed with a solution of nitro-4-phenol at a concentration of 13.9 mg l−1 enriched with nutrients buffered at pH 7.9; this led to the development of microbial activity. The biodegradation occured rapidly, three days after the columns have been fed. When equilibrium was reached after a running of 10–15 days, the biodegradation yield stabilized at about 50% for all kinds of GAC, with no significant differences. Oxygen appears to be a determinant factor for a total mineralization of the nitro-4-phenol. Dissolved oxygen and temperature were shown to control the rate of bioactivity. At lower concentrations of nitro-4-phenol, the oxygen demand decreased and thus allowed a larger biodegradability which could reach 100% in yield. This yield fell down to about 50% when the temperature was maintained at 5°C. These observations were quite similar for all types of GAC tested (pinewood based chemically or physically activated - bituminous coal based or coconut based) and for sand. All these experiments have been realized at a concentration in ammoniacal nitrogen of 6.5 mg l−1. A lack in this component did not lead to a reduction of the biodegradation. So far, it seems that physical characteristics of GAC (porosity, raw material,…) do not play an important role in the development of biological activity, suggesting that microorganisms are mainly fixed on the external surface of GAC.