COMPARISON OF BIOLOGICAL-ACTIVITY OF DIFF ERENT TYPES OF GRANULAR ACTIVATED CARBONS

In drinking water production plants, two phenomena occur into the gran ular activated beds : physical adsorption of organic matter, especiall y of micropollutants and biodegradation of a part of the dissolved org anic carbon. In order to get a better understanding of the development of biological activity a comparative test has been studied on four GA C from different origins and way of manufacturing, and with various ch aracteristics (porosity, density, specific surface,...). For that purp ose, preliminary experiments led to the choice of an organic compound, nitro-4-phenol, which presents an average biodegradability. Four mini columns, each one filled with one GAG, plus another one filled with sa nd (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 t he first step of the test, GAC were saturated in batch with an aqueous solution of nitro-4-phenol at a standard temperature of 25 degrees 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 b uffered at pH 7.9;this led to the development of microbial activity. T he biodegradation occured rapidly, three days after the columns have b een fed. When equilibrium was reached after a running of 10-15 days, t he biodegradation yield stabilized at about 50% for all kinds of GAG, with no significant differences. Oxygen appears to be a determinant fa ctor for a total mineralization of the nitro-4-phenol. Dissolved oxyge n and temperature were shown to control the rate of bioactivity. At lo wer 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 maintaine d at 5 degrees C. These observations were quite similar for all types of GAC tested (pinewood based chemically or physically activated - bit uminous coal based or coconut based) and for sand. AU these experiment s 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 deve lopment of biological activity, suggesting that microorganisms are mai nly fixed on the external surface of GAC.