Evaluation of the local self-purification processes in a natural river.

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The nitrification capacity of riffles in a natural river was discussed. The bacteria density in the river bank subsurface layer was estimated together with the seepage flow direction and speed, the porosity of the sand gravel layer and the nitrogen concentrations in the river and seepage waters. On the basis of the exponential growth model for nitrogen oxidation bacteria the nitrification rate of the seepage layer on the river bank was obtained. The spatial variety of bacteria density due to the inhomogeneity of gradation structure, porosity and the input of ammonium and nitrite nitrogen was observed. Without the considerable load of substrate, the density of the ammonia oxidation bacteria per unit volume of the seepage layer was 0.5 - 1.5mg/L and that of nitrite oxidation bacteria was 0.05-0.2mg/L. The nitrification capacity in the subsurface layer of a riffle was estimated more than ten times as large as that of a pool in the same longitudinal length. The maximum density of the ammon … More ium and nitrite oxidation bacteria was 5.35mg/L and 1.46mg/L respectively in the subsurface layer adjacent to the wastewater effluent, in which approximately 20% of the inorganic nitrogen load was thoroughly oxidized.Effect of the physical and chemical conditions of the seepage flow on the bacterial nitrification were discussed in order to determine the nitrification within the hyporheric zone in a natural river. Several kinds of liquid medium were supplied into the cylindrical columns filled with the river sand gravel at different velocities, and the characteristics of the nitrifying capacity varying with both the flow velocity and the inorganic nitrogen concentration were clarified in detail. In batch incubation experiments, more bacterial growth and nitrification were observed in the unidirectional, less turbulent seepage flow condition such as usually seen in the hyporheic zone than in the well-mixed condition generated by the bubble aeration. The nitrified nitrogen within the sand gravel column in unit time was strongly affected by the flow velocity. In the rich nitrogen conditions, the high nitrification was observed which was several times as large as the maximum values estimated by the logarithmic growth model using the corresponding bacterial density. Furthermore, in low concentration conditions, where little bacterial growth was expected by the model, nearly the maximum oxidation, approximately 70% of that, was achieved under the seepage flow speed 3 meters/hour. Less