Pergamon PH: S0273-1223(98)OOO64-X Wal. Sci. Tech. Vol. 37. No.3. pp. 129-137. 1998. C> 1998 IAwQ. Published by Elsevier Science Ud Printed in Great Britain. 0273-1223/98 S19'00 + 0-00 MODELLING BENTHIC ACTIVITY IN SHALLOW EUTROPHIC RIVERS W. Rauch* and P. A. Vanrolleghem** • Depanment of Environmental Science and Engineering, Technical University of Denmark. Building JJ5. 2800 Lyngby. Denmark •• BIOMATH Depanment. University Gent, Coupure links 653. 9000 Gent. Belgium ABSTRACf In this paper a simple modelling approach is presented that allows fast computation of benthic activity in rivers. The approach extends the half-order reaction concept used in biofilm models for use in a multiple substrate/multiple bacterial species system. Moreover. it is compatible with the IAWQ Activated Sludge Model no. I format and has closed mass balances. The conversion of carbonaceous organic matter under aerobic and anoxic conditions and nitrification are represented in the model. The case study to which it was applied revealed that benthic activity is highly influenced by the eutrophic state of the river (presence/absence of algae) leading to diurnal oxygen fluctuations. More specifically the spatial distribution of species along the river was significantly different. resulting in postponed (downstream) nitrification in the eutrophied river. Also, oxygen depletion is found to be more severe and its spatial extension is larger. © 1998 IAWQ. Published by Elsevier Science Ltd KEYWORDS Benthic activity; biofilm; eutrophication; half-order kinetics; modeling; river water quality. INTRODUCfION In river beds often a stable layer of organic material is found consisting of sediments. attached bacteria. algae etc. The conversion processes within this benthic biomass can have a considerable effect on the concentration of soluble compounds in the overlying water column, such as oxygen, organic matter and nutrients. Hence, benthic activity as the exchange of material between the water column and the benthos, is an important component of the processes responsible for water quality in the river, especially in shallow waters. Sediment oxygen demand, for example, may consume a significant amount of the available oxygen in the water phase (see Boyle and Scott, 1984; Rutherford et al., 1991; Hom and Wulkow, 1996). Furthermore. sediments can act both as a source and a sink for nutrients depending on the environmental conditions (aerobic, anoxic or anaerobic). While the importance of benthic activity to the water quality is undoubted, the actual relations implemented in models to describe the phenomena are usually quite simple. Most often user specified fluxes such as the sediment oxygen demand are applied to predict the removal rate for the dissolved components in the bulk liquid above the benthos (see e.g. Bowie et al., 1985). Another simplified approach is to neglect the problem of mass transport inside the benthos. As a result. the conversions by the heterotrophic and autotrophic bacteria species inside the benthos can be calculated in the same way as being suspended, the only 129