Functioning of Biological Filters Used in Drinking-Water Treatment: The Chabrol Model

Although biological filtration, combined with pre-ozonation to transform refractory substances into biodegradable compounds, appears to be a promising technique for reducing the level of dissolved organic matter, and avoiding bacterial regrowth problems in the distribution network without intense chlorine application, some questions remain unresolved. A model was developed to relate the macroscopic functioning of biological filters used for eliminating dissolved organic matter in drinking water treatment to the kinetics of the basic microbiological processes involved. The processes considered are (1) exoenzymatic hydrolysis of macro-molecular dissolved organic compounds; (2) growth of bacteria controlled by monomeric organic substrates; (3) adsorption/desorption of bacteria onto the solid support; and (4) bacterial mortality caused mainly by protozoan predators. The model calculates the vertical distribution of fixed bacterial biomass and the effluent concentration in biodegradable organic matter from the characteristics of influent water for given values of contact time and temperature. The model was calibrated and validated by comparison with observed data set from pilot and full-size filters run in the Neuilly-sur-Marne and Choisy-le-Roi plants (Parisian suburbs). The Chabrol model constitutes a powerful tool to: (1) define the optimal design of new filters as a function of the required water quality or of the influent temperature and biodegradable fraction; (2) define the optimal contact time in existing filters; and (3) point out areas requiring further research into the basic microbiology involved.