Simulation des transferts réactifs multi-constituants au sein des lits d’infiltration percolation – évaluation des capacités d’oxygénation Simulation of multi-constituent reactive transfers within vertical flow sand filter (VFSF) - Evaluation of oxygenation capacities

Résumé

L’assainissement des eaux usées par infiltration percolation appartient à la filière de traitement des rejets polluants à cultures fixées. Dans un contexte géographique spécifique et pour une population avoisinant 500 à 1 000 équivalents habitants, elle paraît bien indiquée. Filière dite rustique, elle n’en est pas moins complexe. L’objectif de cette étude est de contribuer, à travers une simulation numérique, à la compréhension des phénomènes physiques et biochimiques qui s’établissent au sein d’un lit d’infiltration percolation. Les aspects essentiels à l’activité bactérienne que sont l’hydrodynamique du milieu poreux, le développement de la biomasse active, le transport, la consommation et les transferts d’oxygène y sont abordés. À travers des essais d’une vérification méthodique du modèle effectuée à partir des solutions analytiques, il ressort principalement que la dispersion hydrodynamique et le taux de dégradation ont des effets contraires sur le rendement d’abattement des charges polluantes. En outre, un résultat significatif obtenu est la comparaison qualitative et quantitative des apports convectifs et diffusifs en oxygène au sein des lits d’infiltration percolation qui sont à aération naturelle.

Mots clés:

• Biomasse active,
• infiltration percolation,
• milieux poreux,
• modélisation,
• non saturé,
• oxygène,
• substra,
• transfert

Abstract

Wastewater sanitation using infiltration/percolation is part of an approach that uses attached microorganisms to treat pollutant loads. It appears suitable for a specific geographical context, and for population equivalents of approximately 500-1000 people. The aim of this study was to improve, by means of a numerical simulation, the understanding of certain physical and biochemical phenomena observed within an infiltration/percolation bed. All the aspects essential to bacterial activity are examined, including: the hydrodynamics of the porous media; the development of an active biomass; transport; and oxygen transfer and consumption. The latter are of paramount importance in non-saturated porous media, where significant aeration can take place, whereas in saturated soils and aquifers containing nitrogenous and organic compounds, the oxygen in water is rapidly consumed.

The model we have formulated includes seven equations, which describe macroscopic transport, and are coupled and non-linear. The terms “wells/sources” are functions of unknown variables. The resolution of the equations, obtained after discretization of the equations using Euler’s finite difference method, was performed using Thomas’ algorithm and Fortran 95 programming. We used an innovative approach: analytical solutions developed for saturated porous media were modified to take into account a heterogeneous flow field in a non-saturated porous medium. In a systematic approach, we tested two problems that are part of a gradual verification process: one-dimensional convection-dispersion solute without a kinetic reaction; one-dimensional solute with a first-order decay.

The code we have developed insures a very good approximation of the solute transport within a non-saturated porous medium. For a given rate of flow and a given supply period, the greater the dispersion, the quicker the solute will become homogeneous. In other words, a very high dispersion will induce a very low residence time for the solute within the medium. In wastewater treatment within sand beds, the residence time or contact period between the pollutant matter transported by the effluent and the purifying biomass attached to the support is thus a parameter that is linked to the dispersion of the effluent within the medium. It also appears that the reduction in pollutant load is optimized within a biofilm with a high degradation rate, and for an effluent with a low dispersion coefficient. This result is coherent with the link between dispersion and residence time of the effluent in the system.

We also examined the impact that the hydraulic load and the substrate content in the effluent have on the oxygenation capacities of a filtering mass. A qualitative and quantitative analysis of the incoming oxygen flow was performed. Thus we show that, at the beginning of the supply period, convection is more influential than diffusion. We also present several results from the simulations of substrate reduction profiles, which were very closely linked to oxygen content profiles. Thus we observed a rapid decrease in oxygen content due to intensive bacterial activity in the upper part of sand filters, followed by an increase in oxygen towards the bottom of the filter due to the absence of substrate. The main conclusion of this part of the study was that in order to optimize the ability for oxygenation within infiltration/percolation beds, it would be preferable to connect them to the separate sewer networks, which yield more concentrated effluents than do combined sewer networks.

Keys words:

• Active biomass,
• infiltration/percolation,
• porous media,
• modelling,
• non-saturated,
• oxygen,
• substrate,
• transfer