Capacities of infiltration-percolation processes for the treatment of a synthetic effluent
In this article, we present work highlighting the capacity of variously textured porous media to biologically process urban waste water. A comparative study was undertaken that coupled biomass development with general gas transfer mechanisms through two biological engines. Infiltration/percolation beds are biological systems that treat water using a purifying bacterial culture that develops on a mineral support. Used in domestic wastewater treatment, they are regarded as being well suited to rural areas. These infiltration/percolation beds are easy to use and attain a high quality in their output, two factors that constitute assets for small communities. Infiltration/percolation beds were simulated in the present work by columns lined with sands of varying origin and structure. They were fed automatically, at set intervals, with waste water containing 525 mg/L of dissolved organic carbon (DOC) and 54 mg/L of Kjeldahl nitrogen. A balanced development of the biomass, including the phases of growth and regression, was intrinsically related to the physical nature of the material used as a support. First, during the supply period, balanced growth of the biomass was quickly reached within the crushed sand. Secondly, within round sand, the regression of the biofilm was less significant and more regular over several weeks. In both supports, the regression was well correlated with an exponential decay. Lastly, the frequency of the supply periods, the organic loads involved, and the rest periods imposed are all factors that contribute to a lack of accumulation of living and/or inert organic matter in the columns. After obtaining a balanced development of the biomass, the abilities of the columns to reduce the concentrations of carbon, Kjeldahl nitrogen and ammonia were evaluated. Samples of effluent were taken downstream, both before the beginning of the supply period and as it came out of the column approximately thirty minutes after beginning the drainage period. The percentage of suspended matter coming out of the columns gave rather precise information on the scrubbing of the solid mass caused by various shearing speeds or by the structure of the base. Generally, it appeared that : - The reduction of the overall DOC was higher than 70%, regardless of which type of sand base was used. - In the water coming out of the column made up of round sand, the overall DOC content was without exception lower than 125 mg/L, as would be expected for an infiltration-percolation process. - The treatment of carbon in the columns based on stream sand was in general more effective than that obtained in columns with crushed sand. Concerning the treatment of nitrogen, crushed sand yielded outputs with a reduction in ammonia that was a lot higher than 80% (mean ± SD: 92 ± 4%) and was systematically higher than those obtained with stream sand (mean ± SD: 72 ± 7%). The suspended matter content was extremely low in the effluent; since no suspended matter was introduced into the effluent, the concentration coming out of the column was the direct result of the biofilm becoming detached and/or, to a lesser extent, of the transport of the biomass in the liquid phase. As could be expected, because of the roughness causing more shearing, more particles became detached when the interior solid mass was made up of crushed sand. Using core samples taken from the filter's solid mass, as well as oxygen probes inserted at various levels into the biological engines, we showed that the vertical distribution of the biofilm in the columns, as well as the oxygenation of the porous media during the rest periods, were all correlated with the structure of the solid supports (note that all sands had similar average diameters). Gaseous exchanges within the filter's solid mass were dependent on both the type of coating and the depth at which they took place. The oxygen probes were inserted at depths of 14 cm and 18 cm respectively, to determine the percentage of oxygen saturation in the liquid phase within the filter during one week while the columns were operating. Two phases were characteristic of the exchanges observed in the upper part of the filter's solid mass. First we noted an instantaneous reduction in the oxygen content of the liquid phase, linked to the arrival of a batch, followed by a reoxygenation of this portion. Next there was a long phase, lasting approximately 94% of the time between batches, during which time the oxygen content in the residual moisture remained constant. In the lower part of the solid mass, variation in oxygen content was different. Partial deoxygenation in the liquid phase of the porous media here was primarily due to the biochemical oxidation of the organic matter (i.e., consumption by the biomass). It was also due, to a lesser extent, to the augmentation in moisture content that occurs after each batch, and leads to a reduction in porosity and a decrease in oxygen transfer. During the final phase of drainage, a balance was created between the open porous space with air, and the space containing stagnant moisture. The diffusion of oxygen in the former and its transfer towards the latter thus compensated for this consumption, and it therefore remained constant. If the medium was left to dry out longer, the diffusion and transfer phenomena increased, and we observed an increase in oxygen content. With respect to the vertical distribution of biomass in the columns, we found that it was almost homogeneous inside the stream sand, up to a depth of 28 cm (total depth = 40 cm). Generally, the organic matter content at each point of measurement in the column was higher in the stream sand than in the crushed sand. The variation was greater around 28 cm, where there was a more significant quantity of moisture inside the sand.