Oxygen Transfer and Clogging in Vertical Flow Sand Filters for on-Site Wastewater Treatment.

13 million people (about 20 percent of the population) use on-site wastewater treatment in France. Buried vertical sand filters are often built for STE treatment, especially when the soil permeability is not sufficient for treatment and infiltration. Clogging is one of the main problems deteriorating the operation of vertical flow filters for wastewater treatment. The extent of clogging is not easily assessed, especially in buried vertical flow sand filters.

The authors suggest examining three possible ways of detecting early clogging: (1) NH₄-N/NO₃-N outlet concentration ratio; (2) Direct measurement of oxygen content within the porous media. This information can be obtained by gas analysis of the filter's air phase; (3) Outflow measurements.

Two pilot-scale filters were equipped with probes for oxygen concentration measurements and samples were taken at different depths for pollutant characterization. Influent and effluent grab-samples were taken three times a week and analyzed using standardized methods. The systems were operated using batch-feeding of septic tank effluent. The pilots were started during a 6 weeks period (average load: 30 g_COD.m^-2.d^-1 and 8 g_SS.m^-2.d^-1). Clogging was induced using daily pollutant load of 55 g_COD.m^-2.d^-1 and 12 g_SS.m^-2.d^-1 (average), resulting in permanent ponding between two successive batches.

Nitrate outlet concentration and oxygen concentration inside the filter appear to be better parameters for diagnosing clogging in vertical flow filters than Ammonium outlet concentration and outflow. In addition, the high spatial resolution of oxygen, NH4-N, and NO3-N concentrations allowed us to draw a qualitative description of processes occurring under unclogged and clogged conditions.