Nitrogen transformation in two subsurface infiltration systems at pilot scale

Nitrogen removal and transformations were studied in two pilot-scale combinations of a special configuration of a subsurface wastewater infiltration system with vertical flow named symbiotic treatment^®. Both pilot-scale combinations operated in parallel and each one consists of four stages in series, one of them with a vertical distribution of stages and the other one with a horizontal distribution. The main differences between them were the separation between stages (presence (the horizontal distribution)/absence (the vertical distribution) of filtration between steps), the hydraulic load (0.113 m³/m² h and 0.082 m³/m² h for the horizontal and the vertical distribution, respectively) and the depth of the soil filters (1 m each stage in the horizontal distribution whereas the depths in the vertical distribution ranges from 20 cm to 40 cm). Results of both configurations showed elevated dissolved oxygen concentration, and high removal of organic matter and total suspended solids (with mean removal values of 96% for COD for both plants and 90% and 98% for TSS for the vertical and the horizontal distribution, respectively). High total Kjeldahl nitrogen removals were obtained in both configurations (mean removals of 70% and 90% for the vertical and the horizontal distribution, respectively). Whereas the nitrification potential was higher in the configuration with horizontal distribution which includes pumping and filtering between stages and higher depth of the soil filters, both tested configurations showed promise for nitrification of wastewater, ammonia nitrogen was efficiently transformed to nitrate.