Evaluación a escala de laboratorio y planta piloto del potencial de los humedales artificiales para valorizar el escurrido de la digestión anaerobia.

Abstract

[ES] El nitrógeno es un elemento importante para los seres vivos, ya que compone el 78% de la atmósfera y es un componente esencial para los ácidos nucleicos y aminoácidos. Sin embargo, vertidos de aguas cargadas con altas concentraciones de diferentes formas de nitrógeno son dañinas para las masas de agua receptoras. En las aguas residuales se puede encontrar nitrógeno en las formas de nitratos, nitritos, nitrógeno amoniacal y nitrógeno orgánico. En forma de amonio, el nitrógeno es perjudicial para los organismos acuáticos, ya que reduce los niveles de oxígeno disuelto que necesitan estos organismos para vivir y puede provocar eutrofización. En las Estaciones Depuradoras de Aguas Residuales Urbanas (EDARs) normalmente se lleva a cabo la eliminación de nitrógeno mediante procesos de nitrificación y desnitrificación. El nitrógeno amoniacal que llega al tratamiento secundario de las EDAR procede, por un lado, de las aguas residuales urbanas y, por otro, de la corriente líquida generada en la centrifugación de los fangos tratados mediante digestión anaerobia. Esta corriente residual contiene concentraciones elevadas de amonio, cuya nitrificación representa un consumo de oxígeno importante y, por ende, un consumo energético considerable. Dadas estas consecuencias, se están buscando alternativas para eliminar el nitrógeno de las aguas residuales, concretamente por medio de humedales artificiales. Los humedales artificiales son áreas construidas que están temporal o permanentemente saturadas de agua, con el fin de eliminar contaminantes de las aguas residuales mediante el aprovechamiento de la interacción entre microorganismos y la atmosfera. En este estudio, se evaluará a escala de laboratorio y planta piloto, la eficiencia de humedales artificiales para el tratamiento de esta corriente procedente de la digestión anaerobia. A escala de laboratorio se ensayará con columnas de filtración para tratar la corriente residual líquida de la digestión anaerobia procedente de una EDAR de Valencia. Se probarán diferentes sustratos: fango deshidratado generado en una Estación de Tratamiento de Aguas Potables (ETAP) y una mezcla entre grava y arena. A escala de planta piloto, se tendrán dos líneas de humedales artificiales, en la que ensayarán también los mismos dos sustratos. Las plantas piloto tendrán la configuración típica de un sistema francés de humedales de tratamiento, ligeramente modificada. Concretamente, dispondrán de una primera etapa de humedal de flujo vertical que recibe las aguas más cargadas, una segunda etapa de humedal de flujo vertical y una tercera etapa de humedal de flujo horizontal. En las dos primeras etapas se pretende alcanzar un alto grado de nitrificación y en la tercera se pretende reducir los niveles de nitrógeno mediante desnitrificación. Se ensayarán distintas cargas hidráulicas superficiales y regímenes de funcionamiento. Los resultados que se obtengan serán utilizados para diseñar un humedal de tratamiento capaz de tratar todo el caudal de corriente líquida de la digestión anaerobia generado en una EDAR de Valencia.

[EN] Despite nitrogen being the most abundant element in the atmosphere, representing approximately 78% of its composition, discharges of water with high concentrations of various forms of nitrogen are harmful to receiving water bodies. One of the problematic components is ammoniacal nitrogen, which reaches the secondary treatment of Wastewater Treatment Plants (WWTPs) from both urban wastewater and the liquid stream generated during the centrifugation of sludge treated through anaerobic digestion. This residual stream contains high concentrations of ammonia, the nitrification of which involves a significant consumption of oxygen and, consequently, significant energy expenditure for WWTPs equipped with anaerobic sludge digestion. To address this issue, the present research work evaluates the suitability of using artificial wetlands, with a substrate based on dehydrated sludge from the water treatment process (DSP), to achieve: (1) nitrified effluent and (2) reduce the concentration of Total Nitrogen (TN) returned to the head of the plant, thereby reducing the oxygen demand in WWTPs due to the recirculation of the effluent from sludge treatment through anaerobic digestion (DSP). At the laboratory scale, three treatment columns were set up, two of them filled with DSP (columns 1 and 2), and the third one with gravel and sand (column 3), which serves as a "control." The columns filled with DSP served as a reference for the design of the pilot plant. Column 1 was filled exclusively with DSP, while column 2 was filled with a mixture of DSP and gravel. Using the fill-contact-drain method, the exclusive DSP column (column 1) achieved an efficiency of 61%, while the column with the DSP and gravel mixture (column 2) achieved 80% for a Hydraulic Surface Loading Rate (HSLR) of 0.114 m3/m2d. Likewise, with an HSLR of 0.076 m3/m2d, column 1 achieved an efficiency of 44%, and column 2 achieved an efficiency of 88%. A study was also conducted using pulse feeding. In this modality, water was not retained, and it simply passed through the column. Efficiencies of 90% for column 1 and 80% for column 2 were achieved, respectively, for the same HSLR of 0.076 m3/m2d. The pilot plant consisted of a three-stage wetland and two treatment lines aimed at evaluating the efficiency of the dehydrated sludge substrate in ammoniacal nitrogen removal. One line used dehydrated sludge as the substrate, while the other line used silica sand as a reference substrate, referred to as the gravel line. The first stage consisted of three tanks that alternated every two days, so that 1 tank operated for 2 days over an "N" number of cycles and rested for 4 days. They operated using the fill-contact-drain method, distributing the effluent equally to the second stage, which consisted of two tanks operating as pulse vertical flow wetlands without retaining water. The third stage was always kept flooded at a desired level to allow denitrification. The porosity of the materials affected the HSLR, with sludge being less porous than sand, resulting in a lower HSLR for the DSP line (0.1064 m3/m2d) compared to the gravel line (0.1733 m3/m2d). However, the flow rate was unified when dilutions began so that both lines operated under the same flow rate, resulting in the same HSLR (0.095 m3/m2d) when performing two cycles per day. With this HSLR, average efficiencies of 99% were achieved for ammoniacal nitrogen removal in the dilution of effluent with secondary clarifier water at 20% in the DSP line, while the gravel line achieved an efficiency of 62%. For a 50% dilution, average efficiencies were 98% and 59%, respectively. Regarding total nitrogen, the DSP line removed an average of 44% for the 20% dilution, while the gravel line achieved 22%. For the 50% dilution, average values were 20% and 11%, respectively. In conclusion, the results indicate that dehydrated sludge is a more effective substrate for ammoniacal nitrogen removal compared to conventional sand and gravel substrates.