Eliminación de nutrientes mediante tratamientos biopelícula : corrientes actuales y necesidades de investigación

Los tratamientos biopelícula fueron unos de los primeros tratamientos biológicos que se aplicaron en las aguas residuales. Los tratamientos biopelícula presentan importantes ventajas frente a los cultivos en suspensión, sin embargo, el control de los tratamientos biopelícula es complicado y su modelización también. Las bases teóricas del comportamiento de las biopelículas empezaron a desarrollarse fundamentalmente a partir de los años 80. Dado que el proceso es complejo con ecuaciones de difícil resolución, estas conceptualizaciones han sido consideradas durante años como ejercicios matemáticos más que como herramientas de diseño y simulación. Los diseños de los reactores estaban basados en experiencias de plantas piloto o en comportamientos empíricos de determinadas plantas. Las ecuaciones de diseño eran regresiones de los datos empíricos. La aplicabilidad de las ecuaciones se reducía a las condiciones particulares de la planta de la que provenían los datos empíricos. De tal forma que existía una gran variedad y diversidad de ecuaciones empíricas para cada tipo de reactor. La investigación médica durante los años 90 centró su atención en la formación y eliminación de las biopelículas. Gracias al desarrollo de nuevas prácticas de laboratorio que permitían estudiar el interior de las biopelículas y gracias también al aumento de la capacidad de los ordenadores, la simulación del comportamiento de las biopelículas tomó un nuevo impulso en esta década. El desarrollo de un tipo de biopelículas, fangos granulares, en condiciones aerobias realizando simultaneamente procesos de eliminación de nutrientes ha sido recientemente patentado. Esta patente ha recibido numerosos premios y reconocimientos internacionales tales como la Eurpean Invention Award (2012). En 1995 se descubrió que determinadas bacterias podían realizar un nuevo proceso de eliminación de nitrógeno denominado Anammox. Este nuevo tipo de proceso de eliminación de nitrógeno tiene el potencial de ofrecer importantes mejoras en el rendimiento de eliminación y en el consumo de energía. En los últimos 10 años, se han desarrollado una serie de tratamientos denominados “innovadores” de eliminación de nutrientes. Dado que no resulta posible el establecimiento de estas bacterias Anammox en fangos activos convencionales, normalmente se recurre al uso de cultivos biopelícula. La investigación se ha centrado en el desarrollo de estos procesos innovadores en cultivos biopelícula, en particular en los fangos granulares y MBBR e IFAs, con el objeto de establecer las condiciones bajo las cuales estos procesos se pueden desarrollar de forma estable. Muchas empresas y organizaciones buscan una segunda patente. Una cuestión principal en el desarrollo de estos procesos se encuentra la correcta selección de las condiciones ambientales y de operación para que unas bacterias desplacen a otras en el interior de las biopelículas. El diseño de plantas basado en cultivos biopelícula con procesos convencionales se ha realizado normalmente mediante el uso de métodos empíricos y semi-empíricos. Sin embargo, los criterios de selección avanzados aplicados en los Tratamientos Innovadores de Eliminación de Nitrógeno unido a la complejidad de los mecanismos de transporte de sustratos y crecimiento de la biomasa en las biopelículas, hace necesario el uso de herramientas de modelización para poder conclusiones no evidentes. Biofilms were one of the first biological treatments used in the wastewater treatment. Biofilms exhibit important advantages over suspended growth activated sludge. However, controlling biofilms growth is complicated and likewise its simulation. The theoretical underpinnings of biofilms performance began to be developed during 80s. As the equations that govern the growth of biofilms are complex and its resolution is challenging, these conceptualisations have been considered for years as mathematical exercises instead of practical design and simulation tools. The design of biofilm reactors has been based on performance information of pilot plants and specific plants. Most of the times, the designing equations were simple regressions of empirical data. The applicability of these equations were confined to the particular conditions of the plant from where the data came from. Consequently, there were a wide range of design equations for each type of reactor During 90s medical research focused its efforts on how biofilm´s growth with the ultimate goal of avoiding it. Thanks to the development of new laboratory techniques that allowed the study the interior of the biofilms and thanks as well to the development of the computers, simulation of biofilms’ performance had a considerable evolution during this decade. In 1995 it was discovered that certain bacteria can carry out a new sort of nutrient removal process named Anammox. This new type of nutrient removal process potentially can enhance considerably the removal performance and the energy consumption. In the last decade, it has been developed a range of treatments based on the Anammox generally named “Innovative Nutrient Removal Treatments”. As it is not possible to cultivate Anammox bacteria in activated sludge, normally scientists and designers resort to the use of biofilms. A critical issue in the development of these innovative processes is the correct selection of environment and operation conditions so as to certain bacterial population displace to others bacteria within the biofilm. The design of biofilm technology plants is normally based on the use of empirical and semi-empirical methods. However, the advanced control strategies used in the Innovative Nutrient Removal Processes together with the complexity of the mass transfer and biomass growth in biofilms, require the use of modeling tools to be able to set non evident conclusions.

Behaviour of dolomite, olivine and alumina as primary catalysts in air-steam gasification of sewage sludge

Sewage sludge gasification assays were performed in an atmospheric fluidised bed reactor using air and air–steam mixtures as the gasifying agents. Dolomite, olivine and alumina are three well known tar removal catalysts used in biomass gasification processing. However, little information is available regarding their performance in sewage sludge gasification. The aim of the current study was to learn about the influence of these three catalysts in the product distribution and tar production during sewage sludge gasification. To this end, a set of assays was performed in which the temperature (750–850 °C), the in-bed catalyst content (0, 10 and 15 wt.%) and the steam–biomass ratio (SB) in the range of 0–1 were varied with a constant equivalence ratio (ER) of 0.3. The results were compared to the results from gasification without a catalyst. We show that dolomite has the highest activity in tar elimination, followed by alumina and olivine. In addition to improving tar removal, the presence of water vapour and the catalysts increased the content of H2 in the gases by nearly 60%.

Sewage sludge gasification. Dolomite performance under different operating conditions

Gasification is a technology that can replace traditional management alternatives used up to date to deal with this waste (landfilling, composting and incineration) and which fulfils the social, environmental and legislative requirements. The main products of sewage sludge gasification are permanent gases (useful to generate energy or to be used as raw material in chemical synthesis processes), liquids (tars) and char. One of the main problems to be solved in gasification is tar production. Tars are organic impurities which can condense at relatively high temperatures making impossible to use the produced gases for most applications. This work deals with the effect of some primary tar removal processes (performed inside the gasifier) on sewage sludge gasification products. For this purpose, analysis of the gas composition, tar production, cold gas efficiency and carbon conversion were carried out. The tests were performed with air in a laboratory scale plant consisting mainly of a bubbling bed gasifier. No catalyzed and catalyzed (10% wt of dolomite in the bed and in the feeding) tests were carried out at different temperatures (750ºC, 800ºC and 850ºC) in order to know the effect of these parameters in the gasification products. As far as tars were concerned, qualitative and quantitative tar composition was determined. In all tests the Equivalence Ratio (ER) was kept at 0.3. Temperature is one of the most influential variables in sewage sludge gasification. Higher temperatures favoured hydrogen and CO production while CO2 content decreased, which might be partially explained by the effect of the cracking, Boudouard and CO2 reforming reactions. At 850ºC, cold gas efficiency and carbon conversion reached 49% and 76%, respectively. The presence of dolomite as catalyst increased the production of H2 reaching contents of 15.5% by volume at 850 °C. Similar behaviour was found for CO whereas CO2 and CnHm (light hydrocarbons) production decreased. In the presence of dolomite, a tar reduction of up to 51% was reached in comparison with no catalyzed tests, as well as improvements on cold gas efficiency and carbon conversion. Several assays were developed in order to test catalyst performance under more rough gasification conditions. For this purpose, the throughput value (TR), defined as kg sludge “as received” fed to the gasifier per hour and per m2 of cross sectional area of the gasifier, was modified. Specifically, the TR values used were 110 (reference value), 215 and 322 kg/h·m2. When TR increased, the H2, CO and CH4 production decreased while the CO2 and the CnHm production increased. Tar production increased drastically with TR during no catalysed tests what is related to the lower residence time of the gas inside the reactor. Nevertheless, even at TR=322 kg/h·m2, tar production decreased by nearly 50% with in-bed use of dolomite in comparison with no catalyzed assays under the same operating conditions. Regarding relative tar composition, there was an increase in benzene and naphthalene content when temperature increased while the content of the rest of compounds decreased. The dolomite seemed to be effective all over the range of molecular weight studied showing tar removal efficiencies between 35-55% in most cases. High values of the TR caused a significant increase in tar production but a slight effect on tar composition.