Implementation of a selector for sludge settling enhancement in an activated sludge system treating petrochemical wastewater

Abstract

Due to the recent approval of the Best Available Techniques Reference Document for Wastewater Treatment in the Chemical Sector (Decision (UE) 2016/902), the European industry is urged to improve the wastewater treatments, in order to attain more stringent regulations. Particularly, petrochemical activated sludge systems focused on organic matter removal must face two frequent issues, which difficult sludge settling: i) Development of filamentous bulking due to low food-to-microorganism (F/M) ratio in the biological reactor ii) Unintended biological nitrification, which leads to denitrification in the clarifier. These tendencies were confirmed in a pilot petrochemical continuous stirred tank reactor (500 L), where sludge volumetric index (SVI) values of 350 mL g-1 were obtained. Nitrification was studied with an experimental petrochemical bench-scale bioreactor (1L), which provided concentrations up to 350 mg NOx--N L-1. To overcome the filamentous bulking, a one-compartment selector was implemented in the pilot activated sludge system. The selector could be operated as anoxic or aerobic. Both configurations succeeded to solve the excessive proliferation of filaments with average SVI values below 75 mL g-1, provided dissolved oxygen concentrations higher than 2 mg L-1 were guaranteed in the main reactor. However, the aerobic selector showed the more robust and consistent performance, aiming to its full-scale implementation. Its optimum design parameters were determined at hydraulic retention time of 30 minutes and F/M of 35 g sCOD g-1 VSS d-1. Its performance improved with the biodegradability of the wastewaters supplied in the range of 10 to 50 g BOD g-1VSS d-1. Otherwise, the selector was unable to enhance sludge settling when being provided with high concentrations of particulate matter (up from 65 g tCOD g-1 VSS d-1). Nevertheless, the inclusion of the aerobic selector doubled the nitrification rates in the activated sludge system. Since conventional methodologies to limit nitrification may not be convenient for existing installations due to process or site constraints, a new strategy has been bench-scale developed. Two petrochemical bioreactors have been supplied with cost-effective concentrations of 0.4 and 0.9 mg g-1VSS d-1 of folic acid in comparison to a control. Afterwards, the vitamin supply has been stopped in order to

assess about habituation effects. Despite both doses have succeeded to control nitrification, the addition of folic acid has also affected the operational parameters of the biological system. The supply of the lower vitamin concentration provided a reduction on nitrification rates of 93.6% and improved the sludge volumetric index compared to control (17.4 in front of 67.3 mL g-1). However, its full-scale feasibility is conditioned to the availability of spare aeration capacity, since oxygen demand increased 85.7%, probably due to an older sludge age (71.4% reduction in observed sludge yield). Reductions up to 97.1% in nitrification rates were obtained during and 60 days after the dosage of the higher vitamin concentration. Despite other advantages, such as increasing the organic matter removal efficiency (60.0%) and reducing oxygen demand (14.7%) relative to control, the high dosed reactor exhibited a worse sludge settling (93.1 mL g-1) and more sludge production (57.1% increase in observed sludge yield). These results suggest new dosing alternatives, such as supplying a discontinuous folic acid concentration of 0.9 mg g-1VSS d-1. To conclude, this manuscript provides industrial engineers with guidelines to upgrade their existing wastewater treatments. Although the experimentation has been conducted with petrochemical effluents, the methodology and conclusions could be extrapolated to other industrial sectors with similar characterization of wastewater. With regard to future steps, this research sets the basis for the development of the action mechanism for nitrogen heterocyclic compounds, as nitrification inhibitors, which is of interest for the wastewater and the agricultural field.

Con la aprobación de la nueva versión del BREF de aguas residuales, la industria química europea está obligada a mejorar sus procesos de tratamiento de agua para adaptarse a especificaciones de vertido más restrictivas. Particularmente, los sistemas de fangos activos petroquímicos focalizados en la degradación de materia orgánica deben afrontar dos limitaciones frecuentes que dificultan la decantación del lodo: fuerte tendencia al bulking filamentoso por baja razón alimento-biomasa (F/M) y nitrificación biológica no deseada. Ensayos piloto con efluentes petroquímicos han demostrado que la inclusión de un selector aerobio en el sistema de fangos activos permite superar el bulking filamentoso, obteniendo siempre valores de índice volumétrico de fangos inferiores a 75 mL g-1. Los parámetros de diseño óptimos para el selector son un tiempo de residencia de 30 minutos y una razón F/M de 35 g DQO g-1 VSS d-1. Se debe alimentar al selector los efluentes más biodegradables y con el mínimo contenido en materia orgánica particulada. Sin embargo, con la implementación del selector, la nitrificación se ha duplicado. Puesto que las técnicas convencionales de control de nitrificación no resultan adecuadas por limitaciones de proceso y de implementación, se ha desarrollado como alternativa, la dosificación de ácido fólico al reactor biológico. En ensayos de laboratorio con un reactor petroquímico, dosis continuas de 0,4 y 0,9 g g-1 VSS d-1 de vitamina han permitido limitar la nitrificación a concentraciones inferiores a 10 mg NOx—N L-1 sin aportar toxicidad al efluente. La alta concentración, incluso ha mantenido este efecto hasta 60 días después de cortar la dosificación de vitamina. Sin embargo, el ácido fólico ha modificado los parámetros operativos del sistema biológico. La concentración baja ha aumentado el consumo de oxígeno (85,7%). La concentración alta ha aumentado la eficiencia en degradación de materia orgánica (60,0%), la producción de biomasa (57,1%) y ha empeorado la decantación del lodo (93±29 mL g-1). Estos resultados aconsejan ensayar otras estrategias posológicas, tales como dosis discontinuas de 0,9 g g-1 VSS d-1. Esta investigación proporciona las bases para un desarrollo futuro del mecanismo de acción de las moléculas heterocíclicas nitrogenadas, como inhibidores de la nitrificación.