Diseño y evaluación de un tanque séptico de flujo ascendente con un filtro de zeolita para el tratamiento descentralizado de bajo costo de aguas residuales municipales

Abstract

Today, the conventional septic tank is still widely used as an alternative to treat domestic wastewater in individual homes and areas without connection to the centralized wastewater collection treatment system (AR). However, the low quality of the effluent from septic tanks makes them a source of environmental pollution and a potential contributor to the eutrophication of aquatic resources. In the pursuit to preserve the quality of aquatic resources, an Upflow Septic Tank (TSFA) with three compartments was modified installing a zeolite filter in order to improve its performance in the removal of organic matter and ammonia and to produce an effluent with a better quality. To evaluate the performance, two TSFAs of the same dimensions were built and a zeolite filter was installed in the last compartment of one of the TSFAs. This configuration of the TSFA was identified as a Hybrid Upflow Septic Tank (TSFAH). To evaluate the effect of the change in the configuration, when installing the filter, the hydraulic behavior of the TSFA was analyzed using tracers tests and mathematical modeling using the axial dispersion and the tank in series (TIS) models under different TRH (12 h , 18 h, and 24 h) and configurations (TSFA only, TSFA with sludge, and TSFAH with sludge) and the treatment efficiency under five different HRTs (48 h, 36 h, 24 h, 18 h, and 12 h) using kinetic models that correlated the process efficiency with hydraulic and organic loading rate, using synthetic domestic wastewater. According to the TIS model and the RTD curve the TSFA has a non-ideal flow pattern intermediate between plug flow and completely mixed flow. According to the results, the TRH has no significant influence on the flow pattern of the system. On the contrary, the configuration of the system has a significant effect on the flow inducing behavior towards plug flow with the addition of the sludge and the zeolite filter. The Peclet number showed that advection was the predominant factor in mass transport. The axial dispersion model was adequately adjusted to the experimental data for 12 h HRT (R²=0.9262). Meanwhile, the TIS model (N=4) based on ⱷ showed a better fit for 18 h (R2=0.9911) and 24 h HRT (R²=0.9833) and was more adequate to determine the hydraulic behavior of the TSFA. The TSFA and the TSFAH configuration reached organic matter and solids removals that indicate good performance and robustness; although, the efficiency of the treatment was reduced when the reactor operated with TRHs below of 24 h. The evaluation of the kinetic models shows that the modified Stover-Kincannon model fits better to both systems given the experimental data, with an R² greater than 98% in the linear regression analysis. In addition, the COD values predicted with this model were well adjusted to the experimental data, since the mean percentage error was 7.97%. The addition of the zeolite filter increased the viability of the application of this material for the removal of ammonia from wastewater, reaching removals of up to 98% of ammonia and increasing the polishing capacity of the system to increase the soluble COD removal efficiency. However, as time passes, the zeolite filter becomes saturated and ammonium removal decreases significantly.