Otimização das condições operacionais do sistema de tratamento biológico da água residuária proveniente da produção de fécula de mandioca

Abstract

As a direct consequence of industrial development, the generation of organic waste exceeds the levels of the natural capacity for biological degradation. In the field of food production, the cassava starch stands out, because the wastewater coming from the processing of cassava, when not properly treated, results in a deterioration of the ecological balance. The present study aimed to optimize the main operating conditions that directly influence the efficiency of the processes of anaerobic biodigestion, nitrification and denitrification of the wastewater coming from cassava processing. For the anaerobic biodigestion process, an anaerobic reactor of 20 L, operating in a batch system was used. The effects of temperature and total volatile solids from the inoculum (TVSi) on the removal of chemical oxygen demand (COD) and total volatile solids of the reactor (TVSr) were evaluated using a Central Composite Rotational Design (CCRD), being the variables responses simultaneously evaluated by the desirability function. The effluent from the anaerobic biodigestion process was used as affluent for the nitrification of the wastewater. In this stage, an aerated reactor was used, with a useful volume of 4 L and aeration control by rotameter. The air flow and cycle time parameters were controlled, and their effects on the removal of ammoniacal nitrogen and conversion to nitrate were evaluated using a CCRD. The wastewater previously treated under anaerobic and aerobic optimized conditions was used as affluent for the denitrification process, performed under anoxic conditions in a reactor with a useful volume of 2 L, operating in a batch system. The temperature and COD:N ratio were monitored, and their effects on nitrite and nitrate removal were evaluated using a CCRD, with ethanol as the external carbon source. Simultaneous optimization of the response variables were performed using the desirability function technique. For the anaerobic treatment of wastewater, the highest COD removal (96.82%) occurred at 42 °C and 12.0% TVSi. The highest TVSr removal (69.31%) was recorded at 45 °C and 10.0% TVSi. The representative equations of the process are statistically significant at a confidence interval of the 90%. Based on the desirability function, it is observed that the optimum operating condition for the anaerobic reactor is 39.7 °C and 10.8% TVSi. The COD and TVSr removal efficiencies estimated under these operating conditions were 90.45 and 63.12%, respectively. In aerobic conditions, the highest ammoniacal nitrogen removal, of 96.62%, occurred under 24 hours conditions and 0.15 L min-1 Lreactor-1, and the lowest removal, of 24.48%, occurred under conditions of 7,08 hours and 0.15 L min-1 Lreactor-1. The highest conversion to nitrate, of 24.81%, occurred under conditions of 40.92 hours and 0.15 L min-1 Lreactor-1, and the lowest conversion, of 11.65%, occurred under the conditions 7.08 hours and 0.15 L min-1 Lreactor-1. The optimum operating values of the aerated reactor, obtained by the desirability function, are 29.25 hours and 0.22 L min-1 Lreactor-1. The efficiencies of ammoniacal nitrogen removal and conversion to nitrate estimated under these operating conditions were 94.66 and 23.49%, respectively. In anoxic conditions, the highest removals of nitrite and nitrate, 97.38 and 98.70%, occurred under the conditions of 22.50 °C and COD:N ratios of 5.00 and 8.00, respectively. The mathematical models representative of the process are statistically significant for the removal of nitrite and nitrate. The optimum values of temperature and COD:N ratio which result in higher efficiencies of nitrite and nitrate removals, simultaneously, are 24.32 ° C and 5.81, respectively. The nitrite and nitrate removal efficiencies estimated under these operating conditions were 97.46 and 98.70%, respectively. The optimization of the biological treatment of the starch wastewater, considering the interdependence between the steps, proved to be feasible and with satisfactory removal of the different constituents of interest for the process.