Lagoas de polimento em bateladas sequenciais aplicadas ao tratamento de efluente de reator UASB: operação, desempenho e pós- tratamento.

Abstract

The present work aimed to evaluate the operation and performance in sequential batch polishing ponds treating UASB reactor effluent and the application of post-treatment by coagulation, flocculation and sedimentation. The experimental apparatus consisted of a UASB reactor, fed with municipal sewage, an intermediate tank (temporary storage) and 05 polishing ponds (PP). In Phase 01 of the research, the following were investigated: a) variations in environmental conditions (pH, temperature and dissolved oxygen) as a function of seasonality (hot and cold period) and depth (surface and bottom) in the PP, as well as the dynamics of the algal community throughout the treatment as a function of seasonality, by identification and counting of microalgae; b) the influence of the agitation condition (continuous, intermittent and without agitation) on the ponds performance; c) the dissolved oxygen balance in the liquid, including the determination of the processes rates (photosynthesis, respiration and air-liquid transfer); d) the effect of inoculation with microalgae (12.5%, 25% and 50% inoculum volume) on the nutrients removal in the PP. In Phase 02, the post-treatment of the effluent from the UASB-PP system was carried out by coagulation, flocculation and sedimentation (CFS) processes, on a bench scale. The factorial design technique was used to identify, define and optimize the main interfering variables in the processes for the experimental conditions. As a result, it was possible to observe that the PP presented daily cycles of stratification and vertical mixing of temperature and chemical components (pH and DO), whether in hot or cold periods. The dominant microalgae species throughout the treatment was Chlorella vulgaris. The agitation mode did not statistically affect the performance of the PP. Regarding the DO balance, it was observed that photosynthesis is the main source of oxygen in the pond and of the total produced in the treatment cycle, 42.7% was consumed, 42.6% remained in the effluent and 14.7% was lost to the atmosphere. Inoculation favored faster nutrient removal; among the percentages tested, the 12.5% inoculum showed the highest treatment capacity to remove ammonia (117 ± 26.8 L m-3 d -1) and orthophosphate (100 ± 30.6 m-3 d-1), being, therefore, the optimal value to be applied when starting the PP. It was also verified that the coagulant concentration (PAC) and pH were the main interfering variables in the CFS process, considering the removal of color, turbidity and total phosphorus. The optimal operational ranges obtained for the variables were: dosage ≥ 13.5 mg Al L-1 and pH between 6 and 8. Finally, the UASB-PP system presents itself as an efficient treatment alternative, in terms of organic material and nutrients, compared to conventional stabilization ponds, contributing to minimize the environmental problems related to the release of sewage into water bodies. Furthermore, the effluent produced can constitute a source of water for the agricultural and/or urban sectors in the municipalities. In the latter case, the use of post-treatment is necessary.