Produção e consumo de oxigênio e sua influência na remoção de nutrientes em lagoas de polimento

Abstract

Wastewater treatment is fundamental for maintaining public health and promoting environmental health, constituting one of the pillars of basic sanitation. Among the wastewater treatment systems, the association of Upflow Anaerobic Sludge Blanket (UASB) and polishing ponds stands out. In this system, the polishing ponds receive the effluent already clarified by the reactor, increasing photosynthetic activity which results in high consumption of carbon dioxide (CO2) and high pH levels of the water, favoring the removal of ammoniacal nitrogen by desorption and allowing the precipitation of phosphorus salts. Photosynthesis, as well as the efficiency of the treatment itself, is superior when the system is operated in a sequential batch regime in relation to the continuous flow regime. In polishing ponds, the main responsible for the production of oxygen is the photosynthesis process carried out by the algae; while the main responsible for oxygen consumption is the process of oxidation of organic material by bacterias. The rate of oxygen production in LP has great importance for two reasons: the rate of production must be sufficient to supply the oxygen consumption in the pond to oxidize organic material and must also contribute to the increase of pH, enabling removal of nutrients from the wastewater. In this context, this research aims to investigate the influence of the oxygen balance in polishing ponds in a sequential batch regime, fed by effluent from UASB reactor, on nutrient removal processes. For this purpose, an experimental system was developed, it was composed by one UASB reactor with a useful volume of 2,5 m³ operating with a hydraulic retention time of 6 hours and three polishing ponds with volume of 600L with hydraulic retention time of 10 days. The efficiency of nutrients removal was evaluated based on physical-chemical analyzes of Chemical Oxygen Demand (mg / L), ammoniacal nitrogen (mg / L), orthophosphates soluble (mg / L), chlorophyll a (µg / L), total alkalinity (mgCaCO3 / L) and turbidity (NTU) in raw sewage and in the effluent of the reactor and of the ponds. Additionally, polishing ponds were continuously monitored on: the dissolved oxygen (DO), pH and temperature. The results showed that the pond system was efficient in removing nutrients, especially nitrogen, being necessary higher TDH or pH values to achieve better phosphorus removal. The pH was the parameter that directly affects the removal of nutrients, so that the analyzes indicate a strong and opposite correlation between pH and the concentration of residual nutrients. Despite favoring the removal of nutrients, it was found that high pH levels harmed the performance of algaes, so that a brief collapse of the pond was observed in some batches. The DO production by photosynthesis was enough to meet the DO demand for the consumption for the oxidation of organic matter and atmospheric escape by desorption.